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

HR Diagram for nearest16,000 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 = 4πR2σT4

For a given radius, weCan sketch lines of Lvs T on the HR diagram

Red Dwarfs

Luminosity Classes:

Higher density gas > broader absorption lines in spectrumHigher density gas in star’s atmosphere > smaller star > smaller luminosity for a given temp.

L = 4πR2σT4

Lecture 8 Part 2Masses 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 farout you are orbiting Therefore we need to look at binary stars systems

Binary StarsBetween 50% and 67% of all stars are in multiple star systems

Proxima Centauri....

... Orbits Alpha Centauri A and B

Kepler’s third law: MA + MB = a3/P2

a

A Ba = average separation of stars in astronomical unitsP = period of orbit in yearsMA + MB = total mass in terms of solar mass

ra r b

A B

ra:r b = MA : MB

I: Visual Binaries

Rigel

I: Visual Binaries

Mizar

II: Spectroscopic Binaries

III: Eclipsing Binaries

Mass-Luminosity Relation for the Main Sequence: L = M3.5

+5

0

-5

-10

+10

+15

Absolute Magnitude