The Whirlpool Galaxy is one of the most famous galaxies in the sky, having been discovered by Charles Messier in 1774 and designated in the Messier catalog as M51. In 1781 the companion galaxy, NGC 5195, was discovered by Pierre Mechain. The Whirlpool Galaxy and its companion are often designated M51a and M51b. The distance to the galaxy is about 23 million light-years.
This Hubble image of the heart of M51a at right shows its strong spiral character.
The intriguing image below shows the heart of M51a as taken by the Wide Field and Planetary Camera of Hubble. The pattern is interpreted as bands of dust absorption and is taken to locate the position of a black hole at the center of the galaxy.