When you study galaxies you get to look at the most beautiful things in the universe (at least I think so). As late as 1920 the true nature of the fuzzy "spiral nebulae" was not known. The 60-inch telescope at Mt. Wilson began operating about 1907; it was capable of getting good photographs of the nebulae. Their spiral shape was obvious but their nature was not. There was a great debate about what they were. Were they relatively small objects relatively close or huge systems at very great distances? Edwin Hubble attacked the problem using both the 60-inch telescope and the new 100-inch telescope (1917) at Mt. Wilson. By 1926 or so he had identified and studied a number of Cepheid variables in the Andromeda spiral (M31). His results indicated a distance of about 275,000 parsecs, or about 900,000 light years. This discovery answered the question; the spiral nebulae were giant systems at very large distances. Hubble's photographs also began to show individual stars in the nebulae, refuting the notion that they were really nebulous objects not made of stars.
Spiral galaxies vary in appearance. At one end of the range are those with large central bulges and tightly wrapped arms. Toward the other end we find galaxies with relatively small central bulges and more open arms. The first type he called "Sa"(M104 is an example) and the latter "Sb." Type "Sc" galaxies (like M74 here) have very open and less well-defined spiral structure.
Elliptical galaxies look elliptical. Of course, this is a projection of a three-dimensional object into two dimensions, so the actual shape is some spherical or oblong solid. A type E0 galaxy appears nearly round. An E1 galaxy, like M87 here,is slightly flattened. An E7 may resemble a fat cigar.
Ellipticals come in a wide range of sizes. Dwarfs can be as small as 1 kpc in diameter and have less than 1,000,000 stars; that is small for a galaxy. Giant ellipticals can be up to several megaparsecs across. Compare this to the Milky Way Galaxy's diameter of 30 kpc; a giant elliptical can be over 50 times the diameter of the Milky Way!
There's not much star formation going on in elliptical galaxies. Someone has detected evidence of a little star formation, but it is small. This type of galaxy is populated mostly with older stars, mostly low mass stars which are more reddish in color.
Irregular galaxies don't have any identifiable shape to them. That's why they are known as irregular. They tend to be smaller than the spirals but larger than dwarf ellipticals. The irregular galaxy M82 is well-known and can be seen in an amateur telescope as an elongated glow. The Small and Large Magellanic Clouds (SMC and LMC) are small irregular systems that are very close to the Milky Way. The SMC is the system in which Henrietta Leavitt discovered the relation of period to luminosity in Cepheid variables.
Photos of spiral galaxies present a somewhat deceptive picture of the galaxy's makeup. Most of the light in a galaxy comes from the brightest stars (O and B), while the huge number of dim low-mass stars (M) makes them comprise most of the galaxy's mass. So - in that beautiful galaxy picture - the spiral arms are outlined by the light of the blue giants, which are comparatively rare, while the mass is made up of the dimmer stars that you can't see in the photo. Also - spiral galaxies are oriented randomly in space; some are seen face-on (the pretty ones) and some are seen edge-on.
Some of the galaxies pictured in Chapter 15 have "M" numbers, like "M51" 1n figure 15.2 part (b) on page 393. Picture (a) is of M81. The "M" comes from the last name of Charles Messier (messiay) (1730-1817), a French observer who specialized in comets. He also discovered several objects which resembled comets but were not comets. After listing 3 or 4 of these things, he undertook a search for other such things using his own observations plus catalogs compiled by others, and out of these compiled a list of 110 objects that were known to be fixed in the sky and therefore not comets.
Consider the Crab Nebula, which you encountered in Lab 10; it is listed as M1. The Pleiades are known as M45. The nearest galaxy to us, the Andromeda Spiral, is M31.
Finding the distance to galaxies is not trivial. They are over 1000 times too far away for parallax distance measurement. The only way to do it is to locate some known standard candle object in the galaxy. Cepheid variables are excellent since we can know their absolute magnitude and they are very luminous. Dim standard candles would not be useful because they cannot be seen at great distances. Be sure to study the distance ladder in Figure 15.11. Type I supernovae also make good standard candles. Since we think that they are all about the same (carbon or carbon/oxygen white dwarf exploding at about 1.4 solar masses), their peak luminosity should be consistent and knowable. These explosions also have the advantage of being VERY bright, and therefore visible very far away. The only problem is that we must wait until one decides to explode; they don't do it on our schedule. Also - recent work indicates that there is some variance in the luminosity of these blasts, so there is an error bar on the distances calculated from them. Here's a good illustration of the distance ladder.
Note one very important property of the distance ladder: each rung is built on the previous one, and the uncertainty increases with each step. At the greatest distances the precision of the measurement may not be much better than 2 significant figures if that good.
Finally, we noted the concept of a fundamental measurement - one for which no assumptions are necessary. The use of radar to find the AU is one of these. The speed of light has been precisely measured and Kepler's Laws tell us about the planet's orbits. Measuring Venus' distance requires no assumptions. Once you have the AU measured precisely, parallax measurement of stellar distance requires no assumptions. With the baseline known, the limit to the method is our ability to measure the tiny parallax angles.