Download TYPES OF STARS

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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. How do they
make sense of all these stars? 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), 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. Star #1 has a conspicuous Hα transition at 6563 Ångstroms. So, the zoomedin 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?
Question 2. Given your answer to Question 1, is the Hα line in this spectrum an emission
line or an absorption line? How do you know?
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 : Fill in the table below.
Line Strength
Greatest line area





Least line area
Star Number
Spectral Class
Question: Mnemonic
Make up a mnemonic to help you remember the 7 spectra classes.
Question 5. 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 at longest
wavelength
Peak
Wavelength (Å)
Temperature (in K)
based on Wien’s Law
Star Number
Question 6. 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
Hottest
Temperature
Spectral Class
Weakest Hα
Line
Coolest
Temperature
Spectral Class
Are these two classification schemes the same?
Question 7. What two classes of stars have the strongest H lines? Are these the hottest
stars? The coolest?
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