What happens to a star in orbit around a black hole?

[force5]

I'm trying to find some recent work describing the fate of a star in orbit around a BH. Thanks for any help.

 

[force5]

O.K. - I'm going to try to answer my own question.

I think I recall seeing an article recently that stated, a star was observed in an orbit around what was expected to be a black hole. The star was estimated to give up about one percent of its mass in the form of radiation. Then, if I remember correctly, the star moved farther away from the BH, based on how much energy it radiated. This dosn't make since to me and was hoping someone might have some related information.

 

[Nereid]

If you could give a reference, we could check how the author felt 'giv[ing] up about one percent of its mass in the form of radiation' results in 'the star mov[ing] farther away from the BH, based on how much energy it radiated'. As you state, this doesn't seem right; the star should spiral into the BH, eventually being torn apart (tidal forces, depends on how massive the BH is) and swallowed, with belches in the form of polar jets and radiation from a very hot accretion disk.

IIRC, there was an article on the Chandra website recently, announcing observations which were interpreted as a star being shredded by a galactic core BH (presumably it was in orbit about the BH before it succumbed). However, this having happened in a galaxy far, far away (and a long, long time ago), by now it's truly ancient history

 

[force5]

Thanks Nereid, I'll keep looking for the article. I hope I'm not just recalling comments that I read on another forum or thread. Since I only keep track of those threads I get involved in, my records don't indicate the source of this information.

 

[force5]

This might be the article I saw. It indicates that about three percent of the star was consumed by the BH. And the rest was "forced" away. I think this puzzled me since I thought of black holes as major consumers and not producers of forces extending outside of the event horizon.

===================================================
The World's No.1 Science & Technology News Service

Giant black hole caught devouring star

19:00 18 February 04

NewScientist.com news service

A giant black hole in a galaxy a billion light years away has been caught in the act of butchering a star - the first time this has been seen, according to astronomers. It means that black holes all over the Universe must be eating stars, and that may be the main way they grow.

A powerful flare of X-rays was the star's final scream. The flare, from the centre of a galaxy called RXJ 1242-1119, was thousands of times as bright as all the stars in the galaxy put together.

Its beginnings were seen back in 1992, when the ROSAT observatory picked up emission as strong as that from many active galaxies. Active galaxies contain a giant black hole feeding off a constant gas supply, and usually have a bright blue pinpoint in the centre.

"Yet in visible light, RXJ 1242-1119 is just a normal, inconspicuous galaxy," says Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.


Murder scene

Komossa suspected that these X-rays might be a brief flare from a dying star, rather than constant emission, but she needed follow-up observations to be sure. In 2001, Komossa, Günter Hasinger and their team looked at RXJ 1242-1119 again with two more space-based telescopes.

The Chandra observatory showed that the flare has almost subsided. Komossa's group also used XMM-Newton to show that the X-ray energies have just the broad spread expected when gas is being consumed by a black hole.

Komossa and her group are now able to reconstruct the murder scene. A star about the size of our Sun ventures too close to the black hole. "It then feels enormous tidal forces exerted by the black hole, which finally rip apart the whole star," she says.

Some of the debris circles the black hole for a while, heating up so much that it shines brilliantly in X-rays, before falling below the event horizon from beyond which no light can escape. But the black hole is a very messy eater - only a few per cent of the star actually goes in. The rest gets flung outwards again by the force of the flare.

 

[Nereid]

Here's the Chandra PR of the same story.

So, how did ~99% of the unfortunate star's mass escape being swallowed? Here's the magic phrase in the PR: "This small amount [~1% consumed] is consistent with predictions that the momentum and energy of the accretion process will cause most of the destroyed star's gas to be flung away from the black hole."

So, to understand better, we need to look into 'the accretion process'. IIRC, some of the important processes are:
- the 'ripping apart' will result in much of the matter being in orbits that escape the BH (remember that what was observed is the tidal disruption of a star, which was well outside the event horizon)
- as matter falls towards the BH, it will settle into an 'accretion disk'; a more-or-less flat disk of material in more-or-less circular orbits about the BH. The formation of the accretion disk will generate huge amounts of radiation, through conversion of gravitational potential energy into KE, then through collisions into EM. This radiation will heat the star remnants, and will exert tangential pressure on most of it, so it will be 'blown away'. BH accretion disks can be incredibly efficient in converting mass to EM (and a small % of the star's mass will escape as EM!).