Assignment 1: Stellar motions near the center of the Milky Way

Note to self 9/26/2000: see updated proper motion measurements in recent Ghez paper, Nature, Sep 21, 2000 .

Due Friday, Mar 9, 2000, at the beginning of class

Your job is to measure the speed of stars in a cluster near the center of the Milky Way. You must determine the relative speeds of at least 3 different stars in units of kilometers per second (km/s).

Evidence for very large mass near the center of the Milky Way -- watch the stars in this cluster move over just 4 years! The pictures span a region about 0.3 light years across. I've added colored lines which indicate the separation between some of the stars in the first image ... which is obviously NOT the same as the separation of those stars in the second image.

Separate versions of the two images are available below.

The units marked on each graph are "arcseconds" -- an arcsecond is a tiny angle on the sky. You can't use them to figure out speeds in kilometers per second. In order to convert the distance a star moves in these pictures to kilometers, use the fact that the entire width of each picture is about 0.3 light years. Convert that into kilometers. Then pick a star, and figure out what fraction of entire width it travels from one image to the next; you can then calculate how many kilometers it moves. You also know how many seconds passed between the two pictures, so ...

Submit a copy of one graph with your three stars clearly labelled. Show all your measurements and calculations for each star, ending up with its speed in km/s.

Question: did you measure the entire motion of the star, or only part of it? Explain.

Last modified by MWR, Mar 5, 2000