Diamond in sky no science-fiction fairy tale
By Amanda Jermyn

About 900 light-years away, near the constellation Aquarius, an ancient white dwarf star has been discovered that is so cold its carbon has crystallized, forming a diamond about the size of the earth. This may sound like the beginning of a science fiction fairy tale, yet it is in fact a real discovery made by Prof. David Kaplan and colleagues at the University of Wisconsin-Milwaukee, and recently published in the Astrophysical Journal.

A white dwarf is the extremely dense final state of a star like our sun that has collapsed to form an object about the size of the earth. This is thought to be the fate of all stars whose mass is not high enough to form a neutron star. White dwarfs are comprised mainly of carbon and oxygen, and fade and cool over billions of years.

As the light they emit is very faint, it is extremely difficult to detect them. This particular one is about 11 billion years old, and was found inadvertently while researchers were studying a pulsar, a fast-spinning super-dense neutron star formed in a supernova explosion that sends out powerful beams of radio waves. The pulsar, named PSR J2222-0137, was discovered two years ago by Jason Boyle, then a graduate student at West Virginia University. It was then observed over a two-year period by Adam Deller, an astronomer at the Netherlands Institute for Radio Astronomy, who was able to pinpoint its location exactly. While observing the pulsar using the National Radio Astronomy Observatory’s Green Bank Telescope in West Virginia, researchers noticed that its radio signal was sometimes delayed because a companion object was passing by it, warping space-time. As Einstein’s theory of general relativity indicates, this occurs because light (or radio waves) slows in the gravitational field of a massive object. By studying the delays, using the Very Large Baseline Array of radio telescopes, they were able to determine that the pulsar had a mass 1.2 times that of the sun, and a companion with a mass 1.05 times that of the sun. The observations also revealed that the pulsar and its companion are gravitationally bound in a binary system, orbiting each other every 2.45 days.

Suspecting the companion to be a white dwarf, the team looked for it in optical and infrared light, using the Southern Astrophysical Research (SOAR) telescope in Chile and the Keck telescope in Hawaii. However neither of these instruments was able to detect the white dwarf. According to Bart Dunlap, a graduate student at the University of North Carolina at Chapel Hill, “Because of the radio observations, we know exactly where to look, so we pointed SOAR there and collected light for two and a half hours. 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. If there’s a white dwarf there, and there almost certainly is, it must be extremely cold.” Of course, “cold” is a relative term, given that the white dwarf is burning at about 4,900 degrees Fahrenheit, yet that’s 5,000 times cooler than the center of the sun. A white dwarf at this low temperature would be comprised largely of crystallized carbon, in effect, a diamond the size of the earth.

Last year I reported on the discovery of a diamond planet 40 light years from earth orbiting the star 55 Cancri, and there may well be other exoplanets like it. There may be other diamond stars in the universe too. However, given how faint they are, they would be difficult to detect. We can only hope that more of them show up in binary systems, giving us a better chance of finding them. Meanwhile, the recent discovery may require a minor adjustment to the nursery rhyme: “Twinkle, twinkle little star.” Listen up, children, there really is a diamond in the sky!