This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Classification of Galaxies

by Dr Jamie Love Creative Commons Licence 1997 - 2011

As you know, you don't need a powerful telescope to see a galaxy. The closest ones appear as glowing blurs to the naked eye and with only a modest telescope you can see several more. To the untrained observer these dim fuzzy lights can be very confusing and might be mistaken for comets - except comets will move against the starfield over the months but galaxies won't.

A French Astronomer named Charles Messier was a "comet hunter" during the 18th century and he had become irritated by several objects that had lead him astray. Just to set the record straight, and to warn other comet hunters about these confusing patches of light, in 1781 he published a book describing the position and characteristics of about a hundred "fuzzy" objects in the night sky. Messier had no idea what they were so he assigned them numbers. Later astronomers, using powerful telescopes, identified these "Messier objects". Some are stars clusters, some are nebulas and some are galaxies.

We still use Messier's catalogue numbers as a "short-hand" for these objects and you have met many of them already. The star cluster of Praesepe (in CANCER) is called M44 and the Pleiades star cluster (in TAURUS) is M45. The Orion Nebula (in ORION, I hope ) is M42 and the Ring Nebula (in LYRA) is M57. Naturally, Messier saw the Andromeda Galaxy. He assigned it number 31. He noted another small light next to M31, so he called it number 32. We now know that M32 is a galaxy too - a companion galaxy of M31 (the Andromeda Galaxy). Messier saw another fuzzy dim light and assigned it number 33 but today we know it as the Triangulum Spiral Galaxy. And so it went on. The Messier numbers have been used for centuries by astronomers and all of the M's have now been properly identified. Astronomers usually keep with tradition and amateur astronomers find "M numbers" particularly fun because you need only a modest telescope to see them all. Many Messier objects are galaxies.

Of course, the more galaxies we can see the more we can learn about galaxies in general. Professional astronomers need bigger and better telescopes so they can see further into the universe. And that brings us to Hubble.

Edwin Hubble first studied law but at the age of 25 he switched to astronomy. He used several large telescopes to contribute more than any other man (in my opinion) to our understanding of galaxies and the universe. It's nice to know that his name still generates great science from his namesake, the Hubble Space Telescope (HST), which is orbiting our world. The HST can see objects billions of times fainter than the stars you see with your naked eye and it can see galaxies billions of light-years away. (Recall that the galaxies in our Local Group are at distances of millions, not billions, of light-years.)

In 1999 astronomers used the HST to conduct a detailed survey of a very tiny part of the sky chosen at random. They counted between 1,500 and 2,000 galaxies within an area of sky only 1/25 of a degree in size. That's about the amount of sky covered by a grain of sand held at arm's length. If the rest of the sky has similar numbers of galaxies, then there are about 50 billion galaxies in the universe! That works out to 7 or 8 galaxies for every person on Earth.
Hmm, I lay claim to the Andromeda Galaxy (M31) and its two companions (M32 and M110), the Triangulum Spiral (M33) and, of course, the Milky Way Galaxy plus both its companion galaxies - the Magellanic Clouds. I called them first so I own the best galaxies in the Local Group!

Ah, yeah, right.
Why don't the Magellanic Clouds have M numbers? And what's the M number for our own Milky Way Galaxy?

Think back to what I said about Messier. He named these objects from his home in France so he never saw the Magellanic Clouds. All the M numbers can be seen only from the Northern Hemisphere.
And as for the Milky Way - Messier never bothered to name it. Remember, he wanted to classify things that might be mistaken for a comet and even the most amateur astronomer would quickly understand that the Milky Way is not a comet!

But enough about Messier. I want to tell you about Hubble.
Hubble (the man) produced the first systematic attempt to classify galaxies and it's based upon their shape. It turns out that galaxies of similar shape also have similar gas and dust content as well as similar internal motions. Hubble's galaxy classification gives us a way to describe the diversity of galaxies while making their relative properties obvious and easy to compare. In a sense, Hubble did for galaxies what Hertzsprung and Russell did for stars - provided a framework. Hubble's classification is used today as a way for astronomers to communicate with each other about the different kinds of galaxies.

Hubble divided galaxies into three general types and then divided these into subcategories. Astronomers use abbreviations for all of these types.

  1. Spirals (S) have a spiral shape (obviously) from barely spiral at all (Sa) to fairly spiral (Sb) to very spiral (Sc). Astronomers use math to distinguish the amount of "spiralliness" and assign the "a", "b", and "c" using an equation that defines the degree of spiraling but we don't need that kind of detail here.
    If you look carefully at an Sa galaxy, the kind that's "barely spiral", you'll see that the reason it's barely spiral is because it has a very large central bulge and its arms are so tightly wound around the bulge that you can barely make them out at all. Careful analysis of Sa galaxies shows that they tend to have very small amounts of interstellar gas.
    All Sc galaxies, the "very spiral" galaxies, have a small central bulge and their spiral arms are wrapped very loosely. It just so happens that Sc galaxies have large amounts of interstellar gas.
    The Andromeda galaxy is classified as Sb so it is a fine example of an intermediate spiral galaxy.

