6 Galaxies

The Sun is one of about a hundred billion (10¹¹) stars in our galaxy. It is difficult to determine the structure of the Galaxy as we are located inside it. As well as stars, it contains vast clouds of gas and dust, which can obscure our view in certain directions. However, if you observe the night sky from a dark site on a clear moonless night, you will see the Milky Way, a band of light circling the sky, that comes from many faint stars that cannot be individually distinguished (Figure 13). It reveals the most obvious characteristic of our galaxy, that it has a flattened shape. Careful analysis of the distances and motions of stars in space are required to reveal the true nature of our galaxy, called the Milky Way galaxy.

Figure 13 The night sky as seen from the Atacama Desert in Chile, showing the Milky Way in the direction of the centre of the Galaxy.

The human eye is extremely sensitive, but even with the aid of a telescope it is not ideal as an astronomical detector, because it does not record images. Until the use of photography in the late nineteenth century, astronomers recorded their observations with drawings made at the telescope. Despite the fact that photographic plates were much less sensitive than the human eye, they had one additional critical advantage - they could accumulate the light from a faint object for as long as a telescope could track it (the human eye retains an image for less than a tenth of a second). Photographs, and more recently electronic imaging detectors, reveal a huge variety of galaxies (Figure 14). They revealed that what appeared as faint smudges of light to the eye were in fact vast systems of stars like our own galaxy. (Our galaxy is sometimes referred to as 'the Galaxy' (with a capital 'G') to distinguish it.)

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Figure 14 Examples of different types of galaxy. (a) An elliptical galaxy. (b) A spiral galaxy. (c) An irregular galaxy.

Galaxies are not distributed uniformly in space. Our own galaxy is a member of a small group of about 40 galaxies. Larger galaxy clusters may have more than a thousand members (see Figure 15) and these clusters themselves appear to be arranged into even larger structures.

Figure 15 The Virgo Cluster of galaxies. The dark spots indicate where bright foreground stars were removed from the image. Messier 87, which is visible through a small telescope, is the largest galaxy in the picture (lower left).

Our understanding of the Universe is, not surprisingly, derived largely from the light emitted by stars and galaxies. However, our understanding of the properties and evolution of these stars and galaxies comes from applying scientific principles and mathematical models. As new observational techniques developed and this understanding grew it became more apparent that the objects that can be seen represent only a fraction of the matter in the Universe. The majority of matter, called dark matter, is not visible but is required to understand the properties of the Universe. Some dark matter may simply be in the form of dead stars, but most appears not to be made up of the familiar elements but of some so far unknown constituents. This material is called non-baryonic dark matter. Figure 16 indicates what is currently thought to be the material composition of the Universe.

Figure 16 The matter in the Universe. Chemical elements account for less than about one-sixth of all matter. The majority is believed to consist of non-luminous 'dark matter', the nature of which is still uncertain.