The image shows two colliding galaxies known as NGC 2207 and IC 2163. It’s a false-color image, where infrared is shown as dark red, visible is shown as normal, and x-ray is shown as purple. The first impression you might have is that the image looks awfully purple, and that means there are lots of x-ray sources in these two galaxies.
Quintet
In 1877 Édouard Stephan discovered a cluster of five galaxies in the constellation Pegasus. It came to be known as Stephan’s Quintet. The quintet is probably most famous for its use in the opening scene of It’s A Wonderful Life, so it is likely the best known galaxy cluster.
Galaxy Quest
The categorization of galaxies was first made by Edwin Hubble in 1926, and became known as Hubble’s tuning fork. It was so-named because the variety of galaxies could be laid out along a scale of ellipticals which then branched into two types of spirals. When Hubble proposed this scheme, some suggested that it indicated the nature of galactic evolution, where round, elliptical galaxies gradually changed into flat, spiral galaxies. Hubble himself was cautious not to assume too much about the classification scheme. Still, it does raise an interesting question about these types of galaxies. Why do some become spirals and others not?
Riding the Wave
Just how do spiral galaxies form? It’s all due to a stellar traffic jam, and now the process has been observed in our own galaxy.
Galactic Scale
How do you weigh a galaxy? With planets we can measure their distance from the Sun and their orbital speed. By observing their motion in detail we can calculate their mass very precisely. For binary stars we can use a similar method. Observe the size of their mutual orbits and their orbital period, and by Kepler’s laws you can determine their mass. We can’t do that with galaxies, so what do we do?
Learning Curve
It’s often said that the shape of a spiral galaxy follows the curve of a golden spiral. You can see this, for example, in the image above. While it’s often implied that this curve matches exactly, that isn’t the case. The spirals of a spiral galaxy do tend to approximately follow a logarithmic spiral (of which the golden spiral is a special case), it is typically only a rough approximation.
Spirals With A Twist
Yesterday’s post on spiral galaxies raised some questions about the rotation of galaxies. Do galaxies rotate with the spiral trailing the motion, or does the spiral lead the rotation of a galaxy? The answer may surprise you.
The Gripping Hand
The image above shows a pair of colliding spiral galaxies known as Arp 274. What’s interesting is not that they happen to be colliding, but that the two galaxies are spiraling in opposite directions. The one on the left spirals in a clockwise direction, while the one on the right spins in a counterclockwise direction. Sometimes we’ll refer to the left galaxy as left-handed, while the right one is right-handed. The reason is that if you hold hands up with your thumbs pointing at yourself, you’ll see the fingers on your left hand curve clockwise, and the fingers on your right hand curve counterclockwise.
I’ll Fly Away
A couple days ago I wrote about how one of the properties of a galaxy is that its stars are gravitationally bound. This means that the stars don’t have enough speed to escape the gravitational attraction of the galaxy, and so a star by itself can’t escape the galaxy. But this doesn’t mean that stars can never escape their galaxy. In fact, given enough time, many of them will.