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Observed 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. The SDSS plans to measure the spectra of about one million objects. Here is a spectrum of a star seen by the SDSS:

Click on the image to see it full size

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.

Explore 5. 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?

Launch the Plate Browser

A Word of Warning

Question 6. 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 7. 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 34,970,000 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 30,000 stars, while it has taken digital images of more than 35 million. How can astronomers find temperatures of those other 34,970,000 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.