An Etymological Dictionary of Astronomy and Astrophysics

A type of → gravitational lens, where the foreground
→ lensing object is of low mass, and the multiple images produced are too close together on the sky to be observed as separate images.

Gravitational microlensing occurs when a foreground star happens to lie very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the background source star into two or more images, which are typically unresolved. However, these images of the source are magnified, by an amount that depends on the angular separation between the lens and source. If with the passage of time the lens moves across the Earth-source, the amount of brightening changes. Typically the source will appear to brighten, reach a maximum and then fade symmetrically back to normal over the course of a few weeks or months; this is called a → microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will also act as a lens, further perturbing the images and resulting in a characteristic, short-lived signature of the planet.

Microlensing is used in the search for → dark matter in the → Milky Way galaxy and its nearest neighbours, as well as for → extrasolar planets

(e.g. B. S. Gaudi, 2010, arXiv:1002.0332).

See also: → micro-; → lensing.