Milky Way's mass is drastically reduced

The Sun resides in a large galaxy called the Milky Way. As in other galaxies, most of the Milky Way's mass is in the form of dark matter, a mysterious, invisible substance that only reveals its presence by the way its gravity tugs on stars and gas.

In order to measure the mass of the galaxy, astronomers measure the speeds of stars as they orbit around it. The faster they move, the greater the mass of the galaxy must be to keep them from escaping into intergalactic space.

Two trillion Suns

Previous studies of the speeds of 50 to 500 stars suggested the Milky Way's mass is about 2 trillion times the mass of the Sun. Other studies have measured the speeds of dwarf galaxies thought to be orbiting the Milky Way and arrived at about the same figure.

But a new study that measured the velocities of nearly 2500 stars suggests that the true mass is just under 1 trillion Suns.