Image galaxy

Spiral galaxies

There are numerous spiral galaxies, one of these is our own galaxy, the Milky Way, and they have a mass that goes from 1 to 100 billion times that of the Sun and an average diameter of 70000 light years.

The galaxy M51 is a typical example of this category of galaxy and is shown in the figure.

They are disc-shaped and have quite a prominent globular nucleus in the middle. From this centre a number of spiral arms branch out and wrap themselves around the galaxy. The older stars are concentrated in the nucleus, while the younger stars are concentrated in the arms. The nucleus and the disc rotate around the disc’s axis, at a speed that changes from its centre to its limits.

The spiral galaxies, as opposed to the elliptical ones, have a great quantity of gas mixed with dust where stars are still forming with varying intensity.

Our nearest spiral galaxy is the Andromeda Galaxy, which is about 2 billion light years away; to reach it we would have to walk for 400000 billion years without stopping.

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