Star Formation Near Black Holes

by Paul Gilster on August 21, 2008

Simulations showing how giant gas clouds evolve — clouds as large as 100,000 times the mass of the Sun — have demonstrated that stars can form in the neighborhood of supermassive black holes, the kind of black holes found at the center of galaxies. As you would expect, the clouds are disrupted when they move close to the black hole, but only part of the cloud is captured, with the rest contributing to the formation of massive stars that move about the black hole in eccentric orbits. Usefully, the results match what we see near the center of the Milky Way.

These are short-lived stars, says Ian Bonnell (St Andrews University), which in itself may be telling us something:

“That the stars currently present around the Galaxy’s supermassive black hole have relatively short lifetimes of ~10 million years, suggests that this process is likely to be repetitive. Such a steady supply of stars into the vicinity of the black hole, and a diet of gas directly accreted by the black hole, may help us understand the origin of supermassive black holes in our and other galaxies in the Universe.”

The disc that shapes the nascent star, made up of surviving material from the original gas cloud, is itself elliptical in shape. The spiral patterns formed in it by gravitational disruption transfer energy to gas further away from the black hole. Crucial to this work is the modeling of the heating and cooling process within the gas that allows star formation. The work, performed on the Scottish Universities Physics Alliance (SUPA) SGI Altix supercomputer, took over a year of computer time as it modeled the gas clouds in their movement toward the black hole.

Thus we wind up with highly massive stars on eccentric orbits around the huge black hole thought to exist at galactic center. It would take a science fiction writer par excellence (a veritable Benford!) to describe what that scenario would look like from a safe but still relatively close distance. In fact, I’d be interested in hearing from readers about any writers who have tried to model this deeply mysterious part of the Milky Way.

The paper is Bonnell and Rice, “Star Formation Around Supermassive Black Holes,” Science Vol. 321, No. 5892 (22 August 2008), pp. 1060-1062 (abstract).

Talking about astrophysics in Scotland reminds me that this October 8-10, the Royal Observatory Edinburgh will play host to a workshop titled Habitability in our Galaxy, discussing (among other things) exoplanet habitability, possible venues for life in the Solar System, and prospects for SETI. Should be interesting, especially as our ideas of a galactic habitable zone have been undergoing fruitful growth in recent years.

Benjamin August 22, 2008 at 2:02

As for galactc-core science fiction, I might point out Greg Egan’s fantastic new novel Incandescence, much of which takes place on an ark orbiting and deriving its energy from the accretion disk of a black hole or neutron star. The novel is half occupied with the efforts of the aliens dwelling on this ark trying to rediscover general relativity based on only tidal forces without access to astronomy – looking outside when you’re in the accretion disk of a black hole can be dangerous – and, aside from the occasional desperate need for diagrams (which he fortunately provides on his website), is quite a fascinating account of how you could derive general relativity without knowing about electrodynamics or astronomy.

The other half of the novel is concerned with an AI derived from humans – a very Egan character, more or less indistinguishable from the protagonist of, say, Schild’s Ladder – navigating the data networks of much more sophisticated aliens dwelling in the galactic core, and discovering an ark of this sort. There are some brilliant descriptions of how self-replicating technology might work, among other things.

Incandescence is almost more fictionalised science than science fiction, as the important part of the novel is the scientific progress of these arkdwellers, and of the interstellar species discovering about one another, and there is unfortunately a shade too little characterisation. Certainly very good, though.

Administrator August 22, 2008 at 14:25

Benjamin, thank you for this recommendation. I’ll put Incandescence at the top of my reading list!

ljk March 31, 2009 at 18:40

http://www.technologyreview.com/blog/arxiv/23259/

A black hole with a view

Physicists have calculated what the universe would look like from inside a black hole, and not just for fun

Tuesday, March 31, 2009

Ever wondered what it’s like to fall into a black hole? Wonder no more!

Andrew Hamilton at JILA at the University of Colorado and a pal, Gavin Polhemus, have created a video showing what it might look like (10MB .avi). And it’s an impressive peice of work.

Hamilton provides a commentary for this and other videos which dismisses some of the myths that have grown up around black holes, such as the notion that falling inside one would engulf you in darkness.

Not by any means. According to Hamilton and Polhemus, inside a black hole the view in the horizontal plane is highly blueshifted, but all directions other than horizontal appear highly redshifted.

The work is not just for fun. In fact, calculating what the universe looks like from inside a black hole is an important exercise because it forces physicists to examine how the laws of physics behave at breaking point.

Take the principle of locality, which seems to be severely tested inside a black hole. This is the idea that a point in space can only be influenced by its immediate surroundings. But when space is infinitely stretched, as physicists think it is at the heart of a black hole, the concept of “immediate surroundings” doesn’t make sense. So the concept of locality begins to lose its meaning too.

And that provides an interesting “thought laboratory” in which physicists can ask how ideas such as quantum mechanics and relativity might break down

It also throws up some entertaining corollaries. For example, space is so heavily curved inside a black hole that ordinary binocular vision would be no good for determining distances, says Hamilton.

But trinocular vision could the trick instead, he adds.

Ref: http://arxiv.org/abs/0903.4717: The Edge of Locality: Visualizing a Black Hole from the Inside

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