That's a superb image of one of my favorite galaxies!
You can almost count the individual brilliant blue stars in it, as well as the large and small pink emission nebulas. And you can marvel at the incredible mess of the torn, tattered and phenomenally broken central dust lane. Humpty Dumpty
, you've got nothing on the dust lane of NGC 4631!
Note the peanut-shaped yellow bulge of NGC 4631. We can tell that this is a barred galaxy.
The blue (and non-blue) gas streamers and tidal tails are fantastic, too. There is a long tail disappearing down to the right. But my favorite out-of-plane tidal features are the "breakers" above the disk of NGC 4631 to the right. They have formed to the right of where satellite galaxy NGC 4627 appears to be about to hit. Surf's up!!
Faint little dwarf galaxies can also be seen here. There is one at about 12 o'clock and another at about 7 o'clock.
Photo: Matthew T. Russell
I love the color balance of today's APOD, but other color balances are possible. In Matthew T. Russell's image at left, where the overall color balance is less blue, the satellite galaxy, NGC 4627, is now neutral-colored instead of blue. Fascinatingly, the neutral color might be the correct one. The B-V index of NGC 4627 is 0.630, and its U-B index is 0.100. Amazingly, that is almost exactly
the same as the B-V and U-B index of the Sun!!!
Okay, the Sun is a little
less ultraviolet than NGC 4627.
It is probable that NGC 4627 underwent a tremendous burst of star formation some time in the past, but now all the brilliant blue stars have died, and only some A- and many F-stars remain from that blast from the past. And there are of course many old and red stars inside NGC 4627, too.
As for NGC 4631 itself, is a mature galaxy with a large yellow bulge whose entire disk is just popping with blue stars and recent and ongoing star formation. If anyone out there knows of another mature yellow-bulged galaxy whose disk is so brilliantly blue and pink and "unsettled" from fantastic star formation, please post the picture here! (NGC 4945
? No. But maybe NGC 2442
, although that galaxy isn't edge-on.)
NGC 4631/4627 and NGC 4656.
Finally, I can't resist posting this picture of the possible interaction between NGC 4631 and the quite nearby NGC 4656. Unfortunately, I have not been able to find out who took the picture.