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
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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.
7 'We are stardust'
How did the atoms in our bodies, the Earth,