How Galaxies Are Formed

I wanted to take a quick break from discussing the mysteries of energy to share a mesmerizing video. This NASA-created computer simulation illustrates how moving clouds of dust created in the Big Bang could accrete into the spinning collection of billions of stars that we call a galaxy.

“This cosmological simulation follows the development of a single disk galaxy over about 13.5 billion years, from shortly after the Big Bang to the present time. Colors indicate old stars (red), young stars (white and bright blue) and the distribution of gas density (pale blue); the view is 300,000 light-years across.”

Meanwhile, over at his Bayesian Biologist blog, Corey gives us an update on his attempts to find dark matter in simulated skies.

6 thoughts on “How Galaxies Are Formed

    1. Well, at the moment it’s believed that all (or at least most) galaxies have one or more supermassive black holes in the center. These sources of strong gravitational attraction are what attract all of the dust and debris in the first place. That dust will eventually accrete into stars because of its own internal gravitational attraction, and the stars will be surrounded by their own smaller dust clouds, which will then accrete into planets…

      I’m not sure at what point in the video the black hole appears, but I think it must be there from the start.

  1. Suuuuper cool. I think I’ve watched that like 10 times now…

    What I don’t get though is the constancy of gravity. I get that it’s an effect of mass, but it seems to do work (moving the galaxy around), so doesn’t that kind of make it perpetual motion? What about all these conservation laws you’ve talked about??

    1. The short answer is that energy is indeed conserved in this video. Far-away matter had ‘potential’ energy in the same way that a ball at the top of a hill has potential energy. As either object falls, it loses that potential energy but gains kinetic energy by speeding up.

      Once something is spinning, it doesn’t take any extra energy to keep it moving (because it basically just follows the law of inertia).

      As for perpetual motion, that’s a bit of a sticky issue once you start talking about the entire universe – individual objects have to lose energy due to the law of entropy (which I’ll discuss soon!), but an entire universe doesn’t lose any energy at all – it just spreads out.

      If you played this video over the universe’s entire history, everything would slowly spread out farther and farther, things would stop spinning, stars would go cold, and the average energy density of space would eventually approach zero.

      1. “… the average energy density of space would eventually approach zero.”

        That’s kind of what I was thinking. Things have to cool down eventually, right?

        Also, this whole “potential” energy thing really sticks in my craw. Isn’t that just a cute math trick to make it all work out? I know you discussed this a little in the gravity section, but I still feel like I’ve seen the magician palm a coin…

        1. You totally did see that! That’s what’s so crazy about energy. We’ve set up these rules that seem almost arbitrary, and yet this thing called energy is actually conserved all the time in the real world. Energy is nothing more than a cute math trick, and yet, bizarrely, it seems to always work. That’s why no one can point to something and say, “That’s energy” – it’s nothing but math. Weird.

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