ASTR 1200 Announcements First Problem Set Assigned Due

ASTR 1200 Announcements. . First Problem Set Assigned. . . Due next Tuesday in class. Observatory Sessions all now at 8: 30 pm Lecture Notes going up on the website Schedule has been updated. Exam dates set. Text reading still needed Website http: //casa. colorado. edu/~wcash/APS 1200. html

Orbits Falls Freely Hits Surface Misses Surface From Sideways Motion

Physics of Orbits sideways velocity central acceleration If you balance things just right, you have an orbit. Too Little Sideways V = Crash Too Much = Escape to Infinity Earth

Circular Orbit circle implies Kepler’s Law

Example Calculate the period of a planet that is the same distance from the star Sirius that the Earth is from the Sun. r=1. 5 x 1011 m G=6. 7 x 10 -11 M=4 x 1030 kg About 9 Months

Non-Circular Formula 2 r Focus Same Formula for P Use half orbit length for r Fast Slow

Escape Velocity R Fall from Large Height Same Energy Needed to Reverse and Fly Away Escape velocity is the speed at which object must be thrown upward to escape and never come down.

Example Earth R=6 x 106 m M=5 x 1024 kg

Surface Gravity Force of Gravity at Surface of Body is So or The acceleration of a body near the surface. Independent of mass. Remember Galileo and the Leaning Tower of Pisa

Gravity on Earth 1 gee is 9. 8 m/s per second

Spin Up When a body is spinning “Angular Momentum” is conserved For spherical body: As R decreases P decreases Spin up of ice skater. Same for Collapsing Stars

Escape Velocity R Fall from Large Height Same Energy Needed to Reverse and Fly Away Escape velocity is the speed at which object must be thrown upward to escape and never come down. G=6. 7 x 10 -11 in mks units

Question What is the escape velocity of the Moon? M=7. 35 x 1022 kg, R=1738 km a)2000 km/s b)200 km/s c)20 km/s d)2 km/s e)0. 2 km/s

Answer: D Moon R=1700 km M=7 x 1022 kg

The Sun

The Sun Falls into “Disk Stability” 99. 9% Ended in Sun (0. 1% in Jupiter) Probably the Same Around All Stars -- Planets are Common Shoots Planet-Size Bullets into Space

Most of Mass Forms Ball in Center A Star Is a Hot Ball of Hydrogen (plus 11% Helium) One Million Miles

Question • Why is the Sun spherical while the planets orbit in a disk? • A) They actually orbit in a sphere • B) The planets hit each other if they don’t all lie in the same plane • C) The Sun has lost spin over time, while planet orbits cannot • D) The Sun will eventually evolve to a disk

Answer: C • The surface of the Sun is orbiting at far less than orbital speed. (just like the Earth) Back when it was first forming it spun much faster and was closer to a disk.

What Stops the Fall? Gravity Gets Stronger As Material Gets More Dense R smaller implies F greater The smaller it gets, the faster it falls in! Why doesn’t it just become a black hole? Or worse yet, a point-like singularity of mass?