1. Stars…Giants, Supergiants, Dwarfs….
How we can tell a lot about stars from starlight. (You need a lot of physics)

2. Here are more…every star tells a story

3. Last week’s thrilling episode… you can learn a lot about stars from their spectra
The spectrum of the Sun

5. Starlight…application of spectroscopy to stars
Continuous spectrum gives surface temperature (Wien’s Law)
Spectral lines give chemical composition, temperature (also), speed of rotation (How?) and other properties
Examples of stellar spectra…what can we say?

6. Spectral classes of stars: O,B,A,F,G,K,M
What can you say about the temperatures of these stars?

7. So what have we learned about stars from their spectra?
Temperatures (real hot)
Recipes (what elements they are made of)
Speed of rotation
More neat things
We can also tell which ones are big and which ones are not

8. The physics of pressure broadening
What can we learn from observing whether a spectral line is narrow or broad?

9. A tale of three stars (a black one, a pink one, and a red one)
What does it mean?

10. How does pressure modify the shape of a spectral line?
Think first of an atom emitting (or absorbing) light in isolation

11. When the pressure goes up, atoms “feel their neighbors” and have identity crises. The atomic energy levels, instead of being crisp and unique, get “fuzzed out”. “Fuzzed out” is technical terminology for a change in the energy which depends on how close the neighbors are, how many of them there are, what direction they are,……

12. Broad lines from pressure-broadened atoms

13. What determine the pressure in a star’s atmosphere?
The weight of the gas on top of you
Weight determined by two things:
How much gas (density and height of column)
The acceleration due to gravity
= g = GM/r2

14. Pressure in a stellar atmosphere
The larger the star is, the bigger its radius r
If the mass is the same as a smaller star, the acceleration of gravity will be smaller
The atmospheric pressure will be less, as well as pressure broadening.

15. Just from the spectra (no idea of distance) we can tell big stars (giants) from little stars (dwarfs)

16. A real question: if two stars have the same color, and pressure broadening of the spectral lines tells you that one is much larger than the other, what can you say about the relative brightnesses?

17. Summary
From spectra, we can determine that some stars are much larger, and thus much more luminous than others of the same color (=temperature).
There is a wide range in luminosity (and size) for stars of the same temperature.
Leads to the classification of dwarfs, giants, and supergiants

18. Disclaimer
Historically, this conclusion was reached by measurement of distances to stars. It is also easier to show that stars must have different luminosities (given their distances). However, you can conclude this just from the spectra.

19. With information provided by spectroscopy, we can search for correlations between stellar properties

20. With information provided by spectroscopy, we can search for correlations between stellar properties

21. What the data show: the Hertzsprung-Russell Diagram
Highest quality data from the Hipparchus spacecraft

22. The Hertzsprung-Russell Diagram and the Types of Stars
See Figure 16.20
Types of stars, important terms
Main Sequence
Giants
Supergiants
White dwarfs
What does it all mean?

23. Hertzsprung-Russell Diagram