1. Photometry and Spectroscopy
Astronomy 315
Professor Lee Carkner
Lecture 7

2. Quiz #1 Next Monday (March 26) Covers lectures 1-9
About 16 multiple choice (~50% weight)
About 4 short answer/problems (~50% weight)
Equations and constants provided
But unlabeled
You must bring pencil and calculator!
No cell phones/PDAs
Observing list 1 due this Friday

3. Studying for Quiz #1 Study Can you: Study guide posted on web page
lectures
exercises
homework
readings
Can you:
Identify the key concepts of the class?
Write a paragraph explaining key concepts?
Solve math problems from exercises and book without help?
Study guide posted on web page

4. Studying Stars Spectra are studied through spectroscopy
Stars are too small to see structure
Spectra are studied through spectroscopy

5. Spectroscopy If we take a spectrum of a star, what does it look like?
What do the lines and their strength tell us?
Strength of line depends on:
The temperature being such that the transitions can occur

6. Spectral Signatures
An atoms electron’s can be in a number of states from 1 (the ground state) to removed from the atom completely
At higher temperature they are in higher states
Ionized atoms are represented with roman numerals (e.g. Ca II, calcium with 2 electrons missing)

7. Hydrogen Transitions

8. Spectral Lines in Stars
Most stars have very similar compositions
The spectrum we take only covers a certain energy range
Three reasons:
Temperature so high that electrons only produce higher energy transitions

9. The Balmer Series
All stars are made primarily of hydrogen, but many stars have weak H lines
In what stars do we see Balmer lines?
Not in cool stars (electrons all in ground state)
Only in medium hot stars are the Balmer lines strong

10. Spectral Types The spectral types are (from high to low temperature):
Each spectral type is divided into 10 sub classes (from high to low T)

11. Temperature Dependence of Stellar Spectral Lines

12. Stellar Spectra -- Image

13. Spectral Typing Spectral type gives us temperature
O and B stars T ~
A and F stars T ~
G, K and M stars T ~
Spectral typing is accurate to about 2-3 subcategories or a few hundred degrees

14. The Spectral Types
Stars were first classified by strength of the H Balmer line
Eventually it was determined that this sequence did not reveal anything of astrophysical significance

15. Photometry
We want to get an accurate quantitative measure of brightness
Our system is composed of two things:

16. Magnitude
The magnitude scales is logarithmic and is related to the flux by:
m2 – m1 = 2.5 log10 (f1/f2)
where the flux is defined as the amount of energy received from the star per unit area per unit time (watts/m2/s)

17. Notes on Magnitude Magnitude scale runs backwards
Scale is semi logarithmic
A star that is n less magnitudes has 2.5n times the flux
5 magnitude difference is factor of 100 difference in flux
Magnitude is sometimes indicated with an “m”

18. Magnitudes of Selected Objects
Sun:
Moon:
Venus:
Sirius:
Faintest star you can see:
Faintest star with small telescope:
Large telescope and CCD camera:
Hubble Space Telescope:

19. Filters Use a set of standard filters, such as the UBVRI scale
e.g. V = nm, B = nm
We report the magnitudes with the letter of the filter

20. Standard UBVRI Passbands

21. Color Index The color index gives an estimate of the temperature
Example B-V:
Negative B-V means smaller B magnitude, which means more blue light, indicating a hot star

22. Star Names Only the brightest stars in the sky have proper names
e.g., Rigil Kentaurus from Rijl al-Qanturis meaning “Foot of the Centaur”
Bright stars also have a Bayer designation
Alpha (a) Centauri, Beta (b) Centauri, Gamma (g) Centauri, etc.

23. Next Time
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