This star ‘survived’ a black hole

^{NASA / CXC / M. Weiss}

A team of astronomers has just identified a star (now a white dwarf) that endured being swallowed entirely by a black hole, becoming trapped in an elliptical orbit of just nine hours. Details of the study were published in Monthly Notices of the Royas Astronomical Society.

What’s going on?

It is common to observe that a black hole expels flares of X-rays after it swallows large amounts of matter. This is because matter, as it approaches the black hole, heats up at high temperatures.

This is the common behavior of a black hole. However, what is not common is to observe that these ejections are observed periodically. This puzzling behavior is observed in GSN 069: an X-ray ejection is detected every nine hours.

“This white dwarf is locked in an elliptical orbit near the black hole, orbiting every nine hours,” explained Andrew King of the University of Leicester. “At its closest approximation, about 15 times the radius of the event horizon of this black hole, gas is drawn from the star in an accretion disk around the black hole, releasing X-rays.”

Possible explanation

According to King’s model, this fairly light black hole (400,000 solar masses) encountered a red giant on its way, one of the final stages in the life of stars. Quickly, the black hole undid the star’s outer layers, speeding its evolution into a white dwarf, the remnant that is generated when the star runs out of nuclear fuel.

At this time, the white dwarf was captured in an orbit around the black hole, so it continued to feed on it. The nine-hour orbit would explain the periodic detection of the same X-ray time.

^{White dwarf orbit around the black hole. / NASA / CXC / M. Weiss}

The fact that GSN 069 has a reduced mass makes it possible for the star to travel to a closer orbit. To survive a much more massive black hole, the orbit would have to be much larger.

What will happen to the white dwarf? It will remain in its orbit, as it continues to be stripped of its mass for billions of years more. Over time, its size and mass will decrease until it possibly becomes a gas giant.

“He will do his best to escape, but there is no escape,” said King. “The black hole will eat it more and more slowly, but it will never stop.

Adrian Diaz

This news was originally published in N + 1, science that adds.
About N + 1: It is the first online magazine of scientific and technological dissemination that allows the total or partial reproduction of its contents by the media, bloggers and influencers, mentioning the text and the link to the web: “This news has originally published in the magazine N + 1, science that adds: www.nmas1.org“