Spinning Propeller Star Slingshots Plasma at 7 Million MPH

(Inside Science) -- Astronomers have detected the fastest-spinning white dwarf star found yet -- one that researchers say acts like an extraordinarily powerful magnetic propeller, a new study finds.

White dwarfs are stars that have burnt up all their fuel and shed their outer layers, leaving behind their cool, dim cores. Our sun will one day become a white dwarf, as will more than 90% of the stars in the Milky Way.

In the new study, researchers analyzed the white dwarf in the binary star system LAMOST J024048.51+195226.9. The white dwarf is a stellar remnant about 2,015 light-years from Earth that is roughly our planet’s size but at least 230,000 times its mass. The researchers imaged the spinning of the star using the highly sensitive HiPERCAM instrument on the largest functioning optical telescope in the world, the 10-meter-wide Gran Telescopio Canarias in Spain’s Canary Islands.

The white dwarf’s powerful gravity yanks plasma off its larger companion, a red dwarf star. In the past, this material fell onto the white dwarf’s equator at high speed, resulting in its extraordinarily fast spin.

The white dwarf also possesses a strong magnetic field, which acts like a protective barrier that causes most of the plasma falling onto it to blast out from the dead star at speeds of roughly 6.7 million mph. This makes this stellar remnant only the second “magnetic propeller” white dwarf ever found, more than 70 years since the first.

Any plasma that doesn’t get propelled away from J0240+1952 flows toward the white dwarf’s magnetic poles. It gathers in bright spots on the white dwarf’s surface, and as these rotate in and out of view from Earth, astronomers observe pulses of light, which they use to measure the white dwarf’s rate of spin.

All in all, the white dwarf completes one full rotation in a record-breaking 25 seconds. That is nearly 20% faster than the next fastest-spinning white dwarf, which completes a revolution in just over 29 seconds. “Put into Earth’s perspective, it is like the day only lasted for 25 seconds,” said study lead author Ingrid Pelisoli, an astrophysicist at the University of Warwick in England.

These findings help support theoretical predictions of how magnetic propellers should behave, which scientists had not been able to confirm until they found a second example of one. “I find it very satisfactory to find exactly what you expected,” Pelisoli said.

The scientists detailed their findings online Nov. 22 in the journal Monthly Notices of the Royal Astronomical Society.

Editor’s note (11/24/2021): A previous version of this article stated that LAMOST J024048.51+195226.9 is the name of the white dwarf. The article has been updated to clarify that LAMOST J024048.51+195226.9 refers to the star system the white dwarf is part of.