We know quite a bit about other galaxies by using information about the Milky Way and applying it to them as well as using our own observations of other galaxies to figure out what is going on out there. The reverse is also true - characteristics of other galaxies can be applied to the Milky Way. Remember, we have a hard time seeing various parts of our own galaxy, so checking out other galaxies gives us an idea about what distant parts of our galaxy look like or what is probably happening in those places.
What do other galaxies look like? To the eye they are fuzzy patches in the sky. With long exposures on film, various features such as spiral structure and star clusters are visible. Though it is possible to see trends in the general shapes of galaxies, they are all unique - sort of like snowflakes.
But when you go to a telescope and look out in the sky and you see these fuzzy things, how can you tell it is a galaxy? All little bits of fuzz look sort of similar through a telescope, though there are certain pattern to some of them. And just by looking at them with your eyes through a telescope doesn't really help. Even the invention of photography which helped astronomers get more detailed images of galaxies, wasn't enough to tell them what these things were and how far away they were. That was a major problem in astronomy around the beginning of the 20th century. There were all of these fuzzy things out there (which we called nebula, plural is nebulae) located all over the place. But what were they? We thought some of them were in our galaxy, but we were not sure which ones were and which ones weren't. Perhaps none of the fuzzy things were outside of our galaxy? Perhaps our galaxy is the only galaxy - who says there really are any other objects out there any ways? Or perhaps our galaxy is just like one of those fuzzy things - but like which type of fuzzy things? Does it have a spiral shape like some of the spiral nebulae? Or an oval shape like some of the nebula have? Or maybe some other kind of shape? According to the best data at the time, we didn't really know where we were located even in our own galaxy, so how was it possible to figure out what those darn fuzzy things were?
This is really a conundrum. How will the astronomers be able to solve this problem and save the day (well, at least solve this problem)? They didn't really solve the problem at first. Initially there was just a lot of talking about the problem and trying to show who was right, who was wrong, and who had the worst breath. One of the big discussions about the whole nebula problem took place in Washington D.C. in 1920 at the National Academy of Science. The two people involved in the debate were Heber Curtis and Harlow Shapley, so this debate is known as the Curtis-Shapley Debate. Now you've already run into Harlow Shapley, who was the fellow that figured out the distance to the center of our galaxy back in 1915, but some people had their doubts about his work. The other fellow, Heber Curtis (1872-1942), studied the spiral nebulae quite a bit, so he was really the expert when it came to those things. You have two guys - one an expert about the size of our galaxy, the other an expert on the spiral nebulae - debating each other about various things like what are those spiral nebulae? What is it really like out there? Just how big are the spiral nebulae? How far away are they? How big is our galaxy? What is the Universe like?
It's sort of a draw as to who won the debate. Since Shapley was an expert on the Milky Way, he was able to put forth his idea about the distance we are from the center - which was pretty big. This was correct. (Shapley actually thought that the Milky Way was about 100 kpc wide, which isn't really correct, but he was along the right track). Then he proposed that since the Milky Way was so huge (at least, bigger than anyone ever thought it could be), those spiral nebulae were probably not very far away objects. After all, our galaxy is so huge - or so he thought.
Curtis believed that the spiral nebulae were actually distant objects, not part of our Milky Way, and not even very close to it. He also thought that our galaxy is just like them in terms of size, shape and structure. He was correct on those points. Then he put forth that the Milky Way was not very large (he thought it was only about 10 kpc in size, which is much too small), since it was not large enough to contain the spiral nebulae. He sort of missed that one.
In a way, since each of them was an expert in one aspect of the problem, they each got one thing right. They didn't know that at the time, since just debating about something doesn't help figure out what the actual answer is. Shapley was generally considered the winner of the debate due to his charisma (imagine that, an astronomer with charisma!). The debate really didn't solve the problem. What did solve the problem was good, solid science, in this case when Edwin Hubble provided real evidence for the distances to galaxies in 1924. Hubble had an advantage over Curtis and Shapley, since he had the use of the new, big telescope on Mount Wilson in southern California. At that time, the big scope was the 100" (2.5 meter) telescope located there.
