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Physics, Astronomy & Cosmology / Re: Are distant galaxies receding owing only to space expansion?
« on: 23/03/2018 21:44:41 »
(Oops! overlap with chris...)
Within our own solar system, motion of the planets is dominated by gravity.
Within our galaxy, motion of stars is dominated by gravity, but each star has its own independent speed and direction within the general flow.
Within our local galaxy group, motion of galaxies is dominated by gravity, but each galaxy has its own independent speed and direction within the general circulation. Andromeda galaxy is heading towards the Milky Way galaxy at about 100 km/second.
See: https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision
The Hubble "constant" is currently estimated at around 73 km/s per Megaparsec.
So to see an expansion that overwhelms the random motion we see between Andromeda and our galaxy, you would need to look at least 3 Megaparsecs away (Andromeda is only 0.8 Megaparsecs away).
It is only when you look at other groups of galaxies that you start to see signs of the general expansion, so the M81 galaxy group is 3.5 Megaparsecs away, and has a recession velocity of about 300km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_groups
The Virgo Galaxy Cluster is 18 Megaparsecs away, and has a recession velocity of around 1100km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_clusters
Within our own solar system, motion of the planets is dominated by gravity.
Within our galaxy, motion of stars is dominated by gravity, but each star has its own independent speed and direction within the general flow.
Within our local galaxy group, motion of galaxies is dominated by gravity, but each galaxy has its own independent speed and direction within the general circulation. Andromeda galaxy is heading towards the Milky Way galaxy at about 100 km/second.
See: https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision
The Hubble "constant" is currently estimated at around 73 km/s per Megaparsec.
So to see an expansion that overwhelms the random motion we see between Andromeda and our galaxy, you would need to look at least 3 Megaparsecs away (Andromeda is only 0.8 Megaparsecs away).
It is only when you look at other groups of galaxies that you start to see signs of the general expansion, so the M81 galaxy group is 3.5 Megaparsecs away, and has a recession velocity of about 300km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_groups
The Virgo Galaxy Cluster is 18 Megaparsecs away, and has a recession velocity of around 1100km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_clusters
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