Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

August 29, 1998

Orion Star Colours
Credit and Copyright: David Malin

Explanation: What determines a star's colour? Its temperature. Red stars are cool, around 3,000 kelvins (K), while blue stars are hotter and can have temperatures over 30,000 K. Our own lovely yellow Sun's temperature is a comforting 6,000 K. Differences in star colours are dramatically illustrated in the above photo of the constellation Orion, made using a "star trail step-focus" technique. In this technique, a time exposure is used to create star trails, but during the exposure, the focus is changed in steps. For the brighter stars, the blurred image produces more saturated colours in photographs. At the upper left, the cool red supergiant Betelgeuse stands out from the other, hotter, bluish stars composing the body of the constellation. Bright Rigel, a blue supergiant, is at the lower right.

Tomorrow's picture: The Sun Erupts

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
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