Beyond the Milky Way lie hundreds of billions of other galaxies. In 1926 Edwin Hubble sorted the ones he could see by shape, and his scheme is still used. Shape turns out to tell a lot about a galaxy's gas, its stars and its history.
- Elliptical (E0 to E7): smooth, featureless ovals of stars, with little gas or dust. The number gives how flattened the image looks: $n = 10\left(1 - \frac{b}{a}\right)$, from E0 (round) to E7 (most stretched). The largest galaxies of all are giant ellipticals.
- Spiral (Sa, Sb, Sc): a bright central bulge in a flat disk with spiral arms, rich in gas and dust. From Sa to Sc the bulge shrinks and the arms open up. Barred spirals (SBa to SBc) have arms that start from the ends of a bar of stars through the center. The Milky Way is a barred spiral.
- Irregular: no regular shape, often small and gas-rich, like the Large and Small Magellanic Clouds.
Why it works
A galaxy's colors come from its stars. Hot, massive blue stars live only a few million years, so a galaxy that looks blue must be making new stars, which needs gas. Spiral arms and irregulars are full of gas and glow with young blue stars and pink nebulae. Ellipticals have used up or lost their gas long ago; with no new stars, only long-lived red and yellow stars remain, so they look reddish. Many large ellipticals are thought to have grown through mergers of smaller galaxies, which scrambles orderly disks into rounded swarms.
Key idea
Galaxies come as ellipticals (smooth, old red stars, little gas), spirals and barred spirals (disks with arms, gas and young blue stars) and irregulars (no clear shape, gas-rich). The Milky Way is a barred spiral.
Worked example
Worked example: Classify an elliptical
An elliptical galaxy's image is $8$ cm long and $4.8$ cm wide. What is its class?
- $\frac{b}{a} = \frac{4.8}{8} = 0.6$.
- $n = 10(1 - 0.6) = 4$: an E4 galaxy.
- This describes only how it looks from here; a round E0 image might be a flattened galaxy seen face-on.
Answer: E4.
Common mistake
Pictures of galaxies tend to show beautiful spirals, so many people think every galaxy is a spiral like ours. Ellipticals are common, especially in crowded clusters of galaxies, and small dwarf galaxies, mostly faint ellipticals and irregulars, are the most numerous of all. Check: our own neighborhood, the Local Group, has three big spirals (the Milky Way, Andromeda and Triangulum) and dozens of dwarfs.