Galaxies

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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.

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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?.

• 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 Messiers catalogue published in 1773.
  
  M51

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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

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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.

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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.

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The expanding universeWhen 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 Hubbles Law.
• (The picture here is of the most distant galaxy
  so far observed where the variable stars called
  Cepheid variables are still visible.)
• 
  
  

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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!

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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.

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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.

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Hubble Heritage Galleryhttp//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.
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