The centre of the Galaxy

Galactic Centre
IMAGE: A HKL colour composite of the Galactic Centre region. The central black hole is located in the centre of the box. CREDIT: Max-Planck-Institut fur extraterrestrische Physik.

At the centre of the Milky Way sits a supermassive black hole. But how exactly do you see something that captures all light that goes near it? The answer is by inference; you watch what happens nearby and infer what is there using some standard physics like you might do in school.

Our galaxy, the Milky Way, appears as a faint white band of 100 billion stars across the sky. It is actually a spiral galaxy but looks like a band because we live inside it. In one of the arms to be precise. The centre of the Galaxy is in the direction of the constellation Sagittarius. The very centre, the supermassive black hole, was properly identified in the 1970s by the detection of radio waves from an area named Sgr A*. Within a few light years of the centre are ten thousands of stars. This makes it a very dense region indeed as near the Sun the typical spacing of stars is about 4 light years.

Astronomers at the Max-Planck Institute have been observing this region, using the New Technology Telescope (NTT) and the Very Large Telescope (VLT), since 1992. They study the stars motions which are possible to see, with a suitable telescope, as the stars are moving quite quickly near the centre. They have created a nice mpg movie showing the motion of the stars in a region of space about 20 light days across (about 40 times the distance of Voyager 1 from the Sun).

By splitting the light into its constituent colours - a technique called spectroscopy - they can identify the elements that make up the outer layers of the stars they observe. This is a bit like cosmic fingerprinting. Doing this, they have found that there are three types of stars near the centre; those giving out light from helium (He I), those which contain carbon monoxide (CO) which absorbs some of the light and relatively boring stars with featureless spectra. This seems to imply that at least two periods of star formation have occurred in the centre of the galaxy.

Using all their observations, at different parts of the spectrum from radio light to X-ray light, they suggest that the central black hole has a mass of about 3.6 million solar masses (a solar mass is 1 million times the mass of the Earth) and is spinning about half as fast as the theoretical maximum spin rate.

Posted in astro blog by Stuart on Tuesday 12th Jul 2005 (15:56 UTC) | Permalink
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