Galaxies shaped like the Milky Way are everywhere in the universe. A thin disc, with arms wound through it. These are spiral galaxies.
The arms look bright because young stars are lined up along them. Young massive stars are blue-white, and they are short-lived. They burn out before they can travel far from where they formed, so they glow like marks left at their birthplaces.
Making stars takes gas. The disc of a spiral galaxy still holds plenty of it, and it goes on making stars to this day. The arms are where that gas is crowded.
There is a bulge at the centre and a halo spread outside the disc. The structure of the Milky Way built up in the previous article is exactly this shape, seen from outside.
Some galaxies look nothing like that. They have no arms and no disc, appearing as featureless ellipses. These are elliptical galaxies.
There are almost no young stars here. Old red stars and little else, with little of the gas that stars are made from. The business of making stars finished long ago.
They are held up differently too. The stars of a spiral galaxy turn together in one direction through the disc. The stars of an elliptical do not. Each runs an elongated orbit of its own, and although the whole does not turn, it does not collapse either.
Think of a swarm of bees. Every one of them moves quickly, and yet the shape of the swarm holds. That the directions of motion are scattered is itself what holds the galaxy up against gravity.
Hubble laid these two out on a single diagram. On the left go the ellipticals, ordered from round to flat. The numbers from to are how flattened they are.
Further right, the diagram splits in two. Above are the ordinary spirals, below the barred spirals with a bar across the centre. Along either branch, left to right, the bulge shrinks and the arms unwind.
At the junction sits , with a disc but no arms, halfway between a spiral and an elliptical.
The diagram is shaped like a tuning fork, and it goes by that name. As a tool for classification it is still used exactly as it stands.
Hubble took the sequence for an order of evolution: a round elliptical flattens and in time grows arms. That is why he called the left early type and the right late type.
We now know the order runs the other way. When spiral galaxies collide, the disc is destroyed and the directions of stellar motion are scattered. The gas is used up or blown away. What is left is an elliptical.
Only the names early and late have stayed in use. They no longer stand for an order, so they are nothing but labels for shape.
The classification itself was not wasted, though. A shape says what its galaxy has been through. A spiral grew up quietly; an elliptical came through a violent merger.