Stars form perilously close to Milky Way’s black hole

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 09 January, 2009

Two stars have been located just a few light years from the galactic centre, confirming that stars can form perilously close to a black hole.

Artist concept of young, blue stars encircling a supermassive black hole at the core of a spiral galaxy like the Milky Way. New observations demonstrate that stars can form at our Galaxy's core, despite the powerful forces generated by the black hole. Image: NASA, ESA, and A. Schaller (for STScI).

At the centre of our Milky Way Galaxy, and suspected to lurk in the centres of all galaxies, is a black hole. Our Galaxy hosts a four million solar-mass black hole, and as a result, the galactic core is subject to powerful gravitational tides strong enough to rip apart clouds of molecular gas. Gas clouds, that is, from which new stars are born. This presents something of a paradox, however, since astronomers using the Very Large Array of radio telescopes have identified two protostars located only a few light years from the galactic centre, blissfully unaware of the monster that lurks in the neighbourhood. The findings categorically demonstrate that stars can indeed form very close to a black hole, and don’t, for example, ‘fall in’ after forming elsewhere.

“We literally caught these stars in the act of forming,” says Smithsonian astronomer Elizabeth Humphreys, who presented the findings at the American Astronomical Society meeting earlier this week. In order to probe the central region of the Milky Way, which is shielded by dense blankets of dust and gas, Humphreys and her colleagues searched for water masers, radio signals that point to the locations of nascent stars still cocooned within their blankets of star-forming material.

The two new protostars were located at seven and ten light years from the galactic centre, and along with another protostar identified in an earlier study, show that star formation is still taking place near the Milky Way’s core. The finding also corroborates recent theoretical work, in which a supercomputer simulation predicted that star formation could occur within a few light years of the Milky Way’s central black hole.

This 0.6 by 0.7-degree infrared photograph of the galactic centre shows a large population of old, red stars, but a new study reveals two protostars within 10 light years of the Galactic centre. Image: 2MASS/E. Kopan (IPAC/Caltech).

But in order for this to be possible, the molecular gas at the centre of our Galaxy must be denser than previously believed. A greater density would make it easier for a molecular cloud’s self-gravity to resist the tides exerted by the black hole, allowing it to not only hold together but also to collapse and form new stars.

“We don’t understand the environment at the galactic centre very well yet,” says Humphreys. “By combining observational studies like ours with theoretical work, we hope to get a better handle on what’s happening at our Galaxy’s core. Then, we can extrapolate to more distant galaxies.”