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Why is space and time spoken of as one thing
I've never understood why it's spacetime and not space and time. In other words why aren't space and time spoken of as two different things. Can someone explain to me why this is

There are different words for different aspects of space. For example, consider: length, width, and height. Other words include depth and breadth. We can speak of them as different things if we choose to, but we generally consider them to be part of unified concept of space. Why?
It's because we understand that these words just pick out measurements along specific directions in space set by context or the speaker's viewpoint, and that there is no intrinsic difference between those directions. Depending on which way they are facing, what's length to one person is width to another.
More formally, there is a symmetry between directions in space. We can rotate freely and the intrinsic properties of space look just the same, e.g. the Euclidean metric (infinitesimal distance formula)
$$\mathrm{d}s^2 = \mathrm{d}x^2 + \mathrm{d}y^2 + \mathrm{d}z^2$$
is invariant under the orthogonal group of rotations, $\mathrm{O}(3)$. Together with spatial translations (which are like rotations about a point at infinity), this forms the Euclidean group of isometries, $\mathrm{ISO}(3)$. Because of this symmetry that "intermixes" directions of space, we think of those different directions as being unified one into one thing: space.
In special relativity, spacetime has a Minkowski metric (in units of $c=1$)
$$\mathrm{d}s^2 = -\mathrm{d}t^2 + \mathrm{d}x^2 + \mathrm{d}y^2 + \mathrm{d}z^2\text{,}$$
and it is invariant under the Lorentz group $\mathrm{O}(1,3)$ that act like rotations in spacetime; together with the translations (in space and in time), the full isometry group is the Poincaré group. In fact, the usual Lorentz transformation (say, boosting along the $x$-axis) is exactly a rotation with by a hyperbolic angle in the $tx$ plane.
That's why people began to think of spacetime as one unified whole. The reasons are essentially the same as why people think of space as one unified whole despite it having different directions: there is a symmetry that "intermixes" those directions, albeit slightly differently in spacetime because of the different sign of the temporal and spatial directions in the metric.
In general relativity, this is generalized further, but the details aren't immediately relevant here.

Why is space and time spoken of as one thing?

Because it's how Minkowski presented it in 1908. See Space and Time where you can read this: "Henceforth, space for itself, and time for itself shall completely reduce to a mere shadow, and only some sort of union of the two shall preserve independence".

I've never understood why it's spacetime and not space and time.

You're right to do so. Because spacetime is actually a mathematical abstraction that people mistake for reality. The Earth is surrounded by space, not spacetime. Light moves through space, not spacetime. See this physics stack exchange answer where relativist Ben Crowell says "Objects don't move through spacetime. Objects move through space". When you move fast through space your measurements of space and time change, but it's you that's changed, not space, and not the things in space. For example a star doesn't actually length-contract to a discoid shape just because you step on the gas.

In other words why aren't space and time spoken of as two different things.

They are. The thing to appreciate is that whilst a lot of popscience articles blithely talk about spacetime, people like Einstein were very particular about saying space when they meant space. See for example Einstein's 1920 Leyden Address where he referred to the continuum of space-time and yet described a gravitational field as space that's neither homogeneous nor isotropic. The distinction between the two tends to get lost. In a gravitational field space is inhomogeneous, and the inhomogeneity diminishes with distance in a non-linear fashion. When you plot it using say light-clocks at different elevations, you see a curve on your plot. Your plot depicts curved spacetime. That's the map. But inhomogeneous space is the territory. See Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime.

Can someone explain to me why this is?

By and large it's because people don't have a clear understanding of relativity. Space and time are not on an equal footing. I can hop forward a metre but you can't hop forward a second. There is no real rotation when you accelerate, it's just an abstract mathematical rotation, wherein your "local motion" is of necessity reduced by your macroscopic motion through space because the total rate of motion can't exceed c, because of the wave nature of matter. See my time dilation answer here on stack exchange. Since when you look at what clocks do  you appreciate that the time is a cumulative measure of local motion, you're actually better off thinking in terms of space and motion rather than space and time.

Objects move through time as well as space.

@Rob Jeffries : no, they don't. That's a figure of speech. See for example this answer. You don't literally climb to a higher temperature, and in similar vein you don't literally move through time. You can appreciate this via the gedanken stasis box. No motion of any kind occurs in the stasis box. So when I shut you inside for five years then open the door, you think I opened it immediately. You "travelled to the future" by not moving at all whilst everything else did.

I'm doing it right now. x,y,z and t coordinates are always relative to something else.

If a particle has a spatial position $\mathbf{x}(t)$, we can say that the particle moves in space, even though it also has some curve as its trajectory over its entire history (e.g., an ellipse for a Keplerian orbit). Similarly, in spacetime, we can have $x^\mu(\lambda) = (t(\lambda),x(\lambda),\ldots)$, so what's the problem in saying it moves in spacetime, even though it also has a worldline in spacetime over its entire history? (I agree with Ben Crowell regarding speed, but this more general point seems to needlessly insisting on forbidding something totally harmless and straightforward.)

@Rob Jeffries : you are not moving through time. When you measure the motion of something you can see that it gradually changes position in space, you can see that it moves through x y z space. Motion through space is evidential and empirical. But when you measure time t you refer to a clock. And a clock is a device that features something else moving through space, such as a pendulum, in a regular cyclical fashion. There is no scientific basis for the notion of moving through time. Objects don't move through spacetime. Objects move through space. And they don't move through time either.

@Stan Liou : the worldline plots the particle's motion through space "over time", which is in empirical terms with respect to some other motion through space, such as the Earth orbiting the Sun, or the regular cyclical motion of cogs and gears inside a clock. We speak of "the time", but it isn't something the particle moves through, nor is spacetime. It isn't moving along that worldline, because that's just an abstract line in a static "all of time" block universe. IMHO moving through spacetime isn't harmless, and it's important to understand this point to correctly understand relativity.

@JohnDuffield if all "moving through spacetime" means is that a worldline can be parametrized by time (which is what I content), then it's not only harmless in understanding relativity and completely consistent with any 'block universe' viewpoints, but also just plain trivially true.

You cannot measure your motion in xyz without reference to something else either.


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How do we know that there is no border at the end of "infinite" space?
It has been suggested that space and beyond is infinite, but admittedly the same hypothesis was made by virtue of the fact that it is physically impossible to prove otherwise.
How do we know that there is no border at the end of "infinite" space?

I'm presuming "it has been suggested" means that science has suggested it.  The correct way to read phrases like that are terribly exacting, and might be better phrased "We have not falsified the theory that space is well modeled as an infinite space in all directions."  The steps it takes to go from there to "space and beyond is infinite" are simplifications, and they are exactly the kind of simplifications that are causing you grief.

@PatJ: Or on Physics - the subject matter tends to overlap.  But the OP would do better to read a recent popular book on cosmology - there are probably a number available at your public library - since a good answer is going to be longer than most people would care to type.

If you are interested in the storytelling aspect rather than strict theory, try http://worldbuilding.stackexchange.com/questions/30957/what-if-we-lived-near-a-boundary-of-the-universe

Funny, I always thought that part of the definition of infinity is that it has no end. So how can you suggest that infinity can have a border?

What is beyond the known observable universe? No-one knows. We do not know what lies beyond that which we can see.

@Aabaakawad What do you mean?

We do not know what lies beyond the limits of our instruments, by definition.
For all we know, there could be a big sign saying "This area under construction."
However, that doesn't seem very probable.
The most probable answer is "more of the same".  After all, there is no reason the bit of the universe we can see should be different from other places.
If you keep adding "more of the same" to the universe, you get the infinite universe you have seen described.
It is just a theory, but it seems like a pretty probable one.
It is also easier to do calculations on. Having a border introduces all kinds of problems like galaxies near the border only getting gravitational forces from one side only.  With an infinite universe these forces come from all directions and cancel each other.
It is still just a theory, but it is one that is (relatively) easy to work with.

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Could there be a brown dwarf in our solar system?
Is it possible that a brown dwarf object could be orbiting our star and undetected so far by wise surveys?

The nemesis theory proposes that a low mass star or brown dwarf in highly elliptical orbit is a companion to our sun as a solution to the cyclical mass extinction problem (http://www.theatlantic.com/science/archive/2015/11/the-next-mass-extinction/413884/) and (http://www.space.com/22538-nemesis-star.html). Scientists noticed that some mass extinctions follow a seemingly cyclical pattern, so researchers proposed that the orbit of a small companion star (or a brown dwarf) was disturbing comets or asteroids as it approached aphelion and flinging them towards the earth.
As interesting as this would be, WISE's infrared scans of the sky have revealed nothing and there is currently no evidence supporting the theory. Furthermore the orbit of any companion would have to be very large and would therefore be very unstable and would likely be detectable. The Astrobiologist David Morrison stated of the theory: "(T)he Sun is not part of a binary star system. There has never been any evidence to suggest a companion. The idea has been disproved by several infrared sky surveys, most recently the WISE mission. If there were a brown dwarf companion, these sensitive infrared telescopes would have detected it."

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What is the first recorded reference to the Moon being a satellite of the Earth?
I saw a list recently of the moons of the various planets and the discoverer(s) and date of discovery. Earth's Moon was listed as Unknown.
I know you don't 'discover' a huge disc in the sky. But still, what is the first recorded reference to the Moon being a satellite of the Earth? That someone knew it is smaller and orbiting? Celestial Sphere does not count.

The moon's orbital period was probably the second thing that ancient farmers figured out.  The sun being the first. Babylonians had discovered the lunar saros by 1000 BC.  As far as who had the original idea that Earth was the center of the Earth/Moon system - it came during the middle ages when the moon was accepted as a sphere.
Galileo figured out moons orbit parent bodies, and Newton developed laws of motion and celestial mechanics.
It wasn't one single person...

