A group of astronomers headed by the Max Planck Institute for Astronomy has measured the velocity of the stars in the galactic halo and thereby derived the most accurate value to date for the total mass of the galaxy: The region within a radius of 200 000 light years contains 4×1011
solar masses. An extrapolation to 800 000 light years leads to 1012
solar masses. This result shows that the mass of the Milky Way has previously been significantly over-estimated. It also proves that our Milky Way has been extraordinarily efficient at forming stars.