Astronomers have discovered a precious find: a star so old, and so cold, that it is made up of crystallized carbon — better known as diamond.
David Kaplan, an astronomer from the University of Wisconsin-Milwaukee, and colleagues reported the discovery of this remarkable star in the Astrophysical Journal. The star is a white dwarf — the dying remains of a once-active star. White dwarfs are no longer able to fuel reactions that generate light and heat, meaning that the star cools as it ages.
“What’s particularly interesting in this case is that the white dwarf is extremely, extremely cold,” Kaplan said.
Cold, that is, for a star. Such stars can start out around 100,000 degrees Celsius but this one is less than 3,000 degrees — the coldest white dwarf star ever discovered. Scientists know the star must be very cold because it is invisible to their telescopes, implying that it is radiating little light or heat. But how does one detect an invisible star? By its effects on the stars around it.
The white dwarf is in orbit with a pulsar — or pulsating star. These exotic stars are born in the cataclysmic explosions of aging stars. A pulsar emits a beam of radiation as it rotates, much like the rotating beam of a lighthouse, giving rise to a pulsing signal in a telescope as the beam repeatedly sweeps over the earth.
The signals should come at regular intervals. But for this pulsar, the signals would come a bit off the mark, like a drummer with questionable rhythm.
This indicated another massive object had to be present. It was a dead giveaway that the pulsar had a companion star — the white dwarf. The reason for this lies with a quirk of Einstein’s theory of general relativity. Signals from a pulsar passing by a massive object will be delayed because of the warping of the fabric of space that is caused by the massive object.
The discovery built on the work of multiple researchers, stretching over many years of research. The pulsar was discovered by Jason Boyles of Western Kentucky University. The observations of the pulsar were performed by Adam Deller, of the Netherlands Institute for Radio Astronomy. These observations allowed the scientists to estimate the pulsar’s distance from Earth — about 900 light-years — and measure the rate of the system’s orbit.
“The key thing why this is such a beautiful and nice work is that they didn’t see anything, yet they can make really strong conclusions,” said Gils Nelemans, of Radboud University Nijmegen in the Netherlands, who was not involved with the research. “We’re really almost forced to conclude that this is really the only explanation.”