The largest galaxies in the universe are giant ellipticals – huge clouds containing trillions of stars whose overlapping individual orbits create an enormous, fuzzy-edged ball. These monsters can grow to be ten times the size of the Milky Way, but even by these standards, IC 1101 stands out: it has a diameter more than 50 times that of the Milky Way, and is roughly 2,000 times heavier.
IC 1101 lies at the heart of a galaxy cluster called Abell 2029, over a billion light years from Earth. The cluster has an overall mass of around 100 trillion Suns, though most of this is invisible ‘dark matter’. Only the galaxy’s central region is bright enough to be seen in visible light (it was discovered in 1790 by William Herschel). Despite its relative brightness and early discovery, however, IC 1101’s true scale was only realised in 1990 when astronomers detected the faint stars orbiting in its outskirts for the first time. More recent images from the Hubble Space Telescope have confirmed that it is roughly 5 million light years across, while the Chandra X-ray Observatory has revealed an extended halo of hot gas spread across a similar region.
Giants such as IC 1101 are only found at the centre of old, densely packed galaxy clusters, and astronomers think they form from the collisions and mergers of smaller galaxies. Over time, these collisions heat up the star-forming gas within the galaxies, giving it enough energy to escape their gravity. This robs giant ellipticals of the ability to form new stars, so as their more massive, shorter-lived stars age and die, they end up containing only lower-mass, sedate red and yellow stars. The orbits of individual stars also become more chaotic until the kinds of structure seen in spiral galaxies disappear and only a ball of stars in overlapping orbits remains. At the centre of the galaxy, a supermassive black hole provides a gravitational anchor around which each star orbits. Meanwhile, the overall mass of this giant star cloud is still enough for its gravity to keep a loose hold on the surrounding hot gas, creating a halo of X-ray-emitting material around the giant elliptical galaxy, at the centre of the cluster.