ASTR 1102 Lecture : Lecture31
Document Summary
Earth"s orbit around the sun - 30 km/s. Sun"s orbit around the galaxy - 220 km/s. Measure relative to globular clusters and other galaxies. Orbital radius: r = 8000 pc = 2. 5x1020 m. Speed: v = 220 km/s = 2. 2x105 m/s. The sun is about 23 galactic years" old. If it did not, everything would fall into the center. Rotation is also why it is flattened into a disk. All stars move at about the same speed. Use radio to measure velocity of neutral hydrogen. 21cm radio emission and the doppler shift. Stars do not rotate around a point mass at the center. Gravity comes collectively from matter inside the stars orbit. Rotation velocity tells us how much mass is inside the orbit. Rotation curve tells us distribution of mass. High velocities in outer disk need a lot of mass in outer disk. Rotation curve indicates more than 90% is further out than sun.
