The tidal forces of the Milky Way slowly pull apart the Canis Major dwarf galaxy (shown here in red). The stars ripped off in this fashion, surround the galaxy in a vast ring. Image: Nicolas Martin & Rodrigo Ibata, Observatoire de Strasbourg, 2003 Astronomers have found a companion in collision with our galaxy, the Milky Way. The new galaxy is the closest ever found – just 25,000 light years from our Sun and 42,000 light years from the centre of the Milky Way.

The visible part of our galaxy is about 100,000 light years across and the Sun is about two-thirds of the way out from the centre. So the new galaxy is a celestial next-door neighbour – in comparison the Andromeda galaxy is at a distance of 2.5 million light years. It was found in an infrared survey – this light allows scientists to see through the clouds of gas and dust in the disk of our galaxy that normally block views of more distant objects. For this reason the new galaxy has remained hidden from astronomers until now.

M-class giant stars trace out the shape of the new galaxy. These shine brightly in the infrared and look red to the eye – a more familiar example is the star Betelgeuse in the constellation Orion. The galaxy turns out to be a dwarf with its core in the direction of the constellation Canis Major and long trails of stars stretching above and below the disk of the Milky Way. Astronomers now believe that the Milky Way is steadily growing through consuming this and other small galaxies.