Astronomers have identified the smallest known black hole. The puny object weighs only 3.8 times the Sun's mass and spans just 24 kilometres across.
The black hole is believed to have formed from the collapse of a massive star when it ran out of fuel.
Astronomers are not sure what the smallest possible mass is for black holes formed this way, but they estimate that it is somewhere between 1.7 and 2.7 times the Sun's mass. Less massive objects are expected to collapse into dense neutron stars instead of black holes.
"This black hole is really pushing the limits," says Nikolai Shaposhnikov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, who carried out the study with Lev Titarchuk, also of Goddard. "For many years, astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question."
The black hole they studied is part of a binary system called XTE J1650-500, where the black hole and an ordinary star orbit around each other. Gas stolen from the ordinary star heats up and emits X-rays as it spirals into the black hole.
As in other similar systems, the intensity of the X-rays rises and falls in a semi-regular way, producing what are known as quasi-periodic oscillations (QPOs). The reason for this is still debated, but some scientists say it is due to gas repeatedly piling up near the black hole and then getting swallowed.
Regardless of how they are produced, the frequency of the QPOs appears to be closely related to the black hole mass. Shaposhnikov and Titarchuk have previously used this relationship to calculate the mass of three other black holes that had already been weighed using well-established techniques and found good agreement between the different approaches.
Now, the two have used the Rossi X-ray Timing Explorer satellite (RXTE) to study QPOs from XTE J1650-500. They calculate the black hole weighs 3.8 times the Sun's mass, with an uncertainty of 0.5 times the Sun's mass. The next smallest black hole with a well-measured mass is called GRO 1655-40. It weighs about 6.3 times the Sun's mass.
Exotic black holes
Even smaller black holes may have been produced in the violence of the universe's earliest moments. Some of these primordial black holes could be microscopic, but so far none of the exotic black holes have been detected.
There is also a chance that microscopic black holes could be created in the Large Hadron Collider particle accelerator due to begin operating in 2008. But these black holes should evaporate with a flash of radiation in a fraction of a second.
At the other extreme are black holes with billions of times the mass of the Sun that reside at the centres of some galaxies. The heaviest known weighs as much as 18 billion Suns. By contrast, the black hole at the centre of our Milky Way galaxy weighs about 4 million Suns and is about 20 million kilometres across.
A black hole's size is defined by a boundary called its event horizon. Anything that passes closer to the black hole's centre than the event horizon - including light - is doomed to be swallowed by the black hole.
The new results were presented on Monday at a meeting of the American Astronomical Society's High-Energy Astrophysics Division in Los Angeles, California, US.
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
Have your say
Only subscribers may leave comments on this article. Please log in.
Only personal subscribers may leave comments on this article
Tue Apr 01 21:39:54 BST 2008 by Kate
I have no idea
Tue Apr 01 22:38:53 BST 2008 by William Critchley
What has Kate no idea about?
Wed Apr 02 13:56:46 BST 2008 by David Smith
I have no idea... :-)
Wed Apr 02 19:58:15 BST 2008 by Iggy
...and it does take two ideas to have one idea per Einste and that makes it harder to answer. Good Luck Kate!
Mini Black Hole
Wed Apr 02 00:03:22 BST 2008 by Benjamen
I want my own lil black hole
Wed Apr 02 00:18:00 BST 2008 by No One
About time and not too soon I might add.
All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
If you are having a technical problem posting a comment, please contact technical support.