# Astronomy Problems 561: 02-15-11 HR Diagram part II.

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Astronomy Problems 561: 02-15-11 HR Diagram part II

HR Diagram for nearest 16,000 stars: Most stars lie along the Main Sequence – Simple relationship between temperature and luminosity – Stars spend most of their lives converting hydrogen to helium, and this is what occurs when the star is on the main sequence

L = 4πR 2 σT 4 For a given radius, we Can sketch lines of L vs T on the HR diagram Red Dwarfs

Luminosity Classes: Higher density gas > broader absorption lines in spectrum Higher density gas in stars atmosphere > smaller star > smaller luminosity for a given temp. L = 4πR 2 σT 4

Lecture 8 Part 2 Masses of Stars; Binary Star Systems

How do you weigh a star?

Orbital Motion: An object in orbit around another object is in continual free-fall How fast you orbit an object depends on your mass and how far out you are orbiting Therefore we need to look at binary stars systems

Binary Stars Between 50% and 67% of all stars are in multiple star systems Proxima Centauri....... Orbits Alpha Centauri A and B

Keplers third law: M A + M B = a 3 /P 2 a AB a = average separation of stars in astronomical units P = period of orbit in years M A + M B = total mass in terms of solar mass

rara r b AB r a :r b = M A : M B

I: Visual Binaries Rigel

I: Visual Binaries Mizar

II: Spectroscopic Binaries

III: Eclipsing Binaries

Mass-Luminosity Relation for the Main Sequence: L = M 3.5

+5 0 -5 -10 +10 +15 Absolute Magnitude

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