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.
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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?.
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.How many galaxies?
In 1912 in an area just 1/30th of the moon. In all, this telescope has imaged over 3,000 galaxies deep in space. 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 observerat 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 variablesare still visible.)
The expanding universeWhen astronomers look deep into the universe they are looking back in time towards the Big Bang when the universe began.
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 Waythrough 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!The Milky Way - our own galaxy
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.M100 - a galaxy similar to our own
In 1918 about 200 billion stars.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 Sagittariusthan elsewhere.
This shows that the centre of the galaxy is in
The Earth is 30,000 light years from the
centre and the galaxy is 100,000 light years
You can work this out for yourself using a
star map program which shows the clusters.How do we know where we are in our galaxy?
Hubble Heritage Gallery about 200 billion stars.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.