1. Dark Stuff – an interlude Toby Burnett

2. How do we measure mass? Weigh? Count the number of atoms? Gravitational effect?

3. Can we “weigh this?”

4. Measuring mass of distant galaxies If know v and R, then use F=ma =mv 2 /R = GmM/R 2 to get M=v 2 R/G M is enclosed mass How do we measure R, v? M v R

5. A typical “rotation curve” Dark Matter!!

6. The enclosed mass around Sag A

7. Stirring up the stars If one looks at this region with big telescopes and near-infrared cameras one can see lots of stars. If one takes pictures every year it seems that some stars are moving very fast (up to 1500 kilometers per second). The fastest stars are in the very center - the position marked by the radio nucleus Sagittarius A* (cross).