1. Stellar Spectra
AST 112 Lecture 7

2. Stellar Spectra
The interior of a star can be considered a “hot dense object” that emits a continuous spectrum.
The interior is surrounded by a much cooler atmosphere.
What type of spectrum (emission, absorption or continuous) would you expect to see from a star?

3. Stars show an absorption spectrum.
* Some stars show emission features as well.

4. Stellar Spectra
Stellar spectra tend to resemble one of these seven spectra

5. Stellar Spectra
Why do stars have different spectra?

6. Pioneers of Stellar Spectroscopy

7. 1 2 3 4 5 6 7

8. The Old Classification
First classified by strength of hydrogen lines
Old Classification: A, B, C, D, …

9. The Old Classification
Maybe not perfect?
17,000 oF
30,000 oF
15,000 oF
60,000 oF
12,000 oF
10,000 oF
7000 oF

10. The Harvard Classification
More natural order followed (found by Annie Jump Cannon)
This order follows temperature
Everything’s great, right??
Higher Temperature

11. The Balmer Series These absorption lines are called the Balmer Series.
They occur when the electron in hydrogen is in the first excited state and absorbs a photon

12. The Balmer Series
So in order to see Balmer lines, some fraction of the hydrogen atoms must have their electrons in the first excited state.
The more hydrogen atoms in the first excited state, the darker the absorption lines.

13. A Discrepancy Quantum Mechanics:
H absorption lines should get stronger as temperature goes up
The order of the old scheme looks right
But temperature is wrong!

14. The graph does not agree with the spectra!
17,000 oF
30,000 oF
15,000 oF
60,000 oF
12,000 oF
10,000 oF
7000 oF

15. The Harvard Classification
Saha: Electrons detach at these temperatures!
If the electron of H is detached, absorption lines don’t happen

16. Multiply the top graphs to get the bottom graph.

17. This graph peaks at 10,000 K (17,000 oF)
This graph peaks at 10,000 K (17,000 oF). At what temperature would you expect the darkest H absorption lines? What happens to the H absorption lines above or below this temperature?

18. Combine Graphs “A” Star 17,000 oF
Graph of line darkness peaks at 17,000 oF

19. OBAFGKM OBAFGKM is correct!
The physics of the absorption lines agrees completely.
Spectral type is determined by temperature.

20. Structure of Matter vs. Temperature
Warmer:
Molecules Break
Cooler: Molecules Form
HOT:
Atoms Ionized
Same atoms. Different interactions. Different spectra.

21. Stellar Spectra
We can do this for all of the elements. This is how the line strength behaves (with temperature) for each element. And it agrees with stellar spectra.

22. Stellar Spectra
Decreasing temperature: O B A F G K M

23. So what are stars made of?
Chemical composition does not vary much from star to star
Cecilia Payne:
All stars made up of:
About 75% Hydrogen
About 25% Helium
About 1-2% heavier elements

24. Stellar Spectra If stars have little variation in composition,
states of elements determine spectrum.
Temperature determines the state of the elements, and therefore the spectrum.

25. Measuring Stellar Temperature (Spectra)
M type stars: 5000 oF
O type stars: 70,000 oF
Each spectral type subdivided by number
B0, B1, … , B9
Larger number, cooler star
Sun is G2
Sirius is A1
Betelgeuse is M2

26. Measuring Stellar Temperatures
Recall Stefan-Boltzmann Law and Wein’s Law
Brightness = sT4
Peak Wavelength
depends on T
When do these laws apply?
Do they apply to stars?

27. Measuring Stellar Temperatures (Color)
Rank the following stars according to temperature, hottest first:
Yellow
Red
Blue

28. Measuring Stellar Temperatures (Color)
Rank the following stars according to temperature, hottest first:
Blue (Sirius: 16,500 oF)
Yellow (Sun: 10,000 oF)
Red (Betelgeuse: 5600 oF)