Chapter 23

Here is the final chapter, Chapter 23 – Beyond the Milky Way… 😦 So, for the last time, if something looks like it might be wrong…. let me know! 🙂

1. a) How did Edwin Hubble measure the distances of galaxies?
He measured the distance by finding Cepheid variables in the unknown ‘starry nebulae’. Using the variables, he could work out the periods and the distances; realising that the ‘starry nebulae’ could not be in our galaxy.

1. b) Why was his value for the distance of the Andromeda Spiral much too low?
Hubble didn’t know that there were two types of Cepheids, Population I and Population II. His calculations were based upon the Population II kind, when in fact they should have been based on Population I.

1. c) Name any three members of the Local Group of galaxies.
Andromeda
Milky Way
Large Magellanic Cloud

Hubble's Galaxy Classification

2. a) ‘All galaxies are receding from the Milky Way galaxy.’ Is this a true statement?

Yes. But the Milky Way is also moving. The whole universe is expanding and therefore receding.

2. b) Draw a diagram to illustrate Hubble’s system of classifying galaxies.

2. c) Draw a diagram to illustrate the areas of radio emission in a radio galaxy.

Radio Galaxy3. What is the recessional velocity of a galaxy if the wavelength of a spectral line at 8000 Å is increased by 200 Å?
∆λ / λ = v / c            ∆λ = 200 Å
v = c (∆λ / λ)            λ = 8000 Å
v = 3×10^5 (200/8000)    v = ?
v = 3×10^5 x 0.025        c = 3×10^5 km/s
v = 7500 km/s = 7,500,000 m/s

4. What are the main characteristics of quasars?
Quasars are incredibly bright objects that look stellar in appearance. In fact, these are so far away, that they cannot be stars. They are very red-shifted and are receding very quickly. They can be a strong source of radio waves, however, not all of them are. We think that the centre of each galaxy may be a similar object to a Quasar.

5. How will we eventually be able to tell whether or not the present expansion of the universe will continue indefinitely?
This is all based around the idea of the Big Bang and the Big Crunch. The mass of the current amount of matter in the Universe is not large enough for the gravity in the centre of the Universe to pull it all back in. However, it does not mean that at some point we will be.So Long and Thanks for All the FishIf you don’t get it…. either ask your parents or watch ‘Hitchhikers Guide to the Galaxy’!

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