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Astronomy/Star colors vs red/blue shift

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Expert: - 1/15/2008


Question
If red stars are cooler and blue stars are hotter, how can we tell which stars are approaching, and which are receding by analyzing their red/blue shift?

Answer
Hello,

Neither the color of stars (or their temperature) has anything to do with red or blue shifts. These refer to displacement of specific lines in the spectrum.

T%he displacement of line position has nothing to do with a star's basic color.

If displaced toward the red end (longer wavelength) we say "red shift". If displaced toward the blue end (shorter wavelengths) we say "blue shift". Irrespective of the shift, the star's color remains the same. The shift refer to the motion.

Those stars which exhibit "blue shift" of their spectral lines (say, hydrogen absorption lines) are the ones approaching since their light waves are shortened in our direction.

Those stars which exhibit "red shift" (say of their hydrogen absorption lines) are receding from us, since their light waves are being lengthened as they move away.  

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Philip Stahl

Expertise

I have forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics as applied to astronomical investigations.

Experience

Astronomy: more than forty years experience starting with construction of my own simple telescopes. Worked at university observatory in college, doing astrographic measurements. M.Phil. degree in Physics/Solar Physics and more than ten years as researcher.

Organizations
American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union

Publications
Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'.

Education/Credentials
B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research

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