NGC 6946 - Fireworks Galaxy

NGC 6946 is a rather nearby spiral galaxy, on the border between the constellations Cepheus and Cygnus. NGC 6946 is both a spectacular and dim galaxy, and called the "Fireworks Galaxy" for reasons which will become obvious.

NGC 6946 was discovered by William Herschel on September 9, 1798. At one time, it was suspected to be an outlying member of the Local Group (Hubble 1936).

This galaxy is faint but obvious, its diffuse oval halo brightening considerably toward the center. NGC 6946 is a nebulous object in a well populated star field and forms a nice pair with the star-rich open cluster NGC 6939 located 39' NW.

This beautiful face-on spiral spans nearly 40,000 light-years across, and is seen through a field of foreground stars from our Milky Way Galaxy. NGC 6946 appears quite close to the galactic plane, and is highly obscured by interstellar matter in the Milky Way. At a distance of 10 million light years, this galaxy relatively nearby, but not a member of the Milky Way's Local Group.

Nine supernovae (SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et, and SN 2008S) have been observed in NGC 6946 as of March, 2009. With this number of supernovae, NGC 6946 now has three more than follow-ups M 61 and M 83 (with six each).

Why is this galaxy so active? NGC 6946 is undergoing a tremendous burst of star formation with no obvious cause. In many cases spirals light up when interacting with another galaxy, but NGC 6946 appears relatively isolated in space. A suggested explanation for the high star formation rate is the recent accretion of many primordial low-mass neutral hydrogen clouds from the surrounding region.