@EastOfJupiter Long before that we had Aristotle that proposed that the moon (and 54 other things) orbited the earth.  Or, more precisely, that there are 55 crystalline spheres surrounding the earth that everything was attached to, and the moon was on #1.  I'm not sure why the OP wants to specifically exclude this.

Otherwise I would expect that for western cultures the discovery of Jovian satellites is a good approximation for this date.  But for eastern societies?  Not so sure.  Mythologies can be pretty poetic and heavily subject to interpretations and translations.  Chinese mythologies apparently have a story about the moon which mentions the sun (and it's 9 now-murdered siblings) circling the Earth, so hypothetically the Chinese of ~2170 b.c. considered the moon a satellite, by some vaguely modern definition of "satellite".

Thank you for the edits and comments. I was trying to make the question somewhat surprising and amusing, in the sense that we do not always think of obvious things... To me, a celestial sphere is not a satellite, it misses the point of seeing the moon as a round body in space like the earth (differing shadow lengths on different locations at once confirm that Earth is a ball: ancient Greeks). So, I guess the prize goes to the Chinese in 2170 b.c., for making a recognizable statement that aligns with the facts. We can now amend our store of knowledge.

@zibadawatimmy - I excluded the spheres proposed by Aristotle because the OP specifically asked that celestial spheres not be considered an appropriate explanation of the moons' motion through our skies.

I think it's an interesting question - the folks over in History of Science and Math Stackexchange do en excellent job. I think a question along the lines of "What is the first recorded evidence that the moon was understood to be, or at least though to be a satellite orbiting the earth?" might be well received. You could specify if want to know if it was thought to be gravitationally bound to the earth, or just circling the earth for unknown reasons. Of course if you don't specify, you might get both answers!  Check to see if it's already been answered.

If what you want is "seeing the moon as a round body in space like the earth (differing shadow lengths on different locations at once confirm that Earth is a ball: ancient Greeks)", then I think the answer would have to be classical Greece (e.g., 5th or 4th C BC). Lots of mythologies have both the Sun and the Moon going "around" the Earth, but with the Earth as e.g. a disk. This probably applies to the Chinese case, since Chinese science never conceived of the Earth as a "ball" (and the "2170 BC" date for that folk tale is pure fantasy).

@PeterErwin OK, well, maybe there is not a hard answer, but the question is valid as a way of considering when we learn facts and how amazingly long it takes for them to be accepted!

While his method was off for the Moon and accuracy off for the sun, Aristarchus of Samos was the first person to somewhat accurately measure and figure out that the Moon was smaller than the Earth and the Sun, quite a bit larger.    http://www.astro.cornell.edu/academics/courses/astro201/aristarchus.htm  He came after Aristotle, so Aristotle had no way of knowing about his work, which is too bad because if he had, he might have liked it and the history of science might have avoided the whole geocentric argument that lasted for nearly 2,000 years.

@userLTK Right, but then we would have gotten stuck on Aristarchus' ideas instead. It is just that we would be arguing about them while living on the moon and Mars by now. Which is my point. The Catholics could be on New Ireland, and the Protestants on NewNew Ireland, and they would be too far apart to throw bombs at each other while arguing about which star God lives on.

@nocomprende  Not to turn this away from the subject at hand, but I agree.  History is very good at repeating itself.

From before the dawn of history people naturally assumed that the sky was a solid dome above the flat earth.  The dome was assumed to rotate once a day, so the stars were assumed to be lights attached to the dome.
Anyone who assumed that the sky dome was opaque had to assume that the sun and the wandering planets were nearer than the sky dome.  I think that it was usually assumed the sky dome was opaque and colored blue, thus making the sky seem blue when it reflected the light of the sun during the day, thus making the sun closer than the sky dome.
Those who kept records of astronomical observations soon noticed that the moon occulted or passed in front of stars, planets, and the sun (solar eclipses), and thus was closer.
Everyone assumed that the sun and the moon were tiny, until traveler's reports showed that they had the same apparent diameters every place that as visited, and so were about equally far away from every place that was visited.  Thus as the size of the known world grew larger and larger, the minimum possible size and distance of the moon and the sun grew larger and larger.
The idea of a spherical earth was proposed and gradually accepted by Greek philosophers between the sixth and third centuries BC.  Thus the earth became known as Earth, a sphere instead of a flat disc, and the dome of the sky became a hollow spherical shell around it.
In Hellenistic times the diameter of the Earth was calculated with reasonable accuracy, as well as the distance to the moon and thus its diameter.  So it became known that the moon was about a quarter the diameter of the Earth and over sixty Earth radii distant.
Because of solar eclipses, it had been known since prehistoric times that the moon was closer than the sun, and keepers of astronomical records of events when the moon occulted (passed in front of) stars and planets knew that the moon was closer than planets and stars.
Anyone who assumed geocentrism, that the Earth was the enter of the universe, naturally assumed that every object revolved around the Earth, including the moon, and that the moon was thus the closest satellite of the Earth.  A few ancient Greek philosphers supported the Heliocentric theory, that the Earth and the planets revolved around the sun.  Some of them could have believed that the moon also revolved around the sun, but as far as I know all heliocentric believers also had the moon revolve around the Earth.
By the last few centuries BC there were many educated persons who believed that the Earth, the moon, and the sun were giant balls of rock (and thus somewhat similar objects), that the sun was a giant burning rock, and that the stars and planets might also be giant burning rocks far, far away from the Earth and appearing as dots as seen from Earth.  Since everybody already since prehistoric times believed that the moon revolved around the Earth, by the last few centuries BC educated people in Western civilization believed that the moon was what we now call a natural satellite of the Earth, though some of them believed and some did not that the sun and planets were also natural satellites of the Earth.

Very nicely written, and probably the best answer I am likely to get. I just keep wondering why, when all this was pretty well known so long ago, that it took so long to be accepted and why 1500 years later people were still being burned at the stake for having what amounts to common sense? But I guess that is a question for Religion.SE.

Not a criticism, but I've always felt the flat Earth assumption was overstated.   The curvature of the Earth is visible if people get the right point of view, even in antiquity and the "on the back of a turtle", seemed to me to be a statement of the recognition of Earth's shape, as much as it is a believe in giant turtles. A fable built on a real observation, which doesn't change your answer, but I wanted to put that out there.

@User11722  Common sense is a dangerous term to use in the history of science.   Today, we have the benefit of evidence that they didn't have in the 16th century.   I asked a related question here, and you might enjoy some of the answers.  It wasn't clear, based on the evidence that geocentrism was wrong.  The geocentric model worked.  It made accurate predictions.   https://hsm.stackexchange.com/questions/1979/why-didnt-aristarchus-theory-of-heliocentrism-stick

I believe the earliest known depiciton of the Moon is that on the 'Nebra Sky Disc' which is said to be 4000 years old. Also, it could be (but not confirmed) depicted on the Tal-Quadi slab which is dated over 5000 years old.

but how are these "recorded reference(s) to the Moon being a satellite of the Earth" specifically. Did they somehow convey that the Moon is a body in space orbiting the Earth?

References, please.

They are documented by the most primitive means of the time. How else would they be documented 4000-5000 years ago?

In the geocentric system, the Moon is still, in a sense, a satalite of the Earth, as is the sun, the planets and the stars as they all orbit the Earth.  Aristotle is often credited for the geocetric model, but there are writings that preceed him that describe it.  The idea that the Moon orbits the Earth may be even older than that, but I couldn't find anything on that.
I think one of the interesting things the ancient Greeks came up with, was they may have been the first society for whom the Moon was no longer a god.  Their gods were human like, not the Sun, the Moon, Nature, etc.  The Sun and Moon became objects in the sky instead of part of their mythology.   But I digress.
The first written and worked out calculation that the Moon was smaller than the Earth and orbited the Earth as a "moon", and that the Earth was smaller than the Sun and orbited the Sun came from Aristarchus of Samos, who came not long after Aristotle in the 3rd century BC.
If Aristarchus had been a few years earlier, perhaps his analysis would have swayed Aristotle, but that's just speculation on my part.  Aristotle's ideas came first and he was well respected and well read.
Ptolomy of Alexandria who'd writings follow about 3 centuries later, knew of both Aristotle's and Aristarchus' models, but he found Aristarchus' model troubling because he thought that the Earth's rotation speed would create high speed winds not seen on Earth, so he leaned towards Aristotle's model and, in addition to that, using epicycles, the Ptolemaic model was highly predictable.  It explained planetary motion and people like that.
Predictions, a model that fits what we see as well as the respect for Aristotle's teachings and the Church saying "this is so" may have all played a role in the 15 centuries that the Ptolemaic model endured.  We shouldn't judge that as stupid, because there was a logic to it and a lack of evidence to say otherwise.  If a system works and can't be shown to be false, it's likely to stick around.
There may have been the occasional scholar who read Aristarchus during the middle ages and agreed with him, and I've read that Copernicus credits Aristarchus, but that wouldn't be enough to sway the accepted theory.   Tycho Brahe, for example, who came after Copernicus but preceded and later coincided with Galileo, went back to a revised geocentric model.  See Tychonic system where the Moon, Sun and Stars orbit the Earth.
Galileo's telescope, however, provided proof to the contrary of the Ptolemaic system.  He discovered Jupiter's 4 large moons, later named the Galilean moons and he observed the phases of Venus and later, he timed the tides to the Moon's and Sun's orbit.   Galileo was the first to provide proof that the Moon was a satellite of Earth and that other planets could have moons.

