US 708 is a hyper-velocity class O subdwarf in Ursa Major, in the halo of the Milky Way Galaxy.

[3][4][5] US 708 was first discovered in 1982 by Peter Usher and colleagues of Pennsylvania State University as a faint blue object in the Milky Way halo.

[6] In 2015, Stephan Geier of the European Southern Observatory led a team that reported in Science that the velocity of the star was 1,200 km/s (4,300,000 km/h; 2,700,000 mph), the highest ever recorded in the galaxy.

[3][7][8] The star's high velocity was originally suspected to be caused by the massive black hole at the center of the galaxy.

Then its companion acquired enough mass to go supernova, which triggered US 708 being flung away at its high velocity, not by the black hole at the center of our galaxy.