AndrewJ
member
Reged: 08/21/09
Posts: 39
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I have a general question for deep space enthusiasts (albeit which relates to observable DSOs).
Although our Local Group is currently receding from the Virgo Cluster I have heard that we may eventually get drawn into it, presumably when the Virgo cluster gets more massive by drawing in close-by groups. This suggests that all of our Local Supercluster, including the Local Group, will collapse in on itself.
Alternatively, I have read that 100 billion years from now all galaxies bar the Local Group (us, M31, M33, LMC etc) will be beyond the boundary of the observable universe due to the expansion of space. This implies that the Local Group won't get drawn into the Local Supercluster and such galaxies as M82 will continue to recede.
So at the scale of our Local Supercluster who wins? - gravity or expanding space?
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Bill Barlow
sage
Reged: 12/03/07
Posts: 426
Loc: Overland Park KS
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Andrew...
I have read that our local galaxy group is actually moving toward the Virgo supercluster at nearly 1 million MPH while the Andromeda galaxy is moving toward the Milky Way at 200,000 MPH.
But an even larger massive group of galaxies known as the "Great Attractor" lies some 250-300 MLY from earth and is known by Abell 3627. This massive cluster has several thousand galaxies and has such a strong gravitational pull that the Milky Way and Virgo supercluster is moving toward it at some 14 million MPH. So even though the overall universe is expanding, the force of gravity within these large clusters of galaxies seems to still counteract this expansion somewhat.
But we won't be around to witness this eventual merger. Humankind will likely be extinct long before this happens. Take care.
Bill
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GlennLeDrew
Carpal Tunnel
Reged: 06/18/08
Posts: 1678
Loc: Ottawa, Ontario, Canada
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Eventually, expanding space will 'win'. Over long time scales, individual members in 'local' clusters tend to successively merge into one ever-larger galaxy. So what was formerly a cluster becomes a singleton. Of course, it's possible that some of the dwarf galaxies of a group may escape absorbtion, but they will then become isolated units in the ever-expanding universe.
Will the Local group merge with the Virgo cluster? My suspicion: no. If discrete groups are currently separating at a rate which gravity will not halt, then they will continue to do so. This is because the mass of each of these groups should not change significantly because they can primarily feed only upon themselves. The mass of a system will become more centrally concentrated as mergers lead to an ever-growing, dominant member. But as far as other groups lying *outside* its boundary are concerned, the gravitational potential is no different.
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Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover
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AndrewJ
member
Reged: 08/21/09
Posts: 39
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Quote:
I have read that our local galaxy group is actually moving toward the Virgo supercluster at nearly 1 million MPH while the Andromeda galaxy is moving toward the Milky Way at 200,000 MPH.
Yes, M31 is coming our way (and eating M33!). Do you know where you read that the Local Group is moving toward the Virgo Cluster? Wiki states that the Cluster is merely slowing our recession from it. Interestingly, M86 and M90 in the Cluster are both blueshifting to us (although the other galaxies are redshifting).
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AndrewJ
member
Reged: 08/21/09
Posts: 39
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Quote:
Will the Local group merge with the Virgo cluster? My suspicion: no. If discrete groups are currently separating at a rate which gravity will not halt, then they will continue to do so. This is because the mass of each of these groups should not change significantly because they can primarily feed only upon themselves.
Are we aware of any groups moving in the direction of the Virgo cluster that would increase its mass and cause groups that are hitherto receding from it comparatively slowly to get drawn in? Essentially, I am wondering if a run-away increase in the gravitational strength of the Virgo Cluster might overcome expanding space throughout the Local Supercluster. I don't know if this is improbable or if there is any galaxy group near to the Cluster that might add to its mass.
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glava2005
super member
Reged: 04/12/09
Posts: 159
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our local group IS NOT MOVING toward the Virgo cluster but the mass of it is slowing the rate of recession from it about 100-200 km/s. and it is highly unlikely that the Virgo cluster will be able to gather enough mass so fast to counteract the expansion of space.
andrewJ the blue shift of M86 and M90 is caused by their motion within the cluster.
and yes the Milky way and Andromeda will begin their merge in about 2.5 billion years and in about 5 join with the M33 and probably in 10-15 billion our whole local cluster will be 1 enormous galaxy.
who knows in 100 billion years humans might evolve so much to be able to bend space and time so it won`t matter 
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tatarjj
Pooh-Bah
Reged: 04/20/04
Posts: 1316
Loc: Austin, TX
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Quote:
Quote:
Will the Local group merge with the Virgo cluster? My suspicion: no. If discrete groups are currently separating at a rate which gravity will not halt, then they will continue to do so. This is because the mass of each of these groups should not change significantly because they can primarily feed only upon themselves.
Are we aware of any groups moving in the direction of the Virgo cluster that would increase its mass and cause groups that are hitherto receding from it comparatively slowly to get drawn in? Essentially, I am wondering if a run-away increase in the gravitational strength of the Virgo Cluster might overcome expanding space throughout the Local Supercluster. I don't know if this is improbable or if there is any galaxy group near to the Cluster that might add to its mass.
This will not happen. As is easy to see from redshift data, we are moving away from the Virgo cluster. Since we are receding, any other galaxies at the same distance from the Virgo cluster would also be receding. Because we are outside the boundry of galaxies gravitationally bound to the Virgo cluster, the gravitational effect we feel from it and all the galaxies that will ever fall into it is the same as if all that mass were concentrated into a single point. This is an approximation that assumes a radially uniform density of mass, however. If a huge mass of galaxies were concentrated on the other side of the Virgo cluster, and were falling into it, this approximation would be a little inaccurate. But there isn't such a mass, and even if there were, we are so far outside the boundry of space that is gravitationally bound to the Virgo cluster that it wouldn't matter.
