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Lesson 5 - Galaxies and Dark Matter

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Luke Bohni

on 26 February 2013

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Transcript of Lesson 5 - Galaxies and Dark Matter

Dark Matter So what does Dark Matter have to do with Galaxies??? What is a Galaxy? Galaxies and Dark Matter A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction Meet Edwin Hubble Edwin Hubble was a well known astronomer from the early 1900's who did a lot of work in discovering and classifying galaxies as distinct objects.
Prior to his discoveries astronomers thought that these objects that could be seen were simply nebulae in our own Milky Way but he showed that they were in fact much farther away and moving away from us. The Ellipticals These galaxies tend to be made up mostly of old stars and don't have any obvious structures or features instead looking like big yellow blobs Hubble's Tuning Fork Hubble came up with the following diagram to help astronomers classify galaxies The Spirals Lesson 5 Spiral Galaxies have amazing spiral arms that seem to emanate from their center and twist around. Their classification is determined by how tightly wound up their arms are and how large their nucleus is. The Barred Spirals Similar to Spiral Galaxies, Barred Spirals have arms that wind around their nucleus. The difference is that they appear to have a bar that crosses through their nucleus from which the arms emanate. So what causes these spirals? Contrary to what you might think, the spiral arms don't rotate around galaxies the way you might think.
If they did, they would get all tangled up but we don't see this happening. Pressure Waves In fact what causes the spiral arms is something called a Pressure Wave.
As this wave travels around the galaxy, it compresses the gases near it causing large blue stars to be born.

These stars however don't live for very long and so as the wave moves on, they die out and new stars are born where the wave now is

As a result, the spiral arms are only giving the impression of moving when in actual fact they are just showing where the pressure wave is at that time Quasars The name Quasar actually stands for Quasi-Stellar-Radio-Source. It is believed that Quasars are essentially the first galaxies that were born in our Universe. They contain super-massive black holes at their center and as matter falls in to these black holes, the massive amounts of energy that we see are emitted. They are huge objects which we only observe at very large distances (suggesting that they existed in the early Universe) and that emit massive amounts of energy. Well as it turns out, galaxies are the reason that we know about this mysterious 'Dark Matter' In the early 1970's astronomers noticed that the speed at which stars were rotating in galaxies was faster than expected. At the speed they were travelling, they should have been flying away from the galaxies a bit like water drops off a spinning wet ball. So how does this lead to Dark Matter? So what is dark matter?
The only way that astronomer could explain these observations was to say that either the laws of gravitation were incorrect or there was a whole lot more mass present in these galaxies.
They were reluctant to say that Gravity was wrong since it has worked so well for us so far, hence Dark matter was suggested. If you can answer that you might just win a Nobel Prize.
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