Articles

Systems of Stars
Sir Patrick Moore for Galaxy Zoo

What is a Galaxy? The short answer is that it is a system of stars – each of which is a sun. Our Sun, together with the Earth and the planets, belongs to a galaxy made up of around 100 thousand million stars. It is a flattened system, and if we look along its main plane we see many stars in almost the same direction producing the lovely band of light which we call the Milky Way. It is natural to refer to our system as the Milky Way Galaxy; it is slightly above average in size and mass, but it is no means exceptional. Our nearest really important neighbour, M31 in the constellation of Andromeda, is considerably larger than our system. (M31 means that this was the 31st object in a famous catalogue drawn up in the eighteenth century by the French astronomer Charles Messier. The M numbers are still used, but official we turn to the New General Catalogue or NGC; M31 is also NGC 224).

The galaxies are widely spaced. They are found in definite groups or clusters, some sparse and others populous; we belong to the Local Group, which consists of three large systems – our Milky Way, the Andromeda Galaxy M31 and the Triangulum Galaxy M33, plus few of medium size and over two dozen dwarfs. Around 50 million light-years from us we find the Virgo cluster, with over a thousand galaxies of which the largest, M87, far outranks even M31. Because the galaxies are so far away, few are visible without optical aid, and indeed only three can usually be glimpsed; M31, which is on the fringes of naked-eye visibility, and the far south Clouds of Magella, which are satellites of the Milky Way and are ‘only’ around 170,000 light-years from us.

It is widely supposed that all the galaxies are receding from us, so that the entire universe is expanding, but this is not the whole story. The galaxies inside a definite group moving randomly with respect to each other, and indeed M31 is approaching us; in the remote future (certainly over 1,000 million years hence) there will be a collision. The individual stars will seldom hit each other – we may draw a comparison with two orderly crowds moving in opposite directions – but the dust and gas will be colliding all the time, triggering star formation. However, each group of galaxies is racing away from each other group, so that the ‘expanding universe’ concept is correct. During the 1920s the American astronomer Edwin Hubble (after whom the Space Telescope is named) used spectroscopic methods to show that the galaxies really are external systems rather than parts of the Milky Way, and also that the greater the distance of a galaxy, the faster it is moving away from us. This does not mean that we are in a privileged position; the expansion is universal in every sense of the term.

Galaxies are of many kinds. Some are spiral in form like graceful Catherine-wheels; others are elliptical, some more or less spherical, others irregular in outline. There is endless variety; for example we have ‘barred spirals’, where the arms issue from the opposite ends of a bar running along the central plane. Our Milky Way is a barred spiral, although the bar itself is not very obvious, and of course measurements of the exact shape are not easy to make simply because we lie inside the system, around 26,000 light-years from the centre. The centre itself seems to contain a massive Black Hole and this is also true of most large systems. The Milky Way, like other spirals, is rotating; the Sun takes about 225 million years to complete one circuit – a period often called the cosmic year. One cosmic year ago, even the dinosaurs had yet to make their entry. It is interesting to speculate about what conditions will be like one cosmic year hence…

We have found out a great deal about the galaxies, but we cannot claim that our knowledge is at all complete, and there are so many of them that even classification is a problem. This is where Galaxy Zoo is so helpful. It has been organised by professional astronomers, and demonstrates yet again that in astronomy professionals and amateurs can work closely together to their mutual benefit. And – who knows? – while taking part in the programme you may be luck enough to make some unexpected and spectacular discovery. The universe, with its majestic star-cities, is indeed a wonderful place.