That is a beautiful picture of spiral galaxy M 66.
M66 is one member of the Leo Triplet:
In the picture of the Leo Triplet, you can see M 66 at bottom right. M 65 is at top right, and edge-on NGC 3628 is at top left.
This galaxy trio shows signs of having danced around each other for such a long time that they have spent most of their star forming material. It is typical of galactic interaction that it tends to compress the gas inside the galaxies and cause this gas to start forming stars at a high rate. Famous examples of galactic interaction that leads to intense star formation are the Antennae interacting galaxies and M 51 with companion NGC 5195:
M 51 and NGC 5195. Note the blue color of M 51, which results from huge numbers of newly formed hot stars. Also note the much more neutral-colored arcs and loops surrounding NGC 5195. They are streams of mostly old stars that have been thrown out of NGC 5195 because of tidal interaction with M 51. NGC 5195 has no star formation itself.
Like NGC 5195, M 66 is surrounded by arcs and loops of old to intermediate stars that have been thrown out of the galaxy because of tidal interaction with M 65 and NGC 3626. Like M 51, M 66 has star formation, too. There are obvious blue patches inside M 66, which are sites of recent star formation. There are also pink patches of emission nebulosity in M 66. They are not so obvious in this (otherwise very beautiful and colorful) image, but they show up better in other color pictures of M 66.
So M 66 has a large population of old stars, but it has ongoing star formation too. This sets it apart from its "Messier neighbour", M 65. Although one bluish cluster or association is visible in high resolution images of M 65, no emission nebulae are detected. Star formation appears to have ceased in this galaxy. Typically, M 66 and M 65 have noticably different color indexes: +0.70 for M 66 versus +0.90, which is much yellower, for M 65.
Edge-on Leo Triplet member NGC 3628 has no detectable bluish clusters or associations at all, and no emission nebulae. It is however possible that such clusters and associations exist, which are hidden from our perspective deeper in the disk of the galaxy. But we can see that NGC 3628 has a very puffed-up disk and a long tidal tail of stars emanating from one end of its disk:
http://astronomy.qteaser.com/images/NGC-3628-LRGB.jpg
The Leo Triplet creates an environment with a lot of tidal tugging and stretching. The Leo Triplet is a galaxy group with a small total amount of star formation. It is also a part of the sky where most of the gas and dust has already been spent, and there is little star formatin material left to set up stellar maternity wards. M 66 is the member that won't give up, and which keeps producing young blue stars born in rose-colored emission nebulae.
Sleep, blue little star baby (or rather, big blue star baby, and I don't know what the pink star is doing there) in your lovely pink crib!
Ann
Spiral Galaxy M66
