Have you ever seen the Milky Way? If you stare at the sky on a clear moonless night, far away from city lights, you will notice a faint milky fuzziness that looks like clouds. This is the Milky Way, our galaxy. Galaxies are gigantic stellar islands that contain millions of stars. However, they are so far away that we cannot see most of them, even with a telescope. Astronomers must use extremely powerful telescopes. Galaxies are classified by shape. There are four primary classifications of galaxies: elliptical, spiral, lenticular, and irregular. Elliptical galaxies have an oval shape and do not contain much dust or many gas clouds. Spiral galaxies appear almost as flat disks, with multiple spiral arms and plenty of interstellar matter. The lenticular galaxies are something in between, having a globular central part and a disk without spirals. The stars in irregular galaxies seem to be scattered about randomly, rather than being arranged in an organized manner. While the smaller galaxies contain only several hundred stars, the largest could reach up to several billion stars.
The Sun is really a regular star among the 400 billion stars in the Milky Way. If you could see the Milky Way sideways it would appear saucer shaped, like two plates put together. The flat part called a disk is only 10 light years wide, however the diameter is 100,000 light years. The bright globular central part of the disk is called a bulge. The bulge is heavily populated with old stars. The spiral arms, starting from the bulge gently curve around the dark areas of dust and gas clouds. The dust clouds cover the view to the galactic center, located in the constellation Sagittarius. However, scientists can study it using radio waves and infrared light which pass through dust. Scientific observation revealed that the Milky Way has a highly populated place with a strong magnetic field which might be evidence of a black hole.
The Milky Way is only one of the 30 galaxies forming the Local Group of galaxies that measures about 5 million light years across. The Milky Way has three galaxies satellites: Sagittarius Dwarf Elliptical Galaxy, Large Magellanic Cloud and Small Magellanic Cloud. The closest one is the Sagittarius Dwarf Elliptical Galaxy, discovered in 1994 and located only 80,000 light years away. The Large Magellanic Cloud, an irregular galaxy, is also our neighbor and visible from the southern sky. While the Large Magellanic Cloud orbits our Galaxy at distance about 179,000 light years, the Small Magellanic Cloud is a little bit further, about 210,000 light years. Andromeda is the biggest galaxy in the local Group.
Galaxies don't stand alone. They form clusters and super clusters of galaxies. While clusters are comparable with our Local Group, super clusters could contain thousands of galaxies. For example, the Virgo cluster contains more than 1000 galaxies. It is in the center of the local Super cluster, which is 50 million light years away. Astronomers have created a 3-dimensional model including thousands known galaxies. Surprisingly, this model shows that the clusters and super cluster build up to even more complex structures. It is like going from single atoms, to molecules, to cells, to organs, to organisms to communities, and finally to groups of communities.
Quasars, or quasi-stellar radio sources, are exotic objects that really puzzled astronomers when first discovered in the early 1960's. They look exactly like stars; however their spectra show very high red shifts, indicating that they are a great distance away. Astronomers now think that the quasars are the central parts of very distant, young galaxies. It is difficult to explain the tremendous amount of energy that the quasars emit into space. Since they are such a strong radio source, some astronomers believe that at their centers they have massive black holes, exceeding billions of times the mass of the Sun. The surrounding dust, stars, and debris are sucked in by the gravity and create an accretion disk, similar to those around black holes formed by massive dying stars. As the electrons spin toward black hole they are accelerated to the speed of the light and start to emit strong radio waves. The fact that most of the quasars are found in colliding galaxies, infers the idea that additional gas, dust and star debris falling into the central black hole help drive the tremendous luminosity or brightness.
Astronomers have detected an x-ray source in the center of the Milky Way. This supports the hypothesis of the existence of a massive black hole in the center of our Galaxy. According their estimation the black hole has a mass about million times the mass of our Sun.
Some of the biggest galaxies could eventually swallow their smaller satellites. Astronomers call them galaxy cannibals. For example, the Andromeda galaxy is coming toward the Milky Way with a speed of 300 km/s. After 5 billion years the two galaxies will pass though each other and probably will merge into one large galaxy.
The Great Attractor is the biggest known structure in our Universe. This large concentration of mass (about 10^35 solar masses) is located in the constellation Centaurus. As a result, The Milky way is flying in this direction with an amazing speed of 600 km/s.
A location in space with very high mass and density; gravity prevents anything, including light from escaping such a region.
One of several satellite galaxies of the Milky Way. Some scientists believe that the Sagittarius Dwarf Elliptical Galaxy will eventually be absorbed by the Milky Way.
It is about 82,000 light years away.
Viewable from the southern hemisphere, the LMC is an irregular-shaped galaxy that is one of several satellite galaxies of the Solar System. It is 157,000 light years away.
A dwarf galaxy orbiting the Milky Way and containg several hundred million stars. It is located 210,000 light years away.
A spiral galaxy located about 2.5 million light years away, in the Andromeda Constellation.
A cluster of almost 2,000 galaxies, located in the constellation Virgo.
A structure formed when material in space is pulled by the gravity of a massive object, such as a star or a black hole, slowly compressing it into a spiral shaped disk as it's pulled in.
A type of electromagnetic radiation with long wavelengths that put it outside of the range of the visible spectrum.
The amount of light put out by a star.