About 70% of galaxies are spirals. They are more interesting than ellipticals, since they are places old stars die and new stars are formed continuously. As we will see, the death of a star produces dust which is the material from which planets form. We guess that planets are essential for the development of life, so spiral galaxies are good places to search for life. But note that, as with ellipticals, we do not have a good understanding of how spiral galaxies formed.
Shaped rather like a fried egg, a spiral galaxy has a flat round outer disc surrounding a small central bulge. The outer disc contains stars, gas and dust. The central bulge contains stars. Around the central bulge there is a halo of globular star clusters.
Diagram of a spiral galaxy
As their name suggests, spiral galaxies have arms which spiral out from the centre like a Catherine wheel firework.
In the rest of this story we are going to focus down on a single spiral galaxy, the one we live in.
Size of Galaxies
Let us use the PennySystem to try to get a feeling for the size of a galaxy. The galaxy where we live (which we will meet next) is about 100 thousand light-years across which, if the Solar System were the size of a penny, would be about 2.5 km meters (1.6 miles) across.
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