disks. Microlensing was used in 2018 to detect Icarus, then the most distant star ever observed. How it works Microlensing is based on the gravitational lens effect. A massive object (the lens) will bend the light of a bright background object (the source). This can generate multiple distorted, magnified, and brightened images of the background source. Microlensing is caused by the same physical effect as strong gravitational lensing and weak gravitational lensing but it is studied by very different observational techniques. In strong and weak lensing, the mass of the lens is large enough (mass of a galaxy or galaxy cluster) that the displacement of light by the lens can be resolved with a high resolution telescope such as the Hubble Space Telescope. With microlensing, the lens mass is too low (mass of a planet or a star) for the displacement of light to be observed easily, but the apparent brightening of the source may still be detected. In such a situation, the lens will pass by the source in a reasonable amount of time, seconds to years instead of millions of years. As the alignment changes, the source's apparent brightness changes, and this can be monitored to detect and study the event. Thus, unlike with strong and weak gravitational lenses, microlensing is a transient astronomical event from a human timescale perspective, thus a subject of time-domain astronomy. Unlike with strong and weak lensing, no single observation can establish that microlensing is occurring. Instead, the rise and fall of the source brightness must be monitored over time using photometry. This function of brightness versus time is known as a light curve. A typical microlensing light curve is shown below: A typical microlensing event like this one has a very simple shape, and only one physical parameter can be extracted: the time scale, which is related to the lens mass, distance, and velocity. There are several effects, however, that contribute to the shape of more atypical lensing events: Lens mass distribution. If the lens mass is not concentrated in a single point, the light curve can be dramatically different, particularly with caustic-crossing events, which may exhibit strong spikes in the light curve. In microlensing, this can be seen when the lens is a binary star or a planetary system. Finite source size. In extremely bright or quickly-changing microlensing events, like caustic-crossing events, the source star cannot be treated as an infinitesimally