A possible white dwarf star, the faintest and coldest one ever detected, is likely so cool its carbon is crystallizing, creating what could be considered an Earth-sized diamond floating through space, astronomers say.

Made up mostly of oxygen and carbon, such objects are stars similar to our sun that are at the end of their lives and have collapsed down to about the diameter of the earth to fade and cool slowly over time.

The white dwarf at the center of the current study is probably as old as our Milky Way galaxy, around 11 billion years old, the researches say.

They estimate the star's temperature is only about 5,000 degrees Fahrenheit; the center of our own sun is about 5,000 times hotter than that.

The white dwarf, 900 light years from Earth in the direction of the constellation Aquarius, is so cool it is difficult to detect, and might never have been identified if not for the discovery of a companion pulsar, a rapidly spinning neutron star, which was observed first.

That object proved to be gravitationally bound with some sort of companion object, either a second pulsar or, as it turned out, an extremely cool example of a white dwarf.

The pair are orbiting around each other about once in 2.45 days, the researchers say; the pulsar's mass is about 1.2 times the mass our sun, whereas the suspected white dwarf's mass is almost exactly that of our sun.

Believing the white dwarf might be observable in both infrared and optical light, astronomers used telescopes in Hawaii and Chile to try and detect it, with no luck.

The physics says it's there, but it's too dim to detect, although the researchers say they aren't finished in their attempts to confirm its existence.

"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." University of North Carolina researcher Bart Dunlap said.

"If there's a white dwarf there, and there almost certainly is, it must be extremely cold."

At the estimated temperature, the collapsed white dwarf would consist largely of crystallized carbon similar in makeup to a diamond, the researchers say.

It's not the first diamond-like star ever found, they say, and they're not exactly rare; but they're so dim, with low intrinsic brightness, they're extremely difficult to find.

Just like diamonds.