Hello there; I’ve got a question for the astronomers: how sparse is our local cluster?

Submitted by Steel-Rains t3_11jsqkl in askscience

I read a post stating that the solar system is in a remote part of the Milky Way Galaxy and that the Milky Way is in a remote part of the universe. I know that collections of galaxies are called clusters; how sparse is our cluster? Do we know how many galaxies in a certain proximity is “average” and how do we differ from this? This is the question I can’t seem to find an answer too. Thanks!

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morningcoffee1 t1_jb5gs3a wrote

Our "cluster" is called the "local group" and contains about 30-35 galaxies. The two largest are our own Milky Way and the Andromeda Galaxy. The radius of the local group is ~5 million lightyears.

For comparison, the Virgo Cluster is relatively close by at 60 million lightyears and is a bit larger in radius: 7.5 million lightyears *but* contains between 1300 and 2000 galaxies.

So, using that comparison, as well as most other local clusters, our local group is indeed quite sparse.

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Heittovaihtotiedosto t1_jb6gf03 wrote

”30-35 galaxies”

Why is there a range? Is our own galaxy blocking the view and we can’t be certain what is out there, or is it a matter of definition or consensus on what galaxies are considered part of the local group?

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morningcoffee1 t1_jb6sbml wrote

Yes exactly, our Milky Way blocks a lot of the view. Plus most of them are irregular and small (compared to our Milky Way at least) and then it's easy for things to "get lost"

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lifeontheQtrain t1_jb61sia wrote

Are the galaxies on average much smaller in the Virgo cluster?

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morningcoffee1 t1_jb67te6 wrote

nope... a similar smorgasbord of large and small. However, the Virgo cluster does have several elliptical galaxies which "we" do not have. We have two large spiral galaxies and a bunch of irregular ones, but no elliptical.

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[deleted] t1_jb5kmeq wrote

[deleted]

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morningcoffee1 t1_jb5qe33 wrote

No, that has nothing to do with that. We're talking WAY different scales here.

For life to develop you do need some stability of course, I mean, a supernova going off next door won't be helpful. :-) But for life we're talking about the local environment. Things like: how far is the nearest star? What is the chance of trouble nearby?

As for a crowded set of Galaxies, the chance of galaxy collisions is indeed higher, but when galaxies collide, in general, stars do not collide. You do get a BUNCH of new star formation, but locally things can still be stable. Looking at individual galaxies within the Virgo cluster we're not seeing much different structures like what we see in our own Milky Way...

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rootofallworlds t1_jb6pqcw wrote

The Local Group is what's known as a galaxy group, with a few dozen galaxies, in contrast to larger galaxy clusters with hundreds or thousands of galaxies. So yes, it's sparse compared to the giant clusters, but not exceptionally sparse. The more massive clusters are much rarer than the small groups.

https://www.mpe.mpg.de/2040034/clusters_and_groups_of_galaxies

What seem particularly rare are isolated "void galaxies", but there are a few known. https://esahubble.org/images/potw1545a/

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