Follow this Link to a NASA Web site on supernovas. It has a very nice animation and a description of what these objects are.

NGC 4088 is an 11.1 magnitude galaxy in the constellation of Ursa Major. It has a very small, bright nucleus in a complex, broad lens; there are two main spiral arms, with many bright knots, that spring from the ends of the bar. The galaxy seems to be a transition case between normal spiral galaxies and those with bars. It is a member of the Ursa Major Galaxy Cluster and has a close companion spiral galaxy NGC 4085 (just outside the picture above) that it may be interacting with. Also known as Arp 18, it is in Arp's class "spiral galaxies with detached segments…… End of one spiral arm partially disconnected" (Arp).

This galaxy, the supernova, and the companion galaxy NGC 4085, were easily seen visually in Kopernik Observatory's 20-inch telescope.

Based on the published red shift, (and a Hubble Constant of 73 Km/sec per Mpc) a rough distance estimate for NGC 4088 and SN 2009dd is 34 million light years, with a galaxy diameter of about 56,000 light years.

MCG 9-20-92 is a small Irregular or Spiral Galaxy (magnitude 17.6) seen to the lower left of NGC 4088 in the Kopernik picture. It is far more distant than NGC 4088 - a background system.