A team of scientists have discovered a hypervelocity star that defies current astronomical theories by breaking the galactic speed record. The hypervelocity star they found blasts through space at around 1,200 kilometers per second – about 2.7 million miles per hour. This speed allows the star to reach escape velocity to exit the Milky Way Galaxy, something that has never been seen before. To put this into a better perspective, the team said at this speed a spacecraft could reach the moon in just five minutes.
The astronomers said that contrary to a few other known stars escaping the galaxy, this new star, identified has US 708, was expelled from an intensely tight binary system by an immense thermonuclear explosion, or supernova. Their results were published in the current issue of the journal Science.
A hypervelocity star is one that surpasses a velocity of 200 kilometers per second. According to scientists, the speed of hypervelocity stars could be around 1,000 kilometers per second. Current theories state that there are around 1,000 hypervelocity stars in the Milky Way Galaxy. Although this sound like a lot, there are over 100 billion stars in the Milky Way. Even if all potential hypervelocity stars were found, this equates to only 0.000001 percent. Most astronomers believe hypervelocity stars achieve either speeds by coming close to a black hole.
Medium-size stars like the Sun are gravitationally bound to the Milky Way by its orbiting around the center of the Galaxy, at a relatively normal velocity on the galactic scale of 120 miles per second. Until now, only a few stars have been observed to reach speeds that do not hold them in orbit with their galactic hosts. These hypervelocity stars travel at speeds hitherto unknown to astronomy. Though, astronomers believe that the most plausible mechanism for these stars to escape the grasp of the Milky Way is to come close enough to the supermassive black hole at the center. This would jettison the star at hypervelocity by its mass interacting with extreme forces in gravity around an orbital point.
Stephan Geier, an astronomer at the European Southern Observatory, led the research that discovered US 708, the aforementioned hypervelocity star. By using the Echllette Spectrograph and Imager, two distinct systems on the Keck II telescope, they were able to measure the radial velocity of US 708, the speed at which it travels either away or towards a point of view. By consolidating the coordinates measured and the speeds at which the star was traveling in between them, they measured the tangential component of acceleration of the star, or the speed at which it moves perpendicular to a point of view.
The team calculated the total velocity of US 708 at 1,200 kilometers per second – about 742 miles per second. This is much higher than all other stars that have been observed in the Milky Way Galaxy. Beforehand, they theorized that the hypervelocity star was powered by a potential close encounter with the supermassive black hole in the center of the galaxy. However, by using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) to measure its trajectory, they were able to determine that US 708 did not achieve such an extreme velocity by coming close to the black hole.
Eugene Magnier, one of the led astronomers on the study, explained they used Pan-STARRS1 to deduce the location and speeds of US 708. This system observes stars on a continuous basis and maps their movements through space. He said this system allows scientists to make a “movie” of the motions of stars. Therefore, they are able to study and analyze unbelievably rare and extraordinary hypervelocity stars like US 708.
US 708 has additional unusual characteristics that put it in a different category in regards to other hypervelocity stars: it is a rapidly rotating, extremely compact helium star. Maginer said helium stars like this are a result of a stellar interactions from nearby companion stars, remnants from a previous super giant that lost its hydrogen chromosphere.
The team stated US 708 could have possibly been linked to a compact binary star system, which transferred helium to a white dwarf companion, inevitably causing a massive thermonuclear reaction, known as a type Ia supernova. As a result, US 708 was violently blasted from the former binary system, and is now speeding through the galaxy at hypervelocity.
These findings provide actual evidence to the theories of hypervelocity stars and the origin of their unusual speed. Moreover, it provides a link between helium stars and the thermonuclear reactions that destroy them. Magnier stated this is a crucial step towards understanding the theoretical properties of hypervelocity stars and supernovae.
By Alex Lemieux
Photo by Kevin Gill – Flickr License