Yeah! That’s right! We’re going to classify galaxies and I’ll explain what all the numbers and letters mean. Take that Mike Goad! More like Mike TOAD!
Just kidding, Mike’s a great lecturer of mine. I’m just here to reinforce my learning about galaxies, and add a couple extra things I learnt about elsewhere.
The most often used system is purely based on looks, and the most common scheme to sort galaxies is by using Hubble’s tuning fork. This one groups 3 types of galaxies: Elliptical, Lenticular, and Spiral. Anything that’s not these is usually called irregular.
Hubble’s tuning fork was initially thought to show the evolution of a galaxy, with ellipticals being a new galaxy or “early” type, and spirals being a developed and “late” type. This is completely false! That incorrect conclusion came from the fact that ellipticals tend to have some older stars and spirals have a lot of younger/newer stars. In reality we see a large amount of spirals in the early universe, so it seems that we could have gotten it the wrong way round! Don’t take my word on that yet, galaxy formation is an extremely undiscovered field of cosmology!
But anyways let’s look at the 4 galaxy types!
Ellipticals are mostly featureless and hazy looking, and are in the shape of an ellipse. They can be extremely large, hosting trillions of stars! These galaxies are not as common as spirals and irregulars, but they tend to be found in the centre of galaxy clusters.
Galaxies that are elliptical are written using an E, followed by a number from 1 to 7. Those that appear circular or “face on” are given the number 1, and the most squished looking ones which are almost edge on are given 7. For example, NGC 7332 is an E7 galaxy, and is edge on. Messier 105, the image on the right, is the complete opposite and is E1.
Spirals are just neat, and come in many forms! Spirals and irregulars make up most of the galaxies in the observable universe. These galaxies will have their stars and gas clumped together in spiral arms that wind around the centre. They are made up of a supermassive black hole surrounded by a bulge of densely packed chaotic stars, then a disk in which the spirals reside. Surrounding these is a halo of stars and then an enormous dark matter halo.
Spiral galaxies are written as S type, and those that have a bar going through the bulge are called Barred spiral galaxies and are written as SB. If astronomers aren’t sure, they’ll write SAB instead.
Additionally, we add a lowercase letter or two from a to d to describe the tightness of the spirals and the size of the central bulge. Galaxies with a tightly wound spiral and large bright bulge are written as a, followed by ab, b and so on until d, where the spirals are loose and the bulge is small and dim.
To add more pain
On top of this, we also classify the termination of the spirals. If they finish right in the centre of the galaxy they’re given the label (s), which of course stands for sentre of galaxy. Those that terminate in the inner ring are given (r) and (R) for the outer ring. Don’t worry about these rings, they’re determined on a case-by-case basis and many will have no label or just be an (s)!
Spirals might also get a luminosity classification which classifies their luminosi- I’m joking it classifies their spiral definition! That’s confusing isn’t it? Type I have well defined arms, followed by type II up to V which have the least defined arms.
These describe spirals pretty well, but there are so many morphological types that this classification system doesn’t fully express and will need a post for itself. For example, there are grand design spirals and flocculent galaxies or Seyferts which have interesting features but could very well have the same S designation as a rather boring spiral (although spirals are seldom boring anyway!)
Let’s take a mental break! Lenticulars are like a fusion of elliptical and spiral. They have the galactic bulge structure of a spiral, but have a disk of stars like an elliptical instead of spiral arms. They’re written as S0 or SB0 if they’re barred. They may also get a subscript number from 1 to 3, for S01 to S03 this depends on dust absorption in the galaxy with 1 meaning little to no dust. In the case for SB01 to SB03 the numbers depend on how prominent the bar is, with 1 being the most prominent.
There’s just something about astronomers numbering things backwards it seems!
Anything wild and crazy, or that doesn’t fit the other groups is classified as irregular. These galaxies, on average, tend to be similar to Spirals in terms of star composition so they will often appear a bit more blue.
Irregulars are written as Irr, but if the galaxy has something resembling diffuse and broken spiral arms or a bar, then astronomers may also add Sd/SBd/SABd. Those that are irregular and have no bulge are given Sm/SBm/SABm, and the truly irregular are given Im.
Hoag’s object is an example of a wildly irregular galaxy.
I think this is already a madly long post, and we haven’t even got to other weirder sub-classifications! That’s a topic for next time!