Hubble and the telescope operator, Milton Humason used this telescope to find and analyze Cepheid stars in the Andromeda Galaxy (of course, at that time, it was still known as the Andromeda Nebula). What good are Cepheids? - plenty good! If you remember from the previous section, Cepheids are those Red Giant stars that pulsate and you can use them to determine distance. The longer the period, the greater the average brightness (the Leavitt Law) so if you can find a Cepheid with a period similar to one in our own galaxy, you can compare their apparent magnitudes (how bright they look to your eye), and the difference in brightness is directly related to their different distances. If you can find a Cepheid in a galaxy, you can find the distance to that galaxy. That is exactly what Hubble and Humason did. By using the Cepheid Leavitt Law to determine the distance to the Andromeda Galaxy (one of our closer neighbors), they found that it was 900,000 light years away! This distance was much greater than anyone even suspected, and this is one of the close ones! Actually, they were a bit off - it is actually about 2,250,000 light years away - our current distance estimate methods are a little bit better now.
Not only did Hubble figure out that those fuzzy spiral nebulae (like Andromeda) were actually very distant, separate objects, he started a whole new field of astronomy, the search for objects that could reveal distances to remote galaxies. To find the distances to far away galaxies, it is necessary to use objects whose brightnesses we know fairly well (or some other property that is well defined) and also objects that are bright enough to be seen at a great distance. Objects that fit both these criteria are referred to as Standard Candles. That's just sort of a cute nickname for bright, well behaved objects. What sort of things are Standard Candles? Here are some examples of them -
Once an astronomer can determine how far away a fuzzy blob in the sky is using a Standard Candle, they'll know if the fuzzy blob is in our galaxy (a few thousand parsecs away) or is a distant galaxy (millions or more parsecs away).
Astronomers are able to get distances for galaxies within about 1 billion light-years fairly reliably, but there tend to be greater and greater uncertainties in the values for greater and greater distances. If you were to look up all of the distances to even nearby galaxies (like Andromeda or the Large Magellanic Cloud), you'd see a range of values, not just one single value. For very distant galaxies (say, more than 1 billion light years away) the distances that we derive are much more imprecise and can always be improved. This is one of the reasons that bigger and better telescopes are being built all of the time, to measure more and better distances. We need to know how far away things are so we can figure out what the Universe is like.
Once it was determined that many of those fuzzy things were actually quite distant galaxies, astronomers had to classify them. Why? We are sort of compulsive about doing things like this, but really it is because we could learn more about them if we could group them together in a way that was scientifically meaningful. This is sort of like how we can group stars into bins like Main Sequence, Red Giant, and so forth. We know that objects in such groups share common characteristics, and we can use that information to learn more about galaxies that we don't see very well or that are too distant to measure all of their characteristics with much certainty.
Figure 1. The Hubble Tuning fork diagram showing the different forms that galaxies come in. The main groups are the ellipticals, the spirals and the barred spirals. A transitional form is the Lenticular type (labeled S0). Anything that can't be placed on the Tuning fork due to unusual structures is simply labeled as Irregular.
What do they look like? Do they all look the same? No, of course not; that would be too easy. Most often we classify galaxies based upon their appearance, since that is the most easily observed feature. The basic classification scheme that is used is known as the Hubble Tuning Fork Diagram (I wonder what clever astronomer thought that up). Yes, good old Eddy Hubble set down the framework for the primary classification scheme. There are some other schemes used, and there have been slight alterations to the guidelines that Hubble used, but it is pretty much still the same thing used today. It should be noted that this scheme is based only on appearance - the shapes of galaxies. It doesn't account for how they got into those shapes or the differences in sizes that exist.
