About 900 light-years away, an ancient white dwarf star has cooled into a crystallized chunk of carbon — a diamond. But this isn’t just any old diamond hiding in space: It’s the size of Earth, and it’s 11 billion years old.

The diamond-star, described in a study published in The Astrophysical Journal, is among the coldest white dwarfs astronomers have found. In fact, it’s so cool and dim that it can’t even be seen — its feeble light isn’t nearly powerful enough to pierce the darkness of the cosmos, even from relatively nearby.

Instead, teams inferred the presence of the crystallized dwarf based on the way its gravity perturbs the normally steady radio pulses coming from a spinning companion star.

Yep, the system gets even cooler. The dwarf is orbiting a pulsar — a rapidly rotating neutron star — known by the charismatic moniker PSR J2222-0137. The system is similar to another that was described in 2011, with a crystallized white dwarf orbiting a pulsar — but this new diamond is bigger (there’s also a planet hypothesized to be somewhat diamond-like).

If you were to look in the sky in the direction of the constellation Aquarius, you’d be looking in roughly the right direction to see the system, which is actually a pair of dead stars: The spinning neutron star is the extremely dense remnant of a formerly huge star that ended its life in a supernova. A white dwarf is all that remains of a formerly Sun-like star, contracted into a clump the size of Earth. Left on their own, dwarfs will slowly cool and fade to black over billions of years (but sometimes, with the help of a stellar companion, they can detonate and create dazzling supernovas the outshine entire galaxies).

View Images A field of ancient white dwarf stars, spied by the Hubble Space Telescope in 2002. (NASA/H.Richer)

Anyway, astronomers first spotted the pulsar in 2007. Then-graduate student Jason Boyles began studying the spinner, using the Green Bank Telescope at the National Radio Astronomy Observatory in West Virginia. Normally, pulsars spin so steadily that they rival the best atomic clocks. Boyles measured this pulsar’s spin rate at a relatively pokey 30 times per second — some can spin around themselves in only a few thousandths of a second.

But something was different. The pulses arriving at Earth were periodically delayed, as if some unseen companion were causing the pulsar’s radio emission to take a somewhat circuitous route to Earth. This can happen when an orbiting, massive companion’s gravity messes with the fabric of space, causing things like light and radio waves to travel along twisted pathways.

Boyles and his colleagues suspected a dense, hidden object was paired up with the pulsar; more observations suggested the unseen body orbited the pulsar every 2.4 days, and that it was roughly as massive as the Sun. The team guessed they were dealing with either another neutron star (which would be a very rare pairing) or a dense white dwarf star.

To identify the missing object, astronomers needed to know more accurately how far from Earth the pulsar and its friend were. So, Adam Deller at the Netherlands Institute for Radio Astronomy led the charge to determine that distance, using the Very Long Baseline Array telescopes.

Two more years of measurements, beginning in 2010, helped the team pin that distance at 267.3 parsecs, or about 871 light-years.

With that information in hand, and with the delay time measurements, astronomers could now begin to un-mask the pulsar’s friend. They calculated that the companion must be about 1.05 times as massive as the Sun (within the range of both a white dwarf and a neutron star), and that the pulsar was slightly more massive, at 1.2 Suns. But scientists also determined that the pulsar and its friend were in a roughly circular, rather than elliptical (or eccentric) orbit. That suggested the system hadn’t been walloped by something like a second, neutron star-forming supernova.

“If there were two neutron stars that means two supernova explosions. And a supernova explosion should make the orbit pretty eccentric,” says study coauthor David Kaplan of the University of Wisconsin, Madison. “We see that the orbit is very circular.”

That suggested the team was looking for a white dwarf. At that measured distance, astronomers reasoned, such a star should be visible from Earth. So, the team tried to get a visual on the object, using telescopes in Chile and Hawaii. They searched the region in multiple wavelengths, in the infrared and visible.

And failed.

The trouble was, no matter how hard they tried, scientists just couldn’t coax the dwarf to reveal itself.

The only way that’s possible, the astronomers write, is if the dwarf is cooler than 3,000 Kelvin — which would make it among the chilliest white dwarfs ever discovered. And the only way the star could have cooled to that temperature and NOT be older than the Milky Way galaxy is if it were already crystallized into diamond.