November 4, 1999: The Hubble telescope has caught a cosmic dance between two spiral galaxies. The larger galaxy, NGC 2207, is on the left; the smaller one, IC 2163, is on the right. Their dance has already caused quite a stir. Strong gravitational forces from NGC 2207 have distorted the shape of its smaller dance partner, flinging out stars and gas into long streamers that extend 100,000 light-years toward the right-hand edge of the picture. Eventually this dance will end. Billions of years from now the two galaxies will become one.
Two galaxies don't have to bump into each other to meet. They merely have to pass close enough to get caught up in each other's gravitational stranglehold. And "close" could mean 100,000 light-years, the distance between the galaxies in this Hubble picture. Galaxies possess gravitational forces that can slowly pull close objects toward them. More massive galaxies have stronger gravitational forces. In this picture, the strong gravitational forces of the heftier galaxy, NGC 2207, lock both objects in an orbital embrace. These galaxies are the closest they've been in 40 million years. They are destined to continue swirling around each other, slowly falling closer together until the two become one beefy galaxy.
Spiral galaxies contain large concentrations of gas and dust. A merger between two spirals condenses the gas clouds, igniting star birth. Astronomers believe that many of today's galaxies, including the Milky Way, were assembled in the same way. The galaxies in this image reside 114 million light-years from Earth in the direction of the constellation Canis Major.