  2. Barred Spirals (SB) - NOT to be confused with Sb galaxies - have a "bar" of stars through the center and from each end of the bar can be seen some spiral characteristics. Those with very tight spirals (hardly distinguishable as a spiral at all) are assigned the letter SBa, while the barred spirals with a very loose spiral pattern are designated SBc. A galaxy described as "SBb" would be a barred spiral galaxy intermediate in its spiral looseness.
    There are roughly equal numbers of spiral (S) and barred spiral (SB) galaxies. Some astronomers say that SB galaxies are just a subtype of the ordinary spiral (S). OK, if it wasn't for the bar, these galaxies would be just spiral galaxies (S) of some sort. It's only the conspicuous bar in SB galaxies that differentiates them from S galaxies - these two types of galaxies are identical in all other properties. Many astronomers refer to S and SB galaxies as disk galaxies meaning they have an obvious flat disk - it may have a bar-bulging center (SB) or not (S). All disk galaxies are rapidly rotating.

  3. Elliptical (E) galaxies appear to be elliptical in their two dimensional image and are assumed to be ellipsoidal in three dimensional space. You'll recall that an ellipse is a squashed circle - well, an ellipsoid is a squashed sphere. Elliptical galaxies have no spiral or bar patterns. Instead they range from nearly perfect spheres (E0) to highly flattened ellipses (E7).
    These galaxies have very little dust and what little gas they have is very hot and diffuse. These properties suggest that there is no new star formation going on in elliptical galaxies. (Stars in elliptical galaxies are Population II stars and we will talk about what that means shortly.) Also unlike S and SB galaxies, these E type galaxies are NOT rotating rapidly. While disk galaxies tend to be of moderate size, elliptical galaxies span all sizes from giant elliptical galaxies containing over a trillion stars to dwarf elliptical galaxies with only a few billion stars or perhaps only a few million stars. (And any group of a million or fewer stars is a star cluster, not a galaxy. Right?)
    Both of the Andromeda Galaxy's companions are elliptical. M32 is type E2 and is a "typical" elliptical dwarf with a compact geometry that gives it a high brightness. M110 is type E6 and it is obvious that it is a pretty flat elliptical.

Of course you must remember that we can only see a galaxy from our own place in space. It would be easy to mistake an E7 for an E0 if it was pointing its axis at us. We cannot tell the real three dimensional shape of the object. Regardless, we try to classify them on what we assume is their three dimensional shape.

Hubble arranged these complex patterns into a diagram that has come to be known as the "tuning fork" (because of its shape). As you can see, classifying a galaxy isn't easy. You must always be aware that you are seeing it from only one perspective and that could mislead you about its true shape.

Hubble also added an intermediate classification which links the elliptical galaxies with the disk galaxies.

"Elliptical-spirals" (S0 and pronounced "S zero") have very large central bulges but a weak disk and no spirals. "Elliptical bars" (SB0) have a bit of a bar structure but no distinct bars. These galaxies have the qualities of a rotating disk galaxy with a central bulge but they have very little gas and dust and no spiral arms. Recall that elliptical galaxies do not have a disk and do not rotate while disk galaxies have those features as well as arms and dust and gas in their disks - although the Sa and SBa have the least amount of gas and dust. My point is that S0 and SB0 galaxies are smack in the middle of the divide.

Please take a good look at the fork and make an effort to understand it. Like the H-R diagram, Hubble's tuning fork is at first a bit overwhelming but, with close inspection and a little thought, you'll see that it is logical and easy to understand.

Hubble thought that the tuning fork was not only convenient but he also thought that it gave a clue as to the way galaxies age. He called the galaxies on the left side "early types" and those on the right "late types", believing there was an evolutionary sequence here. However, careful observations of the number and types of stars in these galaxies have caused the modern astronomical community to believe that Hubble was wrong about galaxy evolution. OK, even a giant like Hubble can be wrong!

Besides, Hubble's tuning fork and evolutionary sequence left out some other galaxies.

Yeah, where are the Clouds of Magellan on a diagram like this?

