What happens when a star reaches the end of its life? Well, if it's a massive enough star (about 8 times as massive as our Sun), it will explode in a brilliant flash called a supernova (there are a few different ways to get a supernova, and this one is called a Type II supernova). What's left behind is a stellar remnant—either a neutron star or a black hole depending on it's mass. If the remnant mass is less than about 8 times the mass of the Sun, then it will be a neutron star; more than 8 times the Sun's mass and it's a black hole (this is called the Tolman-Oppenheimer-Volkoff limit, analagous to the Chandrasehkar limit for accreting white dwarf stars if you're familiar). To make a stellar remnant above the T-O-V limit, you need a supernova progenitor to be about 20 times the mass of the Sun. Stars this massive aren't very common, so black holes formed this way should be relatively rare.

How can we test if the T-O-V limit is true? Last year, NASA's Chandra X-ray Observatory observed a supernova in the galaxy M 100 named SN 1979C. The interesting thing about this supernova is that we know which star did it! It's so close that we can look in archival images and see which star was there before the explosion. From this, we know that the star was approximately 20 times the mass of the Sun. New observations from Chandra indicate that there is an accreting compact object where the star was. This means that there is either a neutron star or black hole that is consuming the gas that was thrown out in the explosion. However, the Chandra observations can't distinguish between it being a black hole or a neutron star! While this seems like a failure on science's part, it's not really because it confirms that the T-O-V limit is around 20 times the Sun's mass. Any more massive, and we should clearly see the effects of a black hole. Any less, and we should see an accreting neutron star.

If it is a black hole, it's interesting to note that it would be the youngest one we've ever discovered. While we observed the explosion of the star, and therefore the formation of the black hole, in 1979, it really occured 55 million years ago and it took all that time for the light to reach us from the distance of M 100.

Photo: X-ray/Optical/Infrared composite image of galaxy M 100. The accreting compact remnant resulting from SN 1979C is the bright X-ray point indicated with an arrow. (Image Credit: X-ray: NASA/CXC/SAO/D.Patnaude et al, Optical: ESO/VLT, Infrared: NASA/JPL/Caltech)

It appears that every massive galaxy hosts a supermassive black hole in its centre.  A recent article in the journal Nature described the discovery of black holes as massive as 10 billion Suns.  Even by supermassive black hole standards, that's pretty big.  For some perspective, the black hole in the centre of our Galaxy is about 4 million times the mass of the Sun (or about a 2500 times smaller).  Is there some upper limit to the mass of black holes?  Probably, because they only seem to grow to be a fixed fraction (about 0.3%) of the mass of the galaxies they live in.  We're still working on figuring out how black holes and their host galaxies can grow in tandem.

Here is the orginal Nature article, and a discussion of the result in the New York Times.

This is an optical image of NGC 4889, an elliptical galaxy that hosts one of the big black holes profiled in the article.