Light from Stars
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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.
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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.
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