Dead star makes a lens for its companion

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Astronomers have discovered a unique pair of stars. One is sunlike in size. Around it orbits the tiny, but extremely dense core of a dead star. Yet what’s left of this dead star is so hot that the compact material glows white. As this white dwarf passes in front of its companion (as seen from Earth) every 88 days, its gravity magnifies light from the other star.

This represents the first clear sign of a gravitational lens in a binary star system. Astronomers report its discovery in the April 18 issue of Science.

The white dwarf is not much bigger than Earth. Still, it has a mass about 60 percent as great of as our sun’s. Ethan Kruse and Eric Agol of the University of Washington in Seattle discovered the binary star system. It lurks 2,600 light-years away in the constellation Lyra. The astronomers turned it up while they were sifting through data collected by the Kepler space telescope.

Agol and another researcher had predicted Kepler might find about a dozen self-lensing binaries. His is the first to turn up.

Self-lensing binaries provide a rare opportunity to directly measure the mass of some stars that would otherwise be impossible (see video below). Doing so might help scientists unravel the exotic role of energy and mass in white dwarfs. It might also shed light on the evolution of binary star systems. They are home to nearly 40 percent of the sunlike stars — at least in our Milky Way.

Power Words

astronomy    The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

constellation  Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

gravity  The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

lens   (in optics) A curved piece of transparent material (such as glass) that bends incoming light in such a way as to focus it at a particular point in space. Or something, such as gravity, that can mimic some of the light bending attributes of a physical lens.

mass  A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

Milky Way  The galaxy in which Earth’s solar system resides

star  Thebasic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

white dwarf  A small, very dense star that is typically the size of a planet.It is what is left when a star with a mass about the same as our sun’s has exhausted its nuclear fuel of hydrogen, and collapsed.