Starship Asterisk*

by **ta152h0** » Sun Sep 14, 2014 4:06 am

has it ever been decided that the Milky Way galaxy is M0 ????

by **smitty** » Fri Sep 12, 2014 12:03 pm

Buddy wrote:
smitty wrote:What is the significance of the red arrow associated with the blue dot which, according to the explanation, represents our location? The red arrow is not mentioned in the explanation. Thanks in advance for any help. Overall, very interesting!
Based on the video at the bottom of this page https://public.nrao.edu/news/pressrelea ... luster-gbt, I would say it's an axis. It's very interesting looking at the 4-D structure.

Thank you, Buddy; that short video really is helpful. Must admit to not totally "getting" the whole story, but obviously some excellent work going on here regarding our understanding of our physical location in the universe.

by **J.Gielen** » Fri Sep 12, 2014 7:50 am

DMcCartney wrote:Are anyone else's suspicions roused by what appear to be radiants formed of galaxy points, both inside and outside the boundary, that seem to originate from our galaxy? Is this maybe an indication of a perspective error?

I noticed that too. To me, It seems like artifacts in the data so one could ask how accurate and complete is this data?

by **Nitpicker** » Fri Sep 12, 2014 7:11 am

On re-reading some of the links and earlier comments, I suppose it could be that the white streamlines we see in the APOD are the vector-potential of the "peculiar velocity", that is, streamlines indicating the motion that cannot be explained by Hubble's law (aka the metric expansion of space). If so, I don't really know how to interpret the fact that they all appear to converge on the Great Attractor, nor whether it has very much physical meaning. (I think this is what alter-ego may have been saying in an earlier post, but I didn't realise it at the time.)

On the plus side, if the Laniakea model doesn't have very much physical meaning, my inherent need to understand it suddenly vanishes in a puff of logic (which might even be expressed in 2-D at the quantum scale).

by **Ann** » Fri Sep 12, 2014 4:40 am

Thanks for the suggestion, Nitpicker. Maybe I actually will write to the Stellarium people, unless I'm feeling too lazy.

On a different note, what will really happen to the galaxies closest to the center of mass of the Laniakea supercluster? Perhaps some of them will eventually collapse into a single galaxy, while others just might be flung away. (Or not.) And then after trillions of years, maybe that super-duper galaxy will collapse into one single inconceivably massive black hole.

(And then maybe that black hole will tear a hole into the fabric of spacetime so large and full of energy that a whole new baby universe may burst forth from it.

Or not.

)

Ann

by **Nitpicker** » Fri Sep 12, 2014 2:34 am

Ann wrote:
Nitpicker wrote:
Acrux is a multiple star system with at least two components, probably more. Stellarium and Wikipedia each only list details of two of the components, and they contradict each other somewhat in the characteristics of both components (which I find to be reasonably common when comparing different star catalogues).
The two bright components of Acrux are both early B-type stars, and both are blue. Not only are they intrinsically blue, but they are sufficiently unreddened for the light that actually reaches us from this mulitple star to be blue. Yes, different sources disagree as to exactly how blue the light that reaches us from Acrux is (in other words, they disagree on exactly how negative the B-V index of these components are), but believe me, the combined color of these components is blue and has a negative B-V index. The B-V index is close to -0.2, which is really quite blue as stars go.

Ann

Well, the components of Acrux look a bit blue to me in Stellarium and have comparable B-V indices to Beta and Delta Crucis. But Acrux is brighter than Beta or Delta, so perhaps the crazy Europeans good people who write Stellarium, choose to consider that, when rendering the stars. Or maybe it is just a mistake. Perhaps you should report it to them as a bug. I've found them to be reasonably responsive.

by **Ann** » Fri Sep 12, 2014 1:58 am

Nitpicker wrote:
Acrux is a multiple star system with at least two components, probably more. Stellarium and Wikipedia each only list details of two of the components, and they contradict each other somewhat in the characteristics of both components (which I find to be reasonably common when comparing different star catalogues).

The two bright components of Acrux are both early B-type stars, and both are blue. Not only are they intrinsically blue, but they are sufficiently unreddened for the light that actually reaches us from this multiple star to be blue. Yes, different sources disagree as to exactly how blue the light that reaches us from Acrux is (in other words, they disagree on exactly how negative the B-V index of these components are), but believe me, the combined color of these components is blue and has a negative B-V index. The B-V index is close to -0.2, which is really quite blue as stars go.