I notice that, as of this date, nobody has answered your question, directly.  OP will appreciate the entire lack of clarity to be found in 'outer space'.
This, then, is the earliest reference I have found:
[X-Info] The Moon : considered as a planet, a world, and a satellite (John Murray, 1874), by James Nasmyth and James Carpenter.

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When was the nearest star discovered?
When historically did we realize that the Sun is a star, like all the others?
An answer was posted, then the question was put on hold. I would like to answer:
If indeed someone asserted that the Sun was a star 2500 years ago, that is rather humbling. That it took until about 100 years ago for it to be proven is even more humbling!
People level a lot of claims against science and scientists. But if a field of knowledge takes over 100 generations to prove a true statement, perhaps they have a right to criticize. So, I think that science needs to do a better job of proving its theories. Either that, or people need to have more common sense, not to recognize something that has been staring billions of people in the face for all time. Perhaps, we should all be a bit more humble about the state of knowledge, even in obvious areas?
Maybe if we had taken the people who proposed this fact, and the concept of atoms, seriously at the time, we would be on our way to other stars by now instead of still having only visited that big white thing nearby (forgot the name just now) and that not for the past 40 years.

About 8 minutes after it started shining... haw haw haw.

And was it actually discovered by professionals, or amateurs?

These are two different questions.

While your "who discovered the Moon" question was mildly humorous, and was serious enough to possible to keep, please do not continue with this type of questions. This question is not of any value, neither for us nor you. The joke aspect was depleted after the first time.

Nor a real question, probably intended as a joke, but does not manage to be funny.

Boy, you people are way too serious. I thought that history of Astronomy would be important. It tends to reduce hubris. So at what point should we not consider history? More than 100 years ago? 10? Maybe the Holocene never happened...

This question might be better suited to the History of Science and Maths SE: http://hsm.stackexchange.com/

@FJC Already answered there: http://hsm.stackexchange.com/questions/123/when-how-was-it-known-that-our-sun-is-the-same-thing-as-the-night-time-stars

@nocomprende: If your actual question is "When was the sun recognized as star?", then I'd argue somewhere around 1930, when stars where really understood for the first time, due to progress in nuclear physics.

@AtmosphericPrisonEscape A classic Self Inquiry question is: "What do you know for sure?" and many people find that there are very very few things that they actually know for sure. Do we understand stars even now? If we don't, do we "know" anything about the Sun, other stars, etc? What if the state of knowledge changes? I guess I wasn't going that far, I was aiming for the idea in the Answer below, which I accepted. But you have a point. A friend once said, "We can never know anything, really." Another friend replied: "Then I might as well slash my wrists right now." Over to you...

@nocomprende: Well then you've just understood a part of what's wrong with your answer: "until about 100 years ago for it to be proven..." Nothing is ever proven. Even if a judge in a murder case might take a certain amount of evidence as proof, we're not judgdes here, we're detectives.

@AtmosphericPrisonEscape Maybe it is the concept of knowledge that is suspect then. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be still able to be untrue." Or something like that.

After a quick google, This page pops up showing that Anaxagoras was the first documented person to make the link between the sun and stars being the same thing.

Hypothesised, rather than discovered but close enough.

The referenced page says "around AD 1590, Giordano Bruno suggested the same thing, and was burnt at the stake for it." So, pretty serious business, I think. Better get it right. It further states: "finally in the 19th century the distances to stars and other things about them could be measured by various people. Only then was it proved that the Sun is a star." Not so long ago, really. I would want to know this, as an Amateur Astronomer. Or was Bruno a Professional? Either way, he was the pig in the breakfast, not the chicken.


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Solar Pressure and Gravity
I recently read some articles and pages on Solar Sails, and the possibility of using lasers to propel spacecraft, and I had a relatively outlandish idea.
Would it be possible for a star to be outputting enough light that planet-sized (or smaller) objects caught in it's gravitational pull would not cross a certain threshold?
A sort of 'reverse blackhole' if you will, where the event horizon is an area that the matter(not light) can not proceed further into the star.
I would assume this to look like a star with a large ring around it, equidistant from any point of the star's center, where matter would coalesce.
Furthermore, would it be possible that an object could remain 'stationary' (ie. not in orbit and no acceleration in any direction beyond the pull of gravity) around this star without fear of gravity pulling it into the star?

Possible duplicate of Will a Ball placed close to Sun fall into it?

I'd like to point out that my question is related, not a duplicate, of that and it's related answers.
The Q&A you've suggested is too specific, as it deals with only our star, the sun. My question is less specific, asking if it is possible for a star (of any kind or size) to exhibit this (specific) behavior within the realm of reasonable expectations.

Okay, but you can actually use exactly the same formulae. Since radiation pressure is proportional to the luminosity of the star, the most luminous known star R136a1 — which has $L = 8.7\times10^8,L_\odot$ and $M = 315,M_\odot$ — exerts $8.7\times10^8$ times the pressure, so from the fifth equation in my answer, $m/r^2 \sim 7,\mathrm{g},\mathrm{cm}^{-2}$. For the Sun, the maximum size of a ball of rocky material is of the order 0.1 to 1 $\mu$m. For R136a1 the maximum size is then $\sim1,\mathrm{cm}$ — far from a planet.

While I'm lacking the formulae, the applicable surface area of a body would have to be great enough, and it's density low enough that it's gravity remains below that threshold. So a smaller body is much more likely. If you think about it, most dust from near the sun gets pushed out by the pressure.
There wouldn't be a specific point where you would get the matter buildup, it'd be more like the Roche limit where the gravity and density are the factors of the distance from the larger body.
Without running calculations I'm also unsure as to whether it would in fact be a stable arrangement but more like an L1 style Lagrange point, where it requires some energy every now and then otherwise it drifts.
Edits are welcome, I feel that I haven't used the best language available here and may have rambled.

Wouldn't other bodies at the limit pull the smaller objects into them anyway? Thus, the system would be relatively "clean"?

That depends on how much more massive they are and what the radius of their orbit is. I also forgot to mention that if the body is stationary and not orbiting the star, the pressure would have to be significantly stronger because there is less centripetal force

Both the radiation and gravity follow an inverse square law, so there is no "horizon" at which gravity would overcome light radiation, they both get weaker at the same rate.
Radiation pressure affects smaller objects disproportionately. This is one factor in the formation of comet tails: dust and gas is pushed away from the heavier nucleus, which is relatively unaffected.
Very small objects can already be pushed from our sun, I did a quick estimate and found that dust with a mass of less than $10^{-16}\,kg$ the mass of a bacteria, would be more affected by the light of the sun than the gravity.
For a planet to be affected more by the radiation than the gravity of a star would be beyond realistic (though might make a good "xkcd 'what if'". Not only would be be enough to vaporise a planet, it would be enough to unbind the star. It would be a supernova.

Do these numbers hold with a much larger or luminescent star, such as UY Scuti or R136a1? I'd 'plug and chug' but I'm unfamiliar with the second half of your equation and pretty sure any results I came across would be inaccurate at best.

The second half is very rough. It assumes a mass of 5000 kg/m3 and uses the numbers from the radiation pressure wikipeida


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How come the northern and southern lights are the same?
Not an expert, hence the question. To my knowledge the magnetic field of the earth travels south to north. Wouldn't there be a bias on whether the charged particles causing the auroras are negatively or positively charged? If each pole attracts the opposite, how come they're the same?

I don't know about the tag. Concerning your question: I'm not entirely sure if you're asking that, but you are aware that magnetic poles and electric charges do not attract/repel each other?

@ymb1 Aurora is a tag in Earth Science.  Granted, it's a bit on the border between Earth science and Astronomy, but that's probably why it's not tag-able here.

As most people are aware when they play with bar magnets, magnetic poles repel magnets of the same polarity and attract opposites. Magnets have two polarities: north and south.
Electric charges are classified as positive and negative, and also attract opposites and repel charges of the same sign.
However, magnetic poles and electric charges do not match up in the way your question suggests - it's not like North is positive and South is negative and they attract electric charges. Electric and magnetic fields are connected and interact (in fact, both are produced by moving electric charges) but the signs/polarity do not interact in the way you are imagining..
Aurorae (northern and southern lights) are produced because charged particles are streaming toward the Earth all the time in the solar wind. Charged particles (positive or negative) are blocked by magnetic fields - the easiest way to imagine this is to draw the Earth's magnetic field like a bar magnet and the charged particles cannot cross the field lines. At the poles the field is not parallel to the Earth's surface (blocking charged particles) but perpendicular to it, so the charged particles can move into the atmosphere without crossing magnetic field lines. In the atmosphere they interact with particles in the air and create the beautiful lights that we see.
So, in short, the lights are the same at the north and south poles because it is the same kind of particles interacting with out atmosphere.


text:
How did the Earth come to be in orbit around the sun?
I'm not sure if this post should be in the physics subject forum, but this seems to fit here too. I have been reading a book about chemistry and how the universe came to be with the theory of stars creating most of the elements we know of. I began wondering how the Earth came to be in orbit around the Sun. I understand that we are in a constant free fall towards the Sun, but our motion causes us to "miss" the Sun stopping us from burning up. However, I'm confused how we were put into motion in the first place. My best guess is that we were captured by the Sun's gravity, and were put into orbit, which would mean the Earth was once a meteor, right? If someone could please explain this, it'd be very helpful because I can't seem to find the answer anywhere.
Thank you
P.S. I'm just a 9th grader, so I don't have much knowledge in astrophysics.

Hi bob. It seemed to me this would indeed fit better on another site, but I decided that site was Astronomy.

