Light From Stars
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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. You will see the spectrum of a computer-simulated star. On the left, you will see a simulation of what the star would look like. You can click on any of the temperature buttons to see a simulated spectrum, or you can enter you own temperature in the box.

Do you notice a relationship between the spectrum's peak wavelength and the simulated star's temperature? What temperature gives a peak wavelength in the blue region of the spectrum? What color is the star? Do the peak wavelength and color match?

Find a star with a peak wavelength in the red region of the spectrum. What color is it? Find the peak wavelength of a star that appears red. Where is the peak wavelength?

Human body temperature is about 310K. Where is the peak wavelength of a human body? Why don't you glow like a star?
HINT: try decreasing the temperature slowly until you get to 310 K. How does the simulated color change?

Challenge Question: Can you find any temperature that would appear green? Why or why not?

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