Cambridge Cosmology: Galaxies

Our own Galaxy - the Milky Way

Our own galaxy consists of about 200 billion stars, with our own Sun being a fairly typical specimen. It is a fairly large spiral galaxy and it has three main components: a disk, in which the solar system resides, a central bulge at the core, and an all encompassing halo.

The spiral galaxy M83 which is believed to be similar in size and shape to the Milky Way (AAO)


The disk of the Milky Way has four spiral arms and it is approximately 300pc thick and 30kpc in diameter. It is made up predominantly of Population I stars which tend to be blue and are reasonably young, spanning an age range between a million and ten billion years.


The bulge, at the centre of the galaxy, is a flattened spheroid of dimension 1kpc by 6kpc. This is a high density region where Population II stars predominate---stars which tend toward red and are very old, about 10 billion years. There is growing evidence for a very massive black hole at its centre.


The halo, which is a diffuse spherical region, surrounds the disk. It has a low density of old stars mainly in globular clusters (these consist of between 10,000 - 1,000,000 stars).The halo is believed to be composed mainly of dark matter which may extend well beyond the edge of the disk.

A view of the Milky Way from the southern hemisphere - towards the centre of our galaxy (AAO)

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