Download Classifying Spectra PDF version - the Home Page for Voyager2

Classifying Stellar Spectra
Printable form
Stellar spectra tell us the temperatures and the luminosities of the stars. In this exercise, you will be
looking at the data that lead people to classify stars based on their spectra and doing some of the
classification yourself. Definitely read about spectral types in your text before you start.
Most stars produce absorption spectra, dark lines on a background of rainbow colors, like the ones
shown below. In this picture, each of the colored horizontal bars is due to a different star. The stars
names, e.g. HD 12993, are on the right. HD stands for the Henry Draper catalog and the number is the
number for that star within the catalog. These spectra are arranged in order from hotter to colder going
from top to bottom
The colors correspond to wavelength of the light, with longer wavelengths for the red light and shorter for
the
blue.
S
t
e
l
l
a
r
S
p
e
c
t
r
a
l
T
y
p
e
s
:
O
B
A
F
G
K
M
C
r
e
d
i
t
&
C
o
p
y
r
i
g
Stellar Spectral Types: OBAFGKM
Credit & Copyright: KPNO 0.9m Telescope, AURA, NOAO, NSF APOD May 30, 2001
The spectral classes are specified by the letters O, B, A, F, G, K, M, L, T going hotter to colder. Each
letter is subdivided by assigning a number 0 through 9 following the letter and going from hotter to colder.
So B0 is colder than O9 and hotter than B1. Obviously not every type is shown.
Originally only black and white photos of the spectra (using film) were available and people decided on
the spectral type from the absorption (dark) lines. The color images are nice, but the spectra can be
analyzed more precisely by plotting the intensity of the light at each wavelength as here. There is a figure
with many plots of stars’ spectra below. The mount of light is on the y-axis and the wavelength is on the xaxis.
The hottest star is on top. The stars get colder going down. The plots have been shifted so that they don’t
overlap. Short wavelength is on the left, the same way as for the color photo. The places where the lines
jut down are where the dark spectral lines occur, where there is less light.
Please notice the way that plots of the hotter stars’ light rise toward shorter wavelengths. This is because
the colored background of the spectrum is due to a black body curve, like the ones shown in your
textbook and like the ones in the figure below. The shapes of these black body curves all look the same
because it is a logarithmic plot, i.e. the axes are in powers of 10. The ploits in the text are linear on both
axes. Regardless of the axes, the hotter the object, the shorter the wavelength at the peak of the curve.
Black bodies (and other
dense objects) emit light at all
wavelengths. But our plots
include only wavelengths
from 350nm to 750nm. So the
hottest stars have the peak of
their black body curves off
the plot on the short
wavelength side. This is quite
obvious in the hottest three
stars plotted below. The last
spectrum plotted is so cool
that the peak of the curve is
off the plot on the long
wavelength side. In between,
the peak of the curve moves
toward longer wavelength as
you go to cooler stars.
Please print out the spectra
for the stars that you were
assigned and put them in
order, hottest to coldest based on the peak of
the curve. There is a quiz in WebCT that assigns
your stars by number. The plots of stellar spectra
are found from links at the end of this lab.
To decide on the spectral type, we look at the
absorption lines and apply specific criteria.
Each spectral line always appears at the same
wavelength (except for Doppler shift which is not
happening here). Not every star shows the same
spectral lines. Although most stars are made of
same materials, temperature is the main factor
determining which absorption lines are seen and
how strong they are.
Use the Example spectra to see where the lines are. Two spectral traces are shown, so that features of
both hot and cold stars can be identified.
The symbols show where the lines are and ALL the lines from the same symbol will show up at once (e.g.
all the hydrogen lines are either present or not).
At the end of this lab are links to sets of reference spectral plots. Each of these plots is identified by a
number in the sequence (irrelevant), then a letter and Arabic number for the spectral type and a Roman
numeral specifying the luminosity class. For example, if you open O/B III the first plot will be labeled 62
O6.5 III. The number 62 is irrelevant, O6.5 tells the temperature of the star and III is the luminosity class,
giant.