Ann

by **Nitpicker** » Fri Sep 12, 2014 1:28 am

Chris Peterson wrote:
Nitpicker wrote:Edit: and the whole basin/streamline analogy would fall apart if there were streamlines moving away as well as towards. Still confused.
I don't think the analogy would fail. Drop a bunch of marbles in a shallow bowl and many will be rolling away from the center. But I agree that the limited information available in the captions is confusing.

The problem with the basin analogy is that it implies a final solution for every element at the "drain" (or beyond the boundary), however curved the path. I think the analogy has confused a lot of people in this topic. The streamlines need to be shown circulating in some way (if that is what the observations actually suggest), rather than converging on the attractor region.

by **Chris Peterson** » Fri Sep 12, 2014 12:51 am

Nitpicker wrote:Edit: and the whole basin/streamline analogy would fall apart if there were streamlines moving away as well as towards. Still confused.

I don't think the analogy would fail. Drop a bunch of marbles in a shallow bowl and many will be rolling away from the center. But I agree that the limited information available in the captions is confusing.

by **Nitpicker** » Fri Sep 12, 2014 12:40 am

Chris Peterson wrote:
Nitpicker wrote:
Chris Peterson wrote:the material isn't so much moving towards a center as it is moving around a center.

APOD Robot wrote:white lines indicate motion towards the supercluster center
The above two quotes would appear to contradict each other at first glance. If the white lines in the APOD are in fact indicative of motion towards the Great Attractor (therefore streamlines, or velocity vector-potential), and material is moving around, not just towards the Great Attractor, then I can only conclude that there must be another set of streamlines not shown, which would indicate motion away from the Great Attractor. I am therefore beginning to lose my tenuous grasp on understanding the Laniakea model, as presented in this APOD.
I think there is material moving away. The caption for the original image even talks about blue flow lines for those galaxies. Darn if I see any in the image, though.

That said, there may well be a net motion towards a central region. But it isn't a central point. It's going to be a diffuse region determined by the net mass of all the material in the supercluster.

Interesting. I never noticed the original caption, which I think is shown under the top image on this page:
https://public.nrao.edu/news/pressrelea ... luster-gbt

However, I think perhaps the caption is in error, and should instead be referring to the blue streamlines outside the boundary of Laniakea, as shown in the image of a video frame at the bottom of the page.

(Whether one considers it a point or region makes no difference to my understanding. For instance, a point with uncertainty defines a region. But I am still confused by all the streamlines heading inwards within the boundary.)

Edit: and the whole basin/streamline analogy would fall apart if there were streamlines moving away as well as towards. Still confused.

by **Chris Peterson** » Fri Sep 12, 2014 12:19 am

Nitpicker wrote:
Chris Peterson wrote:the material isn't so much moving towards a center as it is moving around a center.

APOD Robot wrote:white lines indicate motion towards the supercluster center
The above two quotes would appear to contradict each other at first glance. If the white lines in the APOD are in fact indicative of motion towards the Great Attractor (therefore streamlines, or velocity vector-potential), and material is moving around, not just towards the Great Attractor, then I can only conclude that there must be another set of streamlines not shown, which would indicate motion away from the Great Attractor. I am therefore beginning to lose my tenuous grasp on understanding the Laniakea model, as presented in this APOD.

I think there is material moving away. The caption for the original image even talks about blue flow lines for those galaxies. Darn if I see any in the image, though.

That said, there may well be a net motion towards a central region. But it isn't a central point. It's going to be a diffuse region determined by the net mass of all the material in the supercluster.

Edit: or has nothing yet reached the Great Attractor (apart from maybe the Norma Cluster) from our current perspective in spacetime?

Sure it has. There's material around it right now. But again, it isn't a point, just a region. Galaxy clusters that pass through this region will continue outwards, just like any orbiting body does.

by **Nitpicker** » Fri Sep 12, 2014 12:05 am

Ann wrote:
Nitpicker wrote:
Ann wrote:
I note that the colors of some of the Crux stars have been flipped. It's a problem only for Color Commentators.)
Have they? I can't see which ones based on my own raw, RGB images of Crux. (The map was created with Stellarium.)
My semi-bad. All the colors are correct except that of Acrux, Alpha Crucis, an early B-type star which is quite blue in color.

