Classifying Stellar Spectra
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Stellar spectra tell us the temperatures and the luminosities of the stars. In your last homework,
you plotted the apparently brightest and the nearest stars. In this exercise, you will be looking at
the data that lead people to classify stars based on their spectra.
Most stars produce ABSORPTION spectra, the ones with dark lines on a background of rainbow
colors. As you can see the hydrogen and helium lines dominate at high temperatures. At lower
temperatures, there are LOTS of lines visible. These lines are from larger atoms, ones where the
negatively charged electrons are further from the positively charge (attracting) nucleus. Since
these electrons are far from the nucleus and there are lots of electrons in between them and the
nucleus, it takes less energy to cause these electrons to absorb light. So we see the lines of
these larger atoms in cooler stars.
Stellar Spectral Types: OBAFGKM
Credit & Copyright: KPNO 0.9m Telescope, AURA, NOAO, NSF APOD May 30, 2001
Each band of color is a different star. The stars have been put into order from hot at the top to
cool at M5. All of the photos include the same range of colors, but as you compare the spectrum
of the M5 star with the spectrum of the O6.5 star, you can see that the blue color background is
much brighter in the O6.5 star than in the M5 star. This is because the M5 star is so much
cooler. If you look at the red end of the picture, the O6.5 star doesn’t look very bright, but it is
actually brighter than the red end of the M5 spectrum. The intensities have been adjusted to
keep the picture from being too bright at either end.
The “F4 metal poor” star has little of the elements heavier than hydrogen. To astronomers,
anything except hydrogen and helium is a “metal”. The temperature is very close to the
temperature of the F5 star you have further up in the picture. If you look at the colors in the
background of each, the balance is about the same. This indicates the same temperature. But the
metal poor star has few lines.
The emission line stars are ones that have extended atmospheres. The gas in these
atmospheres is seen against the dark background because it extends so far from the star. The
Classifying Stellar Spectra
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emission lines are brighter places in the spectrum. In the M4.5 emission line star, there are some
emission lines in the yellow part of the spectrum, to the blue (left here) of the g band of iron (an
absorption line about 30% of the way in from the
right) . They are NOT the only ones in the sample
that have emission lines.
As nice as the colored pictures are, it is both hard
to get accurately-printed colors and unnecessary.
A prism (triangular piece of glass) and a diffraction
grating (like the plastic you used) spread out light
with a different position for each wavelength. So
rather than showing lots of colored pictures, people
are using plots of the amount of light as a function
of wavelength.
In the following figure to the right, the amount of
light has been measured at each wavelength and
plotted. The plots have been shifted vertically so
that they do not overlap. Don’t worry, there are
larger, clearer copies of this kind of plot in the
reference files.
If the stars were at the same distance as one
another, then we would see MUCH more energy
from the hotter stars because the dense parts of
the stars emit with a black body spectrum. The
black body spectrum was discussed in chapter 3
and plots of the amount of energy can be made.
The figure below shows one form of the black body
curves.The peak of the black body curve depends
on the temperature of the body. The peak of the
Black Body Curves
18
101.E+18
Visible Light
30,000K
14
101.E+14
6000K
10
101.E+10
curve is related to the temperature of
2000K
Curves the body by the formula
6
10
1.E+06
500K
2
10
1.E+02
Temperature (in Kelvin)
=29,000,000/(peak wavelength
Ångstroms)
300K
100K
in
-2
101.E-02
-6
10
1.E-06
-10
101.E-10
1.E-09 -9
10
1.E-08-8
10
1.E-07 -7
10
1.E-06-6
10
1.E-05-5
10
1.E-04-4
10
1.E-03 -3
10
Wavelength, meters
Classifying Stellar Spectra
1.E-0
10
You will be getting some experience
with “classifying” spectra. That is
deciding the temperature and spectral
type of a sample of stars. You will
have plots of the spectra of reference
stars and plots of “unknown” stars. It
2
will be your challenge to find the type and temperature for each of your stars.
To decide the spectral type, compare the appearance of the plotted star data with the appearance
of the reference star. Use the underlying shape of the curve to find the temperature by estimating
the peak of the curve and solving
Temperature (in Kelvin) =29,000,000/(peak wavelength in Ångstroms)
Decide the spectral type by comparing the plot to the plots for other stars. Use the shape of the
curve to get the approximate temperature. Look at the hydrogen line strengths to decide the
spectral type for B A and hotter F stars. Use the Calcium h and k line to decide the type for A5 to
about G0. Use the iron g band (at about 4300Ångstroms) compared to the calcium h and k to
decide in the range F5 to about G5. Use the molecular TiO band strength to get the spectral type
in the M star range.
Reference Spectra with Lines Identified
To decide on the luminosity class for each star, look FIRST at the underlying black body curve
and decide a temperature (or nearly) . Then look at the reference stars with different luminosities
near the correct temperature. GENERALLY the giants (luminosity types I, II or III) have narrower
lines than do the dwarf (V) stars. In the hotter stars (B and O) there will be more lines visible in
the giant and supergiant stars. This is because the lower surface pressure allows us to see
skinny little lines that would be wiped out in a star with higher pressure. Cool stars, like M stars,
often have emission lines in the giants. The emission line is a part of the spectral plot that sticks
up.
It is not very easy to tell the luminosity class from the plots. Just do your best. The idea is to get
into how it is done. It isn’t a matter of life or death.
Example spectrum with lines identified. Alt example
More instructions about how to classify spectra Alt source for more instructions.
Star assignments for each person are on the WebCT homepage.
Links to the comparison spectra are below (alternate set)
O/B V
O/B III
KMII, OI
A/F V
A/F III
B,A I
G/K V
G,K III
F,G I
M V, and FG IV
MIII, BFG II
K,M I
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) Temperature calculated from 2 if you have a value. Otherwise estimate it from the spectral type
4) Spectral Type, Letter and number
5) Luminosity class, I, II, III, IV, or V
6) A couple of sentences about how you decided on the spectral type and luminosity class
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Things like, “ The hydrogen lines were stronger in the unknown than in the A3V star and
weaker than in the A4V star,” would be appropriate.
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.
Links to the spectra to classify
1
13
25
37
49
61
73
85
97
2
14
26
38
50
62
74
86
98
3
15
27
39
51
63
75
87
99
4
16
28
40
52
64
76
88
100
5
17
29
41
53
65
77
89
101
6
18
30
42
54
66
78
90
102
7
19
31
43
55
67
79
91
103
8
20
32
44
56
68
80
92
104
9
21
33
45
57
69
81
93
105
10
22
34
46
58
70
82
94
106
11
23
35
47
59
71
83
95
107
12
24
36
48
60
72
84
96
108
11
23
35
47
59
71
83
95
107
12
24
36
48
60
72
84
96
108
Alternate sources
O/B V
O/B III
KMII, OI
A/F V
A/F III
B,A I
G/K V
G,K III
F,G I
M V, and FG IV
MIII, BFG II
K,M I
Unknown spectra
1
13
25
37
49
61
73
85
97
2
14
26
38
50
62
74
86
98
3
15
27
39
51
63
75
87
99
4
16
28
40
52
64
76
88
100
Classifying Stellar Spectra
5
17
29
41
53
65
77
89
101
6
18
30
42
54
66
78
90
102
7
19
31
43
55
67
79
91
103
8
20
32
44
56
68
80
92
104
9
21
33
45
57
69
81
93
105
10
22
34
46
58
70
82
94
106
4