Astronomers have recently observed an ultrafast star racing out of galaxy at 26m mph.
A team of astronomers, including University of Hawaii at Manoa astronomer Eugene Magnier, used the 10-meter Keck II and Pan-STARRS1 telescopes in Hawaii to find a star that breaks the galactic speed record. It travels at about 1,200 kilometers per second (about 2.7 million mph), a speed that will enable the star to escape from the Milky Way galaxy.
The team showed that unlike the half-dozen other known escaping stars, this compact star was ejected from an extremely tight binary by a thermonuclear supernova explosion.
Stars like the sun are bound to Earth's galaxy by its gravity and orbit its center at relatively moderate velocities, tens to a few hundreds of kilometers per second. Only a few so-called hypervelocity stars are known that travel so fast that they are unbound.
A close encounter with the supermassive black hole at the center of the Milky Way was usually considered the most plausible mechanism for enabling these stars to escape from the galaxy.
Stephan Geier (European Southern Observatory, Garching, Germany) led the team that observed the known high-velocity star, US 708, with the Echellette Spectrograph and Imager on the Keck II telescope to measure its distance and radial velocity component, that was, how fast it was moving away from Earth.
By carefully combining position measurements from digital archives with newer positions measured from images taken during the Pan-STARRS1 survey, they were also able to derive the tangential component of the star's velocity, or how fast it was moving perpendicular to Earth.
By putting all the measurements together, they determined the total velocity of the star was 1,200 kilometers per second, much higher than the velocities of the other known stars in our galaxy. More importantly, the trajectory of US 708 allows them to rule out the Galactic Center, and therefore, its supermassive black hole, as the possible cause of US 708's extreme velocity.
US 708 has additional peculiar properties that are in marked contrast to other hypervelocity stars: it was a rapidly rotating, compact helium star that probably formed as a result of interaction with a close companion. Helium stars are the remnant of a formerly massive star that has lost its hydrogen envelope.
Thus, US 708 could have originally been part of an ultracompact binary system in which it transferred helium to a massive white dwarf companion, ultimately triggering a thermonuclear explosion known as a type Ia supernova. As a result of this explosion, the surviving companion, US 708, was violently ejected from the disrupted binary, and was now traveling at an extremely high velocity.
These results provided observational evidence that there was a link between helium stars and thermonuclear supernovae, and are a step toward understanding the progenitor systems of these mysterious explosions.
The study is published in the journal Science.
