Milky Way's fastest stars circle each other at 500km a second

Astronomers have confirmed that two extremely dense stars in an intimate dance are spinning around each other in just 5.4 minutes at about 500 kilometres a second, making them the fastest known stellar partners in the galaxy.
The whirling duo, known as HM Cancri, also has the tightest orbit of any known "binary" star system.
Both stars are white dwarfs-the dense, white-hot remnants left behind when sunlike stars die.
The stellar corpses are separated by no more than three times the width of Earth.
In such tight quarters, hot gases flow between the two stars, releasing huge amounts of energy.
"This is the most extreme example of one of these double white dwarf systems we have so far," study co-author Danny Steeghs of the University of Warwick in the UK, told National Geographic News.
Study leader Gijs Roelofs, of the Harvard-Smithsonian Centre for Astrophysics, was part of the team that first detected periodic x-ray emissions from HM Cancri in 1999.
Initial observations had suggested a 5.4-minute orbit, but the researchers weren't sure if the pulses of light were coming from two circling stars or one superfast spinner.