Download Project 2 – Spectral Types of Stars

Spring 2011
Astr 221: General Astronomy II– Star, Galaxies & Cosmology
Project 2 – Spectral Types of Stars
This project is your Homework #2.
It is due on 2/3/2011 at the start of class. I strongly encourage discussions before the due date.
Forword:
If thermal radiation were the only source of light from a star, the star’s spectrum would be a nice
smooth curve. However, actual spectra observed from stars have a series of peaks and valleys as
shown in the spectrum below, meaning some of their light comes from ”non-thermal” radiation light emitted or absorbed by a process other than random jostling of atoms. With this project,
you’ll explore in more detail this process.
Main Tools:
1. The SDSS SkyServer’s Explore:
http://cas.sdss.org/astrodr7/en/tools/explore/obj.asp?id=587731511532060697
(you’ve been here before with the previous project.)
2. another colleague to team up with.
Explore 1.
Pretend for a moment that you are an astronomer living in the early 20th century, before the
OBAFGKM star classification system was developed. You are one of the first astronomers who
ever looked at spectra of stars, and it is up to you to develop a classification system.
The table below shows a list of the first stars you are trying to classify. Look at their spectra
and divide them into several groups. There is no set number of groups you should try to achieve,
and the groups do not have to have equal numbers of stars (because not all types of stars are
equally common). If you find a spectrum that has nothing in common with any of the others, a
group of one is OK (but on the other hand, 16 groups of one is probably not useful either!)
plate
mjd
fiber
66
266
266
273
273
273
281
266
266
266
498
273
281
281
51630
51630
51630
51597
51597
51597
51614
51630
51630
51630
51984
51597
51614
51614
21
275
365
2
157
589
4
173
314
513
538
245
3
133
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Spring 2011
Astr 221: General Astronomy II– Star, Galaxies & Cosmology
Spectra in the SDSS are sorted by Plate, MJD (Modified Julian Date) and Fiber number. Using
the Explore Tool, search by Plate-MJD-Fiber, and you will have the spectra of these stars, one
by one.
Record your groups in a table. Each star should fit in one of your groups. Make notes detailed
enough that another classifier can duplicate your work.
Now partner up another classifier (one of your colleague), and compare your spectral classification systems.
1) Do you have the same number of spectral types? If not, what distinctions did one draw that
the other group did not?
2) What do your classification systems have in common? What makes them different?
3) Try to combine the best features of each classification system. Repeat the exercise with your
improved system.
4) Using the information included in the following two tables, comment on how does your classification system compare to the OBAFGKM spectral type classification? What are the similarities?
What are the differences?
The table below shows some of the characteristic absorption and emission lines of each star type
of the OBAFGKM classification:
Spectral Type
O
B
A
F
G
K
M
Temperature (K)
28,000 - 50,000
10,000 - 28,000
7,500 - 10,000
6000 - 7500
5000 - 6000
3500 - 5000
2500 - 3500
Spectral Lines
Ionized He (He II)
He, some H
Strong H, some ionized metals
H, ionized Ca (labeled H and K on spectra) and Fe
Neutral and ionized metals, especially Ca; strong G band
Neutral metals, Na
4 strong TiO, very strong Na
The chart below lists some of the more common ones and their approximate location in the
electromagnetic spectrum.
Spectral Lines
Ha, Hb, Hg
Ionized Calcium H and K Lines
Titanium Oxide
G Band
Sodium
Helium (neutral)
Helium (ionized)
Wavelengths (Angstroms)
6600, 4800, 4350
3800 - 4000
lots of lines from 4900 - 5200, 5400 - 5700, 6200 - 6300, 6700 - 6900
4250
5800
4200
4400
Present your answers/conclusions as a report. Enjoy!
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