ASTRO-110 Lecture Notes - Lecture 27: Binary Star, Elliptic Orbit, Star System
Document Summary
Knowing a star"s mass is very important but harder to measure than temperature or luminosity. We must use newton"s version of kepler"s third law of planetary motion which relates the masses of a pair of objects in an elliptic orbit to the period and semi- major axis of the orbit. So we can only measure masses when we find binary star systems!! Hard part is measuring the size of the orbit types of binary systems: visual binaries, spectroscopic binaries, eclipsing binaries. Spectroscopic: if we cannot visually resolve the stars in a binary, we can usually detect a periodic doppler shifting in its spectrum. The orbital period is the period of the doppler shifts; often difficult to get accurate measure of orbital size. This data can be used to determine the orbits size. As a bonus, knowing how long each star eclipses the other, and how fast the stars are moving, lets us determine the radii of the stars.
