Spiral Galaxy Rotator
The Java applet below requires the Java 1.4.x plugin, available from java.sun.com (try this page for direct information about the Java Plug-In if you don't know where to go.) 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?
Suppose that spiral arms are "material arms". That is, suppose that most of the stars in a spiral galaxy are "in" the spiral arms. A star that is in a spiral arm always stays in that spiral arm. As the stars in the galaxy orbit around the center of the galaxy, the arms should rotate along with them. In this model, we would see the spiral arms because that's where most of the stars are.
What would happen to the spiral arms in this case?
Two things are plotted below. One is the image of a spiral galaxy, looking "down" on the disk.
Below that 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?