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Galaxies, planets, black holes and Angular momentum October 21, 2008

Posted by jcconwell in Astronomy, physics.
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I was originally going to make this blog about black holes and jets, as part of a multi-part series on black holes in general. It began when a friend asked me how can jets come from black holes and also why spiral galaxies are flat? They both have a common concept that appears over and over in astrophysics, and ice skaters…

Conservation of angular momentum.

Conservation laws are very useful to understand what’s going on in a system. Charge, mass, energy, linear momentum, and today’s topic, angular momentum, are all conserved in an isolated system.

If a quantity is conserved, it means it’s unchanged. Look at it before or after, and it’s the same.

Angular momentum is like linear momentum, it says an isolated body in motion will remain in motion, only this concept talks about rotational motion. Now angular momentum depends on 3 things, how fast is the rotation, how much mass there is, and the final, the sneaky one, the distribution of the mass, or how far away the mass is from the axis of rotation.

so the angular momentum L ~ (mass)(rotation)(distance of mass from axis)2

Why the third quantity is squared goes into more physics than needed here.

You can see the dependence on the third factor , “distance of mass from axis” , with an ice skater. When she completes her routine, she starts a slow rotation, then she brings in her arm and spins up. As she brings in the mass of her arms, the third quantity goes down, so for L to be constant (conserved), the rotation MUST increase.

Now most astronomical objects, stars , planets, galaxies are formed from big balls of gas.

This gas has some small random rotation. As the gas contracts, due to its gravity, it starts to spin up faster. Because it’s a fluid (think of spinning a ball of jello on a toothpick ) as it spins up it flattens out along its equator. If you look at pictures of Jupiter, which spins 3 times faster than Earth, you’ll see it’s an ellipse not a sphere. During the creation of a star, some of the mass of the gas cloud that forms the star rotates so fast it flattens out into a pancake, or disk, and may begins to form planets. Which is why it’s thought all the planets orbit in the same direction and almost in the same plane in our solar system. The same process occurs to form accretion disks around black holes and neutron stars.

Some think a spiral galaxy is thought to be just a scaled up version of this disk formation.

Now for the giant elliptical galaxies, these are thought to form out of galaxy collisions. which strip them of gas, and after the merger of several galaxies the angular momentum/mass is reduced because the rotations of the individual galaxies are random, some left, right, some up, some down, for a large collection of galaxies. This leaves a much rounder looking object.

The formation of galaxies is an ongoing area of research. There is still much debate and the details may change. In a few years the large infrared telescopes like the Spitzer space telescope, and Hubble’s successor the James Webb space telescope will be able to see the formation of galaxies within the first billion years after the big bang, and give new insights into these processes.

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