Astronomy/Calculation

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Expert: - 6/26/2011


Question
What would be your "shorthand" calculation of the angular momentum of our galaxy?

Answer
Hello,

Basically, taking the galaxy as a thinnish disk, for which the moment of inertia is:

I = ½ mR^2

and m = 1.4 x 10^42 kg (according to Wikipedia stats)

with R = 5 x 10^4 Ly = 5 x 10^20 m

Then, the angular momentum L = I w

where w (angular speed) = 2 pi/ T

and T = 50 million years = 1.57 x 10^15 s

(This is taking the time for the spiral arms to make a complete revolution)

Then: w = 3.9 x 10^-15 rad/s

and:

L = I w = ½ (1.4 x 10^42 kg )(5 x 10^20 m)^2 (3.9 x 10^-15 rad/s)

L = 6.9 x 10^68 kg-m^2/s

Again, this will vary, most certainly depending on the value one would adopt for T. (Others may have different opinions on that!)

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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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