The Hubble Galaxy Classification System
Published on Oct 14, 2006 at 2:01 am.
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Filed under galaxies.
Every now and then, you’ll hear an astronomy talking about a galaxy, and the galaxy may be referred to as “an elliptical”, or an “Sb”, or perhaps and “SBa” galaxy. So, just what is all this about? This goes back to a system of classifying galaxies by their appearance developed by Edwin Hubble in the 1920’s. Though based on morphology, or shape, of the galaxy, there turns out to be some deeper astrophysics to this system of classifying galaxies. Different types of galaxies have similar characteristics. That is one reason that the Hubble system is still used today, even though there have been serveral other systems developed since that time.
Hubble’s system is based on the appearance of a galaxy. Looking at various galaxies, Hubble realized that some galaxies seemed to have spiral structure, and others appears simply as ovals of fuzz. Mysteriously, it was always easier to photograph the spiral galaxies than the ellipticals. The reason for this was discovered by Walter Baade in the 1940’s — the elliptical galaxies tend to have only yellow, orange, and red stars in them, while the spiral galaxies also had numerous hot blue and white stars. The film emulsions of that day had very poor sensitivity to red light, so the elliptical galaxies were very difficult to photograph. This showed that there is something special about the different galaxy types other than just their shapes.
But, Hubble also noticed that not all ellipticals looked alike, and the spiral galaxies had even more variation than the ellipticals. So, he decided to extend the classification beyond just “spiral” and “elliptical.” The spiral galaxies were further subdivided into those that had spiral arms wrapping outward from the core, and those that had a bar-like structure through the middle of the galaxy, with the spiral arms hanging off of the ends of the bar. These were “normal spirals” and “barred spirals.” Within each of those categories, he further subdivided based on shape.
Ellipticals he further classified by just how elliptical they appeared. He assigned a number to each galaxy given by an equation given by: # = 10(1-a/b). The term “a” is the length of the galaxy, and “b” is its width. For perfectly round galaxies, the number equals zero, so those galaxies are called E0 galaxies. Slightly elongated would be E1, or E2. The more elongated, the bigger the number. You tend not to find any galaxies more elongated in appearance than E7. Now, I should point out than an E7 is not really cigar shaped, like some introductory textbooks make it out to be. Rather, it is more a fat pancake seen from its edge. Seen from the top, it would look like an E0. But, we can’t say that all E0 galaxies are flattened galaxies seen from their poles. Some are, in fact, rather spherical galaxies. Regardless of the number, all ellipticals share certain properties. All ellipticals tend to be very poor in intersteller medium (the gas between the stars), with few nebulae or dust clouds. Also, most of the stars in elliptical galaxies are very old, ancient in fact. That is not surprising, given that elliptical galaxies lack the gas needed to make new stars. Since the redder stars live longer, elliptical galaxies are redder than spiral galaxies, as a rule of thumb. As with any statement in astronomy, you have to realize that the universe is BIG. So, if an exception can be found, it will exist somewhere. Indeed, there are a handful of anomalous blue elliptical galaxies. I know some of the people who research these things, and I plan on posting another blog about them later.
Spiral galaxies are a bit more complicated. As I said, Hubble differentiated between barred spirals and non-barred spirals. For a long time, the bars were thought to be anomalies, but now it seems that they are actually pretty normal things. In fact, our own Milky Way Galaxy apparently has a bar in it! The modern version of Hubble’s system has a way of categorizing how big or prominent a bar is. But, for brevity, I’ll stick to just what Hubble did. If a galaxy was a “normal” spiral, it was just S, and if a barred spiral, it was SB. But, not all normal spirals or barred spirals looked alike. Some had very prominent central cores, and very small spiral arms, and others had tiny cores and were mostly spiral arms. So, Hubble assigned a letter to each spiral denoting how prominent the core was compared with the spiral arms. 
The letter “a” meant that the core was very prominent, “b” meant that both the core and spiral arms shared the stage, and “c” was for spirals in which the core was rather insignificant. The Andromeda Galaxy is an Sb galaxy. Our own Milky Way is an SBb galaxy, under this scheme. Hubble also noted that the cores of galaxies looked a lot like elliptical galaxies (and Baade showed that there were, in fact, a greater proportion of red stars in the core than in the spiral arms). Spiral galaxies consist of a disk, in which the spiral arms are the most obvious part, and a central bulge at the core. Hubble noticed that the more prominent the spiral arms, the moe imporant the disk appeared to be in the galaxy. So, he proposed a sort of in-between galaxy, called an S0, which was like an armless spiral.
Now, not all galaxies fit this system, so he created a provision for those that didn’t fit as either elliptical or spirals. They are called irregulars. Often, the Hubble system is portrayed in a “tuning fork” sort of diagram, as at the top of this post, or here below. The ellipticals are at one side, and the spirals are at the other. 
Normally, the E0 spherical ellipticals are placed at one extreme end, and they are arranged from E0 to E7, and then there is an S0 galaxy connecting the ellipticals to the spirals, and two arms leading from the S0: Sa, Sb, Sc or SBa, SBb, SBc, in decreasing order of prominence of the central bulge (and increasing prominence of the disk and spiral arms). Later, another type of spiral was added: Sd, which has almost no discernable central bulge. Also, more modern versions of this have gradations between the different forms of spirals (barred or not). I have seen the spirals represented as a cylinder rather than as two tines of a fork.
In the early days, this was thought of as a sort of evolutionary diagram: galaxies started as E0, got flatter through centrifugal force, and then pinched off to form disks, in which the spiral arms developed. However, this is most definitely NOT what happens. It can’t be, because spirals and elliptical have different compositions. The spiral galaxies have the dust and gas needed to make new stars, and consequently they have young stars in them. The ellipticals don’t have much dust and gas with which to make stars. Now, interestingly enough, though, if you were to strip the dust and gas from a spiral, it would look like either an elliptical or an S0. I’ll say more about that in a later blog entry. As it turns out, the modern way of thinking about galaxies is that this is how you make most ellipticals.
Hubble’s galaxy classification scheme was devised in the years immediately after the realization of what galaxies were. The first step in science for organizing data is to catoragize things, and the simplest and fist step is to catoragize things by their appearance. Later, we normally find that this is not the best way to organize things. However, with galaxies, it turns out that the appearance is determined by what is going on inside the galaxy, so this method works. That is why Hubble’s first system of classifying galaxies is still used today.
-Astroprof
(Images courtesy NASA, GSFC)
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