Not only do the galaxy shapes vary, but also the content of the galaxies varies - different types of galaxies can have quite different types of stars in them and different environments. This can result in galaxies having different colors, different things happening (or not happening) in them, different ages, different evolutions, and so on. Remember the different stellar populations -
Remember, galaxies are very far away, so you generally can't see individual stars, but you can see large groups of stars. That is why we talk about stellar populations, since the characteristics of groups of stars is what we are able to measure. Let's start checking out the different types of galaxies that are out there.
As the name implies, these are elliptical in shape, though some are
not very elliptical at all but look like circles. To distinguish the different
shapes we use a numerical designation along the lines of E0, E1,
E2...all the way up to E7. The "E" is for elliptical, while the number
describes the degree of ovalness. The number is found by measuring the
long (a) and the short (b) axis, and taking those values and putting
them into the following formula
Figure 3. The method used for defining the different elliptical galaxies is illustrated here. The longer axis length is compared to the shorter length and a number based upon this value is used to distinguish the range of elongation. In the first case both axes are the same length so the type is E0, while in second the value of 4.7 is found using the formula, which becomes 5, making that elliptical an E5.
Ellipticals tend to look rather yellowish or orangish. This indicates that they are made up of mainly Population II stars. Observations of them show that there is no new (or significant) star formation occurring. There is not much star formation occurring, which means that there must not be a lot of gas and dust in them, since this is what stars are made from. This also gives us a clue concerning how they were made - but I'll get to that later. If you were to look at how the stars in elliptical galaxies move, you'd tend to see rather random motions (sort of how globular clusters move around our galaxy).
Figure 4. A group of galaxies with a large cD (Giant Elliptical) in the center of the group. Image from the Hubble Space Telescope.
The biggest of the ellipticals are often just called Giant Ellipticals and these are the largest of all galaxies. They get a special designation rather than the E designation; they are labeled as cD galaxies - don't ask me why they're called that, they just are. These tend to be very spherical in shape, so I guess they don't need the "E" designation scheme - but there are other features that make them distinct. They can have masses of up to 10 trillion solar masses (1013 Msolar). They are so big that they tend to be found in the center of groups or clusters of galaxies. It is likely that these big brutes weren't always that big but have gotten bigger over time by eating up little galaxies that got too close to them (what we call Galactic Cannibalism- really, we do).
On the other end of the scale, one finds the Dwarf Ellipticals and Dwarf Spheroidals. These are among the smallest of all galaxies, typically with masses around a few million solar masses (106 Msolar). Dwarf Ellipticals and Spheroidals can be best described as galaxy groupies, since they tend to hang around much larger galaxies. If you look at a picture of the Andromeda Galaxy, you'll see two little dwarf galaxies (one is an elliptical the other spheroidal) around it. Due to their wide range of masses, ellipticals are sort of hard to figure out. Sometimes it is hard to determine if you are looking at a nearby dwarf elliptical or a distant larger elliptical.
Figure 5. Several different spiral galaxies. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.
Spirals show a much greater range of structure than ellipticals, so their classification is a bit more complex. First there is the letter "S" designating the galaxy as a spiral. Then there are the cases where there is a Bar going through the center of the galaxy. If so, you need to add a "B" to the designation. Then there are the other characteristics - how big the bulge is compared to the entire galaxy, and how tightly wound up the arms are. There is a tendency that when the bulge is large the arms are wound up pretty tightly, and when the bulge is really small the arms are really spread out. The letters a, b, c and d are used to categorize this characteristic. The various designations for spirals are Sa, Sb, Sc, Sd, SBa, SBb, SBc and SBd. Some people are a bit indecisive about a galaxy being in a particular group, so sometimes a spiral can be designated as a Sab or Sbc, since they're not sure which group it belongs in.
Spirals are easy to identify since they have a spiral structure or flat disk shape (if seen edge on). Of course, they have the spiral arms due to the star formation that is occurring there, but remember, there is material between the arms; it is just not as exciting or as easy to see as the arms. The arms stand out so well because they have all of those hot, big stars to light them up as well as the H II regions in the area. The masses of spirals are typically a few billions to a trillion solar masses. There is the added complication that they aren't made of the same stellar populations. The populations of stars vary depending upon where you are looking - in the disk you find Population I stars and in the bulge and halo you find Population II. This is why in color pictures of some spirals you see the disk looking bluish while the bulge looks yellowish-orangish.
