Orbits of star in the Galaxy


by Master J
Tags: galaxy, orbits, star
Master J
Master J is offline
#1
Nov23-09, 02:11 PM
P: 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!!!
Phys.Org News Partner Science news on Phys.org
Better thermal-imaging lens from waste sulfur
Hackathon team's GoogolPlex gives Siri extra powers
Bright points in Sun's atmosphere mark patterns deep in its interior
Matterwave
Matterwave is offline
#2
Dec2-09, 10:01 PM
P: 2,043
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.
Chronos
Chronos is offline
#3
Dec3-09, 12:30 AM
Sci Advisor
PF Gold
Chronos's Avatar
P: 9,180
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.

Jolb
Jolb is offline
#4
Dec6-09, 11:27 PM
P: 419

Orbits of star in the Galaxy


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.


Register to reply

Related Discussions
Finding the mass of our Galaxy and the amount of stars in our Galaxy Precalculus Mathematics Homework 1
star light from a dead star ? Astrophysics 3
circular kinematics... a star in orbit around another star. Introductory Physics Homework 4
How do we know the Sun orbits the Galaxy? General Astronomy 13
Orbits of Andromeda and Milky Way Galaxy About Each Other Cosmology 1