Gravitational microlensing
The microlensing (English microlensing ) referred to in astronomy not measured in the case of gravitational lensing, in which the distance between the different generated by the gravitational lens images of the background object is so small that they can be observed by today's telescopes are not separated and the deflection of light can be. The effect of gravitational lensing shows up then the fact that the total light of the unresolved images of the background object appears brighter than it would be without the lens. Such reinforcement would in itself does not easily recognizable, since the actual brightness and distance of the background object are not usually known. But move the lens and the background object in the sky very close to each other over, then the brightness during such a microlens event in a characteristic way and again, while the given by the Einstein radius area high gain is crossed.
Microlenses in the Milky Way and nearby galaxies
In the best studied case of the micro- lensing background stars are observed in the bulge of our Galaxy or in the Magellanic Clouds. Crosses an object of planetary to stellar mass before such a star, the background star is markedly enhanced, while it is near or within the Einstein radius. This is for stellar masses and distances of several kiloparsecs of the order of a thousandth second of arc. Depending on the mass of the object lens and the relative movement of the lens and the background object takes the microlens event for several days to months. Since the lens effect depends only on the mass, it also occurs for low luminosity lenses such as old white dwarfs, neutron stars and brown dwarfs, which are too weak for direct observation at a great distance.
The probability that a particular background star at a specific time is just such an event is very low, of the order of 10-6. So there must millions background stars can be monitored simultaneously to find microlens events. Among these stars but are also about 1% of variable stars of various types, of which the microlens events must be distinguished. Criteria for this are the shape and symmetry of the brightness rise and fall, the uniqueness of the microlens event as opposed to the repetition of many changes in brightness of stars and the fact that gravitational lenses for all wavelengths reinforce the same, while internal changes in brightness of stars are often associated with color changes.
In the direction of distant galaxies of the local group such as the Andromeda Nebula also microlens events can be observed. Because of the greater distance they showed the increase in brightness of a picture element ( ' pixel '), in the light of the star is actually enhanced mixed with the other stars.
Search the constituents of dark matter
The first search for the microlensing subsequently resulted in a work of Bohdan Paczynski from the year 1986., When a large part of the dark matter in the halo of our Milky Way would consist of weak or non- luminous objects with planetary to stellar mass, could such Massive Astronomical Compact Halo Objects ( MACHOS ) can be discovered by searching for microlens events toward background stars in the Magellanic Clouds. Several programs, such as OGLE discovered around 1993 the first events. Essentially, however, they are explicable by known stellar populations, there is no evidence that a predominant portion of the dark matter consists of MACHOS. A previously popular model in which the dark matter is made up of objects with only 10-6 to 10-2 solar masses, similar planets could thus be excluded, since then a lot more short microlens events should occur.
Search for exoplanets
If a star that caused a microlens event, a planet has, and this exoplanet very close to the line of sight to the background star moves past, the light curve of the event is modified. In addition to the slow brightness increase and decrease, caused by the lens effect of the foreground star, there will be a short peak brightness of only about one day's duration. Such events were actually observed (see exoplanet ).
Gravitational lenses in which a galaxy as a lens images a compact distant object such as a quasar, show a further manifestation of the microlens effect: the variable micro-lens effect of the individual stars of this galaxy contributes to the variations in brightness of the images of the background object at. This effect Kyongae Chang has already been described in 1980 in her dissertation. Macro - gravitational lenses with additional microlensing by a single star, which is in line of sight of one of the images of a background object are, for her, and Sjur Refsdal also called Chang- Refsdal lenses.