Ann

Acrux is a multiple star system with at least two components, probably more. Stellarium and Wikipedia each only list details of two of the components, and they contradict each other somewhat in the characteristics of both components (which I find to be reasonably common when comparing different star catalogues). Unlike you, I'm not particularly sensitive to the colour blue, so the simulation presented by Stellarium looks close enough to me, when compared with a range of images of Crux available from a Google search.

by **chuckster** » Thu Sep 11, 2014 11:55 pm

Uh, OK. Maybe I'm imagining too much force behind that accelerating dissipation of the universe. Maybe it's more like a net effect than a burgeoning runaway freight train effect. Plenty of time and opportunity for gravity to attract things together as you'd expect, with the dissipating acceleration gently superimposed ? The general descriptions I get from reading, documentaries, Dr Tyson, etc aren't always put in context with everything else, and stress the same things over and over as sound bites, which is why I am here on the APOD forum today. When I see a big simulation like Laniakea I naturally wonder how such a big chunk of Creation behaves in the context of dark gravity.

by **Nitpicker** » Thu Sep 11, 2014 11:34 pm

Chris Peterson wrote:the material isn't so much moving towards a center as it is moving around a center.

APOD Robot wrote:white lines indicate motion towards the supercluster center

The above two quotes would appear to contradict each other at first glance. If the white lines in the APOD are in fact indicative of motion towards the Great Attractor (therefore streamlines, or velocity vector-potential), and material is moving around, not just towards the Great Attractor, then I can only conclude that there must be another set of streamlines not shown, which would indicate motion away from the Great Attractor. I am therefore beginning to lose my tenuous grasp on understanding the Laniakea model, as presented in this APOD.

Edit: or has nothing yet reached the Great Attractor (apart from maybe the Norma Cluster) from our current perspective in spacetime?

by **Chris Peterson** » Thu Sep 11, 2014 10:11 pm

chuckster wrote:What makes the galaxies move toward a "center" ? Doesn't gravity become a minor player over these distances, due to dark energy, dark matter and space expansion ?

Again, the material isn't so much moving towards a center as it is moving around a center. And no, over these distance scales (within galactic clusters and superclusters) gravity is the dominant force, easily overcoming the expansion of space.

by **chuckster** » Thu Sep 11, 2014 8:00 pm

What makes the galaxies move toward a "center" ? Doesn't gravity become a minor player over these distances, due to dark energy, dark matter and space expansion ? Intuitively, you'd think gravity is acting to bring things together, but hasn't that been proven to not be the case, over large distances ? Can the momentum from space expanding behind a cluster, as well as that left over from the Big Bang close the distance between it and another cluster faster than space expansion can increase the distance between them, or am I lost in circles, here ? Wouldn't be the first time. Dimbulb minds want to know !

by **DavidLeodis** » Thu Sep 11, 2014 7:43 pm

I get a distinct feeling that I may be the only person that does not understand what is going on in regard to this APOD

. My brain hurts

.

(I just like that latter smilie).

by **Ann** » Thu Sep 11, 2014 2:48 pm

Nitpicker wrote:
Ann wrote:
I note that the colors of some of the Crux stars have been flipped. It's a problem only for Color Commentators.)
Have they? I can't see which ones based on my own raw, RGB images of Crux. (The map was created with Stellarium.)

My semi-bad.

All the colors are correct except that of Acrux, Alpha Crucis, an early B-type star which is quite blue in color.

Ann

by **Nitpicker** » Thu Sep 11, 2014 4:47 am

alter-ego wrote:
DougStern wrote:So if everything is migrating to the centre of the supercluster, what happens at the super cluster centre eventually?
What is at the "center" is not known. It's not clear that anything different happens if the center is reached. Keep in mind, the Laniakea watershed model displays boundaries as streamlines with cosmic expansion removed. While the animation shows streamline arrowheads all colliding and disappearing, nothing like that is occurring or will occur. Like a globular cluster, stars are orbiting and passing through the center without special events for the most part. On the scale of a supergalactic cluster, the dynamics are probably similar for galaxies (at least the ones that are already in the attractor "valley". Unlike globular clusters, whose spherical size is not affected by cosmic expansion, Laniakea will expand, changing the galactic density and times it takes for a specific member to reach a "center".

I think Laniakea's shape and size will change on the cosmic time scale, galaxies will get further apart on average, and what galaxies do swing through the attractor, things not change much. For static-sized Laniakea 160Mpc, a maximum member velocity ~15000km/sec, the one-way time to reach the center from the edge ~5 billion years, or about 1/3 the age of the universe.

