Stellar motions near the center of the Milky Way

Due Friday, Dec 8, 2000, at 3:00 PM

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).

A group of astronomers from UCLA have used the Keck Telescope to take infrared images of the very center of the Milky Way over the past few years. You can find a description of their work in Ghez et al., Nature, vol 407, p. 349 (2000). The picture shown below spans a region about 0.041 parsecs across (click on the picture to see a larger version). The ellipses drawn for stars S0-1 and S0-2 are the astronomers' best guesses for their orbits; if we continue to watch this area for another few decades, we may see if they are correct.

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 0.041 parsecs. 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 much time passes from the first to last position for each star, 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.

Convert the speed of one of the stars into the familiar units of miles per hour. Compare this to the speed of the fastest object on Earth for which you can find a reference (in a magazine, newspaper, book, etc.). Be sure to include the reference with your answer.

Question: does this picture show all of the motion of the stars, or is it possible that some of their motion might not appear in it? Explain.

Last modified by MWR, Nov 29, 2000