Last week, Jamie (my significant other) came home from work and told me about a conversation she had with her coworker, Chris. This week she asked another one, Miguel, whether he had any questions about Astronomy, Physics, space, etc. This week’s question comes from Miguel:
What is a galaxy, anyway? Why does it look like a big bright fuzzy star? And why are there different types of galaxies; shouldn’t they all be the same?
This might come as a surprise, but 100 years ago, it was pretty much accepted that we were the only galaxy in the Universe. In fact, there was a great debate in 1920 on whether some funny-looking things in the sky were other galaxies, or whether we were the only one.
So here’s the deal. You look up at the sky, and all those points of light you see are either planets (left) or stars (right).
But with a telescope, you find that there are some fuzzy things in the sky that don’t quite look like you’d expect them to. There are star clusters, like the Pleiades. And there are the bizarre looking objects known as nebulae. There were planetary nebulae, like the Crab Nebula (M1), but there were also things known as spiral nebulae, like Andromeda (M31), below.
Sure, we know today that Andromeda is a galaxy and the Crab Nebula is the remnant of an exploded star, but how do we know that? The Milky Way is our collection of about 400,000,000,000 stars. Some of those stars are known as Cepheids, rare stars that flare brightly at predictable intervals. By 1925, Edwin Hubble had found 10 Cepheid stars in Andromeda, and measured their periods and brightnesses over long times, figuring out that Andromeda was both very large and far enough away that it was outside our own galaxy.
It didn’t take long for people to figure out that Andromeda also had a few hundred billion stars in it, and that not only were all the other “spiral nebula” actually spiral galaxies, like the Whirlpool Galaxy (M51) shown here:
but that galaxies, these huge collections of millions or even billions of stars, existed with a really wide variety of shapes and sizes. In addition to spiral galaxies, there are also elliptical galaxies, like the galaxy Centaurus A shown below:
So we’ve got spiral galaxies and elliptical galaxies as the two major types of galaxies in the Universe. Spirals are shaped like a disk, usually with a bulge and sometimes with a bar in the center, with big, bright arms spiraling outwards from the center. On the other hand, ellipticals are shaped like an American Football, with a big bright center becoming dimmer and more diffuse towards the edges. Let’s show you how you form a galaxy, and then why some of them wind up as spirals, while others become ellipticals.
When the Universe is very young, it’s also very smooth, but there are regions of space that contain more matter, both dark matter and normal matter, than average. There are also some that contain less matter, and the famous picture above shows you the average places in green, the below-average places in yellow/red, and the above-average places in blue. Each individual overdense (blue) place, where there’s more matter, is going to collapse under its own gravity.
If the overdensity were a perfect sphere, everything would collapse to a point at the center. But one direction, randomly, will always be shorter. Because it has less distance to go, that direction collapses first, and we go from an ellipsoid:
to a “pancake,” or a disk, because gravity squashes it in the shortest direction. When all that matter meets up, the dark matter just passes right through, but all the normal matter (protons, neutrons, electrons) can’t just pass through each other, just like your hands can’t pass through one another. So they stick together, and start forming stars, producing a whole bunch of light in this disk:
The disk then rotates, by the law of conservation of angular momentum, and the rotation makes the spiral structure we see, since the inner part rotates with a higher angular velocity than the outer part. So, what we wind up with is a spiral galaxy surrounded by an ellipsoidal halo of dark matter:
And that’s how we get spiral galaxies! Wait a minute, then, so how do we get elliptical galaxies, and why are there so many of them in clusters, but when we see galaxies on their own, they’re almost all spirals? We think that all galaxies start as spirals, but some of them merge together. When you get a small galaxy merging with a big one, the small one gets destroyed and the large one is mostly unaffected, like the large Magellanic Cloud falling into the Milky Way. But when you get two large galaxies merging together, they destroy the spiral structure and make an even bigger elliptical galaxy, as this video shows what might happen to our Milky Way were we to merge with Andromeda: