Scientists make first ever observation of contracting white dwarf star

White dwarf is visible only because it is accreting material from its binary companion.
By Laurel Kornfeld | Nov 16, 2017
For the first time ever, scientists have directly observed a white dwarf star contracting, a phenomenon theorized to occur early in white dwarfs' lives but never previously seen.

White dwarfs are the stellar remnants of Sun-like stars, left behind when the stars use up all their fuel and expel their outer layers, which form a planetary nebula.

While they are approximately Earth-sized, white dwarfs are extremely dense, with masses comparable to that of the Sun.

White dwarfs, especially those in binary systems, have very high spin rates, which researchers have long attributed to their contracting.

Over their first million years, white dwarfs are believed to contract by as much as several hundred kilometers.

However, astronomers could not observe this actually occurring because most known white dwarfs are old and long ago ended their contraction phases.

Additionally, scientists had no way to measure variations in these stars' radii.

Now, a team of scientists from Moscow State University (MSU) in Russia and the Russian Academy of Sciences, along with Italian researchers, actually observed a white dwarf contracting when they studied an X-ray source in an unusual binary system about 2,000 light years away in the constellation Puppis.

Known as HD49798/RX J0648.0-4418, the binary system contains a massive, rapidly spinning white dwarf that emits X-rays through accretion of matter captured via its companion's star's stellar wind.

"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," said Sergei Popov of MSU, lead author of a paper on the findings published in the journal Monthly Notices of the Royal Astronomical Society.

According to study co-author Sandro Mereghetti, the white dwarf's spin rate has increased over the last 20 years. Because the stellar remnant is only about two million years old, a contraction rate of approximately one centimeter per year exactly matches what scientists expect.

"We should thank the uniqueness of the binary system under study," Mereghetti emphasized. "The white dwarf was literally illuminated due to the accretion of matter from the neighboring star."

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