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Re: 21-centimeter Line?

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Posted by Al Gobeo on February 18, 2001 01:52:16 UTC

I never hear of it before, so I launched "Sherlock" a Macintosh search program and it to me to the following web page:
http://classweb.howardcc.edu/astronomy/Presentations/Chapter18/tsld021.htm
Then I read the following

Rotation Velocity and the Width of the 21-centimeter Line

the galaxy. Because of the Doppler effect, 21-centimeter photons from one side of the galaxy are shifted toward shorter wavelengths, those from the other side are shifted toward longer wavelengths, and those from the central regions are not shifted. The result is a 21-centimeter line that is broadened according to how rapidly the galaxy is rotating. Because the rotation speed is governed by the central mass of a galaxy, the width of the line is related to the mass of the galaxy and can be used to estimate its absolute magnitude for distance determination.

This shows how the rotation velocity of a spiral galaxy is related to the width of the 21-centimeter emission line from hydrogen gas in


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