Galaxies come in many shapes and sizes. Among the largest are the “ellipticals”, massive ball-shaped conglomerations of up to a trillion stars. Where they live and how they are built can teach astronomers a lot about the evolution of the universe and what the future has in store for our own Milky Way.
Ellipticals are one of three main classes of galaxies defined by American astronomer Edwin Hubble in 1936. Unlike the other two classes—spirals and lenticulars—ellipticals have almost no discernible structure. They are about as simple as a gathering of stars can be: massive blobs roughly spherical in shape. Through an amateur telescope, they appear as no more than a round, fuzzy patch of light against the dark background of the night sky.
More than any other galactic class, ellipticals exhibit a tremendous range of sizes and masses. The smallest ones, dwarf ellipticals, are merely a few hundred light-years across and aren’t much larger than globular clusters. The largest extend hundreds of thousands of light-years from one end to the other and dwarf our own galaxy in comparison. The most massive can contain nearly a trillion stars, or about a thousand times more than the Milky Way.
The apparent simplicity of an elliptical galaxy—a massive ball of stars—can be deceptive. Within their uniform halos, ellipticals show a rich and complex history tracing back to the roots of the universe itself.
Ellipticals are old. The stars that live within them are among the oldest in the universe. Unlike their spiral cousins, elliptical galaxies have shut down their star formation engines. Devoid of much of the needed gas and dust, they no longer create new stars. Instead, they hold tightly to ancient stars that have lived for billions of years.
The lack of interstellar gas is a relic from the ellipticals’ violent pasts. The motions of stars within these galaxies, along with sophisticated computer simulations, reveal that they are most likely the product of many galactic collisions. As spiral galaxies collide, much of the gas is stripped off and flung into intergalactic space. What is left behind gets rapidly compressed, triggering a blast of star formation known as a starburst. The wave of new stars further depletes the galaxy’s gas reserves through intense stellar winds and supernovae that blow gas clouds into deep space.
The densest regions of the universe provide fertile feeding grounds for growing ellipticals. In rich clusters, galactic collisions are common. The largest galaxies pull in many smaller ones and build themselves by cannibalizing their neighbors. In dense galactic clusters, like the Coma Cluster located nearly 300 million light-years from Earth, the majority of galaxies are ellipticals. In this cluster, they have all congregated towards the cluster’s center leaving the remaining spiral galaxies out towards the edges. Sitting at the core of the cluster is a massive elliptical galaxy—NGC 4874—ten times larger than the Milky Way and surrounded by a swarm of smaller dwarf ellipticals.
One of the most intriguing aspects of giant ellipticals is what they hide in their cores. Deep in the centers, ellipticals each contain a supermassive black hole. A typical black hole forms from the death of a massive star and weighs at most a few times more than our sun. The black holes in galactic centers, however, can contain the mass of several billion suns. No single star can do that. The formation of these supermassive black holes is an area of active research. Most likely, they reflect the formation history of the galaxy. Each galactic collision funnels material down to the center of the galaxy where individual star-sized black holes can merge and grow.
Elliptical galaxies are among the largest single star systems in the cosmos and preserve a long history of galactic collisions. Housing up to a trillion suns, they may even provide a peek into the future of the universe. Will ellipticals continue to consume all spiral galaxies? Will the distant future lead to a universe of only elliptical galaxies where all star formation has long since ceased? These massive stellar warehouses contain hints at both the past and future of our universe and, for that reason, will draw the astronomer’s gaze for many generations to come.