You can see that the convergent streamlines that come to a sudden end in the animations don't correctly give you insight into the galactic dynamics of the cosmic future.

Good comment. I found it very helpful. I suppose we could think of the model Laniakea as a snapshot in spacetime, of this portion of the universe, as it appears to us now. I imagine that over time, the centre of attraction may shift, as well as the streamlines and the boundary, and some bits may even be flung across the border, incoming and outgoing.

by **Nitpicker** » Thu Sep 11, 2014 4:38 am

Ann wrote:
I note that the colors of some of the Crux stars have been flipped. It's a problem only for Color Commentators.)

Have they? I can't see which ones based on my own raw, RGB images of Crux. (The map was created with Stellarium.)

by **Chris Peterson** » Thu Sep 11, 2014 4:37 am

DougStern wrote:So if everything is migrating to the centre of the supercluster, what happens at the super cluster centre eventually?

I don't think everything is migrating to the center. This model identifies a center of attraction. That's a concept important to orbits.

by **Chris Peterson** » Thu Sep 11, 2014 4:36 am

Guest wrote:When simulated with thousands of given massive structures, virtually every massive object (galaxies) get ejected from the system at high speed. The remaining (typically) 2 most massive structures have a tendency to orbit around a common center of mass, but also the center of mass ends up with a direction and velocity that is derived from the ejection of other massive objects. Run some simulations and you will see. There is nothing homogeneous or spherical about the resulting model unless you consider a massive vast pinwheel of very fast outward moving objects to be somehow homogeneous or spherical. And yes, pinwheel is accurate. Math is a wonderful tool to model these things. Thankfully computers help in visualizing and understand the outcome.

I'm not sure of your point. I agree, most of the mass gets ejected. Homogenized. But in the case of a supercluster, the end result isn't going to be just two galaxies. And there will be no pinwheel.

by **alter-ego** » Thu Sep 11, 2014 4:27 am

DougStern wrote:So if everything is migrating to the centre of the supercluster, what happens at the super cluster centre eventually?

What is at the "center" is not known. It's not clear that anything different happens if the center is reached. Keep in mind, the Laniakea watershed model displays boundaries as streamlines with cosmic expansion removed. While the animation shows streamline arrowheads all colliding and disappearing, nothing like that is occurring or will occur. Like a globular cluster, stars are orbiting and passing through the center without special events for the most part. On the scale of a supergalactic cluster, the dynamics are probably similar for galaxies (at least the ones that are already in the attractor "valley". Unlike globular clusters, whose spherical size is not affected by cosmic expansion, Laniakea will expand, changing the galactic density and times it takes for a specific member to reach a "center".

I think Laniakea's shape and size will change on the cosmic time scale, galaxies will get further apart on average, and what galaxies do swing through the attractor, things not change much. For static-sized Laniakea 160Mpc, a maximum member velocity ~15000km/sec, the one-way time to reach the center from the edge ~5 billion years, or about 1/3 the age of the universe.

You can see that the convergent streamlines that come to a sudden end in the animations don't correctly give you insight into the galactic dynamics of the cosmic future.

by **Ann** » Thu Sep 11, 2014 4:07 am

Nitpicker wrote:On further reading, I've just learned that the Great Attractor -- http://en.wikipedia.org/wiki/Great_Attractor -- is the point to which everything in Laniakea is ultimately "falling" towards, as indicated by the "streamlines" in the APOD. (We are very much on the outskirts of Laniakea). The direction of the Great Attractor in our sky, is near the border of the southern constellations Triangulum Australe (TrA, The Southern Triangle) and Norma (Nor, The Carpenter’s Square).

Using The Pointers, Alpha and Beta Centauri (the two stars we southerners often use to find South on Earth), a line extending from Beta, through Alpha, will point to the Great Attractor, such that The Pointers lie roughly halfway between the Great Attractor and Crux.

The Great Attractor.PNG

Thanks, Nitpicker! I appreciate your helpful map!

I note that the colors of some of the Crux stars have been flipped. It's a problem only for Color Commentators.)

Ann

by **DougStern** » Thu Sep 11, 2014 3:52 am

So if everything is migrating to the centre of the supercluster, what happens at the super cluster centre eventually?

Starship Asterisk*

APOD: Laniakea: Our Home Supercluster of... (2014 Sep 10)

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