Tuesday, August 28, 2012

Even white dwarfs must obey Einstein

The white dwarfs are 1/3 the Earth-Moon distance apart
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.
3,100 light-years away in the direction of the constellation Gemini lurks one of the most extreme pair of stars that we know about.  Two white dwarfs, the remains of ordinary stars similar to the sun, whirl around each other every 12 minutes and 45 seconds.  As they orbit, Einstein's theory of general relativity predicts that their gravity distorts space and time itself, and these distortions (called gravitational waves) carry away some energy from the system, forcing the two white dwarfs to draw ever nearer.  Locked by gravity in a slow death spiral, these white dwarfs are destined to collide and merge in two million years. At least that was the prediction, and today it was confirmed by an international team of astronomers (including many friends and colleagues of mine, though I wasn't involved).

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
Confirming general relativity yet again is cool, but more detailed science waits down the road.  After all, we already knew that Einstein's theory was right in situations like this.  But look again at the picture at the top of this post.  Notice that the distance between the two white dwarfs is only a few times the size of the white dwarfs themselves.  This small distance means that the gravity of the white dwarfs is distorting their shapes - they aren't perfect spheres.  The larger-sized white dwarf (which actually has less matter, because white dwarfs are contrary in that way) is stretched by about 3% away from a perfect sphere and toward an egg shape.

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


  1. Anonymous11:08 AM

    What do we think will happen when these stars DO collide? thanks.

  2. There's a fair amount of debate. Most likely they will not explode as a supernova, but there are rare types of faint supernovae that could be explained by the collision of these two stars. It could be that, when they merge, the low-mass white dwarf will get ripped apart in a matter of seconds and be eaten by the higher mass one. Or the high mass one may sip away like a vampire at the low mass one, eating it over the course of centuries. As of right now, it just isn't certain!

  3. MonkeyBoy9:22 PM

    We don't know if the stars are rotating. If they are the change in orbit period could be fully explained by tidal friction.

  4. Oh, the stars are certainly rotating, but tidally locked like the Moon is to the Earth. The misshapen star causes a change in light that the team has detected and modeled, and the shape indicates that the star is rotating at exactly the same period as the orbit - what we would expect for tidal locking.

    As the orbit shrinks due to gravitational radiation, the stars' rotations must change, which does indeed require tidal friction and would affect the orbital period in addition to general relativity. The predicted level of this effect is about 1/10 that of the general relativity effect, but the exact level depends on the structures of the two stars.

    This is what I was referring to when I said that later changes will allow us to better understand the structure and physics of the white dwarfs.

    So, you are right that tidal friction is important, and in many ways, it is the most useful effect. We just need more time of watching this system to make an unambiguous detection of it. But we do know the stars' rotations.

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  6. I wonder what would happen if these white dwarfs actually collide? My guess is a supernova explosion. And what would be the remainder: neutron star or black hole?