Identifying the Spectral Types of Stars
So, what type of star did you think it was? The star had all the
hydrogen lines, so that narrows our choices down to B, A, and F.
However, it had no helium lines, so that rules out a type B
star. The star did have ionized calcium (the H and K lines), which are
characteristic of a type F star. So the star is a type F star. The
star whose spectrum you identified is shown to the right.
(The picture shows two stars close together; the star you identified is the
larger one on the bottom left.)
Another way to find the temperatures of stars is to find the peak
wavelengths of their thermal radiation curves. Any non-reflective object, including a star,
emits light at a certain peak wavelength depending on its temperature
(see the Color
project for a more detailed explanation).
The temperature of such an object can be found by using the
equation
T = 2.897 x 10-3 m K / lpeak.
The star whose spectrum was shown on the last page has a peak wavelength of
about 4200 Angstroms (1 Angstrom = 10-10 m). This yields a temperature of about
T = 2.897 x 10-3 m K / (4.2 x 10-7 m) = 6900K, close
to the middle of the temperature range for a type F star.
The peak wavelength is not always in the portion of
the spectrum we observe, however. The spectra taken by the SDSS do not cover the entire
electromagnetic spectrum. If a star has a peak wavelength in the
ultraviolet (beyond 4,000 Angstroms) or in the infrared (beyond 10,000 Angstroms), you
cannot see the peak. However, you will still be able to see spectral lines for the star,
so you can still use the OBAFGKM spectral type classification to find the star's
approximate temperature.
Now, it's your turn to start classifying stars on your own.
Exercise 5. Use the Object Explorer tool to look at spectra of the
stars in the table below. When you click on a link, the tool will open in a new window, displaying
complete data on the star you have selected. Click "Spectrum" in the left-hand column
to view the spectrum of the star. You may wish to print out each spectrum so you
can compare them side-by-side.
Classify the stars according to their spectra (note: one or two of the
objects are labeled as "huh" instead of a "star."
These objects have since been identified as stars, and their spectral types
have been determined). Some spectral types may appear more than once.
There is not necessarily one star of each spectral type. Be
careful...some of the spectral types are difficult to tell apart!
Plate |
Fiber |
266/51630 |
483 |
294/51986 |
623 |
266/51630 |
394 |
297/51959 |
316 |
266/51630 |
344 |
273/51957 |
391 |
282/51658 |
527 |
281/51614 |
398 |
268/51633 |
66 |
310/51990 |
178 |
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Exercise 6. Use the Plate Browser tool to choose about a dozen
stars from the SkyServer database. Choose any plate you like, then
click on one of the "star" links to see the star's spectrum.
Determine the spectral type of each star. Your sample is probably too
small to ensure you will find a star of each type, but you may always look
at more stars.
Launch the Plate Browser
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Exercise 7. On the board,
make a histogram showing the data from Exercise 6 for everyone in the
class. Let the bins represent
the spectral types (OBAFGKM). When you make the class histogram,
label each square with the star's plate number
and fiber number. There are a couple of reasons for this.
You don't want to count the same star twice if two groups analyzed the
same star. There is also the possibility that two groups
analyzed the same star and disagree on its type. In this case,
call in your "peer reviewers" to check the findings! Which types
of stars are most common? Which types of stars are the least
common? |
Question 7.
You may have noticed that type O stars are rare, especially in the
SDSS database. Why do you think we see so few of them? |
Question 8. Did you
find any stars that did not fit into any of the spectral types?
If so, what were their characteristics? |
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