The Local Group and Other Galaxies

At about the same time as Harlow Shapley was making his breakthroughs with the Milky Way, another debate was raging. The use of large telescopes over two centuries had revealed a considerable number of celestial objects which were not stars. They became known as nebulae, and the brightest of these non-stellar objects had already been catalogued by Charles Messier in 1781. Many are still known today by their Messier numbers, such as M42, the Orion nebula.

In the early part of the twentieth century, observations had divided the Messier objects into two distinct categories. Some were obviously clouds of gas, and these seemed to congregate near the Milky Way. Others showed an elliptical or spiral shape and seemed to occur at random throughout the sky. This latter class of object became known as spiral nebulae and some astronomers suspected that they were collections of stars, just like the Milky Way, only viewed from a much greater distance.

Controversy raged between these astronomers and those others who thought they were gas formations or hazy stars within the Milky Way. Interestingly enough, Harlow Shapley, who had shown such remarkable insight concerning our own Galaxy, was vehemently (and incorrectly as it turned out) opposed to the point of view which stated that spiral nebulae were in fact galaxies in their own right!

As far back as 1755, philosopher Immanuel Kant had expressed the view that these distant objects were perfect analogies to the Milky Way. Yet the matter was only finally ended in 1924 when Edwin Hubble showed that some of the nearby spiral nebulae contained variable stars similar to those within the Milky Way but much further away.

Hubble went on to classify galaxies according to their appearance. He found three broad types: spirals, barred spirals and ellipticals. Any galaxy which did not fit into this scheme was termed an irregular. Hubble meticulously mapped the sky, looking for faint galaxies in order to understand how they were distributed through space. Statistical analysis of his maps showed him that galaxies tend to clump together into groups and larger clusters.

The Milky Way is part of a small group of galaxies, known as the Local Group (see Fig. 1. 4). It consists of approximately thirty known members. The hierarchy does not stop at groups and clusters, however. Clyde Tombaugh, the discoverer of Pluto, recognised that even clusters themselves tend to cluster together! They form massive collections known as superclusters (see Fig. 1. 5) which, as has been shown in the last few decades, stretch through space in chains and filaments.

Fig. 1. 4. This schematic of the Local Group shows that it is dominated by two spiral galaxies: our own Milky Way and the Andromeda galaxy. Each of these galaxies is attended by smaller satellite galaxies, and a number of dwarf galaxies are also present. M33 is another spiral galaxy, and a recent addition is the barred spiral Dwingaloo 1. (Adapted from Ferris, T., Galaxies, Stewart, Tabori and Chang, 1982.)

Fig. 1. 5. This schematic of the Local Supercluster shows how the Local Group is totally dominated by the other much larger clusters of galaxies. Of these other clusters, the Virgo Cluster is the most conspicuous from Earth because its members can be readily seen in a modest telescopes, set in the direction of the constellations Virgo and Coma Berenices. The figure given for each cluster is its distance above or below the X-Y plane in millions of light years

This represents such a fundamental departure from the random distribution of galaxies, that it provides one of the most stringent constraints which must be placed upon any modern cosmological theory, if it is to be successful in explaining the Universe as a whole. Another important question for cosmology is whether the galaxies, or the clusters which contain them, formed first.

Modern cosmology studies the distribution of matter on scales which are far larger than individual galaxies. This means that for many purposes we can imagine the galaxies as being the most fundamental objects in the Universe. In fact, cosmology often assumes that the matter contained in the Universe can be thought of as a perfect gas. There are so many different types of individual galaxies, however, that the cosmological question of how they became the way they are is still important.