Galaxies This lesson deals with important topics relating to galaxies. Each of these topics represents a great body of knowledge and areas of interest to research. Historically, galaxies were called nebulae meaning clouds and only later was it realised that they were vast collections of stars, gas and dust lying far beyond our own galaxy, the Milky Way. The history of how this was discovered is interesting and shows the important advances made through photography to reveal the structure of galaxies and spectroscopy to study their motion. Later the advent of radio astronomy made possible studies of the structure of our own galaxy, the Milky Way, and the discovery of distant galaxies which were invisible to optical telescopes.

What is a galaxy? Long ago astronomers noted ‘fuzzy’ patches of light amongst the stars. Much later in the late 1800s photography revealed the detailed structure of galaxies, but what is a galaxy?. A galaxy is a vast collection of stars, gas and dust. They contain over 200 billion stars and many are flat like a pizza with a bulge in the centre. They are over 100 light years across. In the galaxy shown (M51)* the stars and gas are arranged in spiral arms, like sparks from a slow moving Catherine Wheel firework. The light from the most distant galaxies observed takes over 11 billion years to get to us. There are billions of galaxies out there deep into space. * M51 refers to the number given to this object in Charles Messier’s catalogue published in 1773. M51

Where can you see a galaxy? If you are outside and the sky is very dark (with no moon) and you have good eyesight you may just see a very faint blob of light in the constellation of Andromeda. It looks brighter through binoculars but still fuzzy. It is hard to appreciate that it is in fact a vast collection of stars. This is M31, the Andromeda galaxy. The light has taken 2.2 million years to get to us and is 200,000 light years across! It was named ‘little cloud’ by the Persian astronomer Abd-al-Rahman-al-Sufi in 964 AD and is one of the local group of galaxies. On a dark night you can also see the Milky Way. This is made up of stars, dust and clouds of gas and is a small part of our own galaxy! M31

What shape, how far? Early photography revealed the shapes of galaxies from all angles for the first time and they were classified into spiral, elliptical and irregular. Recent images from modern telescopes including the Hubble Space Telescope (HST) show much more complicated structures such as these colliding galaxies. The light from the nearest galaxies takes a few million years to get to us. The light from the furthest observed galaxies has taken 11 billion years to reach us. These galaxies appear as they were when the universe was very young. Galaxies are also studied using radio telescopes which have discovered new galaxies deep into the universe. These have a very characteristic shape and extend over vast distances. Many galaxies are thought to have black holes at the centre - even our own Milky Way galaxy is thought to have a black hole at its centre.

How many galaxies? The Hubble Space Telescope has imaged hundreds of galaxies in an area just 1/30th of the moon. In all, this telescope has imaged over 3,000 galaxies deep in space. Radio telescopes have detected galaxies invisible to optical telescopes. Estimates are that there are 100s of billions of galaxies in the universe.

The expanding universe When astronomers look deep into the universe they are looking back in time towards the Big Bang when the universe began. In 1912 V.M. Slipher looked at the spectrum of the Andromeda galaxy, M31. He did not know it was a galaxy but he found the lines in the spectrum were nearer to the red end of the spectrum than expected. This effect is called Red Shift and indicated that the object was moving away from the observer at great speed. This method was later used to measure the velocities of other galaxies. In 1923 Edwin Hubble measured the brightness of a pulsating variable star in the Andromeda galaxy, M31. The rate at which the variable star pulsed was linked to its actual brightness so he could estimate how far away it was. He discovered it was way outside the Milky Way. He then applied these measurements to other galaxies. When Edwin Hubble looked at these other galaxies he observed that the further away the galaxy was, the faster it was moving away from us. He came to the conclusion that the universe was expanding. This was Hubble’s Law. (The picture here is of the most distant galaxy so far observed where the variable stars called Cepheid variables are still visible.)

The Milky Way - our own galaxy The Milky Way is the small part of the galaxy we can see. To us on Earth it appears as a band of light across the night sky. There are many ancient myths and legends and it is a wonderful sight. Looking at the Milky Way through binoculars reveals many more stars but also faint glowing areas called nebulae meaning clouds. These are not all the same type, they may be glowing gas clouds where stars are born, as in the constellation of Orion, or the remnants of exploding stars or a distant galaxy beyond our own galaxy. Clouds of gas obscure the view towards the centre of our galaxy, but it was suggested that our galaxy probably looked like the Andromeda galaxy, M31 and other spiral galaxies. M31 has become the most studied galaxy by astronomers over the years. In fact we probably know more about M31 than our own galaxy!

M100 - a galaxy similar to our own Our own galaxy is 100,000 light years across and contains about 200 billion stars. Our own Sun is in one of the spiral arms about 1/3rd of the way in from the edge of the galaxy. The galaxy has young stars in the flat spiral arms and a bulge in the centre. Around the galaxy is a spherical halo of much older stars. The halo has globular clusters of stars and contains mysterious dark matter.

How do we know where we are in our galaxy? In 1918 Harlow Shapley studied the positions of globular clusters. These clusters surround the galaxy like a collection of footballs orbiting in space. The stars in the clusters are 10 billion years old and formed before the galaxy flattened out. Shapley worked out where the Earth must be in relation to these globular clusters and found that there were more in the direction of the constellation Sagittarius than elsewhere. This shows that the centre of the galaxy is in that direction. The Earth is 30,000 light years from the centre and the galaxy is 100,000 light years across. You can work this out for yourself using a star map program which shows the clusters.

Hubble Heritage Gallery http://heritage. stsci Hubble Heritage Gallery http://heritage.stsci.edu/ Some examples from this site are given in the next slides courtesy of NASA and STcI. Note the lanes of gas and dust in the edge-on images of galaxies.