Kinds of galaxy

A galaxy is an island of stars. Ours, the Milky Way, has a few hundred billion. Between galaxies there is almost nothing at all.

A hundred years ago, nobody was sure that other galaxies existed. Today we know there are at least a few hundred billion of them.

They come in three main shapes.

Spirals are flat discs with bright arms winding out from the middle, like cream stirred into coffee. The Milky Way is one.

Ellipticals are smooth and round or egg-shaped, with no arms. They are mostly old, yellow and red stars.

Irregulars have no tidy shape at all. Most are small.

Galaxies are not scattered evenly. They gather in groups and in great clusters of hundreds or thousands. From far enough away, the clusters join up into long threads with huge empty spaces between them, a little like the inside of a sponge.

How we know there are other galaxies. Until the 1920s, many astronomers thought the faint spiral smudges seen through telescopes were clouds of gas inside the Milky Way. In 1923 Edwin Hubble found a star in the Andromeda smudge of a kind whose true brightness is known from the way it pulses. It looked far too faint to belong to our galaxy. Andromeda had to be a separate galaxy at an enormous distance, now measured as 2.5 million light years. The known universe grew many thousands of times larger in one result.

The main kinds.

Kind	Shape	Stars	Gas and new stars	Example
Spiral	A flat, turning disc with arms and a central bulge, often with a bar	Old stars in the bulge, young in the arms	Plenty. Still making stars	Andromeda, the Milky Way
Elliptical	Smooth ball or egg, no disc	Almost all old	Very little. Star making has nearly stopped	M87, in the Virgo cluster
Lenticular	A disc and bulge without arms	Mostly old	Little	The Spindle Galaxy, NGC 5866
Irregular	No regular shape	Mixed, often young	Often a great deal	The Magellanic Clouds

Hubble arranged these in a diagram shaped like a tuning fork, with ellipticals on the handle and spirals on the two prongs. He did not mean it as a life story, and it is not one. Galaxies do not turn from one kind into the next along the fork.

Sizes. The range is enormous. The smallest dwarf galaxies have a few thousand stars and are fainter than some single stars. Dwarfs are by far the commonest galaxies, though the hardest to see. The largest ellipticals hold more than a trillion stars. The Milky Way is on the large side.

Why the shapes differ. Much of the answer is collisions. A cloud of gas that settles gently keeps its spin and flattens into a disc, which is how spirals form. When two large galaxies collide, their stars almost never hit one another, because the gaps between stars are so wide. But their gravity scrambles the neat orbits into random ones, and the gas is used up or thrown out in a burst of star making. What is left is a smooth elliptical with little gas. This is why the giant ellipticals are found in the middle of crowded clusters, where collisions are most frequent. In the early universe, seen by looking far away, galaxies are smaller, clumpier and more often colliding.

Nearly every large galaxy has a very massive black hole at its centre. When gas pours into one, the surroundings of the black hole can outshine all the galaxy’s stars together. Such an object is called a quasar.

How galaxies gather.

Level	What it is	Size
Group	A few dozen galaxies. Ours, the Local Group, has more than 50 known members, most of them dwarfs	About 10 million light years
Cluster	Hundreds to thousands of galaxies bound by gravity. The nearest large one is in Virgo, about 54 million light years away	10 to 30 million light years
Supercluster	A loose gathering of groups and clusters	Hundreds of millions of light years
The cosmic web	Threads and sheets of galaxies round nearly empty spaces called voids	Fills the universe

Clusters are the largest things held together by gravity. Anything bigger is being pulled apart by the expansion of the universe, which is the subject of the next lesson.

How many. Counting galaxies in the deepest pictures ever taken and scaling up to the whole sky gives at least a few hundred billion in the part of the universe we can see. Some estimates that include faint dwarfs too small to detect yet are several times higher.