Exactly!
The Magellanic Clouds are irregular in shape and they have no place in the tuning fork diagram. These kinds of galaxies are irregular galaxies (Irr). The Large Magellanic Cloud is usually classified as Irr but I saw one book that said it seems to have a small amount of spiral so was called a "sloppy" Sa galaxy. However, the Large Magellanic Cloud has a lot of gas and dust in it so I would argue that it's an irregular galaxy. (See below for an explanation of why I say so.) Obviously there is room for disagreement. (But I'm right! )
Some astronomers divide irregular galaxies into two more subcategories - those that can be resolved into nebulas, stars and cluster are designated type Irr I and those that cannot be resolved into these objects are called type Irr II.

The most important feature of irregular galaxies is that they have lots of dust and gas. As much as 90% of the mass of an irregular galaxy is dust and gas! Because of all this mass, irregular galaxies are the homes of many new, young stars. You see, an irregular galaxy is like a giant nebula. Over time the mass condenses into dense patches and these act as points of new star formation. Among the irregular distribution of mass there are patchy areas of new star formation and this produces a kind of ragged looking (irregular) galaxy.
The stars in irregular galaxies move in very chaotic orbits because the distribution of mass in these galaxies is so, well, irregular!

There are no irregular galaxies in the original version of Hubble's tuning fork but modern astronomers and astronomy students like to place the irregular galaxies at the far right of the fork. That gives the fork more meaning because it arranges galaxies from those with the least amount of gas (and oldest stars) on the left while the galaxies with the most gas (and youngest stars) on the far right.
Unfortunately, some books and astronomers insist on placing the irregular galaxies as a third prong on the fork, between the S and SB galaxies. This (stupid) way of representing the irregular galaxies destroys the value of the diagram in arranging the galaxies to show a progression, from "nondusty" to "dusty" as you move to the right, and it turns the two-prong tuning fork into a three-prong pitchfork!
Some astronomy teachers say that the "irregular" classification is just a garbage bin classification - if it looks weird and it isn't an ellipsoid or disk than it must be irregular. I disagree. While it's true to say that the shape of irregular galaxies makes them hard to define, it's the excessive amounts of gas and dust, along with the new star formation, that makes these galaxies unique.

    Here's a summary, including relative numbers, of the three "superclasses" of galaxies ...
  1. Irregular galaxies are weird in shape and have lots of dust and gas leading to lots of new stars which are in chaotic motion. They account for about 20% of galaxies. Imagine them at the far right of the tuning fork.
  2. Disk galaxies (both Spiral and Barred Spiral) account for another 20% of galaxies. Stars in their rapidly rotating disk are in well-organized orbits. They have intermediate amounts of dust and gas with the tidiest galaxies placed in the "a" subclass. They are in the middle of the tuning fork.
  3. Elliptical galaxies make up the remaining 60% of galaxies. Their stars move through an almost dust-free zone in slow, elongated orbits. These galaxies are on the left side of the fork, making up its handle.

We can learn a great deal about our own galaxy (the Milky Way) by observing distant galaxies.
We can't see our own galaxy from any other perspective than our current position so we have difficulty figuring out its shape. By observing other galaxies we can learn what are the most common shapes and from that we might be able to deduce the shape of the Milky Way Galaxy. We know from studies of hydrogen generated microwaves (last month's lesson) that our Milky Way Galaxy is a spiral galaxy. Some estimate that it is a fairly average spiral (Sb) while others believe it is a more "spirally" spiral (Sc) so some astronomers called it "Sbc" meaning it is between the two types (Sb and Sc). As if that were not enough to confuse you, many astronomers now think the Milky Way has a bar structure at the center that would cause it to be called an "SBbc" galaxy!

So all this is silly!

Not at all. We are just in a bad position to "see" our own galaxy and we may be living in a galaxy with an unusually complex shape. On the other hand, maybe it's the importance of the Milky Way Galaxy (to us) that causes astronomers to give it an unusual amount of thought and observation - causing them to constantly rethink it.

We use galaxy classification in order to learn more about the universe and we usually don't have much difficulty assigning galaxies.
The Pinwheel galaxy in TRIANGULUM is a beautiful spiral with very open arms so it is Sc.
In URSA MAJOR there is a spiral galaxy (which Messier numbered 81) and its arms are more tightly wound so it is classified as Sb.
M87 in VIRGO is so round that everyone agrees it is a perfect E0.

The point is, we must learn to accept the complexity of the universe as we try to understand it.
Along the way we might learn something.

I mentioned earlier that Hubble was wrong about the evolution of galaxies. And you now know that he left out irregular galaxies, although we can think of them as being on the far right of his tuning fork. Also, I agree that it is not easy to classify galaxies. However, I hope you will agree that there are still a lot of good reasons to understand the tuning fork method of classification because it helps us to understand and communicate about galaxies. In our next lesson we will explore how galaxies come about, how they interact and how they evolve.



This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.