Fact Impact - Galaxies

Fact Impact: A high-speed run-down of facts about Galaxies
26 June 2011

Interview with 

Andrew Pontzen


Figure 1: The Andromeda Galaxy


  • Galaxies are vast collections of gas and stars
  • The Milky Way - the galaxy we live in - is about 100 thousand light years across...
  • ...and is estimated to contain 200 billion stars.
  • Since early last century we've known that there are other galaxies beyond our own
  • And now it's thought the visible Universe contains several hundred billion galaxies
  • The Andromeda GalaxyBy looking at the movement of gas and stars in individual galaxies we can measure the forces of gravity...
  • ...and typically the forces are very strong, far stronger than we would have initially expected...
  • That's led us to believe all galaxies are surrounded by clouds of invisible stuff known as dark matter, which accounts for the unexpected strength of gravity.
  • Actually we now think it's down to the dark matter that the galaxy forms in the first place
  • In the early Universe, dark matter clumped together and then pulled gas in...
  • ...the gas turned into stars, forming the first mini-galaxies...
  • ...but those first galaxies would have been much smaller than our own Milky Way, perhaps ten thousand times smaller.
  • Bigger galaxies are formed over time as small galaxies merge together...
  • ...and this merging is still going on today - the Milky Way is likely to merge with our neighbour galaxy, Andromeda, starting in about three billion years from now
  • Galaxies come in many sizes and shapes,
  • The faintest and tiniest are called 'ultra-faint dwarfs' and shine with the light equivalent to only a few thousand Suns
  • Whereas the brightest shine with the light of a trillion or more Suns.
  • Some appear to be flat disks of cold gas and stars
  • Barred Spiral Galaxy NGC 1365Whereas others are almost spherical and contain mainly stars and very little cold gas
  • Despite some progress in recent years, it's not fully understood why there are these two different types
  • In some parts of the Universe, galaxies have formed close together
  • We call these regions galaxy groups - we live in the local group, which contains around 40 galaxies
  • Really large galaxy groups are known as 'clusters' and can contain up to a thousand galaxies, all bound together by their gravitational attraction
  • ...and these clusters are the most massive gravitationally bound objects in the Universe, containing up to a thousand trillion times the mass of the sun


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