Edwin Hubble guessed wrong about what elliptical galaxies turn into
Edwin Hubble thought elliptical galaxies were an early stage that later evolved into spirals. That idea turned out to be false. Ellipticals are mostly old, red swarms of stars that have run low on the gas needed to make new ones, though they can grow a disc by swallowing gas and smaller galaxies.
Hubble set out three main galaxy classes, elliptical, spiral and lenticular, in his sequence and his 1936 book The Realm of the Nebulae. Ellipticals are roughly egg-shaped or spherical with smooth, almost featureless images. Together with lenticular galaxies they make up what astronomers call early-type galaxies, a name that outlived the evolutionary guess behind it.
With little gas or dust between their stars, ellipticals form few new stars, few open clusters and few young stars, which gives them their reddish colour; their stars are on average far older than those in spirals. Bursts of star birth can still flare up when they merge with other galaxies. Instead of orderly discs, their stars follow somewhat random orbits in three dimensions. Large ones are ringed by many globular clusters, typically in two groups: one redder and rich in metals, the other bluer and metal-poor.
No other galaxy type spans such a range. Dwarf ellipticals may be no bigger than a globular cluster, yet hold substantial dark matter; supergiants, or cD galaxies, can pack over a hundred trillion stars and dominate their clusters. Diameters run from 3,000 to more than 700,000 light-years. Famous examples include M87, whose central black hole was the first imaged by the Event Horizon Telescope, IC 1101 at the heart of Abell 2029, and Maffei 1, the nearest giant elliptical.
Every massive elliptical has a supermassive black hole at its centre; a survey of 46 ellipticals along with dozens of galactic bulges found one in each, and the black hole's mass tracks the galaxy's closely through relations such as M-sigma. Ellipticals account for roughly 10 to 15 percent of galaxies in the Virgo Supercluster, so they are not the most common type overall, but they crowd toward the centres of galaxy clusters and compact groups. Their structure closely resembles the bulges of disc galaxies, hinting, though not settling, that similar processes built both.
Source: Elliptical galaxy
