A team of astronomers led by Gijs Roefols of the Harvard-Smithsonian Center for Astrophysics recently examined this pair of stars known to astronomers as RX J0806.3+1527 or, HM Cancri. The two stars are both white dwarfs—the hot cores of dead, sun-like stars. They squeeze as much mass as half our Sun into a globe the size of the Earth. A teaspoon of white dwarf material would weigh about five tons.

This artist’s video depicts a pair of white dwarf stars known as HM Cancri, swirling closer together, traveling in excess of a million miles per hour. As their orbit gets smaller and smaller, leading up to a merger, the system should release more and more energy in gravitational waves. This pair of stars might have the smallest orbit of any known binary system. They complete an orbit in 321.5 seconds–just over five minutes.
(Credit: GSFC/D.Berry)

Scientists knew HM Cancri’s brightness varied on a five-minute timescale, but debated whether that variation was due to a tight orbit or other causes. In-depth studies were difficult because HM Cancri is very faint: about a million times fainter than what can be seen with the unaided eye. The team used the giant 30-foot Keck I telescope in Hawaii to gather enough light to confirm that the varying brightness was due to the speedy orbit of these two stars.

The stars move quickly because they are very close to each other, separated by only about one-fourth the distance from the Earth to the Moon. As a result, they share strong gravitational forces. They were once farther apart but have spiraled closer together over time. Billions of years from now, they will crash together and merge.

The stars drag together because they are gradually losing energy. Einstein’s General Theory of Relativity predicts that they are emitting gravitational waves, or ripples in the fabric of space-time. Those ripples carry energy away from the system, as shown in the artist’s conception accompanying this articles.

Future observatories like the proposed Laser Interferometer Space Antenna should somedy be able to detect gravitational waves coming from HM Cancri.

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These supernovae, called Type Ia, serve as cosmic mile markers to measure expansion of the universe because they can be seen at large distances, and they follow a reliable pattern of brightness. However, until now, scientists have been unsure what actually causes the explosions.

This NASA/Chandra X-ray Observatory animation shows two white dwarf stars merging into a supernova.

“These are such critical objects in understanding the universe,” said Marat Gilfanov of the Max Planck Institute for Astrophysics in Germany and lead author of the study that appears in the Feb. 18 edition of the journal Nature. “It was a major embarrassment that we did not know how they worked. Now we are beginning to understand what lights the fuse of these explosions.”

Most scientists agree a Type Ia supernova occurs when a white dwarf star—a collapsed remnant of an elderly star—exceeds its weight limit, becomes unstable and explodes. Scientists have identified two main possibilities for pushing the white dwarf over the edge: two white dwarfs merging or accretion, a process in which the white dwarf pulls material from a sun-like companion star until it exceeds its weight limit.

Image left: X-ray, optical and infrared composite image of galaxy M32, one of six galaxies used in a study to examine properties of Type Ia supernovas

Because these two scenarios would generate different amounts of X-ray emission, Gilfanov and Bogdan used Chandra to observe five nearby elliptical galaxies and the central region of the Andromeda galaxy. A Type 1a supernova caused by accreting material produces significant X-ray emission prior to the explosion. A supernova from a merger of two white dwarfs, on the other hand, would create significantly less X-ray emission than the accretion scenario.

The scientists found the observed X-ray emission was a factor of 30 to 50 times smaller than expected from the accretion scenario, effectively ruling it out. This implies that white dwarf mergers dominate in these galaxies.

“Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging,” said co-author Akos Bogdan, also of Max Planck. “This is probably not what many astronomers would expect.”

“To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appeared to exist,” said Gilfanov. “Now this path to supernovae will have to be investigated in more detail.”

NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.

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