Action in middle of galaxy

THE past century or so has seen huge advances in astronomers’ knowledge of the huge star system in which we live.

We are well aware that it is a spiral-shaped galaxy, and that it is one of three large galaxies in our part of the universe: the dominant members of the so-called local group of galaxies.

The news this year is focusing on the very centre of our galaxy, where astronomers have recently noted that the galaxy’s supermassive black hole brightened considerably at infra-red wavelengths.

This has resulted in the suggestion that some gas orbiting the black hole may have been disturbed, possibly by the close passage of a star.

Even though it was detected at a wavelength not visible to

the human eye, the brightening was quite dramatic.

It was first noted just over three months ago, on May 12, by an astronomer using one of the two giant Keck telescopes atop Mauna Kea in Hawaii.

He measured the increase in brightness to be a factor of 75.

Not only is this the brightest it has been since observations began, but it is also the most variable.

Something is definitely happening! The centre of the galaxy is a complex region for observation.

Aiming their radio telescopes towards this direction in the constellation of Sagittarius, astronomers first noted in the 1950s that this location was a source of radio emission.

It became called Sagittarius A, because of the nomenclature that was in use at the time.

As radio telescopes improved, it turned out that there were two distinct components to the emission, one to the east and one to the west.

However, there is a particularly interesting source of emission in the western component, which was discovered in 1974 using a radio telescope in West Virginia in the USA.

In 1982 it was named Sagittarius A* (pronounced A-star).

Astronomers now feel sure that the radio source Sagittarius A* corresponds with a black hole in the galactic centre, some 4 million times the mass of our Sun.

Indeed, this was postulated decades ago, and I recall that there was much discussion of this topic in the physics department at the University of Tasmania at the time, when I was studying as an undergraduate.

It was even used as part of an examination question in third-year physics, set by the well-known radio astronomer Professor Graeme Ellis.

There is plenty of evidence for this being the location of a supermassive black hole.

In particular, astronomers have been tracking stars moving near that spot, and have noticed that they have very high speeds: up to several thousands of kilometres per second. In contrast, our Sun moves through the galaxy at only some 20km/sec.

The enormous speeds of objects in the vicinity of a black hole are caused by the extremely strong gravitational field.

The more massive and compact the central object, the faster will be the motion of an object orbiting at a particular distance.

The whole idea often makes me think of insects swarming around an outside light on a warm summer evening, with the insects closest to the light dashing around it the fastest.

We can’t be sure exactly what has happened to cause this dramatic event in the middle of the galaxy, but we can state with certainty that it happened a long time ago.

Because the galactic centre is some 26,000 light years distant from us, the brightening happened that long ago — that is, about 26,000 years ago, because it has taken that long for the light to reach us.

This was long before the beginning of human civilisation, which is generally regarded as having taken place some 6000 years ago. Martin George is Manager of the Launceston Planetarium.