Download Document

ID#: __ __ __ __ __ __
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 fromThe 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), 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 data
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 (don’t worry about those units!). Below each big plot is a
zoomed-in view of the area of the spectrum around 6560 Ångstroms (i.e., 656 nm).
Question 1 (1 point). Star #1 has a conspicuous Hα transition at 6563 Ångstroms. So, the
zoomed-in view shows the Hα line for the spectrum. Does the Hα line represent more
light or less light (relative to the surrounding wavelengths)? [In other words, does it
point up or down?] How do you know?
It represents less light, because it points down, and is superimposed on an
absorption continuum.
Question 2 (1 point). Given your answer to Question 1, is the Hα line in this spectrum an
emission line or an absorption line? How do you know?
It’s an absorption spectrum.
We can 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 are A, B, F, G, K, M, and O. In the third column of the table below, write
one of these spectral class letters for each star in the table. There is only one star of each
class in this data.
Question 3: Classifying by Line Strength (4 points): Fill in the table below.
Line Strength
Greatest line area





Least line area
Star Number
6
2
3
7
1
5
4
Spectral Type
A
B
F
G
K
M
O
Note: we gave a lot of partial credit on this question!
Question: Mnemonic (4 points)
Make up a mnemonic to help you remember the 7 spectra classes.
We gave 4 points for any credible answer.
Question 5. Star temperature (4 points). 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 at longest
wavelength
Peak
Temperature (in K)
Wavelength (Å) based on Wien’s Law
Star Number
900
32,222
4
2850
3850
4600
5000
6000
10175
7532
6304
5800
4833
2
6
3
7
1
9000
3222
5
Question 6 (1 point). 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.
Spectral Type
Strongest Hα
Line
Hottest
Temperature
Spectral Class
A
B
F
G
K
M
O
6
2
3
7
1
5
4
4
2
6
3
7
1
5
O
B
A
F
G
K
M
Spectral Type
Weakest Hα
Line
Coolest
Temperature
Spectral Class
We didn’t give any points on this table, because it’s just a repeat of your answers in
the previous questions, and we didn’t want to take points off twice if you had it
wrong in the first place.
Are these two classification schemes the same?
No, they’re not. The classification based on H line intensity is actually called
spectral type, and it’s different than the one based on temperature.
Question 7. What two classes of stars have the strongest H lines? Are these the hottest
stars? The coolest?
The stars with the strongest Hα lines are A and B stars. These are the second- and
third-hottest stars in the temperature classification.
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?
Class O stars are the hottest. Class M stars are the coolest. Both these classes have
weak Hα lines.