M45 is better known as the Pleiades or Seven Sisters from Greek mythology. The Japanese call it Subaru (ever notice the logo on their cars?).
The Pleiades are a relatively nearby cluster in the constellation Taurus at about 440 light-years away. The Hyades in Taurus are much closer at about 150 light-years distance, but their closeness makes them less impressive a sight because they are more spread out.
The Pleiades are a wonderful sight in a small telescope or binoculars where you can take in the whole cluster at once. The bluish haze across the image is a cloud of dust and gas that the cluster is currently passing through. It was previously believed this was left-over gas from the cluster formation, but the cluster is far too old for any of the gas from that to still be present.
This image is a composite of 20 2-minute exposures which is not nearly enough to bring out the detail present in the blue nebulosity.
