Dead Star Acting Like Magnifying Glass
Image: This artist’s concept depicts a dense, dead star called a white dwarf crossing in front of a small, red star. The white dwarf’s gravity is so great it bends and magnifies light from the red star.
NASA’s Kepler space telescope observed this effect in a double-star system called KOI-256 by monitoring changes in the red star’s brightness. The red dwarf star is cooler and redder than our yellow sun. Its companion is a white dwarf, the burnt-out core of a star that used to be like our sun. Though the white dwarf is about the same diameter as Earth, 40 times smaller than the red dwarf, it is slightly more massive. The two objects circle around each other, but because the red dwarf is a bit less massive, it technically orbits the white dwarf.
NASA’s Kepler space telescope has witnessed the effects of a dead star bending the light of its companion star. The findings are among the first detections of this phenomenon — a result of Einstein’s general theory of relativity — in binary, or double, star systems.
The dead star, called a white dwarf, is the burnt-out core of what used to be a star like our sun. It is locked in an orbiting dance with its partner, a small “red dwarf” star. While the tiny white dwarf is physically smaller than the red dwarf, it is more massive.
“This white dwarf is about the size of Earth but has the mass of the sun,” said Phil Muirhead of the California Institute of Technology, Pasadena, lead author of the findings to be published April 20 in the Astrophysical Journal. “It’s so hefty that the red dwarf, though larger in physical size, is circling around the white dwarf.”
Kepler’s primary job is to scan stars in search of orbiting planets. As the planets pass by, they block the starlight by miniscule amounts, which Kepler’s sensitive detectors can see.
“The technique is equivalent to spotting a flea on a light bulb 3,000 miles away, roughly the distance from Los Angeles to New York City,” said Avi Shporer, co-author of the study, also of Caltech.
Muirhead and his colleagues regularly use public Kepler data to search for and confirm planets around smaller stars, the red dwarfs, also known as M dwarfs. These stars are cooler and redder than our yellow sun. When the team first looked at the Kepler data for a target called KOI-256, they thought they were looking at a huge gas giant planet eclipsing the red dwarf.
“We saw what appeared to be huge dips in the light from the star, and suspected it was from a giant planet, roughly the size of Jupiter, passing in front,” said Muirhead.
To learn more about the star system, Muirhead and his colleagues turned to the Hale Telescope at Palomar Observatory near San Diego. Using a technique called radial velocity, they discovered that the red dwarf was wobbling around like a spinning top. The wobble was far too big to be caused by the tug of a planet. That is when they knew they were looking at a massive white dwarf passing behind the red dwarf, rather than a gas giant passing in front.
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