Spiral Galaxy Rotator
The Java applet below requires the Java 1.4.x plugin, available from java.sun.com. The applet code should be compatable at least with Mozilla, and presumably with anything else that correctly implements the latest HTML standards. It should also work with Internet Explorer if you have the Java 1.4.x plugin installed, I believe.
What are (or aren't) spiral arms?
Two things are plotted below. One is the image of a spiral galaxy, looking "down" on the disk. In this simulation, spiral arms are assumed to be material arms. That is, some stars are "in" the arm, and some are "out" of the arm. Stars that are in the arm stay in the arm. Under this model, you see spiral arms simply because the stars that are "in" the arm are more plentiful and/or brighter than the stars that are "out of" the arm. As the galaxy rotates, wherever the stars that are "in" the arm goes, there go the arms.
Below is the rotation curve of the galaxy. Plotted is the linear velocity (in km/s, going from about -300 to +300) as a function of the radius from the center of the galaxy (in kpc, going from about 0 to 15). The default rotation curve is an approximation of that which has been measured for the Milky Way. Various default options are available at the bottom:
- Milky Way
- Something like our Galaxy's rotation curve.
- Keplerian
- The rotation curve that results when all of the mass of the system
is right at the center. This is the rotation curve of our solar
system (the Sun is much more massive than all the planets).
- Linear
- This is the rotation curve of a merry-go-round or a CD. The
angular velocity is fixed, which means that the linear velocity is a
function of radius.
You can also drag the control points around to make whatever rotation curve you want.
Click "start" to set the galaxy in motion given the current rotation curve. Notice what happens when you let the galaxy rotate using the Milky Way's rotation curve. Yet the Milky Way and may other galaxies show relatively open spiral arms that appear to have mained their shape over billions of years. What does this tell us about the nature of spiral arms in galaxies?