We classify our galaxies into four main types: Spiral, Elliptical, lenticular (a mash between the two), and irregular, but there are sooo many other ways to talk about galaxies and today we’ll focus on my favourite weird ones.
Galaxy classification is mostly based on morphology i.e looks. You may be disappointed by that but unfortunately we are rather far away from pretty much everything in this cosmos, and so the best way of discovering space is by looking. That being said, classification by looks gives scientists a huge range of options when it comes to grouping galaxies, all depending on what characteristics we are looking for.
To commence, let’s briefly establish the four main groups of galaxies. If you want to know more about these types, like how to decipher what the heck all those letters and numbers mean, then check out a post by me which breaks these down as simply as possible.
The Fantastic Four (Galaxies)
The simplest galaxy is an elliptical, these are basically an oval disc of stars. They are slightly rare, usually yellowish, hinting that they’re really old galaxies, and they do of course have a supermassive black hole in their centre. They’re sub-classified based on how they’re orientated to us, from directly bird’s eye view at 0, to edge on at 7.
Next are spirals. They’re more common, usually younger based on the blue stars inside the galaxy, and often have spiral arms. They typically have hot dense gas in the middle, and a supermassive black hole. Sometimes they have a bar, sometimes not. They’ll be grouped based on: 1) the presence of the bar, 2) the tightness of the spirals, and 3) where these spirals terminate.
They may also get a luminosity classification which classifies their – you guessed it – spiral definition. Many of these types we’ll talk about today have characteristics that are missed because of this system.
Lenticulars are a mash of the elliptical and spiral. They can have an ellipse shape but hot dense gas in the centre like a spiral or something else. They’re grouped depending of the presence of a bar, dust absorption, and bar definition.
So now let’s get to the good stuff!
Flocculent is such a sweet word, and I wished it were more frequently used. Flocculent galaxies are, as the adjective describes, fluffy!
These are spirals that have extremely undefined spiral arms, ones that are discontinuous in a way, and this we believe is due to how the stars form in the galaxy. What astronomers think happens is explosions or stellar wind move material and provoke star formation which spreads across the galaxy. Messier 63 is my favourite among all the flocculent galaxies, and is a surprisingly dark galaxy due to dust.
It may be difficult to see how these galaxies are spirals, but when the universe is mainly full of spirals it’s far more likely that these are warped swirls rather than an entirely different rather chaotic structure.
Just under a third of all known spirals are flocculent, so actually they’re one of the most common galaxies in the observable universe so far!
These are spirals are big beyblades morphologically, and have extremely well defined arms that make up a large – you could say grand – potion of the galaxy. These arms wrap around the galaxy by a large degree as well.
From the grand designs I’ve seen, a lot of them have few arms, with two being extremely prominent and giving the galaxy its classical beyblade shape. It’s not very well known why these galaxies have their iconic shape, but the leading theory explains that the arms are made inside density waves. These waves travel at a different speed to the individual stars, and wind them up. This theory isn’t complete and comes with its flaws.
Seyferts are a special type of galaxy called an active galaxy, and they have something called an active galactic nucleus. This means that the nucleus of the galaxy ( i.e. the black hole and hot dense gas directly around it) make up a large portion of the entire galaxies’ brightness.
Now you might be thinking, ain’t that just a quasar? Well you’re absolutely on the right track. Seyferts are almost as powerful as quasars, the main difference being that the active galactic nucleus of a quasar actually overpowers the rest of the galaxy tremendously. That’s why we thought they were distant stars at first. Seyferts are the step below quasars.
If we compare a Quasar to a Seyfert below, I think you’ll see the difference in power output from the nucleus.
There are two types of Seyferts: Type I have a large amount of broad spectral lines and Hydrogen I, Helium II, and Oxygen III in their spectrum, and emit a lot of light in the UV and X ray.
Type II have rather narrow lines and emit strongly in the Infrared. They also have what we call forbidden spectral lines, where electrons jump up or down to usually unfavourable energy levels, maybe through a spin flip or another mechanism. This happens at an extremely low rate.
My favourite Seyfert is NGC 4151, it is also called the Eye of Sauron.
Starburst galaxies are bursting with stars haha! Seriously, starburst galaxies have significantly higher than average star formation, which really shows.
The milky way makes maybe 3 to 10 solar masses worth of “star” per year*, but a starburst galaxy could make around 100 minimum per year. Thinking about it, ten solar masses a year is tiny. That’s like 15 red dwarves, but, that is the scale of the universe. It’s just something humans can’t comprehend.
* Studies have huge variation of values, so I’ve added a range. Many studies calculate this number is through estimation and educated extrapolation, and is likely highly inaccurate so take this as me trying to convey the magnitude of star formation in a starburst galaxy.
A lot of starburst galaxies aren’t naturally star burst, there’s usually a trigger and a typical starburst will be merging or brushing past another galaxy. It’s these events that set off tidal tails and instability within the galaxy which can sort of ramp things up when it comes to general activity and star formation.
Starbursts have a fair few sub-groups, blue compact dwarves, ultra luminous or hyper luminous infrared galaxies (very dusty galaxies), Wolf-Rayet galaxies (which are full of Wolf Rayet stars, these are large stars that burn up quickly), and my favourite and a delicacy among vegan cuisine, green pea galaxies. These are blue compact galaxies, but they just so happen to look like green peas. I love them!
The most starburst galaxy I know of is the baby boom galaxy, which is just a tRuLy fantastic name. It’s the brightest and one of the oldest starburst, and it makes around 4000 stars a year. That’s mad. This galaxy is set to be the biggest ellipticals in the known universe.
These are but a very very small amount of weird and wonderful galaxies in the universe. I hope I could teach you something new and cool, and if you prefer videos I’ve actually made one on this topic very recently on my YouTube channel: