Elliptical Galaxies
An elliptical galaxy is a smooth, oval-shaped galaxy with no spiral arms, no flat disc and almost no clouds of new gas. Ellipticals contain mostly older red and yellow stars, and they form very few new stars. They come in an enormous range of sizes, from tiny dwarf ellipticals with only a few million stars, up to the biggest galaxies in the universe: giant ellipticals with over a trillion stars and diameters of more than a million light years.
- ShapeSmooth, egg-shapedNo disc, no arms
- % of galaxiesapprox. 14%Less common than spirals
- Star colourMostly old red and yellowLittle new star formation
- Size rangeapprox. 3,000 to 6+ million light yearsAcross
- Famous exampleM87 (Virgo A)Site of the first ever black hole photograph
- Largest knownIC 1101Possibly over 6 million light years across
What is an elliptical galaxy?
Where a spiral galaxy is flat with a bulge and arms, an elliptical galaxy is closer to a smooth, three-dimensional egg shape. Stars in an elliptical galaxy orbit in many different directions, all crisscrossing each other, instead of moving together in a flat disc. The result is a featureless smooth glow without spiral patterns or dark lanes of dust.
Edwin Hubble sorted ellipticals into eight types from E0 (round like a tennis ball) to E7 (squashed and cigar-shaped). The shape an elliptical happens to have depends partly on how its stars orbit and partly on the angle you happen to see it from.
Old stars, little gas, slow change
Almost all the stars in an elliptical are old (often more than 10 billion years), and the galaxy contains very little of the cold gas that would be needed to make new ones. The reason is partly that almost any cold gas left in the galaxy gets used up quickly, and partly because the supermassive black hole at the centre of every elliptical heats up nearby gas and keeps it from collapsing into new stars.
Because there are no young blue stars constantly being born, ellipticals look much yellower or redder than the bright blue spiral arms of the Milky Way. They are sometimes nicknamed red and dead: red in colour because of their old stars, and dead in the sense that almost no new stars are being formed.
How elliptical galaxies form
For decades astronomers debated where ellipticals came from. The best current answer is that almost every elliptical is the result of one or more galaxy mergers. When two spiral galaxies collide, their flat discs get torn apart by the gravitational chaos and the stars are flung into a random tangle of orbits in all directions. After billions of years the wreckage settles down into a smooth, oval shape, and most of the cold gas has been used up in a burst of new star formation during the collision. What is left is an elliptical.
This explains why ellipticals are most common in the dense centres of galaxy clusters, where galaxies collide most often. It also explains why giant ellipticals are so much bigger than spirals: they are the descendants of dozens or even hundreds of smaller galaxies that have all merged over billions of years.
Dwarf ellipticals: the most common type of galaxy
The giant ellipticals get most of the attention, but the most common kind of elliptical (and indeed the most common galaxy in the whole universe) is the tiny dwarf elliptical. These small smooth galaxies typically contain just a few million to a few billion stars and orbit close to bigger galaxies. The Milky Way has dozens of dwarf elliptical and dwarf spheroidal satellites, including Leo I, Leo II, Sculptor and Draco.
Dwarfs are so faint and small that we cannot see them from very far away. Astronomers think the universe contains many more dwarf galaxies than big galaxies, by perhaps 10 to 1, but they are easy to miss.
M87 and the first photograph of a black hole
One particular giant elliptical, M87 (also called Virgo A), sits at the heart of the nearby Virgo galaxy cluster about 53 million light years from Earth. M87 became world-famous in April 2019, when the Event Horizon Telescope released the first ever direct image of a black hole, the supermassive black hole at the centre of M87. The image shows a glowing orange ring of hot gas around a perfectly dark hole, exactly as Einstein's theory of general relativity predicted.
The black hole at the heart of M87 is around 6.5 billion times the mass of the Sun, more than 1,500 times bigger than the supermassive black hole at the centre of the Milky Way. It is also shooting an enormous jet of high-energy particles out into intergalactic space, which has been studied with telescopes for over a hundred years.
Deeper dive: photographing a black hole in M87
The image of M87's black hole released in April 2019 was a remarkable feat of engineering. To resolve a black hole 53 million light years away, you would need a telescope with a mirror roughly the size of the entire planet Earth. Since that is obviously impossible, a worldwide network of radio telescopes called the Event Horizon Telescope (EHT) linked up to act as one virtual telescope of exactly that size.
Eight radio telescopes, scattered from Hawaii to the South Pole to Spain, all looked at M87 at the same time over five nights in April 2017. The data was recorded onto half a tonne of hard drives, then flown by aeroplane (the data was too big to send by internet) to supercomputers in the United States and Germany. It took almost two years of careful processing to combine all the data into a single image.
The final image shows a ring of hot gas swirling around the black hole and the dark "shadow" of the event horizon itself. The shadow exactly matched what Einstein's 1915 theory of general relativity predicted. In 2022 the same Event Horizon Telescope project released a second image, this time of Sagittarius A*, the much closer (but much smaller) supermassive black hole at the centre of our own Milky Way.
For the other galaxy shapes see spiral galaxies, or visit specific examples: the Milky Way and the Andromeda Galaxy.