Astronomers have identified what could be the coldest, faintest white dwarf star ever seen, so cold that its carbon has crystallized — not unlike a diamond. The work was published in the Astrophysical Journal this week.
White dwarfs are end-state stars that have collapsed to form hot, extremely dense, Earth-sized objects composed mostly of carbon and oxygen. While they can be as massive as the sun, white dwarfs are so dense they’re only about the size of Earth. After they exhaust their nuclear fuel, white dwarfs will continue to cool and fade slowly over billions of years. This is what will ultimately become of our sun.
Objects this dim are really hard to spot. Combining observations from a variety of sources, including the Green Bank Telescope in West Virginia and the Very Long Baseline Array at Mauna Kea, a team led by David Kaplan at the University of Wisconsin-Milwaukee was able to do so because the object was located in a binary system.
The 11-billion-year-old, uncommonly cool white dwarf was discovered along with its pulsar companion, PSR J2222-0137. Pulsars are rapidly spinning neutron stars, the remains of massive stars that exploded as supernovas. As these stars spin, beams of radio waves stream from the poles of its magnetic field, and when one of these beams sweeps across Earth, radio telescopes can capture the pulse of the waves.
Researchers actually spotted the pulsar first. It was spinning more than 30 times a second and appeared to be gravitationally bound to a companion star — which they initially thought was another neutron star. The two orbit each other once every 2.45 days. Observations of the pulsar over the next two years revealed its location and distance: 900 light-years from Earth and in the direction of the Aquarius constellation.
By studying how the gravity of the companion warped space and caused delays in the radio signal, the team was able to calculate both the mass of the pulsar (1.2 times that of the sun) and the companion (1.05 times that of the sun). That’s when they realized the other object couldn’t be a second neutron star: The orbits were too orderly for a second supernova to have taken place.
“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 from the University of North Carolina at Chapel Hill explains in a news release. “If there’s a white dwarf there, and there almost certainly is, it must be extremely cold.”
Comparatively cold, to be clear. The white dwarf couldn’t be more than 2,700 degrees Celsius; the center of the sun is 5,000 times hotter. Astronomers believe such a cool ancient stellar remnant would be largely crystallized carbon — which makes it, in effect, a diamond.
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