Researchers have identified the coolest, faintest, white dwarf star ever observed.
The white dwarf, which is located in the constellation Aquarius, is so cold that its carbon has crystallized; this means it resemble a Sun-sized diamond, a National Optical Astronomy Observatory news release reported. White dwarfs are extremely dense stars in the final stages of their lives. After these dwarfs have used up all of their fuel they collapse from the size of a star to about the size of Earth.
Researchers identified a millisecond pulsar (spinning neutron star), dubbed PSR J2222-0137, spinning over 30 times a second. As it spins its magnetic pole sweeps across Earth, giving off blips of radio waves; this is how the researchers discovered the object.
The observation also revealed PSR J2222-0137 was gravitationally bound to a companion star, which is likely a super-cool dwarf.
"These same observations also pinpointed the position of the pulsar extremely precisely. That's how we can be so certain that all of the other stars in the field are not the pulsar. You could say that we know the position of the pulsar to better than one pixel." This became very important when they searched for the optical evidence of the white dwarf companion," lead author of this paper, Prof. David Kaplan said in the news release.
To determine both object's masses the researchers used Einstein's theory of general relativity, which predicted that light and radio waves slow down in the gravitational pull of massive objects. By measuring the radio beam from the pulsar (and taking the law of gravity into account) the team determined the pulsar had a mass of 1.2 times that of the Sun and its companion had a mass of 1.05 times that of the Sun.
Taking into account the distance of the pulsar, the team was also able to calculate how faint its companion is.
"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," Bart Dunlap, a graduate student at the University of North Carolina at Chapel Hill and one of the team members said in the news release.
This type of star is believed to be made up of mostly carbon and oxygen, meaning it would resemble a giant diamond. Since it's so cold and faint, the team believes the "remarkable" star was born in the earliest days of the Milky Way.
