Orbital Period of the Sun in the Milky Way Galaxy

-- | September 16, 2015

Question: I have read that the Sun is about 15.3 kpc from the centre of the Galaxy and moving at about 230 km/s. I have also read that it takes about 200 million years to orbit the Galaxy. But if the orbit is approx. circular, the period is 2 x pi x radius / velocity which comes out at about 400 million years. What am I missing?  — Bruce

Answer: As you pointed out, the Sun takes about 226 million years to orbit the center of our Galaxy, and it orbits at a speed of about 230 km/s.  The distance from the Sun to the center of our Galaxy, though, is about 8 kilo-parsecs (kpc), which accounts for the factor-of-two overestimate in your orbital period calculation.  As our Galaxy is about 30 kpc in diameter, your calculation would be appropriate for a star on the edge of the Galactic disk.

Jeff Mangum