A spiral galaxy (M51).
Galaxies that have similar shapes also have similar gas and dust content, and similar internal motions. Hubble developed the first attempt at classifying galaxies according to their shape. He divided galaxy shapes into three categories, and then divided these categories into subcategories, as explained below.
Galaxies with a spiral shape are classed as spiral galaxies. Some spiral galaxies have a large central bulge, and tightly wound arms, so it is difficult to make out the spirals. These types of spirals are classed as "Sa". Galaxies with a small central bulge and loosely wound arms which are easy to make out are classed as "Sc". If a galaxy lies somewhere between Sa and Sc, it is classed as "Sb".
Sa galaxies don't have much gas between their stars, but Sc galaxies do.
Barred spiral galaxies have a "bar" of stars through the centre, with spirals attached to the ends of the bar. If the spirals are difficult to make out because they are tightly wound, the galaxy is classed as "SBa". Those with loose, easy to see spirals are classed as "SBc". If the spirals are wound looser than an SBa but tighter than an SBc, the galaxy is classed as "SBb".
Both spiral galaxies (S) and barred spiral (SB) galaxies are referred to as disk galaxies, because they both have an obvious flat disk. In the photo above, you can't see the disk because you are looking at it from "above". If you look at a spiral galaxy edge-on though, you see a flat disk with a bulge in the middle.
The only difference between spiral and barred-spiral galaxies is that barred spirals have a bar of stars across the middle. Both types rotate quickly, and are usually about ten billion times (10 000 000 000 or 1010) as bright as the Sun.
Elliptical galaxies look like an ellipse (an ellipse is a squashed circle). We think they are actually shaped like a squashed ball (or ellipsoid). Elliptical galaxies do not have spirals or bars, but range in shape from a perfect sphere (ball) to a very flat ellipse. The galaxies shaped most like a sphere are called "E0" galaxies, and the ones shaped like a flat disk are classed as "E7" galaxies. Elliptical galaxies shaped somewhere in between a sphere and a disk are classed somewhere between E1 and E6.
Elliptical galaxies come in all different sizes. Some are a million times brighter than Sun, but the biggest ones are a hundred billion times (100 000 000 or 1011) brighter than the Sun. They do not rotate as a whole.
The three types of galaxy described above can be arranged on a diagram like the one below, which is sometimes called a "tuning fork" diagram because of its shape.
The diagram has two extra classifications, S0 and SB0, which represent galaxies which are not quite elliptical (E) or spiral (S or SB), but somewhere in between. "Elliptical spirals", or S0 (pronounced S-zero) have big central bulges but no spirals, and "elliptical bars" (SB0) have a weak bar structure in the middle but no spirals. Both of these two "in-between" type galaxies are called lenticular galaxies.
Some galaxies are completely irregular in shape and do not fit on the tuning fork diagram above. These are classed as "irregular" (Irr) galaxies. Their main feature is they contain a lot of gas and dust, which allows many new stars to form.
Sometimes we place irregular galaxies on the right hand side of the tuning fork diagram, because then the different galaxies are arranged so that the amount of gas and dust they contain (and number of young stars) increases from left to right.
Irregular galaxies contain lots of gas, dust and new stars, and are irregularly shaped.
Spiral galaxies don't have as much dust and gas as irregular galaxies, and the stars rotate rapidly in the disk in circular orbits.
Elliptical galaxies contain very little dust and gas. They are shaped like ellipsoids (squashed spheres).
See if you can classify the following objects according to Hubble's different galaxy types, as described above. Sometimes even expert astronomers don't agree on exactly which type a galaxy is - galaxy classification is largely a matter of opinion, so don't be discouraged if it takes a while to get the right answers at first.
1. M31: Andromeda Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
2. M32 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
3. M33: Triangulum Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
4. M51: Whirlpool Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
5. M64: Blackeye Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
6. NGC292: Small Magellanic Cloud Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
7. M81: Bode's Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
8. M82: The Cigar Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
9. M87: Virgo A Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
10. M88 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
11. M90 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
12. M94 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
13. NGC4881 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
14. NGC253: Silver Coin Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
15. NGC7479 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
16. NGC7331 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
17. Wolf-Lundmark-Melotte (WLM) Galaxy Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
18. NGC5128 Click a letter if you think this galaxy is: a. elliptical (E), b. spiral (S), c. barred spiral (SB) or d. irregular (Irr). |
