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Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light (clouds of gas and dust). These objects make up only a tiny portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light. When a distant star or quasar gets sufficiently aligned with a massive compact foreground object, the bending of light due to its gravitational field, as discussed by Einstein in 1915, leads to two distorted unresolved images resulting in an observable magnification. The time-scale of the transient brightening depends on the mass of the foreground object as well as on the relative proper motion between the background 'source' and the foreground 'lens' object. Since microlensing observations do not rely on radiation received from the lens object, this effect therefore allows astronomers to study massive objects no matter how faint. It is thus an ideal technique to study the galactic population of such faint or dark objects as brown dwarfs, red dwarfs, planets, white dwarfs, neutron stars, black holes, and massive compact halo objects. Moreover, the microlensing effect is wavelength-independent, allowing study of source objects that emit any kind of electromagnetic radiation. Microlensing by an isolated object was first detected in 1989. Since then, microlensing has been used to constrain the nature of the dark matter, detect exoplanets, study limb darkening in distant stars, constrain the binary star population, and constrain the structure of the Milky Way's disk. Microlensing has also been proposed as a means to find dark objects like brown dwarfs and black holes, study starspots, measure stellar rotation, and probe quasars including their accretion disks. ==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 lensing and weak lensing, but it is studied using very different observational techniques. In strong and weak lensing, the mass of the lens is large enough (mass of a galaxy or a 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, a microlensing event is a transient phenomenon from a human timescale perspective. 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 small point of light: the size of the star's disk and even limb darkening can modify extreme features. * Parallax. For events lasting for months, the motion of the Earth around the Sun can cause the alignment to change slightly, affecting the light curve. Most focus is currently on the more unusual microlensing events, especially those that might lead to the discovery of extrasolar planets. Although it has not yet been observed, another way to get more information from microlensing events that may soon be feasible involves measuring the astrometric shifts in the source position during the course of the event and even resolving the separate images with interferometry. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「gravitational microlensing」の詳細全文を読む スポンサード リンク
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