M51: The Whirlpool Galaxy

Explanation: The Whirlpool Galaxy is a classic spiral galaxy. At only 23 million light years distant and fully 65 thousand light years across, M51 , also known as NGC 5194, is one of the brightest and most picturesque galaxies on the sky. The smaller galaxy appearing here below and to the left is well behind M 51, as can be inferred by the dust in a foreground spiral arm blocking light from this smaller galaxy. The Whirlpool, pictured above , is visible with binoculars in the constellation of Canes Venaciti . M51 is a spiral galaxy of type Sc and is the dominant member of a whole group of galaxies. Astronomers speculate that M51's spiral structure is primarily due to its gravitational interaction with this smaller galaxy.