Photographing Orion - Feb. 27, 2014

In introductory astronomy courses one spends a lot of time learning about the many different types of stars in the universe. A quick look at the HR diagram below illustrates some of the main types. Teachers spend a lot of time going through all this but, to the inquisitive student, there is one BIG problem, and that is....

QUESTION: The HR classification diagram says that most stars are blue or red or yellow... but when I look at the stars myself, they all appear white, what's wrong?

ANSWER: Your eyes are deceiving you, stars do have colors, it's just that your eyes are not sensitive to colors when the the light source is very dim. (That's why when you really dim the lights in a room, you only see shades of grey, not color!) To see the true colors of stars in the night sky, we will use a much more sensitive instrument, a DSLR camera. To test this out, we will point the camera at a popular constellation, Orion, and take pictures. Let's see what happens...

These are the photographs each group of students took of the Constellation Orion on Feb. 27, 2014.
  • Group 1 -- 7:45-8:15 pm
  • Group 2 -- 8:15-8:45
  • Group 3 -- 8:45-9:20

Photograph of the constellation Orion. This is a 25 second exposure made with a Canon T4i DLSR camera.  Many stars appear blue, a result of their high surface temperatures. Betelgeuse is the coldest star and it appears yellow/orange.  To the naked eye, which is not very sensitive to color when the light source is very dim, all stars appear essentially white. But a digital camera is much more sensitive to dim lights and the true star colors quickly come out. 

Star trails images, made from stacking together 20 photos taken over 10 minutes. The star trails images enable an easier viewing of the different star colors. Stars make 'trails' because the camera is on a fixed tripod, and the night sky is slowly rotating around us.
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