White Dwarf Magnifies Its Companion Star, Self-Lensing Confirmed

April 21, 2014

By observing an increase in brightness of a Sun-like star caused by its dwarf star companion, astronomers confirm self-lensing phenomenon.

Self-lensing phenomenon in the system KOI 3278.

Self-lensing phenomenon in the system KOI 3278, in the bottom part of the diagram, as the white dwarf passes in front of the regular star. In the upper part of the diagram, white dwarf passes behind its companion star, which results in lower brightness of the system. This led astronomers to originally believe the system had exoplanets. Credit: Eric Agol

Gravitational lensing in a phenomenon where a massive body in the foreground bends time-space to such a degree that it distorts the light from a source in the background, either magnifying it or increasing its brightness. This is useful for observing very distant and very faint objects. Usually, astronomers use relatively close and massive galaxies as lenses to observe galaxies that are so far away and so faint, they would otherwise not be visible.

Gravitational lensing was predicted by Einstein in his theory of general relativity and used for confirming the theory. During one solar eclipse, astronomers observed a star that was actually behind the Sun. This was possible of course, because Sun bent that star's light and it was only possible to observe during a solar eclipse, because otherwise Sun outshines the star we're trying to see.

A type of gravitational lensing called self-lensing is a special case where one star in a binary system acts as a lens for its companion star. Until now, it was only a theory, proposed by astronomer André Maeder some 40 years ago. Last week, physicists Ehan Kruse and Eric Agol from the University of Washington, Seattle, USA, found a white dwarf star that magnifies the light from its companion star.

A white dwarf star is a star the size of the Earth, but much denser, wrapping the space-time around it more than a normal, Sun-like star. Thanks to the Kepler Space Telescope, astronomers have noticed that every 88 days, the brightness of the larger star increases 0.1% due to self-lensing phenomenon. This small increase is the reason self-lensing wasn't detected until now.

At first, astronomers thought that the system, named Kepler Object of Interest (KOI) 3278 contains an exoplanet because of regular drop in brightness, but this is actually because the larger star blocks the light from the white dwarf as the smaller star passes behind it. KOI 3278 is 2,600 light-years away from Earth in the constellation Lyra.

In the animation below, you can see how the much smaller white dwarf distorts the image of its companion star as it passes in front of it. Credit: Eric Agol and NASA/SDO HMI science teams



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