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from the show Star Death, STEREO & South Africa’s SKA bid
- 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
- By 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
- Whereas 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