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83 votes
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Is there a better explanation of this picture showing the very distant star "Earendel"?

I did the annotation of that figure for the press release, so let me start by apologizing for the poor explanation, and then try to dig deeper into what's going on :) (although you already seem to ...
pela's user avatar
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12 votes
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Local neighbourhood and Superclusters

Structure formation Structure in the Universe — galaxies, galaxy groups, clusters, and superclusters — forms from regions of the Universe which are denser than the average; dense enough the overcome ...
pela's user avatar
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10 votes

How do astronomers know that distant galaxies aren't actually nearby star clusters?

Other answers have provided general ideas about how to confirm that individual sources are distant galaxies and not clusters, so I'll focus on the question of how astronomers in the Dark Energy Survey ...
Peter Erwin's user avatar
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10 votes
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Why does the Bullet Cluster have many names?

Most astronomical sources don't have a poetic name such as the Bullet Cluster. Such names are usually coined for objects that may have a particular interest to the public (or the astronomical ...
pela's user avatar
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7 votes

Does the Milky Way belong to a Galaxy cluster

The milky way is not part of a galaxy cluster. The local group is on a lobe of the Virgo supercluster. Not all galaxies are members of large clusters, ours is in a small group. However the only ...
James K's user avatar
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7 votes
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Does the Milky Way belong to a Galaxy cluster

The Local Group contains 54 plus galaxies. Don't know that that counts as much of a cluster. Next up in scale, the Milky way is part of the Laniakea Supercluster That contains about 100,000 galaxies, ...
Wayfaring Stranger's user avatar
7 votes
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Is it possible for galaxy clusters to interact?

Yes. Theoretically, structure is expected to form first on small scales (stars and stellar clusters), and later on increasingly larger scales — galaxies, groups, and eventually galaxy clusters (see e....
pela's user avatar
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7 votes
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Would gravitational waves be subject to external gravitational perturbations?

Firstly, gravitational waves (GWs) are not an echo - we measure the direct signal. The process you describe here is known as gravitational lensing, the deviation of (usually) light rays due to ...
AstroFloyd's user avatar
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7 votes

How do astronomers know that distant galaxies aren't actually nearby star clusters?

We measure the distance. A cluster is a group of stars in close proximity to each other. For relatively near stars you can directly measure the distance via the parallax. That's what the Gaia ...
planetmaker's user avatar
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6 votes
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how to calculate distance between galaxies in the distant past

I'm assuming you're talking about physical distances (as opposed to any of the other distance measures in cosmology). The comoving distance to a galaxy at redshift $z$ is $$ d_C(z) = \frac{c}{H_0}\...
pela's user avatar
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6 votes
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Why is this Einstein ring pink?

OK, guessing rather wildly in the absence of solid information.... The press-release page is here; that provides a bit more information than the news article, and includes two images of (different ...
Peter Erwin's user avatar
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5 votes

Fingers of God effect for galaxy clusters

I think the key point is how the velocity dispersion in a given structure compares with the velocity dispersion you would expect from the difference in the Hubble flow across the linear extent of the ...
ProfRob's user avatar
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5 votes
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Fingers of God effect for galaxy clusters

I'm going to break your question down into two sections. Do galaxy clusters have measurable peculiar velocities? The answer to this depends slightly on what is meant by "peculiar velocity". In the ...
zephyr's user avatar
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5 votes
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Why does a right-handed circular polarized wave get lagged when going through ionized plasma?

What is the mathematical difference between their refraction indices? The conventions on the signs of the direction of magnetic field vectors and of charges. The tutorial specifies several things ...
uhoh's user avatar
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5 votes
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Why does the first measurements of the thermal Sunyaev-Zel’dovich effect from ALMA show a temperature decrease and not an increase at the cluster?

Calling it a "temperature decrease" is kind of misleading. (Possibly this is a side effect of the tendency to use "brightness temperature" in radio astronomy to mean measured ...
Peter Erwin's user avatar
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5 votes
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What is the distance of the closeset galaxy outside our local group and what is its name?

