The Whirlpool Galaxy is a spiral galaxy
that is estimated to be 23 million light-years from
earth in the constellation Canes Venatici.
It was discovered on October 13, 1773,
by Charles Messier, and is designated as M51.
It is one of the most famous galaxies in the sky and
easily observed by amateur astronomers,
and may even be seen with binoculars.
M51's bright circular disk has a radius of about 43,000 light-years.
Its mass is estimated to be 160 billion solar masses.