Andromeda, maybe M33 (I donno), and some of the dwarf galaxies in the local cluster are all that we are bound to. That said, even if you were to stop the Milky Way and Andromeda galaxies, then release them from rest, I don't know if the gravitational attraction between them would be enough to draw them together. We may be colliding with Andromeda simply because we're on a collision course. This would be easy enough to confirm though through some calculations.
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John T.
Austin, TX
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Edited by tatarjj (11/07/09 11:19 PM)
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AndrewJ
member
Reged: 08/21/09
Posts: 39
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Quote:
andrewJ the blue shift of M86 and M90 is caused by their motion within the cluster.
Yep, like the blueshift of M81.
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who knows in 100 billion years humans might evolve so much to be able to bend space and time so it won`t matter
Might not be a threat. We commonly hear that so thinly-spread are stars within a galaxy that if two galaxies merged probably no stars would collide.
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GlennLeDrew
Carpal Tunnel
Reged: 06/18/08
Posts: 1678
Loc: Ottawa, Ontario, Canada
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Quote:
That said, even if you were to stop the Milky Way and Andromeda galaxies, then release them from rest, I don't know if the gravitational attraction between them would be enough to draw them together. We may be colliding with Andromeda simply because we're on a collision course.
If the relative motions of these two nearby galaxies were magically nulled, one can rest assured that they would certainly be mutually attracted and fall toward each other. M33 would to some extent modify the trajectories, of course.
BTW, methinks this question would be more appropriate for the 'Science of Astronomy...' forum.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery (mostly DIY stuff)
Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover
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AndrewJ
member
Reged: 08/21/09
Posts: 39
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Quote:
As is easy to see from redshift data, we are moving away from the Virgo cluster. Since we are receding, any other galaxies at the same distance from the Virgo cluster would also be receding.
Agreed.
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Because we are outside the boundry of galaxies gravitationally bound to the Virgo cluster, the gravitational effect we feel from it and all the galaxies that will ever fall into it is the same as if all that mass were concentrated into a single point. This is an approximation that assumes a radially uniform density of mass, however.
OK. I'm not quite sure about the inclusion of "all the galaxies that will ever fall into it".
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If a huge mass of galaxies were concentrated on the other side of the Virgo cluster, and were falling into it, this approximation would be a little inaccurate. But there isn't such a mass, and even if there were, we are so far outside the boundry of space that is gravitationally bound to the Virgo cluster that it wouldn't matter.
We're beyond space gravitationally bound to the Virgo Cluster given the Cluster's currently observed mass. Is there no chance the Cluster's harvesting of groups maybe 10 mlya from it would allow it to catch groups 30 mlya from it and eventually, when massive enough, ourselves?
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tatarjj
Pooh-Bah
Reged: 04/20/04
Posts: 1316
Loc: Austin, TX
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Quote:
We're beyond space gravitationally bound to the Virgo Cluster given the Cluster's currently observed mass. Is there no chance the Cluster's harvesting of groups maybe 10 mlya from it would allow it to catch groups 30 mlya from it and eventually, when massive enough, ourselves?
No, no NO! Gravity doesn't work that way. Those galaxies near the Virgo cluster that will merge with it are ALREADY pulling us in that direction, and we're STILL receding. The far-field effect of gravity is completely unrelated to density. A planet doesn't care if it's orbiting just outside the surface of a 10 solar mass supergiant or at the same distance from a 10 solar mass black hole, the effect of gravity is the same. The only time that gravity changes by an object getting more compact is when that object is not uniformly dense, and even then, the gravity only changes considerably at very close ranges. We are definately far enough outside the area of space controlled by the Virgo cluster, and the densities are definately uniform enough, that it doesn't matter.
Edited by tatarjj (11/08/09 12:40 AM)
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Hrundi
Carpal Tunnel
Reged: 02/06/08
Posts: 1505
Loc: Estonia
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As far as I know, the virgo cluster is likely capable of stopping the local groups recession and eventually will accelerate us towards the core of the supercluster.
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glava2005
super member
Reged: 04/12/09
Posts: 159
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Quote:
As far as I know, the virgo cluster is likely capable of stopping the local groups recession and eventually will accelerate us towards the core of the supercluster.
there is almost no chance that that will happen. the entire expansion of space would have to stop or be reduced to very low value in order to let the gravity of the virgo cluster to pull us in slowly. given the current data the rate of expansion will only continue to increase.
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Hrundi
Carpal Tunnel
Reged: 02/06/08
Posts: 1505
Loc: Estonia
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Quote:
Quote:
As far as I know, the virgo cluster is likely capable of stopping the local groups recession and eventually will accelerate us towards the core of the supercluster.
there is almost no chance that that will happen. the entire expansion of space would have to stop or be reduced to very low value in order to let the gravity of the virgo cluster to pull us in slowly. given the current data the rate of expansion will only continue to increase.
Seems you're right. I pulled out some more current studies, and it seems that we're heading towards a rather lonely state of existance within the next 100 billion years 
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1995
Loc: Central Texas
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What are the most current thoughts on the "Great Attractor"?
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David
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AndrewJ
member
Reged: 08/21/09
Posts: 39
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Quote:
The far-field effect of gravity is completely unrelated to density.
Ah, this is the knowledge I was lacking. Thanks.
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The only time that gravity changes by an object getting more compact is when that object is not uniformly dense, and even then, the gravity only changes considerably at very close ranges. We are definately far enough outside the area of space controlled by the Virgo cluster, and the densities are definately uniform enough, that it doesn't matter.
This is good news from an observational point of view. I went out of my way to find the Pegasus I cluster as an example of an inhabitant of another supercluster that I thought would definitley be permanently receding. Now it seems as if even humble M51 will sail out of view.
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