Chapter 22

Nearly there! Here’s Chapter 22 – The Milky Way, all about the beautiful Milky Way! If something looks like it might be wrong, drop me a comment in the box. 😀

1. a) What is the difference between an open cluster and a moving cluster?
Both clusters were formed from a large molecular cloud and all have a small gravitational attraction to each other. However, whilst an open cluster doesn’t move; all of the stars move, in a moving cluster, at the same velocity and in the same direction.

1. b) Is the Pleiades cluster made up mainly of Population I or Population II? How do you know?
The Pleiades are mainly made up of Population I, because when they are seen close up with a good telescope, there is still a lot of nebulosity. This means the star formation process is still happening.

1. c) Name any three open clusters.
The Pleiades
The Hyades
The Praesepe

2. a) What exactly is a globular cluster?
A Globular cluster is like a ball of stars, and the further inside the ball, the more stars there are. So when you view a globular cluster, then it will look like a bright object with lots of stars coming off it. The stars that make up a globular cluster are mainly Population II.

2. b) How were the distances of globular clusters first measured?
Some of these globular clusters contained short-period variable stars. These are variables belonging in the group RR Lyrae, when they have a period of Globular Cluster in Herculesabout a day and are about 90 times more luminous than our Sun. When the magnitude and period have been determined, the distance can be calculated.

2. c) The globular clusters are not distributed evenly all over the sky. Why not?
The globular clusters are not distributed around the sky evenly because we are not in the centre of the galaxy. Only being in the centre of the galaxy can you see the even distribution.

3. a) What is the difference between an emission nebula and a reflection nebula?
An emission nebula use light from nearby stars to ‘excite’ the atoms of gas. The light ionises the gas and causes it to light up. A reflection nebula reflects the light from nearby stars.

3. b) A planetary nebula is neither a planet nor a true nebula. What, then, is it?

A planetary nebula is similar to a emission nebula. However, it occurs at the death of a red giant and only last a few thousand years. When a red giant dies, the material is thrown out into neighbouring space. The hot core then sends out powerful UV radiation, which ionises the material and causes it to light up, into a nebula. The ‘planetary’ comes from it looking very similar to a planet when seen through a low power telescope.

3. c) Why does the Coal Sack appear as a dark mass?
It appears to be dark, because there are not enough nearby stars to light it up. Therefore, the nebula hides the stars behind it, causing a dark area.

4. a) Describe the position of the Sun in the Galaxy.
We are no where near the centre of the Galaxy. If you imagine a disk with spiralled tails coming off it, we are near the end of one of those tails; roughly 38,000 light years from the centre.

4. b) Explain the cause of the Milky Way effect?
When we observe the galaxy, we are looking from the outside inwards. This means that all of the stars between us and the centre would be seen. This makes that area very, very bright. However, we also know that the galaxy is a flattened systems, which means it is on a plane. This means we observe a strip of bright stars across the sky. That is the Milky way effect.

4. c) Why cannot we see all the way through to the centre of the Galaxy?
We cannot see the whole way through because there is too much interstellar dust between all of the stars.The Milky Way

  1. We live in the Milky Way itself, so how in the world can you tell that is the Milky Way?

  2. At night, usually in the summer, if you are somewhere that is dark enough, you can see a big band of stars across the sky. This is the milky way. Now because nearly all stars need to live in a galaxy, that means where we are, we are part of a galaxy, but because of the winter sky, we know we must be near the edge of one. Back to the big band of stars. We know that this also must be a galaxy, because of the shape, width and size. But we know because of these things, that this galaxy must be very, very, very close to wherever we are living. And as there is no evidence surrounding us of two galaxies colliding, it must mean the big band of stars we see, must be in the same galaxy as us. This makes us be in the Milky Way.

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