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Note: you only need to hand in the first 3 pages of this handout…
TYPES OF STARS
Modified from: http://cas.sdss.org/dr5/en/proj/teachers/basic/spectraltypes/lesson.asp
When astronomers look through their telescopes, they see billions of stars. What can they learn from
their observations? In class, we’ve learned that the shape of the spectrum (especially, the wavelength
at which it reaches its maximum intensity) can be used to determine a star’s temperature. In addition,
the locations (energies) at which absorption or emission lines occur can be used to “fingerprint” the
star’s composition.
The goal of this problem set is for you to understand that astronomers classify stars on the basis of two
different criteria: (1) the intensity of one of the H absorption lines (called H-alpha, or H), and (2) on
the basis of temperature. At the end of this handout, there are typical spectra of 7 stars. In each case
there is one spectrum showing the part of the data that was acquired over the visible wavelength range
(the x axis is given in terms of wavelength in Ångstroms, which are just an order of magnitude
different than the nm we’ve been using in class) and the y axis is flux (which is essentially a measured
of intensity). Below each big plot is a zoomed-in view of the area of the spectrum around 6560
Ångstroms (i.e., 656 nm).
Question 1. Are these emission spectra or absorption spectra? How do you know?
First, we will classify stars based on the "strength" of their H lines. Look at the spectra of the seven
stars at the end of this worksheet. Using their spectra, rank the seven stars according to the strength of
their Hα lines. If you can't rank them all easily, try coloring in the area between the line connecting the
triangle centers and the actual spectrum. The bigger the colored area, the greater the line strength.
Originally, astronomers classified those stars with the strongest hydrogen lines as 'A' stars, stars with
the next strongest lines as 'B' stars, the next strongest 'C' and so on. Eventually, they realized that some
letters were unnecessary, and dropped them from the classification system. The letter assigned to a star
is called its spectral class. So the spectral classes that remain are A, B, F, G, K, M, and O. There is
only one star of each class in this data.
Question 2: Classification Based on Line Strength : Fill in the table below.
Line Strength
Greatest line area





Least line area
Star Number
Spectral Type
A
B
F
G
K
M
O
Question 3: Which is hotter, a star that peaks at 5000 Å or a star that peaks at 6000 Å? How do you
know?
Question 4. Classification Based on Star Temperature. Look at the spectra of the seven stars again.
This time, you can ignore the zoom-in beneath each panel. For each spectrum, trace the underlying
continuum shape – in other words, draw a line that shows what the spectrum would look like if it had
no emission or absorption lines. In each spectrum, identify the wavelength at which that that
continuum peak is most intense. If the peak is not shown on the graph, then estimate where you think it
might peak.
Rank the stars according to peak wavelength of each star's thermal continuum, using the table below,
and calculate their temperature using Wien’s Law. Remember that Wien’s law is expressed as max =
29,000,000/T(K) if wavelength is in Å.
Peak
Wavelength
Peak at shortest
wavelength
Peak
Wavelength (Å)
Temperature (in K)
based on Wien’s Law
Star
Number
Spectral
Class
O

B

A

F

G

Peak at longest
wavelength
K
M
Question 5. The Connection Between Temperature and Line Strength
Look back to the tables you created above. In the first row of the table below, list the stars (1-7) in
order of decreasing Hα line strength (strongest on the left, weakest on the right) based on your
responses to Question #3. In the second row, list the stars by number, in order of decreasing
temperature (hottest on the left, coolest on the right), based on you answers to questions 5 and 6. In
the third row, add the spectra class that you decided upon for each star.
Strongest Hα
Line
Spectral Type
Hottest
Temperature
Spectral Class
Weakest Hα
Line
Spectral
Type
Coolest
Temperature
Spectral
Class
Question 6. Are these two classification schemes (spectral type vs. spectra class) the same?
Question 7. What two spectra classes of stars have the strongest H lines? Are these the hottest stars?
Question 8. Which class of stars is the hottest? Which class of stars is the coolest? How strong are the
Hα lines for these two classes of stars?
Star #1
Star #2
Star #3
Star #4
Star #5
Star #6
Star #7