mountain profile Institute for Astronomy University of Hawaii

Fastest Star in Our Galaxy Propelled by a Thermonuclear Supernova

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fastest star leaves the Milky Way

Artist's conception of a star (left) being ejected from a galaxy by a supernova explosion. In reality the supernova would have been faded away long before the star reached that position. Credit: ESA/Hubble, NASA.

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 our Milky Way galaxy.

“At that speed, you could travel from Earth to the moon in 5 minutes,” Magnier commented.

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. These results will be published in the March 6 issue of the journal Science.

Stars like the sun are bound to our 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 is 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 is, how fast it is moving away from us. 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 is moving perpendicular to us.

“By observing the sky repeatedly over several years, the Pan-STARRS1 survey, let us make a movie of the motions of the stars in the sky. That enables us to study the behaviors of extremely rare and weird stars like US708,” Magnier explained.

By putting all the measurements together, they determined the total velocity of the star is 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.


Animation of the mass-transfer phase to a white dwarf followed by a double-detonation supernova that leads to the ejection of US 708 from the galaxy.
Credit: NASA, ESA and P. Ruiz-Lapuente, cut and colored by S. Geier


US 708 has additional peculiar properties that are in marked contrast to other hypervelocity stars: it is 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 is now traveling at an extremely high velocity.

These results provide observational evidence that there is a link between helium stars and thermonuclear supernovae, and are a step toward understanding the progenitor systems of these mysterious explosions.


Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Maunakea on the island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems.

The Pan-STARRS1 Surveys (PS1) have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, and the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), and the Los Alamos National Laboratory.