Astronomy 162: Professor Barbara Ryden

Monday, February 24


``What a pleasure it is to mix one's mind with geological
Time, or with astronomical relax it.
There is nothing like astronomy to pull the stuff out of man,
His stupid dreams and red-rooster importance: let him count the star-swirls.''
- Robinson Jeffers

Key Concepts

NOTE: To keep this web page at a convenient size for printing out, I've stuck images of galaxies on a separate page: the Galaxy Gallery. Please refer back to it as you go through these notes, in order to obtain a feel for what different types of galaxies look like.

(1) Our galaxy is one of roughly 100 billion galaxies visible to us.

A galaxy is a large assembly of stars, gas, dust, and dark matter, held together by gravity. By counting the number of galaxies in very long-exposure images of small regions of the sky, astronomers estimate there are, on average, a few hundred galaxies per square arcminute (The Earth's Moon, 30 arcminutes across, blocks about a quarter of a million galaxies from our view.) Multiplying by the number of square arcminutes on the celestial sphere (nearly 150 million) tells us that there are (in *very* round numbers) about 100 billion galaxies accessible to astronomers' telescopes.

Most galaxies, though, are very far away, and hence have a low apparent brightness. If you want to see a galaxy without the aid of a telescope, there are four you can choose from:

In old textbooks, the Andromeda Galaxy is referred to as the Andromeda Nebula. Through a small telescope, the Andromeda Galaxy looks like yet another faint, fuzzy gas cloud, like the Orion Nebula, the Ring Nebula, or the Crab Nebula. It took a long to to figure out which nebulae (like the Orion, Ring, and Crab Nebulae) consisted of luminous gas clouds within our galaxy, and which nebulae (like the Andromeda ``Nebula'') were galaxies in their own right.

In the 19th century, Lord Rosse, using what was then the world's largest telescope, determined that some nebulae were spiral nebulae; that is, as revealed in Rosse's sketch (below left), they have a swirling spiral structure.

[In the above illustration, the left-hand image is Rosse's sketch of the Whirlpool Galaxy; the right-hand image is a bad reproduction of a modern photograph of the same galaxy.]

The true nature of the spiral nebulae was revealed by Edwin Hubble in the 1920's. Using what was then the world's largest telescope, he discovered Cepheid stars in the Andromeda Nebula. From the period-luminosity relationship, he discovered that the Andromeda Nebula was outside our own galaxy, and was a galaxy in its own right. From the Cepheid period-luminosity relationship, the distance to the Andromeda Galaxy is currently determined to be 700 kpc = 2.3 million light years. (When you look at that faint fuzzy oval in the constellation Andromeda, your eye is detecting photons that have sped through intergalactic space for 2.3 million years before ending their journey by slamming into your retina.)

(2) Spiral galaxies have rotationally flattened disks, and contain moderate amounts of gas and dust.

It is useful to classify stars by their temperature and luminosity, with the assistance of the Hertzsprung-Russell diagram. It is useful to classify galaxies by their shape. Galaxies of similar shape, it is found empirically, also tend to have similar gas and dust content, and have similar internal motions.

The first category of galaxies consists of SPIRAL galaxies. Our own galaxy is a spiral galaxy;
so is the Andromeda Galaxy (M31):
and the Whirlpool Galaxy (M51):
Other spiral galaxies are shown in the Galaxy Gallery.

All spiral galaxies have

The class of spiral galaxies is further subdivided into classes Sa, Sb, and Sc. Sa galaxies have big central bulges, tightly wound spiral arms, and a relatively small amount of interstellar gas. Sc galaxies have small bulges, loosely wound spiral arms, and a relatively large amount of gas. Sb galaxies are intermediate between Sa and Sc. Our own galaxy is an Sb galaxy, as is M31. M51, which has a smaller bulge, is an Sc galaxy.

A ``subspecies'' of spiral galaxy is the class of barred spirals. In a barred spiral galaxy, the spiral arms wind away from an elongated central bar rather than from a spherical central bulge. A picture is worth a thousand words: look at the examples of barred spiral galaxies in the Galaxy Gallery. Aside from the presence of the central bar of stars, barred spirals are very similar in their properties to ``ordinary'' spirals.

A ``subspecies'' closely related to spiral galaxies is the class lenticular galaxies. Lenticular galaxies have

The fact that lenticular galaxies lack both gas and spiral arms has led to the hypothesis that interstellar gas is necessary for spiral arms to form.

(2) ELLIPTICAL galaxies are slowly rotating ellipsoids, and contain little gas and dust.

Most bright galaxies are flattened spiral galaxies. However, a significant number of bright galaxies are elliptical galaxies. Elliptical galaxies contain very little gas and dust, and the gas that is present is very hot and diffuse. Consequently, there is no current star formation in elliptical galaxies. The stars in elliptical galaxies are old Population II stars. Unlike the disks of spiral and lenticular galaxies, elliptical galaxies are NOT rotating rapidly.

Elliptical galaxies were given their name because they appear elliptical on the sky. Elliptical galaxies are subclassified according to how flattened they appear. Let ``a'' be the diameter of an elliptical galaxy along its longest dimension (its major axis, in the language of mathematicians). Let ``b'' be the diameter along the shortest dimension. The elliptical galaxy is then given the label ``En'', where ``n'' is a number given by the formula:
n = 10(a-b)/a
For instance, a galaxy which appears circular has a=b, and hence n=0. Galaxies which appear circular are thus given the label ``E0''. Slightly more flattened galaxies are labeled ``E1'', and so forth, up to the most flattened elliptical galaxies, which are called ``E7''. The galaxy M87 (displayed in the Galaxy Gallery) is nearly circular, and hence is classified as E0.

Note that the above classification scheme for elliptical galaxies is fairly lame, since it relies on the projected shape of the galaxy not on the true three-dimensional shape. The projected shape of an object depends on the angle from which we view it, which is totally accidental. Unfortunately, it's impossible to determine the true 3-D shape of an elliptical galaxy - we can't, for instance, take a million-parsec-long journey in order to circumnavigate it and view it from every angle.

Since elliptical galaxies appear elliptical in the sky, they must be ellipsoidal in three dimensions. Just as an ellipse is a distorted circle, an ellipsoid is a distorted sphere. There are three types of ellipsoid:

Elliptical galaxies are probably a mix of all three shapes.

Elliptical galaxies have a very large range of sizes. Both the largest and smallest galaxies in the universe are elliptical. Giant elliptical galaxies contain 1 trillion stars or more. Dwarf ellipticals contains 10 million stars or less. The Galaxy Gallery contains portraits of the giant elliptical galaxy M87 (which is about 200 kiloparsecs in diameter) and the dwarf elliptical galaxy Leo II (which is about 1 kiloparsec in diameter, and contains so few stars you can see right through it). Of the two dozen galaxies closest to us, a dozen are dwarf ellipticals.

The final class of galaxies consists of irregular galaxies. Irregular galaxies tend to contain lots of gas and dust. As a consequence, irregular galaxies contain copious star formation. The star formation is patchy (tending to occur in clusters), as is the distribution of dust. Therefore, irregular galaxies are given their characteristic irregular, patchy, raggedy appearance. The Large and Small Magellanic Clouds, about 50,000 parsecs away from our own galaxy, are examples of irregular galaxies (You can see their pictures in the Galaxy Gallery.)

The class of irregular galaxies tends to be something of a ``garbage bin'' classification. If a galaxy is extremely bizarre looking, it tends to be labeled as an irregular.

Prof. Barbara Ryden (

Updated: 2003 Feb 24

Copyright 2003, Barbara Ryden