One of my favorite minute-physics videos, and it touches on your question.   https://www.youtube.com/watch?v=tmNXKqeUtJM

The picture was so much cleaner 20 to 25 years ago. I'll present that nice clean picture first. Stars form from the gravitational collapse of huge clouds of interstellar gas. Those gas clouds inevitable have some net non-zero angular momentum. This forces the gas cloud to change shape from being more or less spherical to being disk-shaped. (Why? That's a different question. Ask it.)
While this protoplanetary disk continued to feed mass to the growing protostar, it also set the stage for the formation of planets. The gas cloud was mostly primordial hydrogen and helium, but it also contained heavier elements thanks to stellar fusion and supernovas in the billions of years that preceded the formation of our solar system.
Those heavier elements behave quite differently than do hydrogen and helium. They have chemistry. The planets started as microscopic clumps of mass of these heavier elements, bound together chemically. These microscopic clumps occasionally collided, eventually forming larger clumps of mass. These larger clumps in turn collided with one another, forming even larger clumps of mass. Eventually the clumps became large enough that they interacted gravitationally, making them grow even larger. This process continued, eventually forming protoplanets, and then planetary embryos, and finally planets.
Temperature in the protoplanetary disk were high near the forming protostar but dropped precipitously with increasing distance from the protostar. At some point, volatiles such as water, ammonia, methane, and carbon dioxide become as solid as rock. This is the ice line, aka the snow line or frost line. Asteroids inside of Ceres' orbit tend to be rocky. Asteroids outside of Ceres' orbit tend to be icy.
Planets that form outside the ice line can grow very quickly and then they can grow very, very large. The stuff that comprises the protoplanetary disk orbits the growing protostar at something other than the rate suggested by Kepler's laws thanks to the pressure of all that stuff in the disk. Thanks to the square-cube law, larger objects aren't as subject to that pressure. Those larger objects orbit at a Keplerian rate. Planets that form outside the ice line grow very quickly and then sweep up gas and ice because they are orbiting at a different speed than the immediate surroundings. The result is gas giants such as Jupiter and Saturn and further out, ice giants such as Uranus and Neptune. Planetary growth is a much more difficult process and much slower process inside the ice line. That's why Mercury, Venus, the Earth, and Mars are rocky and so much smaller than Jupiter, Saturn, Uranus, and Neptune.

That's the pretty picture. The not so pretty picture:

Why are Mercury and Mars so much smaller than Venus and the Earth?
Simulations suggest that the rocky planets should all be more or less the same size. That is not the case in our own solar system, let alone elsewhere.
How could Uranus and Neptune have formed?
Simulations cannot recreate Uranus and Neptune at their current distances from the Sun. The material in the protoplanetary disk should have been too sparse at those distances to form large planets.
Much, much worse, what's the deal with all the weird exoplanets scientists have found?
Scientists have found Jupiter-sized objects orbiting very close to their sun, Neptune-sized objects orbiting where the simple model would have only rocky planets forming, and planets in highly inclined (and sometimes retrograde) orbits that don't make sense.

These simulations (which have become to be very good) and the plethora of exoplanets have pushed the theory of how planets form back into the “that’s funny” stage. ("The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!” but “That’s funny...”", a quote widely attributed to Isaac Asimov.)

David Hammerman's answer is good, and I just want to augment it. This should be a comment, but I don't have the rep on this stack exchange yet, but I'm trying to save you from asking another question as he suggests. If you want to know why some things in space are flat and others are round, I suggest checking out these videos: https://www.youtube.com/watch?v=Aj6Kc1mvsdo
and https://www.youtube.com/watch?v=tmNXKqeUtJM

@ user13097 - My name has been misspelled and mispronounced from day one (my birth certificate has a corrected spelling error). I don't quite get it. What's so hard about spelling Neilsen, which is supposed to be my last name? My last name was misspelled 55 years before I was born. Your misspelling, however, is a new one.

Earth used to be many, many asteroids and meteorites. The larger asteroids would pull meteorites and smaller asteroids, crash with them, and bundle into a bigger and bigger clump, that eventually became so heavy, that rock began behaving like liquid under the pressures, forming the neat sphere Earth is now.
That all rubble was already orbiting the Sun when that happened; some of it arrived from outside the Solar System, but most was just a nebula of gases and other matter that coalesced into the current solar system.
As for "falling and missing"... that's a semi-accurate description of orbital mechanics. Taking the distances into account, "missing" the central body is actually quite easy; look at the sky at night - some of the brighter stars are actually other planets in our system. That's it - these tiny dots. The Sun is bright, but it's also tiny in the sky. There's a lot of room to "miss", and instead of crashing, to fly in an elliptic trajectory around the central body - passing it by, faster when you're close, then getting ejected "around the other side" to slow down, and fly away, only to return down the same (missing) path.
Now... how comes Earth's, and most planets' orbits are not elongated ellipses, as is most common with most randomly moving objects in gravitational field - but pretty near circles - that's a subject for a different question and a longer debate.

Planetary systems are formed from clouds of gas and dust.
The gravity of the cloud's mass holds it together; the densest part at the center of the cloud collapses until it's dense enough to begin nuclear fusion, turning into a star.
Things further out randomly collide and stick until a few of them become big enough to have significant gravity of their own. When those "planetesimals" collide with each other, on average their orbits become more circular. After billions of years of this, you wind up with relatively few bodies in nearly circular orbits: the planets of a solar system.
Capture of bodies arriving from outside the solar system does happen, but formation from a proto-stellar gas cloud is the more common way planets are formed.

The Nice theory of planetary formation:
Created in Nice, France
Also answers the not so pretty picture (except for Planet 9). In the beginning, the sun, as a protostar, sucked up mass from a nebula and became a rapidly spinning star. The matter flattened out into a disk (Why? See Why are some galaxies flat.) The Nice theory explains the Late Heavy Bombardment, after most matter had already coalesced.
The Grand Tack
Around 5 billion years ago, the Late Heavy Bombardment happened as Jupiter and Saturn locked into a strong resonace, which brought them closer to the Sun. As this happened, icy debris that was still present in the outer solar system was dragged inward to the inner solar system. This prompted a bona fide "Grand Attack" on the inner solar system.
The Grand Attack
The Grand Attack happened after Jupiter got out of the resonance, flinging it (and Saturn) out back into the outer solar system. The debris it dragged into the solar system locked into swarms of debris that would have siphoned enough energy off of any super-Earths to drag them into the sun. The remaining debris coalesced into the four planets of the inner solar system.
The rest of the theory is consistent with what everyone else already states

Why are Mercury and Mars so much smaller than Venus and the Earth?
What's the deal with all the weird exoplanets scientists have found?
If I have made any mistakes in conventions, please tell me. Us sixth-graders are very inexperienced on StackExchange.


text:
Would a viewer see Earth from the rim of Shackleton Crater?
I've been researching this but so far have not found an answer.  If you were standing on the top of the rim of Shackleton Crater at the lunar south pole, would you be able to see any of Earth? I can't find anything about the height of the rim, although nearby Malapert Mountain is apparently visible from Earth so you could see Earth from Malapert.  Shackleton, though, I'm not so sure.  Any insights appreciated. KPRedmond

Nice question, @Redmond! Welcome in this site!

Welcome to the site.  As @Hohmannfan mentioned, this is a pretty strait forward physics astronomy question, if you are looking for a world building view of the question could you elaborate a bit on scenario to help us get what you are looking for?  I am sure you can get an answer here but if you are looking for a strait-forward physics answer you may want to consider flagging the question for a mod to move it over to astronomy.

For more info on the site and how things work around here, check out the [help] and feel free to join us in [chat] once you hit 20 reputation.

According to Physical Environment Of The Lunar South Pole From Clementine Data the rim is sunlit 95% of the time. So, yes. I'd make this an answer, but this is an Astronomy.SE question.

My Stellarium places Earth very low to the South, so if you have a very clear horizon, maybe you should see it. However, it doesn't seem to be always visible.

The moon has a monthly motion called libration Of interest to you is libration in latitude. The orbit isn't exactly in the plane of the ecliptic, and the rotation axis is slightly different from each. This means that about a week after passing its ascending node, you can see up to about 6.5 degrees past the moon's South pole. Two weeks later, the south pole is invisible, but then north pole can be seen.
From the point of view of an observer on Shackelton it would mean that over the course of a month the Earth would slowly rise and then set again.


text:
Can mechanical angular momentum be transferred by gravity?
At PSE there are a few answers that explain that since the moon is responsible for tides and tides slow down spinning, L must be transferred to the moon.
This implies that if earth's L were reduced by an asteroid, this would not influence the moon's orbit.
Can you  clarify:

does the change of $\omega$ of the Earth alter the orbit of moon only when that change is attributable to the moon or in any case?
would there be tides if there were no moon?
what is the mechanism through which angular momenta can exchange magnitude? This is apparently impossible since there is no connection or, we we consider the obsolete Netwonian gravity, the connection is radial and never tangential?

Given your question and answer at physics.SE, I suspect that the main problem here is a misconception based on how $F=GM_1M_2/r^2$ is taught. Strictly speaking, this only pertains to point masses. Newton found that this also pertains to non-point masses with a spherical mass distribution (i.e., Newton's shell theorem). It does not however pertain to objects with a non-spherical mass distribution. Newton himself knew this; he just didn't know how to solve the problem of arbitrarily shaped objects. The mathematical tools to address this weren't developed until ~100 years after Newton died.

Two objects with slightly different orbital radii around the Earth. The "higher" one moves slower. Connect them with a rigid rod - what happens? A torque is exerted.

$mv^2/r = GMm/r^2$, so $v =(GM/r)^{1/2}$, but $L = mvr = m(GMr)^{1/2}$.