Figure 6. Classic examples of Barred spirals. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.
Barred Spirals share pretty much the same characteristics as spirals except for that extended bulge. It is sort of like someone has taken the normally circular bulge shape and stretched it out. The arms then start up on the ends of the bar. Due to this added structure, the arms in barred spirals tend to be wound up a little bit more tightly than in regular spirals. It is now thought that the Milky Way Galaxy is a barred spiral; perhaps it could be classified as a SBb or maybe even an SBc.
Figure 7. A barred spiral, NGC 6744. It is possible that this is what our Milky Way galaxy looks like. Notice how the arms are not very distinct and poorly defined. It is also thought that the Milky Way galaxy has a bar similar to this one. Image © Anglo-Australian Observatory, Photograph by David Malin.
S0 and SB0 types, also known as Lenticular Galaxies, are sort of crosses between a spiral and and elliptical. They are best described as having a flying saucer shape, since they have a disk and a bulge like a spiral galaxy but no spiral arms. The bulge is often pretty big! They don't have any spiral structure, so they don't have much star formation going on (remember, that's why we have spiral structure). The lack of star formation indicates a lack of gas and dust out of which to make stars. Thus, S0 galaxies have mainly Population II stars in them. If they have a bar, then the SB0 designation is used (make sure you don't get the letters out of order for this designation!) In general S0 galaxies are pretty rare.
Figure 8. Lenticular (or S0) galaxies. These look like a cross between an elliptical and a spiral. You might think of them as a type of spiral galaxy without any spiral structure. The one on the left has a dusty plane, but that is unusual for these galaxies. The one on the right is in the same orientation but shows no dusty structure. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.
As with any classification system, there have to be a bunch of objects that don't fit in. For galaxies, these are the Irregulars. Amongst the more famous irregular galaxies are the two neighboring galaxies to the Milky Way, the Large and Small Magellanic Clouds (LMC and SMC). They sort of have a bar-like structure, but there isn't anything else there - no spiral structure, no defined bulge, nothing.
Figure 9. Some typical irregular galaxies. The Large Magellanic Cloud is in the middle, and the Small Magellanic Cloud is on the right. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.
It is thought that if there was some structure to an irregular galaxy at some time, like spiral arms or bars, then those parts of the galaxy could have been stripped off due to collisions or other gravitational interactions with larger galaxies. Galaxies don't have to actually get too close for there to be tidal disruptions of the galactic structure. As previously mentioned, it is possible for one large galaxy to strip off the gas and dust from a small, nearby galaxy and to suck it up. The bigger galaxy is basically eating away the star forming material (gas and dust) from its hapless victim. This is known as Galactic Cannibalism (and is best served with fava beans and a nice chianti). Irregular galaxies tend to be associated with rather tumultuous events, so they tend to have a lot of star formation going on in them, but this isn't true for all of them, since there can be a wide range of stellar populations (I and II) in different irregulars. Generally, Population I is what is seen. Irregular galaxies form from previously normal galaxies, so they tend to have a wide range of masses. They're just irregular!
It is worth mentioning that the two irregular galaxies that are best known are the LMC and SMC (Large and Small Magellanic Clouds). These are not visible from Iowa, but only from fairly far south of the equator. It is possible to see these two nearby galaxies with the naked eye. Since they are so close, they have been studied very extensively - though you have to go to Chile, Australia, or South Africa to study them at all. Because these galaxies appear to be interacting with our own, and it is likely that the Milky Way collided with them in the past, they show a great deal of star formation currently going on. New images showing the hot gas in these galaxies are visible, and can be seen here - for the LMC and the SMC. Features such as hot gas from star formation, novae and old supernovae is clearly visible. The part of the galaxies that we tend to see with our eyes is generally much smaller.