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Light from Stars

A Star with a peak wavelength of 4000 Angstroms

Because stars emit light with different wavelengths, they have different colors. Stars do not just emit one wavelength of electromagnetic radiation, but a range of wavelengths. If you look at the amount of light a star gives off at different wavelengths, you would get a graph like the one shown to the right.

The wavelength at which a star emits the most light is called the star's peak wavelength. The diagram on the right shows that this star has a peak wavelength of 4000 Angstroms.


Question 2. What color would this star appear to your eyes? Would its g-r astronomical color be greater than or less than zero?
HINT: Remember that the magnitude scale is reversed, so brighter objects have lower magnitudes!

So now you know that stars have different colors because they have different peak wavelengths of light. But why do stars have different peak wavelengths? In the next Explore exercise, you will discover for yourself.

A Simulation of Star Light

Imagine you are observing light coming from a star. You use a prism to spread the light out from shortest wavelength to longest wavelength. (If you haven't tried the "Try This" activity where you look at light reflected off a compact disc (CD), you should try it now.) After you spread out the light into wavelengths, you then use an electronic camera to measure how much light of each wavelength (red, yellow, infrared, etc.) is present in the light coming from the star.

The SDSS's spectrograph, viewed from the side

This device - a prism plus an electronic camera - is called a spectrograph, and it is one of the most useful tools in astronomy. A graph created by a spectrograph measures the intensity of light versus wavelength; this graph is called a spectrum (the plural is spectra). By the time the SDSS ends in 2007, it will have measured over 1 million spectra.

The best way to find out what caused a star's color would be to conduct experiments on a single star, changing some of its properties and observing the resulting color. Of course, astronomers can't do experiments on stars, which are huge, complex, and unbelievably far away.

Since you can't do a controlled experiment, you will try a computer simulation instead. The simulation below models what the spectrum and the visual color of a star would look like as you changed the star's temperature.

Explore 3. Open the stellar temperature simulation (it will open in a new tab).

You will see the thermal radiation curve of a computer-simulated star with a temperature of 1000 Kelvin (about 720 degrees Celsius or 1300 degrees Fahrenheit). This is a spectrum that shows the amount of light that star gives off at various wavelengths. The horizontal axis shows the wavelength in microns (one micron equals 10,000 Ångstroms). The vertical axis is a measure of the energy density (the amount of light emitted).

Use the text box below the thermal radiation curve to change the temperature of the simulated star, then click the "set temperature" button to generate a new graph. Try out several different temperatures.

Do you notice a relationship between the spectrum's peak wavelength and the simulated star's temperature?

On the next page, you'll learn more about how a star's temperature determines its color.