I merely offer this as an example of how torques can be imparted gravitationally where rigid objects are of finite size or asymmetric. i.e. where they are not point masses and cannot be approximated as such.

Perhaps this diagram helps, but if not then I'm afraid I can't help either. https://goo.gl/images/A1F5Vl

David Hammen, I replied to that in the chat

does the change of $\omega$ alter orbit of moon only when that change is attributable to the moon or in any case?

Angular momentum is a conserved quantity (more on this later). What this means is that if some object is undergoing a change in angular momentum, that angular momentum must necessarily be transferred to some other object or objects. The Earth is losing rotational angular momentum. It's rotation rate is slowing down. That means some mechanism must exist for transferring that angular momentum.
The Earth's angular momentum would change if the Earth was struck by an asteroid, but that would be a very, very small change. The loss of gases from the top of the atmosphere is also represents a negligibly small transfer of angular momentum, as does the interaction of the Earth's magnetic field with the solar wind. The Earth is far too large to be subject to the YORP effect, and The only mechanism left is gravitation. Almost all of the Earth's loss of rotational angular momentum due to a slowing rotation rate is via gravitation. A small amount is transferred to the Sun and to the Earth's orbit about the Sun. All that is left is the Moon. That vast majority of the Earth's lost angular momentum goes into making the Moon recede from the Sun.
Angular momentum is a conserved quantity. The conservation laws are very important in higher level physics (Lagrangian and Hamiltonian physics); they are assumed rather than derived. Even higher level physics derives the conservation laws from Noether's theorem. However, angular momentum can also be derived from the strong form of Newton's third law. Proving this is standard fare in every undergraduate and graduate classical mechanics text. For example, it's the subject of chapter 2 in Classical Dynamics Of Particles And Systems by Marion et al. and it's the subject of chapter 1 (starting on page 5) in Classical Mechanics by Goldstein et al. There are some nice online derivations as well, such as the week 1 lecture notes for a class on Central Forces at Oregon State University.
Another name for a force that obeys the strong form of Newton's third law is a central force. A central force is one in which the force

Acts between pairs of particles that comprise a system of particles,

Obeys the weak form of Newton's third law (equal but opposite forces),

Is directed along or against the line connecting the two particles, and

Is a function of the radial distance between the particles only.

Examples include Hookean spring forces, Coulomb forces, and Newtonian gravitation. The gravitational force between a pair of particles is given by $\vec F = G M_1 M_2 / r^2 \hat r$, where $\hat r$ is a unit vector pointing along the line that connects the two particles.

would there be tides if there were no moon?

Yes. The Sun also raises tides, but smaller in magnitude than those raised by the Moon. The heights of the tides raised by the Sun are a bit less than half those raised by the Moon.

what is the mechanism through which angular momenta can exchange magnitude? This is apparently impossible since there is no connection or, we we consider the obsolete Netwonian gravity, the connection is radial and never tangential?

There are two issues with this thesis. One is that Newtonian gravity is not an obsolete theory; it remains widely use in space exploration and in astronomy. A better way to look at it is that Newtonian gravity has a limited domain of applicability.
The other issue is the claim that Newtonian is purely radial. This is only true in the case of point masses or objects with a spherically symmetric mass distribution. It is not the case for objects with an aspherical mass distribution. For example, the Earth has a significant equatorial bulge due to its rotation. This equatorial bulge exerts a torque on satellites orbiting the Earth that causes the satellites' orbits to precess. Scientists take advantage of this. There's a special class of satellites, the sun synchronous satellites, whose orbits by design precess by 360 degrees over the course of a year.
With regard to the Moon, the rotation of the Earth makes the tides raised by effectively lead the Moon by a small amount. While this isn't Newton's tidal bulge, it has a similar effect on the Moon. Averaged out over the course of time, the waters on the near side of the Earth pile up slightly in front of the Moon while the waters on the far side of the Earth pile up slightly behind the Moon. Because those far side waters are further from the Moon, the near side waters dominate gravitationally, causing the Moon to accelerate.

Comments are not for extended discussion; this conversation has been moved to chat.


text:
The Moon drifting away from the Earth
wiki says that average distance from earth increases 3.8 cm per year, can you tell how much of that is due to the expansion of the universe?
wiki also says:

Through the action of tidal forces, angular momentum is slowly being
  transferred from Earth's rotation to the Moon's orbit. The result is
  that Earth's rate of spin is imperceptibly decreasing (at a rate of
  2.3 milliseconds/century),[20] and the lunar orbit is gradually expanding.

Can you give some details about this process, is the orbit changing shape, eccentricity? is the average orbital speed decreasing, by what value? is the spin angular velocity increasing, by what value?
If I did not misunderstand it, the current view on tides says that tidal bulges exert a torque on the moon, and L is tranferred to it, if that is correct, that L being transferred to its spin is increasing or decreasing it? Can you clarify also how a torque on the spin can produce a change of the orbit, which only implies a change (a decrease, in fact) of its orbital speed and not of the spinning speed? Can the spin (L) influence the orbital L, and how?
edit

I asked specific questions, which are not asked or answered there. In particular I am asking: if as they say the tidal/equatorial bulge increases spin angular momentum, how is this transferresin its turn to the orbital angulat momentum? Or, alternatively
can gravity directly increase the orbital momentum of a satellite?
Actually the answer, besides being incomplete, may be wrong because momentum/KE is not added , but subtracted to the moon, since its orbit gets larger. There can be no transfer, since both earth and moon are slowing down
torque is not correctly explained

Cosmic expansion has no effect on the Earth-Moon system. You will find numerous Q+As on the topic at Physics SE. http://physics.stackexchange.com/questions/268353/would-order-1au-metric-expansion-of-space-begin-if-the-solar-system-were-not-ins

@user104372 as the moon's orbit gets larger, it looses kinetic energy and gains potential energy.  That's true with all orbits.   It also loses momentum.

"Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon), and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking, usually of the smaller first, and later the larger body. The Earth–Moon system is the best studied case." - explained at https://en.wikipedia.org/wiki/Tidal_acceleration

Can you give some details about this process, is the orbit changing
  shape, eccentricity?

This isn't a simple answer.   Also, do you want to know this for a simple 2 body system or the more complex, essentially 3 body system of Sun-earth-Moon.
If the Earth and Moon were alone in the cosmos, far away from everything else, then over time, the tidal forces would circularize the Moon's orbit as it pushes the Moon away.  The reason for this is that the same force that slowly pushes the Moon away from the Earth pushes hardest when the Moon is closest, and least hard when the Moon is furthest away, so over a long period of time that circularizes it's orbit.  All 2 body systems where the moon is being pushed away tends to circularize.  This can be observed in our own solar-system.   Two body systems that are very far away from orbital disturbances, like Pluto-Charon or Neptune-Triton or even, Sun-Neptune, are highly circular orbits.
But when you bring the Sun into the picture, everything changes.  The Moon orbits the Earth and the Earth Moon system orbits the sun, and in a very real sense, the Moon also orbits the sun, but it's within the Earth's stable region of it's hill sphere, so the Moon's orbit around the Earth will remain stable for billions of years.   But while it's stable, it's measurably inconstant.  The moon's orbit wobbles a great deal, being stretched and contracted based on it's position in relation to the Earth and Sun.  3 body mechanics is quite complex, even with stable orbits.  Isaac Newton was aware of this but he wasn't able to work it out.  Mathematicians who came later were able to figure out orbital interactions with more than 2 bodies, called perturbation theory.
So, your question depends on whether you mean a 2 body system or a 3 body system.
The Earth's eccentricity, for example, changes on roughly a 100,000 year cycle, mostly due to Jupiter, but that effect is pretty slow, on a roughly 100,000 year cycle.

The Moon's apogee and perigee happen every 27.3 days (Sidereal orbit) and it's eccentricity changes measurably every orbit.  In a sense, it's not very accurate to say the Moon even orbits around the Earth in an ellipse, it's too squashed and stretched for the elliptical model to be accurate.
While the Earth takes about 100,000 orbits to go through one elliptical period, the Moon needs only 6 or 7 orbits to do that.   Note how measurably it's perigee and apogee change every orbit.   See table.
There's a fair bit of information available on the Moon's irregular orbit, but I suspect that has almost nothing to do with your primary question.   In general, in the 3 body system, as the Moon moves away from the Earth it's orbit grows more erratic, so in a sense, because of the sun, the Moon's orbit grows more eccentric, over time, where, if the sun wasn't there, it would grow more circular.

is the average orbital speed decreasing, by what value? is the spin
  angular velocity increasing, by what value?

On average, as the Moon moves away it's orbital speed decreases.  More distant orbits have less kinetic energy, but again, with a 3 body system it's more complex.
OK, I think I see what you're asking on Spin.

from Wiki "angular momentum is slowly being transferred from Earth's
  rotation to the Moon's orbit."

and

There can be no transfer, since both earth and moon are slowing down

You should look at the Earth-Moon as a closed system.  For a closed system, angular momentum remains constant, but angular momentum in this sense, is around the barycenter.   The Moon's angular momentum, in the sense that Wikipedia means isn't the Moon's spin but the Moon's orbit.  As the Moon moves away from the Earth, it's velocity decreases but it's angular momentum around the barycenter increases because angular momentum is a product of both velocity and distance from the center.
Jupiter, for example, has about 71% of the total angular momentum of the solar system.   See article, which I think is related to your question.
Finally,

wiki says that average distance from earth increases 3.8 cm per year,
  can you tell how much of that is due to the expansion of the universe?

