Whirlpool galaxy (M51), optical image. The Whirlpool galaxy is locked in a gravitational interaction with the smaller irregular galaxy NGC 5195 (cream, left of M51). It is thought that NGC 5195 passed through the disc of M51 around 500 million years ago, then swung back through the disc to its present position behind M51 in the last 100 million years. Dark lanes of dust in the spiral arm of M51 are revealed in silhouette against the bright NGC 5195. These would otherwise be invisible at visible wavelengths. M51 is around 50,000 light years in diameter, around half the size of our Milky Way. The galaxies lie around 37 million light years from Earth in the constellation Canes Venatici.