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Physics - mass of a galaxy

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A star at the edge of the Andromeda galaxy appears to be orbiting the center of that galaxy at the speed of about 200km/s. The star is about g*10^9AU from the center of the galaxy. Calculate a rough estimate of the mass of the Andromeda galaxy. Earth orbital radius is 1.40*10^8

  • Physics - mass of a galaxy -

    1 a.u. = 1.40*10^8 km = 1.40*10^11 m

    For the mass of the entire galaxy, M, you will have to assume a spherical distribution of mass, with the star in question at the outside, at distance R.

    In that case, the centripetal acceleration of the star is
    V^2/R = G M/R2

    Solve for M.

    M = R V^2/G

    You did not type the distance R correctly. Is the "g" supposed to be a 9?

    G is Newton's universal constant of gravity, 6.67*10^-11 N*m^2/kg^2

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