An international team of scientists led by researchers at Lomonosov Moscow State University (MSU) has observed for the first time a contracting white dwarf. While this phenomenon is expected in very young white dwarfs, this set of observations will help astronomers better understand what’s going on.
The discovery was possible because the white dwarf is in a very special setup. It orbits a companion star and is stealing material. As the material begins to orbit the white dwarf, it's heated up and emits X-rays.
"Thanks to this discovery, astrophysicists will be able to study and evaluate the evolution patterns of young white dwarfs – and successfully look for similar systems in the galaxy," lead author Sergei Popov from the MSU, said in a statement.
The paper has been published in the Monthly Notices of the Royal Astronomical Society.
The astronomers estimate that the white dwarf spins on itself every 13 seconds. Every year, however, this rotation decreases by about 7 nanoseconds. By the law of conservation of angular momentum, if the spin is increasing, the white dwarf must be shrinking.
This estimate was possible thanks to 20 years of observations of the system called HD49798/RX J0648.0-4418, which is located 2,000 light-years away. The white dwarf is heavier than the Sun, but it has collapsed into a much denser object. It has a radius as small as 5,000 kilometers (3,100 miles), which is less than the Earth.
The astronomers estimate that the white dwarf is about 2 million years old and has a contraction rate of about one centimeter per year. This is the expected rate for a white dwarf that's so young.
"For decades it has been theoretically clear that that young white dwarfs are contracting," Popov explained. "Yet, that very phase of contraction has never been observed ‘in real time’. We should thank the uniqueness of the binary system under study: the white dwarf was literally illuminated (due to the accretion of matter from the neighboring star). But it was highlighted so neatly that the accreting matter did not affect its rotation – an extremely rare phenomenon! In other similar systems, accretion is much more powerful: it determines how the white dwarf rotates, which makes it impossible to notice the beauty of contraction."
White dwarfs form when medium-sized stars, like our Sun, have reached the end of their lives and have blown out most of their gas.