Stephan's Quintet is of course an old favorite, but it deserves being shown again. Besides, this processed version of the Hubble data is probably brand new.
Stephan's Quintet was very controversial a few decades ago, when Halton Arp
used this galaxy group to argue that the idea of cosmological redshift must be wrong. Arp argued that all the galaxies in Stephan's Quintet, including bluish foreground galaxy NGC 7320, were interacting with each other and therefore at the same distance. But the redshift of NGC 7320 is completely different from the redshifts of the other galaxies. Arp argued that the galaxies were at the same distance and the redshifts must be wrong, but modern astronomy says that the redshifts are correct and the galaxies are at different distances.
Back in the days of Halton Arp, astrophotography wasn't good enough to bring out many details in Stephan's Quintet. These days Hubble can resolve the galaxy group well enough to clearly demonstrate that NGC 7320 is a very different beast than the other four galaxies. NGC 7320 has a small faint bulge and a small faint nucleus, whereas the other galaxies have large yellow bulges which are so bright that we can't even see their nuclei in today's APOD. (Hubble is of course able to see their nuclei, but a shorter exposure is needed top bring down the brightness of the bulges and bring out the nuclei. Or else, possibly, a different processing of the Hubble data is needed.)
NGC 7320 has also got a bluish disk which looks very grainy in today's APOD. We can see many small clusters, and we may possibly see some individual stars. In fact, we probably do, in view of the fact that there are several faint orange points scattered over the face of NGC 7320. They are probably red giants or supergiants.
The other galaxies, particularly NGC 7318B, have thrown out incredible tidal features which are full of considerably more course-looking blue or blue-green clumps. These are huge clusters and nebulae. There is a lot less graininess in the blue parts of NGC 7318A and B and NGC 7319 than there is in NGC 7320, because at their distance Hubble can't resolve large numbers of small clusters.
In other words, Hubble has given us clear proof that NGC 7320 is a foreground galaxy. Personally I think NGC 7320 resembles nearby dwarf galaxy NGC 300
. (The picture is by J-P Metsävainio.) According to Principal Galaxy Catalog, NGC 300 is about 8.5 million light-years away and about 0.2 as bright as the Milky Way, but according to http://en.wikipedia.org/wiki/NGC_300
NGC 300 is about 6 million light-years away, which would make it even smaller. NGC 300 is a dwarf galaxy, and so is NGC 7320.
This GALEX ultraviolet image shows relatively nearby spiral galaxy NGC 7331 at upper right and Stephan's Quintet at lower left. NGC 7320 is the undisturbed-looking oval there. It could be that NGC 7320 is a satellite galaxy of NGC 7331, or at least that they belong to their own "Local Group".