79 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 ...
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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 ...
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9 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 ...
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9 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 ...
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8 votes
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Is the Hubble Ultra-Deep Field image representative of the entire sky?

Short answer: both. HUDF is representative on the type of galaxies we can find at that distance (and remember that distance also means time ago). On the other hand, we do not really know how the ...
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7 votes
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How many galaxies are there in the Hercules–Corona Borealis Great Wall?

I'm sorry for the long answer. The result is in the bottom. :) The number of galaxies in the "wall", as in any volume of space, is the number density $n_{\mathrm{gal}}$ times its volume $V_{\...
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7 votes

When will the Milky Way "arrive" at the Great Attractor, and what all happen then?

I don't want to make any assumptions here regarding the Milky Way's presence in the Laniakea Supercluster simply because of how recent the discovery is. The findings could very well be accurate, but I ...
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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....
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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 ...
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6 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 ...
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6 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, ...
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6 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 ...
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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 ...
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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}\...
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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 ...
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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 ...
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5 votes

Temperature of the intracluster medium (ICM)

The answer to your first question is "Yes, the temperature referred to is the 'normal' temperature, reflecting the average kinetic energy of the gas particles". The answer to your second question is ...
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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 ...
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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 ...
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4 votes
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What accounts into calculating the Hubble constant?

I think this might answer your question, I have quoted the important paragraph below. If peculiar velocities could have any value, then this would make Hubble’s law useless. However, peculiar ...
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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 ...
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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 ...
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3 votes
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Temperature of the intracluster medium (ICM)

I second Pela's answer, apart from one addition. The CMB and the intracluster gas do interact with each other via the inverse Compton effect, whereby electrons in the gas cool by giving their energy ...
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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 ...
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3 votes

Is our supercluster part of a galaxy filament?

The Laniakea Supercluster contains several galaxy filaments, even just reading the wikipedia articles you provided you can see the difference in size of the two: Galaxy Filament They are massive, ...
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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,...
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3 votes
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Beta profile fit of Virgo cluster gas?

Schindler et al. 1999: Morphology of the Virgo Cluster: Gas versus Galaxies has details for $\beta$ model fits for Virgo and its subclusters.
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  • 1,453
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: ...
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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....
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