Types of galaxies

About two thirds of spiral galaxies have a bar of stars across the middle, including the Milky Way and Andromeda.

Sorting by shape

Galaxies range enormously in size. NASA notes that the largest contain trillions of stars and can be more than a million light-years across, while the smallest hold a few thousand stars and span a few hundred light-years. To bring order to them, Edwin Hubble sorted galaxies by their appearance in photographs. His scheme is often drawn as a tuning fork: ellipticals on the handle, then lenticulars where the prongs meet, and two prongs of spirals, one with bars and one without. ESA notes that the diagram is now thought too simple, but its basic ideas still hold.

Spirals

A spiral galaxy has a flat, spinning disk with arms that curl out from a central bulge. The arms are where gas is being squeezed and new stars are forming; the hottest young stars are blue and short-lived, so they light up the arms. The bulge is made mostly of older, yellower stars. NASA says barred spirals make up about two thirds of spirals, and that the presence of a bar may show that a galaxy has reached full maturity. Both the Milky Way and Andromeda are barred spirals. Pick "Spiral" and then "Barred spiral" in the model to compare.

Ellipticals, lenticulars and irregulars

Elliptical galaxies are smooth balls or eggs of stars with no disk and no arms. They contain little gas and dust, their stars are mostly old, and they form few new stars. NASA's Hubble pages call them the biggest and most common galaxies in the universe: giant ellipticals reach about 300,000 light-years across, while dwarf ellipticals are only a few thousand light-years wide. Astronomers think many ellipticals are the result of collisions and mergers between spirals, which scramble the orderly disks into a rounded swarm.

Lenticular galaxies, labelled S0, sit between the two. NASA describes them as having the bulge and disk of a spiral but no arms, and the older stars and low star formation of an elliptical. Irregular galaxies have no orderly shape at all; NASA says they range from dwarfs of about 100 million solar masses to large ones of about 10 billion. They are often rich in gas and full of young stars. Galaxies can also be sorted by behaviour: NASA says about 10% of known galaxies are active, with centres more than 100 times brighter than all their stars combined, powered by matter falling towards a central black hole. The brightest of these are quasars.

Age and shape are linked. NASA notes that most galaxies are between 10 billion and 13.6 billion years old, some almost as old as the universe itself, which formed about 13.8 billion years ago. Astronomers think galaxies grow by collisions and mergers, so the mix of shapes changes over cosmic time. NASA's Hubble pages suggest that irregular galaxies, colliding and merging over billions of years, develop structure and become the spirals and ellipticals we see today. Looking far away means looking back in time, and deep Hubble images do show many more small, ragged galaxies in the early universe than nearby.