# Orbits of star in the Galaxy

1. ### Master J

225
I am having trouble understanding how stars orbit around a spiral galaxy.

First off, my understanding of the Local Standard of Rest (LSR) is that it is a point which corresponds to the average velocity of all the stars in the defined local neighbourhood. This point moves around the centre of the galaxy in a closed circle.

Stars move around the galaxy in closed elliptical orbits with respect to the LSR, but in circular open orbits with respect to the fixed frame of the galaxy itself. I am having trouble visualing why this is so. And why is the elliptical motion retrograde wrt the frame rotating with the galaxy??

Is the epicycle motion (the ellipse) an elipse in a complete rotation around the galaxy, or does this mean that stars rotate around the LSR in an ellipse while the LSR rotates around the galaxy?

This is all very confusing to me. I hope I can sort all this out... thanks!!!

2. ### Matterwave

3,859
The stars don't orbit around the LSR, since that's an arbitrary point...there's nothing for the stars to orbit around. As far as I know the Stars orbit around the center of the galaxy in a weird non-closed (not a circle or ellipse) orbit since the mass distribution of the galaxy is not simple like the sun-earth system.

3. ### Chronos

10,126
Stars migrate throughout the galaxy. They are not fixed pointe with respect to the center of mass of the galaxy. Local gravitational effects have more influence than the mass of the galaxy, which merely holds them captive.

4. ### Jolb

419
The Sun's orbit around the galactic center is not perfectly circular. In fact, it's not even closed. It makes a "rosette" shaped orbit--like a spirograph design.

The Local Standard of Rest is just a convenient way of simplifying the mathematics--it's defined as the sun's orbit if it were perfectly circular. The radius is just defined as the sun's current radius to the galactic center (Sagittarius A*), but the orbit is simplified to being a perfect circle.