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Astronomical distances

Light travels at an enormous speed (300 000 km/s) but even so the distances between stars and between our galaxy and other galaxies are so vast that even light takes a long time to travel to us from these distant objects. As you know distances in astronomy can be measure in light seconds, light hours or light years. Using that idea the Sun is 8 light minutes away that means it takes light 8 minutes to reach us from the Sun. When we look at the Sun we are seeing it as it was 8 minutes ago. Light from the nearest star takes over four years to reach us which means it is over four light years away. The Andromeda galaxy is more than 2 million light years away and the galaxy in the top photograph (M81) is eleven million light years away while that in the bottom part of Figure 1 (the Sombrero galaxy) is about 50 million light years away!

The table below gives the distances to some other well-known galaxies.

Galaxy name Distance (light years)
Sagittarius dwarf galaxy 88 000
Andromeda galaxy (M 31) 2 300 000
Whirlpool galaxy (M 51) 37 000 000
Sombrero galaxy (M 104) 50 000 000
Galaxy in the Virgo cluster (M 87) 60 000 000



Even these distances are small compared with the distances of the galaxies shown in the next photograph. This was taken by the Hubble Space Telescope and shows a group of the most distance galaxies so far discovered. It contains over 10 000 galaxies in the constellation Fornax (below Orion) some of them over 12 000 million light years away! It is amazing to realise that the light that is reaching us now from the Sombrero Galaxy started on its journey 50 million years ago and that the view we have of the cluster of galaxies in Fornax is a view of how they looked 12 000 million years ago. What they are like now in out twenty first century we will only know 12 000 million years into the future!


Photo credit: NASA, ESA, S. Beckwith (STScl) and the HUDF Team
 

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© Keith Gibbs 2020