1. Spectra and Motion – Doppler Effect
Q. 68 Shifted Spectrum
Q. 69 Doubled Spectrum

2. Summary – What Spectra Tell Us
Temperature
From the peak of the spectrum
Composition
From wavelengths and strength of dark lines
Motion
From the Doppler shift
Multiplicity
From the number of sets of spectral lines
Orbit and masses
From the changing Doppler shift
Pressure and rotation
From width of lines

3. Luminosity, Temperature, and Radius
The spectrum of a star is pretty much a black body distribution
How bright each point on the surface is depends only on temperature
Multiply by the area to get the Luminosity
F =T4
L = AF = 4R2T4
Q. 70: Luminsity, Temperature, and Radius

4. Intrinsic Properties of Stars
To describe stars, we want to talk about intrinsic properties
Luminosity
Composition
Temperature
Composition is almost always the same
Mass is difficult to measure
Radius can be deduced from Luminosity and Temperature
Radius
Mass
Temperature and Luminosity

5. The Hertzsprung-Russell Diagram
A plot of temperature vs. luminosity
Hot on left, cold on right
Luminous at top, dim at bottom
Stars fall into categories:
The Main Sequence contains about 90% of the bright stars
The Giants are rare but very bright
The Supergiants are very rare but extremely bright
The White Dwarfs are not uncommon but very dim

6. Main Sequence Stars
Main Sequence stars have different sizes, masses, and luminosities
But spectral class determines everything else
This diagram shows correct relative sizes and approximate colors of stars
But not correct relative luminosities

7. Luminosity from Spectral Class
Suppose you have a G2 star. What is its luminosity?
90% of all stars are main sequence
G2:
B5:
K5:
For main sequence stars, the spectral type tells you the luminosity
Together with brightness, this tells you the distance
Spectroscopic parallax

8. Spectroscopic Parallax
Another distance method
Has nothing to do with parallax
Works only on main sequence stars
How it works:
Observe the star – determine it’s brightness B
Measure its spectral type from spectrum
Deduce its luminosity from the Hertzsprung-Russell Diagram
Find its distance from:
L = 4d2B

9. Stellar Masses T M T M T M O5 60 F0 1.6 K5 .74 B0 18 F5 1.3 M0 .51
Only some stars can have their masses measured
They need to be in binary systems
The masses of main sequence stars depends pretty much only on their spectral type
T M T M T M
O5 60 F K5 .74
B0 18 F M0 .51
B G M5 .21
A G M8 .06
A K0 .85

10. The Main Sequence The mass of a main sequence star affects everything
Temperature
More massive is hotter
Luminosity
More massive is much more luminous
Radius
More massive is bigger

11. The Life History of Stars – Young Stars
The Importance of Mass
The entire history of a star depends on its mass and almost nothing else
The more mass a star has, the faster it does everything
The stages of a star differ based on what is happening in the core of the star
The properties of a star vary wildly as it passes through different stages
Qualitatively, stars have similar histories, with one big split:
Low mass stars (< 8 MSun) have quiet deaths
High mass stars (> 8 MSun) go out with a bang