The Galaxy. Schematic Representations of Our Galaxy

Originally, it was believed that stars were positioned randomly throughout space. With the advent of the telescope, however, it became obvious that this was not the case. The Milky Way, known throughout history as being a misty band of light which stretched across the sky, was shown by Galileo’s telescope to be composed of hundreds upon hundreds of faint stars. It seemed obvious to him when he peered at them, that stars were grouped together in some way or another.

The suggestion that the Sun was part of a much larger, disc-shaped grouping of stars was first made in 1750 by Thomas Wright in his published work, Theory of the Universe. William Herschel attempted to map the distribution of stars in the 1780s and found differences of up to a factor of 600 in the stellar densities of different areas of the sky.

The breakthrough in our understanding of the grouping of stars in the Galaxy and especially the Sun’s place within it, came at the beginning of the twentieth century, when Harlow Shapley studied the distribution of distant spherical collections of stars, called globular clusters. He discovered that most lay in the direction of the southern Milky Way. If Shapley’s globular clusters were to be enclosed by a gigantic sphere, then the centre of that sphere would not be centred on the Sun.

Instead, it would be some 25,000 to 30,000 light years in the direction of Sagittarius. Shapley made a bold assumption and concluded that this marked the centre of the Galaxy since the globular clusters must orbit around it. This conclusion was, indeed, the correct one to draw and changed our view of the Universe from being heliocentric to galactocentric.

In the mid-twentieth century, serious attempts were made to map the structure of our Galaxy. Many lines of investigation have shown that the disc contains spiral ‘arms’ of stars which swirl around the centre of our Galaxy like an octopus on a turntable (see Fig. 1. 3). More recent work on the rates at which stars orbit the galactic centre has thrown up some startling conclusions about the nature of matter contained within the Galaxy.

Fig. 1. 3. Schematic representations of the side elevation (a) and plan view (b) of our Galaxy show Hull it is spiral in form, and that our position is offset from the centre. The Sun is located near the inner edge of the Orion spiral arm. The sweeping band of stars in the night sky - the Milky Way is our view through the plane of the starry disc. The centre of the Galxy is located in the direction of tile constellation Sagittarius. (Adapted from Ferris, T., Galaxies, Stewart, Tabori and Chang, 1982.)

It appears that the stars are being gravitationally pulled by a vast quantity of matter in a spherical halo surrounding the disc and nucleus which traditionally make up the Milky Way. It has proved impossible to detect this matter via any other means except by the gravitational influence it would appear to exert on the stars in the Galaxy’s disc. Hence, its exact nature is somewhat controversial at present and many theories abound.

What seems certain, however, is that the visible matter in the Galaxy, such as that which is contained in the stars, is vastly outweighed by the invisible material. This inequality may be as much as 99 to I, and so we can no longer think of our Galaxy as being just a collection of stars. Instead, our modern cosmological view of the Universe pictures the Galaxy as being a region of the Universe which has a denser-than-average collection of matter within it.

If we extend this line of reasoning, stars are then nothing more exotic than denser-than-average regions of the Galaxy. Exactly how these galactic regions became so dense is one of the driving questions in cosmology.