Expansion is best understood if objects aren't orbiting around each other.  If the Moon was stationary and the Earth was stationary, and the only way to do that in our thought experiment is to give one of them a mass of zero so there's no orbit and no gravitational attraction.   We're breaking almost every rule in the book with this thought experiment, but bear with me.
If we take the measured expansion of space to be 74.3 km/s over 3 million light years.   Source.
Average lunar distance 384,400 km.  And, 9.461 trillion km in 1 light year, so about 2.8 x 10^19 km in 3 million lightyears.  Roughly 74 trillion lunar distances. which works out kind of neatly, 74.3 km/s divided by 74 trillion gives us one trillionth of a KM/s, 31.5 million seconds in a year gives us about .0000315 km/year expansion between a stationary, no gravitational attraction moon and Earth, which equals about 3.15 cm per year of expansion of space over 1 lunar distance, which is curiously not far off from 3.8 cm per year increase in distance due to tidal forces, but the similarity is purely coincidental as well as irrelevant.
(too long?)

Thanks for this great answer, can you just clarify how the orbital angular momentum increases? some say the tidal bulge (or the equatorial bulge) exert a torque, but that would act on the spin AM, right. The duplicate says it gives tangential acceleration, which is true. Could you link a site where to compare where the moon it at zenith now and where is the highest bulge? Do you know roughly (on average) how many Km the latter is ahead of the moon?


text:
What is the most asymmetric known planet?
Planets are more or less spherical, can you tell which one is the most asymmetric? Do they figure out such a property from an eccentric behaviour, or what?
I have a simple technical question: an asymmetric body gets a torque in a gravitational field, is the rotational energy/acceleration to be subtracted from the regular energy/acceleration? In other words: does an asymmetric planet get less radial acceleration than a perfect spherical one?

Saturn, oblateness: http://www.astronomynotes.com/tables/tablesb.htm I'm not sure what you're asking with the rest of your question.

If we assume identical mass, identical average density and identical angular momentum, then the symmetric planet should rotate faster than the asymmetric one.  The dense core being off-center and lighter side of the planet extending further from the gravitational center would cause more rotational inertia.   As for gravitational torque, assuming the planet rotates, the gravitational torque would work both ways, accelerating and decelerating as the planet rotates.  Tidal forces would tend to draw the heavy side of the asymmetric planet towards the object it orbits.  (all that's pretty basic).

What is the most asymmetric known planet?

That depends on what you mean by asymmetric, on what counts as planet, and on what parts of a planet are to be considered / excluded. The dominant feature of many planets is that they aren't quite spherical due to their rotation. If that counts then Saturn is the winner. Saturn's poles are 5900 km closer to Saturn's center of mass than are points on Saturn's equator.
That oblateness is however exactly the shape an object such as Saturn is expected to take on. It means that Saturn is in an equilibrium shape. Moreover, it has a very definite axis of symmetry, the rotation axis. So that's probably not what is meant in the question as being asymmetric. All of the giant planets are quite symmetric by this standard. We need to look to the terrestrial planets.
One concept that might be of aid is the center of mass - center of figure offset of each of the four terrestrial planets. Do the oceans count? Do the icecaps count? Excluding the oceans but not the ice leads to COM-COF offset of about 2100 meters. Including both leads to a COM-COF offset of about 800 meters, about the same as Mercury and Venus. Mars however has the Earth beat by far. Various estimates place the Mars COM-COF offset between 2.5 km and 3.3 km.
Another approach is to look at how lumpy the planet's gravitational field is. Mars once again is the winner in this regard.

I have a simple technical question: an asymmetric body gets a torque in a gravitational field, is the rotational energy/acceleration to be subtracted from the regular energy/acceleration? In other words: does an asymmetric planet get less radial acceleration than a perfect spherical one?

No. In a system of particles bound by central forces such as self-gravitation, Coulomb forces, and chemical bonds, the acceleration of the system's center of mass is the net sum of the external forces acting on the individual particles, divided by the total mass.

While not a planet, I'd wager the Moon has Mars beat.      The least asymmetric are likely the gas giants, because (wait for it), they're gas and far more flexible.   Much of the asymmetry is in the outer crustal layers (I would think), that's where you get the most measurable mass variation.

@userLTK The comments are now deleted, but I requested that the asker make any questions about satellites or dwarf planets into a separate post.

@userLTK -- In many regards, Mars beats the Moon. Multiple bad things apparently happened to Mars during its formation. Mars' global dichotomy (northern lowlands vs southern highlands) is more stark than is the near side / far side dichotomy of the Moon. Mars' 3+ km center of figure-center of mass offset beats the 1.9 km offset for the Moon. The impact craters on the Moon pale in comparison to those on Mars, and the mountains of the Moon are tiny compared to Mars' volcanoes, particularly Olympus Mons. Io, on the other hand, may be Mars in weirdness.

@DavidHammen Speaking of an additional acceleration of a rotator that is not perfectly spherical or has some surface features, I would mention the subtle Yarkovsky effect. Negligible for planets in the solar system, it is measurable on asteroids. It may, perhaps, play some role for close-in Earths of small size, though it is impossible to warrant this without numerics.


text:
What is the real position of the tidal bulge?
The wikipage on "tidal accelaeration" has this picture of the tidal bulge:

but says that:

The average tidal bulge is synchronized with the Moon's orbit, and
Earth rotates under this tidal bulge in just over a day. However,
Earth's rotation drags the position of the tidal bulge ahead of the
position directly under the Moon. As a consequence, there exists a
substantial amount of mass in the bulge that is offset from the line

On the other hand, this academical article
states the opposite:

as we move with respect to both tidal bulge and moon, the moon
crosses our meridian before we experience the highest tide.] How
early? Some books show misleading diagrams with the symmetry axis of
the tidal bulges making an angle of 30° or more with the moon. In
fact, the angle is only 3°, so the tides are late by about 24(3/360)60
= 12 minutes

through the centers of Earth and the Moon

Can you say what is the real position of the bulge? it is surely a hard fact not open to interpretations. Do you have access to sites that say where is now the moon and where is the tidal bulge?
According to the basic laws of conservation of momentum and energy, a mass moving to a greater radius should be slowed downm and if we add friction the lag of 12 minutes or more makes sense. If the highest point of the bulge is forward wrt to the vertical of the moon, can you explain what stronger force pushes the water in the direction of the spin?
I found this site that gives the position of sun and moon, but doesn't show tides

This animation, on the other hand, shows that there is no bulge
The red areas experience a rise over 1 m, the blue one a depression of over 1 m:

Just making sure, but you understand that they aren't saying the opposite?  The bulge moves ahead of the Moon viewed from space, but the Moon apparently moves ahead of the bulge, viewed from the surface of the Earth.   They're saying the same thing but from two different perspectives.

@userLTK, can you explain this: the bulges move to the right, also the earth is moving to the right, are the bulges moving faster than the earth?

Look at the top picture and imagine the Earth turns, following the arrow.  For practical purposes the Moon is stationary though it does orbit, but much more slowly than the Earth rotates.  The bulge rotates with the Earth.   Now imagine you are standing on the Earth and as the Earth turns (West to East) the Moon in the sky moves East to West.   The view is reversed simply by the position from which you look at it.    It's like feeling the wind blow right to left, then you turn around and you feel it blow left to right.   Both are correct but the change depends on where you view the movement.

Tides move across the Earth, so I'm not sure what you mean when you ask, is the tide moving faster than the Earth.   The Earth turns West to East, Tides move across the Earth East to West, same general direction as the Moon and stars though the shape of land and ocean floor does prevent Tides from being simply longitudinal.

@userLTK, The second comment is right: tides move Eart to West, sure, but the only tide doing that is at the Antarctica. I was asking about the tides beween Australia and Madagascar/ Brasil they go East, and that implies they are much faster than rotation, how is that possible?

That specific question might be better suited for Earth Science.  Tidal movement is complicated and driven by the shape of the Ocean floor and land as well as the apparent movement across the sky of the Moon and Sun.  I'm very far from an expert, just a hobbyist astronomer.

There is no great contradiction. The offset of the tidal bulge is about 3 degrees. It is exaggerated in diagrams for clarity. The diagram is correct, but not to scale.
This causes the tides to be slightly late. Imagine a person standing on the Earth of the diagram, with the moon directly overhead. The tidal bulge is on their left. The rotation of the Earth will take them towards the left (the moon is also orbiting but its motion is much slower), so a little later (12 min later) they will reach the maximum of the tide. The maximum is delayed by about 12 min.
Actual flows of water around the coast are driven by this tidal bulge, but are complex effects of local topography. The actual flows of water are highly non-linear, including multiple locations at which there is no tide.

"It is exaggerated in diagrams for clarity. " No kidding.  the SAME image also exaggerates the height of the tidal bulge to be about 2500 kilometers, which is very slightly more than we normally experience.

As the same wikipedia article states

An equilibrium tidal bulge does not really exist on Earth because the continents do not allow this mathematical solution to take place. Oceanic tides actually rotate around the ocean basins as vast gyres around several amphidromic points where no tide exists. The Moon pulls on each individual undulation as Earth rotates—some undulations are ahead of the Moon, others are behind it, whereas still others are on either side. The "bulges" that actually do exist for the Moon to pull on (and which pull on the Moon) are the net result of integrating the actual undulations over all the world's oceans. Earth's net (or equivalent) equilibrium tide has an amplitude of only 3.23 cm, which is totally swamped by oceanic tides that can exceed one metre.

