Spectra
You have been looking at graphs of intensity (amount) of light emitted
by stars as a function of the wavelength of the light. Astronomers have a name for
such a graph: a spectrum (the plural is "spectra"). Astronomers
frequently measure spectra of stars, and use these measurements to study stars.
So far, the SDSS has observed spectra for 50,000 objects in the sky. Here is a
spectrum of a typical star seen by the SDSS:
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Click on the image to see it full size
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Practice 4. Find the peak wavelength of the spectrum above -
the wavelength where the general trend of the spectrum seems to be at its maximum.
From the peak wavelength you found,
calculate the average temperature of the star.
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Explore 4. Look through the spectra of 10-20 stars in the
SDSS data using the
Plate Browser tool. Choose a plate, then click on the links for
some of the stars on that plate to see their spectra. From the thermal peak of the
spectra, calculate the temperature of the stars. Are most of these stars hotter or
cooler than the Sun?
Use SkyServer workbook to keep track of the
stars you find.
Launch the Plate Browser
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A Word of Warning
Question 4. How does the observed spectrum above compare to the
thermal radiation curves you saw in the last section? What are the similarities
and differences? |
The broad shape of a star's spectrum is caused by thermal radiation, but stars
also emit light for another reason. When electrons
in the star's atoms lose energy, they release a photon of light. All the photons
emitted from electrons have the same wavelength, so stars can produce a lot of
light in just a few wavelengths. These emitted light waves are the source of the peaks
in the spectrum above. Similarly, when electrons in a star's atom gain energy,
they absorb a photon, leaving a valley in their observed spectrum. When you
see a spectrum for a star, you can easily sort out the contributions of thermal
radiation and non-thermal radiation.
However, the color of a star is determined by the wavelengths of all the light the
star gives off, from both thermal and non-thermal radiation. Therefore, if you estimate a star's
color from its thermal radiation curve, you may not get the right answer.
If the thermal radiation curve for a star closely
approximates the observed spectrum - that is, if the peaks and valleys aren't too
high - the star is called a "thermal source." If the thermal radiation curve
does not approximate the observed spectrum - if the peaks and valleys are high -
the star is called a "non-thermal" source. You can see from the spectrum above
that some of the peaks and valleys can be very high, so many stars will be
non-thermal sources.
Question 5. Do you think the star whose spectrum
is shown above is a thermal source? Why or why not? |
In the next few pages, you will learn a simple way to tell thermal sources
from non-thermal sources.
The Other 5,996,000 Stars
So, if you have the spectrum of a star, you can find its temperature from the peak
wavelength. But SDSS has taken spectra for only about 50,000 stars, while
it has taken digital images of more than 5,996,000. How can astronomers find temperatures
of those other stars?
Even without knowing the details of a star's spectrum, astronomers can still draw
conclusions about the star's temperature. In the next section, you will learn how SDSS's
five light filters allow astronomers to learn which stars are hottest, which are coolest, and
which are somewhere in the middle.
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