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.
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.
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
licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.