Cosmic Ice: Companion of distant pulsar may be a massive Earth-sized diamond in space

Scott Sutherland

Thursday, June 26, 2014, 11:46 - Lucy may have been in the sky with her diamonds, but you'd have to go quite a bit further than that for what a team of astronomers has discovered - possibly the coolest white dwarf star ever found, which has very likely formed into diamond the size of the planet Earth!

Roughly 900 light years away from us, towards the constellation Aquarius, there is a tiny, extremely-dense, rapidly-rotating stellar remnant of a dead star much larger than our Sun hanging in space - a 'pulsar' named PSR J2222-0137 that packs a mass greater than that of our Sun into an orb only 10-15 kilometres wide. Observations by astronomers have shown that it's not alone out there, though. From the way this pulsar 'wobbles' back and forth in the view of telescopes, it is obviously locked into orbit with a companion - another massive object that's very likely another stellar remnant. However, no matter what telescope the astronomers used to view this companion, they haven't been able to see it.

There are a few possibilities for this, of course. It could be another pulsar, however they tend to attract a lot of attention from radio telescopes, as their spinning sends out beams of radiation from their poles that sweep the universe like a light-house beacon. A non-rotating pulsar, simply known as a neutron star, is another possibility, as is a black hole. All three of these candidates bring with them a certain amount of astrophysical 'baggage' though, in that they're all formed in the incredibly powerful explosion of a supernova. PSR J2222-0137 was most certainly formed in this way, but if the companion was as well, the shockwave of the second supernova would have thrown the orbit of the two objects into chaos. In this case, however, the orbits were found to be very regular.

That left one remaining possibility - a white dwarf star. These stellar remnants - spheres of 'degenerative matter' carbon and oxygen, roughly the size of the Earth but with a mass equal to that of our Sun - are left over when a star the size of our Sun reaches the end of its lifespan. Rather than going off in a supernova explosion, the star sends off multiple pulses that throw its outer layers into space, and this less-violent death would not disturb the pair as much as a supernova, leaving it in a more orderly orbit. Using the orbits of the pair, the team figured out that PSR J2222-0137 is roughly 20 per cent more massive than our Sun, and its companion is 5 per cent more massive than the Sun - which fits what we know about white dwarf stars. 

There was one complication, though. Although small, white dwarf stars are very hot, glowing with a residual heat of around 100,000 degrees Kelvin, which radiates out into space for billions of years. This means that they can definitely be seen by our optical and infrared telescopes here on Earth. With PSR J2222-0137's companion eluding the best telescopes we have, it must be very faint, and thus very cold - perhaps around 3,000 degrees Kelvin - which is colder than any white dwarf ever discovered.

"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," Bart Dunlap, a member of the team who is a graduate student at the University of North Carolina at Chapel Hill, said in a statement. "If there’s a white dwarf there, and there almost certainly is, it must be extremely cold."

With an estimated age of around 11 billion years, roughly the same age as our Milky Way galaxy, the degenerative matter of this white dwarf would have settled down to a point where the carbon atoms would crystallize, turning this stellar remnant into an Earth-sized diamond that would weigh in at roughly 90,000,000,000,000,000,000,000,000,000,000,000 (90 decillion) carats!

The team's research, which was published recently in the Astrophysical Journal, can be viewed online (click here).

(Image Credit: B. Saxton (NRAO/AUI/NSF))

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