Your question can be interpreted in many ways. Distance from Milky Way. Distance from the geometric center of the Local Group. Distance from the gravitational center of the Local Group The galaxy ...
Schwern's user avatar
  • 514
4 votes

Why does the Bullet Cluster have many names?

Almost every object in space outside the solar system has multiple names (identifiers) because different astronomical surveys use different naming conventions. The Bullet Cluster has at least 25 ...
David Hammen's user avatar
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4 votes
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Is there a consistent missing mass ratio for Galaxy Clusters?

Yes, all clusters, where sufficiently precise measurements available, require dark matter to explain their dynamics. Yes, the dark matter should be concentrated around galaxies (though not as much as ...
ProfRob's user avatar
  • 155k
4 votes

3d distance between the Galaxies inside the galaxy cluster

You can't calculate the distances to galaxies in a cluster like that to any useful precision. The mean redshift will give you an estimate of the mean distance of the cluster, but the individual ...
ProfRob's user avatar
  • 155k
3 votes

false to stable vacuum transition

The standard picture of vacuum decay events is very much like nucleation in a phase transition: it starts somewhere, and then expands at a constant speed - lightspeed in this case. So the future ...
Anders Sandberg's user avatar
3 votes
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What distance is considered 'close' for the 9th nearest neighbor of a galaxy?

If galaxies were randomly distributed (a spatial Poisson process), then the probability of having $N$ galaxies inside a radius $r$ sphere is $\Pr[N]=\lambda^N e^{-\lambda} / N!$ where $\lambda = (4\pi ...
Anders Sandberg's user avatar
3 votes
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Are 3D coordinate data from Sloan DSS-III available & easily accessible to non-pros?

BTW if anyone wants a quick and fast query to solution do the following: Go to https://skyserver.sdss.org/dr12/en/tools/search/sql.aspx. Paste a query like this: ...
kauii8's user avatar
  • 326
3 votes

Are 3D coordinate data from Sloan DSS-III available & easily accessible to non-pros?

SDSS DR12 Catalog Data looks like a good starting point, apparently pretty open to those willing and able to figure it out. Their SciServer Compute site hosts Jupyter notebooks to query CasJobs in SQL....
Mike G's user avatar
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3 votes
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What is the role of the random catalogue in calculating the two-point correlation function?

The point of the two-point correlation function (pun not intended) is to describe how clustered the galaxies in the universe are. Astronomers want to know if they're all bunched up in tight bundles ...
zephyr's user avatar
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3 votes
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Centre (projected) of a galaxy cluster

Short answer, no, they don't. Longer answer, it's complicated. There are, in essence, five different measures for the centre of a galaxy cluster, based on different physical properties of the cluster,...
Alex's user avatar
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2 votes
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Difference in the baryon acoustic oscillation measurements

I think the most important reason is that galaxy clusters more strongly adhere to the BAO shape than individual galaxies do. Galaxies tend to have more dispersion than clusters, making the BAO signal ...
zephyr's user avatar
  • 15k
2 votes

How to find out if a galaxy is in a specific cluster

You want to use NED (NASA Extragalactic Database) rather than SIMBAD, since the former has more galaxy- and cluster-specific information. I'm going to assume you have a reasonably accurate position ...
Peter Erwin's user avatar
  • 17.1k
2 votes

Does the Abell 1689 supercluster have angular momentum?

I have not studied this galaxy cluster specifically. However, I can speak about systems like galaxy clusters that have a large number of bodies that are in close enough proximity to have a non-...
Unique Worldline's user avatar
2 votes
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Generating random galaxy catalogs for correlation functions

Generating random galaxy catalogs for correlation functions ... the random catalog can be just be a uniform distribution of points in the same volume as the real data. However, real-world galaxy ...
Rob's user avatar
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