M 51 is the famous "Whirlpool Galaxy" in Canes
Venatici. This is actually 2 interacting galaxies,
the larger spiral (NGC 5194) and the smaller (NGC
5195), which passed through the main disk of 5194
some 500 million years ago, and now is located just
a bit behind the larger galaxy (37 million light years
away from earth). The gravitational interaction of
these two galaxies has caused contraction of gas and
dust in the arms of the larger galaxy, which led to
intense new star formation. This is the reason for the
spectacularly distinct spiral structure of this galaxy.
In fact, M 51 was the first galaxy whose spiral nature
was discovered (in 1845).

This image was taken while a supernova within M 51 was
visible. It is marked by the arrow. The inset at the
top right corner is an older film image; compare and see
that the marked star was not visible in that earlier image.

Image data:
Camera: DSI-Color
Exposure: 45 seconds x 40
Telescope: 10" Schmidt-Newtonian