The Smallest Known White Dwarf in the Universe Is an Earth-Sized Diamond
Star death isn't a bad way to go.
Image: artist impression of a white dwarf star in orbit with pulsar PSR J2222-0137/B. Saxton (NRAO/AUI/NSF)
When we die, in the absence of some artful mortician intervention, we'll rather quickly become a puddle of gross chemical slime and a pile of bones. That's how we self-destruct. It's nature's way of saying a final fuck you to its most advanced creation, and really the most glamorous way a corpse might wind up is either being coverted to ash or, possibly, being eaten by some majestic wolf (put that in your will). The average main sequence star, on the other hand, goes out with supreme style. Our friendly neighborhood sun will die when it runs out of fuel; as the star burns off its last hydrogen stores as a solar system-destroying red giant, its guts will collapse in on themselves, forming a hot core of carbon.
After the red giant phase of star death closes in a last fiery whoosh of solar wind, we're left with just that tiny, ultra-dense white hunk. A white dwarf. This white dwarf star-corpse will keep cooling, pretty much forever. The eventual outcome, demonstrated by just-released white dwarf observations from the National Radio Astronomy Observatory, is permanant rest as what can reasonably be called a diamond: crystallized carbon. Stars that've reached this state shouldn't be rare in the universe, but the NRAO's detection of one, the smallest and coolest white dwarf yet observed, is an impressive feat given the smallness, coldness, and dimness of the star. It's a bit like trying to find an ice cube on a glacier.
A white dwarf still has mass though, and it's mass that helps us find any small, dark object in the relatively distant universe, like an exoplanet. More specifically, we look for the effect a massive body has on the things around it. When astronomers pinpoint an exoplanet, it's often thanks to gravitational lensing, in which objects are located by the wobble they affect on some other object.
The newly-described white dwarf was first identified thanks to a closely-neighboring pulsar, PSR J2222-0137. This pulsar, the first object identified in the star system in question, spins around 300 times a second and it was clear from the outset that it had a partner of some sort, possibly a neutron star or very, very dead white dwarf. Pulsars are, just by themselves, fascinating as tiny radio oscilliators (cosmic lighthouses, astronomers are fond of saying), but their rotations are perfect tools or detecting other obects. Adam Deller, an astronomer at the Very Long Baseline Array radio telescope, kept tabs on PSR J2222-0137 for two years, collecting the data needed to pinpoint its exact location relative to the Earth. With this information, it was then possible to target the pulsar with the NRAO's Green Bank Telescope, in West Virginia.
Using the rapid-fire pulses of PSR J2222-0137, Green Bank researchers, led by a then-grad student named Jason Boyles, were able to essentially watch the pulsar's white dwarf partner warp space itself as the two spun around each other. Subtle delays in the pulsar's signals as they traveled from there to here gave away the white dwarf as it passed behind -0137, something that happened every couple of days. These delays gave away the mass of both objects, with the pulsar weighing in (massing in, really) at about 12 times that of our sun, and the white dwarf about 1.2 times the mass.
A strange thing about the teams' observations was that, given the distance and the mass of the white dwarf, it should've been visible using optical or infrared light. Not so: both the Southern Astrophysical Research (SOAR) telescope in Chile and the Keck telescope in Hawaii came up dry. No burning ember of an ex-star to be found—just cold, black, and massive. Also old: the white dwarf is estimated to have been around for roughly 11 billion years, making it roughly the same age as the Milky Way.
“Our final image should show us a companion 100 times fainter than any other white dwarf orbiting a neutron star and about 10 times fainter than any known white dwarf, but we don’t see a thing,” said Bart Dunlap, one of the team members, in a NRAO statement. “If there’s a white dwarf there, and there almost certainly is, it must be extremely cold.”
And to be that cold, the star's constituent carbon would have cooled enough to have crystallized, giving us our diamond. The white dwarf remains, however, 900 light-years away. So we an go ahead and continue ripping up northern Canada for our pretty rocks. Or not.