So the cartoon that is drawn is an idealised scenario that gets messed with due to the real physical geography of the Earth and its oceans. The tides are also affected by the Sun. However, any asymmetry in the same sense as is shown in the cartoon will result in tidal torques and tidal accelerations in the sense that the Earth's rotation is slowed whilst the Earth-Moon system is tidally accelerating to larger separations and longer orbital period.
The "bulge" leads the meridian position of the Moon in your diagram because there is a lag in the response to the tidal force. This is akin to a driven oscillator where the damping leads to a lag in the response with respect to the driving force. And the Earth simply rotates by a certain amount during the lag time. The periodicity of the driving force and response is 12h25m. The actual lag and phase shift will depend on local geography but should be longer than 12 minutes. This source (which I strongly recommend you read) says that in the Southern ocean - which has the "cleanest response" to the tidal driving - the bulge rises about 2-3 hours after the Moon crosses the meridian.
I don't understand the rest of your question; the tides are raised by tidal forces due to the Moon (and Sun).

So, the article is right, and the lag is even greater. Why is it represented ahead of the moon? If the main response lags by 12-25 minutes, then the bulge(s) should slow down the moon and not accelerate it (increasing AM and the radius of the orbit). That is my question: if the bulge is ahead, what pushes it ahead?.

@user104372 It is just the rotation of the Earth. The tides raise the bulge under the Moon but the spin of the Earth carries it away (to some extent).

Can you say (roughly) how many Km is the bulge ahead of the vertical?

We have to also consider : friction, inertia, the slowing down due to the greater radius, the depressions ahead of/after the peak, and the fact that for 25' the pull is in the opposite direction, the drag when the moon goes on a continent, etc; after all that, is there enough left to accelerate the moon? Is there any quantitative study to assess the real mass and height of the water ahead of the vertical, or we just guess it must be so, since the moon is receding?

Unfortunately, the key part of that wikipedia article ("Earth's net (or equivalent) equilibrium tide has an amplitude of only 3.23 cm) should have been flagged with "citation needed." I would love to know the source of that extremely precise value. A tenth of a millimeter precision is ridiculous. Current tidal models are not anywhere close to that precise. They're not good to the centimeter level, AFIAK. Nonetheless, plus one.

@user104372 -- Re Does the equatiorial bulge exert any torque on the moon, or the reverse is true? This is a completely different question. It's best not to ask new questions in comments because comments can be deleted. Ask it as a question. BTW, the answer to your question is "yes".

@DavidHammen, you too linked the TPX07 animation above in your post, are you able to find a more recent picture maybe of a new moon? Yesterday Aug 2 for example, or data are available after a long time? Thanks for trying

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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How far can the Sun and the Moon get?
I noticed that last June the Sun was on the tropic of Cancer and moon on Madagascar:

Can you explain how that works and if they can get even farther in latitude?

The Earth's equator is inclined to the plane of the solar system (called the ecliptic) by 23 degrees. In other words, the Sun's position, as seen from the Earth, varies between 23 degrees North (at the northern hemisphere summer solstice) and 23 degrees South (at northern hemisphere winter solstice), appearing to cross the Earth's equator twice (at the equinoxes).
The tropic lines mark the maximum apparent solar move north or south.
The Moon's orbit is inclined by about 5 degrees to the ecliptic so moves over a somewhat wider range of the sky as seen from the Earth. The Moon orbits the Earth every 28 days or so, so moves over that range each month.
At full Moon the Moon is opposite the Sun in the sky, so if the Sun was close to the tropic, then the Moon would be around the opposite tropic when full, and so on.

No, it's more like 52 degrees, but that would be very rare: this may help explain some more https://en.wikipedia.org/wiki/Lunar_standstill

You mean the point on the Earth where the sun and moon are directly overhead?   Just as eclipses can happen, being on opposite sides of the Earth can happen too.   Generally speaking this is a lunar eclipse.  Directly opposite, just like solar eclipses are relatively uncommon because the moon orbits the Earth on a different plane than the Earth's orbit around the sun, so the timing has to be just right for them to line up.  (see somewhat exaggerated diagram).

For example, if you were to carefully draw the equator across the picture you posted, you'd see the Moon and Sun are not at opposite latitudes, which they would need to be to be as "far apart as possible".


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Will the 2016 Perseid meteor shower be visible in Sydney, Australia?
As the title states, will viewers in Sydney, Australia be able to see the Perseid meteor shower in August, 2016? Assume the viewer is well outside of the city and light pollution is a minimal problem.
According to Australian Geographic, the event will be visible in the northern parts of the country – specifically, "head north for the best chance to witness the event" – but it is not clear whether it will be possible to see anything in the southern parts.
If any viewing is possible, what would be the recommended dates and time of night to see the most?

The International Meteor Organization calendar for 2016, page 12, gives the Perseid radiant declination as +58$^\circ$ and says "regrettably, it cannot be properly viewed from most of the southern hemisphere."
At latitude -34$^\circ$ the radiant never rises above the horizon, so there may be fewer Perseid meteors than sporadic non-Perseids.
This does not change year to year as the AG article seems to suggest.
What does vary is interference from the Moon; this year it's full on the 18th, so the article is correct to recommend the morning or two before the predicted peak on August 12-13.
Also check your local weather forecast.
The predawn hours, e.g. 2 hours before sunrise, are generally best for meteors because the morning side is forward as the Earth orbits the Sun.


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Would a spaceship have to evade interstellar clouds?
Apparently, there are clouds of "dust" between the stars. Would a starship have to fly around those clouds, trying to find "tunnels" between clouds, or are the interstellar clouds harmless for a spaceship?
I'm mainly thinking in terms of abrasion or (micro)collisions, not radiation, but would welcome information on the latter also.

I don't think this can be answered. We don't know the speed of the ship, or the nature of its hull.

We would have to hypothesise about the nature of the futuristic technology involved, so this is off topic here

Maybe space exploration stack exchange, they might have some answers.   In a general sense, yes, I think it would be in the interest of a high speed craft to avoid them.   Even simple hydrogen becomes a problem with high enough speed travel.  At the speeds we're currently able to reach, it's not been an issue.

No, you don't have to hypothesize about future technology. The focus of the question is on the nature of the interstellar clouds. If you assume that all materials can and will be abraded, it does not matter for this question if some materials are slightly more resistant to abrasion than others, especially at the speeds necessary for interstellar travel. But you are welcome to move this question to physics.SE, were I could relate it to the question given in Aaron Franke's anwer.

@what As it is right now, you are focusing too much on aspects of engineering for this to be on topic here. Tell me why I shouldn't migrate this to Space Exploration. It may be a question about interstellar clouds, but it is of what effect it would have on a spaceship, which requires a knowledge of engineering.

Well, on Space Exploration they will probably tell me that "it may be a question about what effect interstellar clouds would have on a spaceship, but that requires knowledge of interstellar coulds, so it should go on physics.SE". Which is where I would want it, if you'd ask me.

@what I doubt that Space Exploration would be loathe to handle physics that was relevant to a spaceship engineering discussion.

How fast do you want to go? The answer is that it is no problem at low speed but disastrous at high speed.

Yes.
As has been commented, the amount of damage taken by an interstellar spaceship depends on its velocity $v$, as well as the number of gas and dust particles that it encounters on its way. This number is usually measured per area, in which case it's called column density $N$, and is equal to the total distance $d$ traveled times the particle density $n$, i.e. $N = nd$. For instance, if a spaceship travels 1 lightyear ($10^{18}\,\mathrm{cm}$) through a region with a density of $10\,\mathrm{cm}^{-3}\!$, each square centimeter of the spaceship will encounter $10^{19}$ particles.
That is, the faster you go, the farther you go, and the more dense regions you go through, the more your spaceship is damaged.
The Breakthrough Starshot project aims to reach our nearest-neighbor stellar system $\alpha$ Centauri in ~20 years, with a gram-sized satellite reaching $0.20c$ by means of a light sail. Today, there was a paper by Hoang et al. calculating the amount of damage taken by such a satellite. The total column density of gas from Earth to $\alpha$ Cen is $\sim10^{17.5\mathrm{-}18}\mathrm{cm}^{-2}$, and assuming (fairly) a dust-to-gas ratio of 1% and a carbonaceous/silicate dust grain population with a Weingartner & Draine (2001) size distribution, they calculate that this journey to $\alpha$ Cen will erode the surface of the spacecraft to a thickness of the order of 1 mm.
Most of the damage is caused by dust, not gas, but in principle gas may slowly heat up the spaceship. However, at $v=0.2c$, as long as the density is $\lesssim10\,\mathrm{cm}^{-3}$, the temperature is insufficient to cause any melting.
Molecular clouds — the dense clouds where stars are born — have densities from $10^2\,\mathrm{cm}^{-3}$ and even up to $10^6\,\mathrm{cm}^{-3}$, i.e. many orders of magnitude higher than the roughly $1\,\mathrm{cm}^{-3}$ found in the more dilute interstellar medium. To reach even more distant stars in a tolerable time, you would have to go faster than $0.2c$, and thus it seems that it is in fact a good idea to evade these clouds.

Thank you for that. Beautiful. So a spaceship would have to either (a) go slow, (b) evade denser areas, or (c) replace the forward parts of its hulls regularly. Depending on the travel destination, the travel strategy could encompass all three to differing degrees.

@what: Yes. I haven't done any calculations, but my guess is that speeds much higher than those 0.2c would be too destructive. Your idea with replacing parts is probably good wrt. the body of the ship, but the sail may be more difficult.

@what There's a potentially easier solution, which might be dubbed "deflector shields".  Most interstellar particles are ionized, and so can be deflected by a magnetic field.  I believe some experiments have been done on the idea and suggest a fairly weak (and likely reasonable to achieve in practice) field, generated by releasing ionized particles of its own, is sufficient to deflect most particles away.

@zibadawatimmy Wouldn't that have a breaking effect? Does that deflect fast enought at 0.2c?

