How Many Galaxies exist

Space is populated with countless other galaxies besides our own. Galaxies in their millions, stretching as far as telescopes can see, are the occupants of deepest space. It was the astronomer William Herschel who, two hundred years ago in the late eighteenth century, began to count the number of stars in the sky. His 'gages' as he called them, showed him that there were more stars 'in some regions of the sky than in others. He then put forward the idea that our Sun and all the other stars are part of a large 'island' in space. But was this theory correct?

Was there just empty space outside our star island? More observations made by later generations of astronomers showed that there were some puzzling features. On the one hand there were the clouds of gas or 'nebulae', which Herschel had discovered, on the other there were great spherical or 'globular' clusters of stars and spirally shaped nebulae. Were these all part of our continent, too, or were they separate? Harlow Shapley, a young American astronomer, discovered that the globular clusters of stars did not seem to be part of the island, but opinion was divided over the spiral nebulae. In the early years of the twentieth century, using large American telescopes and, from 1918, the world's then largest and newest telescope, the '100-inch' (2.5-metre) on Mount Wilson in California, photographs of the spirals were taken and measured under a microscope. The astronomer who measured them was Adriaan van Maanen, a Dutchman working at Mount Wilson, and he claimed that by comparing photographs taken a few years apart he could detect an actual rotation in some of the spirals. He argued that they must be inside our island because, if they were outside (and therefore very distant) this movement would mean such a high speed of rotation that the nebula would tear itself apart.

However, later studies, mainly by the American astronomer Edwin Hubble, made it clear that no rotation had been observed. Hubble had detected stars in the spirals that showed they really were distant objects, lying well outside our own star island. In fact the universe was composed of millions upon millions of star islands. How had van Maanen made his mistake? The answer seems to be not that he had faked his results - he was, after all, a very honest man - but that the measurements he was trying to detect were so small that they were about as large as the errors of the equipment he was using.

What van Maanen thought was a shift due to rotation was a shift of the photograph itself or of his microscope eyepiece. At all events we now know for certain that the spiral nebulae and some other nebulae as well lie far outside our own star island. We now recognize this fact by no longer calling them nebulae - a term which we keep for clouds of gas inside our own Galaxy - but instead call them galaxies. They are vast, just as our own Galaxy is vast, for they all contain millions upon millions of stars, and the distances between them really do begin to beggar our imagina tion. As we have seen, setting off from Earth at the speed of light, we should have to travel for over four years to reach the next nearest star to the Sun.

Then, as we move out into deep space, we travel distances of many light-years before we reach other stars, some of which dwarf our own Sun and even parts of the Solar System. Yet if we set out travelling, say, towards the constellation Dorado (The Swordfish) in the southern skies, and go close to the star 8 Doradus, a bluish-white star some 272 times brighter than our own Sun, we shall then have gone 100 parsecs.

But if we continue on and on until we have gone more than sixty times this distance, only then shall we approach the outer boundary of our own Galaxy. When we have travelled some 15,000 parsecs then, and only then, shall we have reached an edge, and approached intergalactic space. Yet although we have now left the main body of our Galaxy we still have a long way to go until we reach another galaxy. In fact we have to travel an additional 46,000 parsecs before we come to the Larger Magellanic Cloud, so named because in 1520 the explorer Ferdinand Magellan and his ship's crew were the first to notice and chart this cloudy looking object.

We now know it to be a galaxy and recent studies show that it seems to be a kind of spiral with a great amount of material running across its central regions. Containing stars, dust and gas, it spreads out over 3,000 parsecs in space. A further 10,000 parsecs away in a slightly different direction - we should have to make for the constellation Tucana (The Toucan) - lies the Smaller Magellanic Cloud, which is only about half the size of the larger cloud. We have now travelled 63,000 parsecs into space, but we have hardly touched the fringe of intergalactic space. Indeed we can consider the Magellanic Clouds as satellite or companion galaxies to our own.

The nearest very large galaxy to our own is the great galaxy in Andromeda, known as M31 (because it is number 31 in the catalogue of nebulous looking objects published in 1774 by the French astronomer Charles Messier). This is a vast spiral galaxy like our own, though it seems not quite as big. Its diameter is about 50,000 parsecs, so it is over half as large again as our Galaxy.

It also has two small companions. Yet although it is the next nearest large galaxy to us, the distance of M31 is no less than 690,000 parsecs (2% million light-years away) though now we are at distances too far for lIS to be precise, even to a parsec. We are now reaching really large distances, yet we have only moved a comparatively little way into intergalactic space, for astronomers have found that our own Galaxy and the Andromeda Galaxy (M31) are both members of a local group of galaxies. This contains at least 30 galaxies, some large and many small, which all lie within a distance of about one megaparsec (a million parsecs). However, groups or clusters of galaxies are not unusual, many are now known, some of them containing several thousand separate galaxies. After our Local Group, the next nearest is the cluster of galaxies in Virgo (the Maiden), which has some 2,500 members.

This lies at a distance of 20 megaparsecs. For all its great distance, the Virgo cluster is not all that far off in space. Some galaxies are known which lie hundreds and even thousands of millions of light-years into space. How far they go astronomers cannot tell, but one galaxy at least has been discovered at a distance estimated at three thousand million parsecs. Truly galaxies extend as far as observations can take us.