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The Question

(Submitted August 21, 1997)

I studied a bit of astrophysics at UCLA. I was trying to recall the estimated rate of speed that our Sun is moving in relation to the center of the milky Way galaxy.

And is the whole Milky Way galaxy moving also? I would assume so, but was wondering if there is some calculations.

The Answer

The Sun orbits the center of the Milky Way at about 250 km/second and it takes about 220 million years to complete an orbit.

The Milky Way is part of a group of galaxies known as the Local Group. All of these are moving relative to each other due to their gravitational interaction with speeds of around 100 km/s or less. Calculating the velocities of the galaxies in the Local Group is difficult because there are probably members that have not yet been discovered because they are too dim or are obscured by the plane of the Milky Way. The radial velocities relative to the Milky Way are found by measuring Doppler shifts in the spectra of stars in the galaxies. You will find more information at
http://seds.lpl.arizona.edu/messier/more/mw.html

The Local Group is also moving at about 600 km/second relative to the cosmic microwave background. There's a nice picture of this at http://antwrp.gsfc.nasa.gov/apod/ap960205.html

Damian Audley and David Palmer
for the Ask an Astrophysicist Team

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