Hmm… I don't think most dust is ionized. Gas is in some region (the "hot, ionized medium") which fills most of interstellar space, but which also is more dilute than the "warm, neutral medium". And in molecular clouds, which are only of the order 100 K, all matter is definitely neutral. If a region is hot enough to ionize dust, it is probably also hot enough to destroy it.

@pela Yes, molecular clouds are mostly neutral diatomic hydrogen, so you'd still be in a lot of trouble with those.  I was thinking mostly in an ionized medium, or otherwise mitigating the damage from the ionized particles in a stellar wind.  I believe these experiments I'm recalling concerned things like missions to Mars.

@what Thanks a lot for your generous donation :)

Related: https://physics.stackexchange.com/questions/26326/how-dense-are-nebulae
Let's compare nebulae to the air density where the ISS orbits, at 400 000 meters. According to Wikipedia, air pressure at a given altitude is given by the equation
$$p = p_0 \left(1 - \frac{Lh}{T_0} \right)^\frac{gM}{RL}$$
or $101.325\left(1 - \frac{0.0065×400000}{288.15}\right)^{(9.80665×0.0289644)/(8.31447×0.0065)}$.
Google Calculator doesn't like this ^ but putting it in piece-by-piece gives -5737666.10745. Then, let's find the density with the equation
$$ρ = \frac{pM}{RT}$$
or $\frac{-5737666.10745×0.0289644}{8.31447×2473.15}$ which is -8.08192432875. Unfortunately Wikipedia doesn't tell me what unit this number is in (just that it's a "molar form" and that it's density) so unfortunately I'm completely stuck here and I can't finish answering the question. Hopefully this partial answer helped someone make a full answer.

voted up for effort.  :-)

This is an (extended) comment, not an answer I'm afraid.

@RobJeffries How so? Of course it does! It definitely makes a large difference whether your ship is heading into an area with almost nothing versus an area of relatively high atmospheric density.

Perhaps my comment is a bit vague. I mean, where have you estimated or quoted the critical parameters - the ISM density and the size distribution of dust particles? What has the ISS travelling through Earth's upper atmosphere got to do with anything?


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Are there bodies in the Universe that are not spinning?
Do we know of a celestial body which doesn't spin at all on its own axis? If not, why is it so?

Non-spinning black holes are a theoretical ideal that is unlikely to be found in nature.http://astronomy.stackexchange.com/questions/11626/how-is-a-non-rotating-black-hole-created

The Universe itself doesn't spin. Except sunday mornings. :-)

This is at least a little interesting, as an inertial reference frame is actually an absolute thing, in contrast to most other types of reference frames.

Every object in the universe--from a basic star to an exotic black hole--spins, and the origin of that spin can be traced back to the very beginning of time. Within instants after the Big Bang, the primordial fireball of energy expanded at an incredible speed, then later cooled and solidified into all the matter in the universe. Had this fireball been uniform in all directions, everything we see today would be completely homogeneous.
http://www.scientificamerican.com/article/what-causes-objects-such/

Thanks, does this explanation apply also to elementary particles, or there are other reasons?

All elementary particles have intrinsic spin associated with them. If we look for a mechanism that links the cosmic angular momentum with the quantum momenta of the constituent elementary particles that make up the object, we will run into trouble, see: http://sciexplorer.blogspot.nl/2014/11/why-do-particles-have-spin_18.html

Thanks, you know that bugs me! I don't get why they deny particles spin, since that seems a fundamental feature of ohysics. Just the fact that the value of L is a little larger than the expected, doesn't mean that they sit still. The fact that L is 500 times bigger could have dozens of explanations, and that would have intrinsic origin, don't you agree?

@user104372 Particles don't actually spin, just as they don't have position or size. However, they do have angular momentum, which is what we call their "spin".

Downvoted, the idea of the "primordial fireball" is very misleading, and it does not actually explain by which mechanisms this supposed fireball would set "everything" in motion.

I don't understand this answer. What does the initial expansion of the universe have to do with the angular momentum of objects that formed after that point? And what is meant by "had this fireball been uniform in all directions, everything we see today would be completely homogeneous." The "fireball" was uniform in all directions and everything is homogeneous (down to quantum oscillations). Just look at the CMB. In any case, none of that has any relation to spinning stars and black holes.

Not to mention, this is a pretty flawed explanation of the Big Bang.

Many red and brown dwarfs must have planets that are in full tidal lock with their host star, that means that the same side of the planet is always facing the star so there is always day while on the other side there is always night. Our Moon and many other moons are also in such a tidal lock with their planet but not with the Sun so that the often-used term "Dark side of the Moon" is verbally incorrect since on the far side there is a day too (when on Earth it is a New Moon).
If you mean non-spinning in the sense that on the same side of the planet it is day for half a year and night the other half I'm quite sure such planets exist too. On SpaceEngine I've seen similar planets. Nothing speaks against the existence of such planets I believe. There also might exist moons whose same side faces their planet for half a year and the other half faces away from it.

Are you implying that tidally locked bodies do not spin? Because they do - In the case of the moon exactly one time for every full orbit around earth.

@Pathfinder I described both options: tidally locked bodies that technically do spin (as you describe) and bodies that don't spin at all.

I see, my misunderstanding. But regarding your second example you could argue that even these planets / bodies are spinning around the center of mass they share with whatever they are orbiting. Not sure though about the exact definition of "spin", not a physicist. Nice try though ;)


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The expansion of the universe and how stars and galaxies still collide
How can it be that all galaxies and stars are moving away from each other yet they still collide?  From what I have researched it as an open universe (or many open universes) yet the idea of stars and galaxies colliding boggles me.

http://www.scientificamerican.com/article/how-can-galaxies-collide/

That being said, in a nutshell, the universe is expanding, but its expansion compared to the proximity between most stars is miniscule.  On a Galactic scale, most galaxies are moving away from the Milky Way because their distance from us is very significant, with one major exception being Andromeda, which is very close, relatively speaking.  For Galaxies that are close to one another, collision is possible only because they are so close and their proximity outweighs the Expansion.

The expansion is overridden on a small scale by gravity. Our galaxy is not expanding, and the stars are bound together.  In fact, a whole group of galaxies don't notice the overall expansion.

Clusters and supercluster are only compressed in comoving coordinates. In physical coordinates they expand.

What is “physical” coordinates?  Which is used by somone in that cluster making a map?

Physical coordinates are the distances you would measure at some point in time, if you could freeze the expansion of the Universe and lay out measuring rods. That is, it is what people normally think of as a distance, but is imposissible to measure in practise, since the Universe keeps expanding while you try to measure. In contrast, comoving coordinates expand with the Universe, so that the comoving distance between two galaxies (that are not too close) is constant (modulus a small change over time from peculiar velocities). The "normalization" is such that today,  [cont'd below]

…the two types of coordinates coincide, but in the past, at a time corresponding to a redshift $z$, the relation was $d_\mathrm{phys} = d_\mathrm{com} / (1+z)$. If you view a cosmological simulation in comoving coordinates (which you almost always do), you don't see the expansion, but only see the dynamics, and then you are right that the voids expand and compress the clusters. But if you view it in physical coordinates (which measure the "true" distances), the clusters also expand. Only on such small scales at groups is gravity strong enough to prevent them from receding from each other.

The article, which I think was in SciAm, is about larger scale structures than were known previously.

In that case it is even more true that the structure expands with the rest of the Universe. Your answer, up to an including the first three sentences, is correct and the only one here that answers the question. But the last sentence is incorrect as it stands. You could make it correct by explaining the difference between physical and comoving coordinates, or you could simply remove it, possibly stating that only on scales larger than groups is the Universe expanding. Either will earn you a +1 from me :)

@pela I think you were better off saying "proper coordinates". Maybe it's just me, but I've never heard someone say "physical coordinates".

@SirCumference: Yeah, maybe you're right. But it is actually a quite common term, and I've always liked it better than "proper coordinates", since I think it's more intuitive what it means. To me, at least…

The galaxies and stars are not all moving away from each other. The space between them is expanding, however this does not affect their trajectories. For instance the Andromeda Galaxy is set to cross paths with the Milky Way, after which we will join together as the unimaginatively named "Milkdromeda". As space expands it grows faster and faster, this will eventually lead to an "island effect", where the space between galaxies will expand faster than light, at which point everything outside our galaxy will go dark forever.

Simple, and direct.  Objects moving away from each other can still cross paths.
"Expansion is a general direction, not a specific flight path.


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Can a Y dwarf star undergo a supernova?
Y dwarf stars have sub-zero temperatures, so I became curious how a star like these dies out and if is could go supernova.

Stars of spectral class Y are dim brown dwarfs, with temperatures of about 300-400 K (For comparison, Earth's average temperature is around 288 K, give or take, the surface of the Sun is 5,800 K, and the core of the Sun is 15 million K). Their temperature range means that they don't fuse hydrogen in the same way that "true" stars do. They may fuse deuterium ("heavy hydrogen"); Y dwarfs may not fuse lithium, as other brown dwarfs do, because they are relatively low-mass.
The fact that Y dwarfs (and brown dwarfs, as a whole) don't undergo normal stellar nuclear fusion means that they'll never undergo core-collapse supernovae, which typically happen when an extremely massive star gets to the point of iron fusion and then can no longer generate energy from fusion. Brown dwarfs work completely different; this is not at all a possibility.
Instead, they will slowly radiate away their heat until they become extremely dim, via the Kelvin-Helmholtz mechanism. For more information, see the basic example evolutionary tracks on this page; if you're really feeling ambitious, you can try going through these notes to get a better feel for brown dwarf evolution.