Science Focus
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White dwarfs are the remnants of stars like the Sun. They also provide some of the best means to measure large distances in the Universe if they explode as "type Ia" supernovae. All of those explosions occur in binary systems consisting either of two white dwarfs or a white dwarf paired with an ordinary star. To understand the whole process, astronomers need to identify progenitor systems before they explode: binaries with one or more white dwarf.
A particularly interesting example was recently identified and described in a Science paper by Ethan Kruse and Eric Agol. In this system, a white dwarf is locked in mutual orbit with a Sun-like star. The orientation of the binary relative to Earth means the two bodies periodically eclipse each other. When the white dwarf passes in front of its companion, gravitational lensing—the focusing of light by a massive body—magnifies the star's light very slightly. This is the first such "self-lensing" system containing a white dwarf, and should allow researchers to better understand understand the behavior of white dwarfs in binaries.
When one star passes in front of another (from our point of view), the gravity of the foreground star magnifies the light of the background object very slightly. This effect is very small, and so it is known as gravitational microlensing (or just microlensing) to distinguish it from the more dramatic form described in the sidebar. Microlensing can be used in some cases to detect exoplanets orbiting around the star in the foreground: the planet provides a tiny extra boost, beyond that provided by its host star, to the light of the star in the background.
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