When people first developed spectral types they identified specific lines that were sensitive to
temperature and that could be seen in the wavelength region available with photographic film. The
wavelength region available in our plots is larger, so the spectra look somewhat different.
The next figure is a flow chart of how to decide upon a spectral type, letter and number. Use the chart to
decide to within a letter. Look at the reference spectra to see how the appearance changes as the stars
get cooler.
Once you have decided upon an approximate spectral type, look at reference spectra with the same
letter, but look at the different luminosity class (I, III, V) examples. Then decide which luminosity class fits
your star best. Be prepared to change the spectral type a little from what you (previously) thought was
best.
How should you decide among the luminosity classes?
Main Sequence stars, luminosity class V, have wider line and more blended lines.
Supergiants,
luminosity class I, have very narrow lines. Often there are more lines seen in supergiants, since the very
weak lines are smeared out and invisible in main sequence stars.
The blended features in G and K stars (like the G band at 4300Å and the blend between 5000Å and
5300Å) will be many individual lines in luminosity class I.
Luminosity class I stars, especially O, B and M stars, have some emission (bright) lines. They show up as
parts of the plot that jut upward (often on the short wavelength side of an absorption line).
Just do your best and make a decision. Don’t spend your life on this.
Finally, use the underlying black body curve to calculate a temperature. This is a form of Wien’s Law.
Find the peak of the black body curve for each of your stars. If the peak is off the paper on the short or
long wavelength side, just say that it is not available and tell whether it is off the paper on the long or the
short side. If you can find the wavelength for the peak, calculate the temperature from:
Temperature (in Kelvin) =29,000,000/(peak wavelength in Ångstroms)
The answers are in the thousands. Don’t change your spectral type from the lines based on this. The
lines affect where you choose the peak and that alters this result (especially for the A and B stars). There
is a table of temperature and spectral type below that you may find interesting as a check.
Your star assignments are on the WebCT quiz. Links to the stellar spectra are below.
Comparison spectra
O/B V
A/F V
G/K V
M V, and FG IV
O/B III
A/F III
G,K III
MIII, BFG II
KMII, OI
B,A I
F,G I
K,M I
Unknown spectra
1
11
21
31
41
51
61
71
81
91
101
2
12
22
32
42
52
62
72
82
92
102
3
13
23
33
43
53
63
73
83
93
103
4
14
24
34
44
54
64
74
84
94
104
5
15
25
35
45
55
65
75
85
95
105
6
16
26
36
46
56
66
76
86
96
106
7
17
27
37
47
57
67
77
87
97
107
8
18
28
38
48
58
68
78
88
98
108
9
19
29
39
49
59
69
79
89
99
10
20
30
40
50
60
70
80
90
100
For EACH star tell
1) Star number
2) Wavelength at the peak of the Planck curve. IF the peak is off the plot, say so and tell whether the
peak will be off the plot on the long or the short end
3) A couple of sentences about how you decided on the spectral type and luminosity class. Use the flow
chart to guide your work and to guide your comments.
4) Spectral Type, Letter and number
5) Luminosity class, I, II, III, IV, or V
6) Temperature calculated from 2 if you have a value
For the lab as a whole, of course you will write an objective and a conclusion.
The results CAN be emailed, preferably without the plots.
Temperatures for various Spectral Types,
from Allen’s Astrophysical Quantities, Fourth Edition , Cox, Arthur ed.
Spectral Type
Temperature,
Giant, III
Supergiants, I
Main Sequence V
O5
42000
O9
34000
32000
B0
30000
B2
20900
17600
B5
15200
13600
B8
11400
11100
A0
9790
9980
A2
9000
9380
A5
8180
8610
F0
7300
7460
F2
7000
7030
F5
6650
6370
F8
6250
5750
G0
5940
5370
G2
5790
5190
G5
5560
5050
4930
G8
5310
4800
4700
K0
5150
4660
4550
K2
4830
4390
4310
K5
4410
4050
3990
M0
3840
3690
3620
M2
3520
3540
3370
M5
3170
3380
2880