Gauging the mass of a black hole is tricky, but astronomers have devised multiple methods to measure the heft of these ...

They rotate too quickly to be held together by all their stars, so there must be an extended halo of invisible mass surrounding the visible stars and gas. The way stars move in a galaxy can be used to ...

Thanks to the ever-improving world of fitness trackers and apps, you've never had more access to personal health data. But which of the dizzying number of markers and metrics should you really be ...