|The Earth-Moon system (top) and binary white dwarf system (bottom) to scale. Click to enlargify. Earth (right) and Moon (little brown spec on left) images from NASA/JPL/Galileo; artwork by yours truly.|
At least that was the prediction when the the pair of white dwarfs, with the ungainly name of SDSS J065133.338+284423.37 (we'll just call it J0651), was discovered the spring of 2011. While most binary stars are separated by millions, if not billions, of kilometers, these two are separated by about 113,000 kilometers (70,000 miles) – less than 1/3 the average distance between the Earth and the Moon, or just 8% of the Sun's diameter.
Clearly these are not ordinary stars! White dwarfs are the burnt out embers of stars like the Sun. Whereas the Sun counteracts the inward pull of gravity with heat and pressure produced by nuclear fusion, white dwarfs have no fusion anymore, and gravity shrinks them to orbs roughly the size of the Earth. So a single white dwarf can have the mass of the Sun in the volume of the Earth. That's cool enough.
The close binary white dwarfs are so close together that the stars that made the white dwarfs must have been almost touching when they were normal stars. Usually, when a star runs out of fuel, it swells up into a red giant. The red giant sun will reach out to beyond Earth's orbit. But if there is another star right there, the companion star's gravity will pull the dying star's outer layers over, beefing itself up. Meanwhile, the dying star becomes a white dwarf.
Since beefier stars live shorter lives, the newly-hefty companion star will now finish its life cycle in a relatively short time and try to become a red giant. But as it swells up, it engulfs the already-existing white dwarf. The complex interplay of gravity, orbits, gas and magnetic fields tosses off the outer layers of they dying star and causes the old and new white dwarfs to spiral close together, just like J0651. The picture at the top of this post is a scale composite drawing of the Earth-Moon system and the binary white dwarf system.
While the Earth and Moon take 27 days to complete one orbit, these stars take just 13 minutes. This extreme situation means that Einstein's Theory of General Relativity rears its ugly head. Einstein's theory states (among other things) that if two large masses are orbiting each other, they will create ripples in the fabric of space and time. These ripples, called gravitational waves, move out at the speed of light and carry away some of the stars' energy. With less energy, the stars must move closer together.
This effect of gravitational radiation has been detected before. Russell Hulse and Joseph Taylor shared the 1993 Nobel Prize in Physics for their discovery of two neutron stars in an orbit that is slowly decaying at exactly the rate predicted by Einstein.
Today, a team of astronomers led by University of Texas Astronomy graduate student J.J. Hermes announced that they had detected this same decay in J0651. From our vantage on the Earth, the two white dwarfs in J0651 pass in front of each other during their orbital dance. This causes an eclipse, which we see as a drop in light from the star, that lasts for less than a minute. These eclipses recur every 13 minutes.
If the stars are spiraling together, the eclipses should occur more and more frequently. Over the course of a year, J.J. and collaborators found that the eclipses are now happening about 6 seconds sooner than they should if we ignore general relativity. That's shown in the figure below. If there were no general relativity, then the middle of the dips should all lie along the dotted red line, though you see that the April 2012 eclipse is a little early.
|Eclipses of J0651, happening just a bit faster than they should. "Phase" is like "fraction of an orbit", so a phase of 0.1 is 1.3 minutes. Image credit: Hermes et al. / arXiv|
The exact distortions can be measured by carefully studying the eclipses and other information from the stars' brightnesses and spectra. The material that makes up white dwarfs is squeezed by gravity to a state called "electron degeneracy", which is a weird effect from quantum physics. By studying the distortions, J.J. and his collaborators hope to learn more about the exact structure and degeneracy in these white dwarfs, which in turn tests a lot of theories about extreme physics. We still need to wait a few years for that information, because the degeneracy physics should interact with the changing orbit to produce measurable deviations from Einstein's predictions in just a few years' time.
So, in short, another proof of Einstein's theory of general relativity will allow astronomers to look for future deviations from that theory to probe our understanding of the insides of stars 3100 light years away and our understanding of the physics of atoms. How cool is that!
J. J. Hermes, Mukremin Kilic, Warren R. Brown, D. E. Winget, Carlos Allende Prieto, A. Gianninas, Anjum S. Mukadam, Antonio Cabrera-Lavers, & Scott J. Kenyon (2012). Rapid Orbital Decay in the 12.75-minute WD+WD Binary J0651+2844 The Astrophysical Journal Letters : 1208.5051