Free Astronomy Magazine May-June 2015

20 MAY-JUNE 2015 SPACE CHRONICLES Thermonuclear supernova ejects galaxy’s fastest star T by Keck Observatory space. A close encounter with the supermassive black hole at the cen- tre of the Milky Way is typically presumed the most plausible mech- anism for kicking these stars out of the galaxy. A team of astronomers led by Ste- phan Geier (European Southern Ob- servatory, Garching) observed the Science . Stars like the Sun are bound to our Galaxy and orbit its center with moderate velocities. Only a few so-called hypervelocity stars are known to travel with ve- locities so high that they are un- bound, meaning they will not orbit the galaxy, but instead will escape its gravity to wander intergalactic S cientists using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii have dis- covered a star that breaks the ga- lactic speed record, traveling with a velocity of about 1,200 kilometers per second or 2.7 million miles per hour. This velocity is so high, the star will escape the gravity of our galaxy. In contrast to the other known unbound stars, the team showed that this compact star was ejected from an extremely tight bi- nary by a thermonuclear supernova explosion. These results have been published in the March 6 issue of A n artist impression of the mass-transfer phase followed by a double- detonation supernova that leads to the ejection of US 708. While this illustration shows the supernova (bottom center) and the ejected star (left) at the same time, in reality the supernova would have been faded away long before the star reached that position. [ESA/HUBBLE, NASA, S. Geier]