Supernova in the Whirlpool Galaxy
20 million light-years away, a massive star catastrophically ran out of fuel and collapsed in an explosion called a supernova. Nova is the Latin word for "new," as the sudden appearance of a star was historically interpreted as a star being born. However, these stars are only temporarily bright enough to be seen as individuals out of the billions of others in their host galaxies and represent the deaths of stars.
A respectable spiral galaxy may produce several supernovae every century. The Whirlpool Galaxy, a.k.a. Messier 51, has produced three since 1994. This over-achieving galaxy probably had help from its nearby companion. The two are clearly interacting. The tidal forces that they mutually exert, stir up the available gas and dust to create new stars, including ones destined to become supernovae. Each of the last three supernovae in M51 (1994I, 2005cs, 2011dh) were in one of the galaxy's spiral arms, which is where you would expect to find a hot, massive, and newly-formed star.
The red, green, and black dots in the graph represent the respective luminosities in red, green, and blue light. Note how the overall color of the expanding fireball has become increasingly more red, as it expands and cools.
SN2011dh.pdf (Posted August 11, 2011)