Free Astronomy Magazine May-June 2015

21 MAY-JUNE 2015 SPACE CHRONICLES known high-velocity star know as US 708 with the Echellette Spectro- graph and Imager instrument on the 10-meter, Keck II telescope to measure its distance and velocity along our line of sight. By carefully combining position measurements from digital archives with newer positions measured from images taken during the course of the Pan- STARRS1 survey, they were able to derive the tangential component of the star's velocity (across our line of sight). Putting the measurements together, the team determined the star is mov- compact binary system, transfer- ring helium to a massive white dwarf companion, ultimately trig- gering a thermonuclear explosion of a type Ia supernova. In this sce- nario, the surviving companion, i.e. US 708, was violently ejected from the disrupted binary as a result, and is now travelling with extreme velocity. These results provide observational evidence of a link between helium stars and thermonuclear superno- vae, and is a step towards under- standing the progenitor systems of these mysterious explosions. A nimation of a white dwarf accumulating matter and the resulting supernova that ejected US 708 at high velocity. [NASA, ESA and P. Ruiz-Lapuente, cut and coloured by S. Geier] ing at about 1,200 kilometers per second – much higher than the ve- locities of previously known stars in the Milky Way galaxy. More im- portantly, the trajectory of US 708 means the supermassive black hole at the galactic center could not be the source of US 708's extreme ve- locity. US 708 has another peculiar prop- erty in marked contrast to other hypervelocity stars: it is a rapidly rotating, compact helium star likely formed by interaction with a close companion. Thus, US 708 could have originally resided in an ultra n