Scientists have spotted a rare hypervelocity star, called CWISE J124909+362116.0 (“J1249+36”), which is passing through the Milky Way at an extraordinary speed of about 600 km/s. This discovery was possible thanks to the citizen science project “Backyard Worlds: Planet 9” and a team of American astronomers. The research, led by Professor Adam Burgasser of the University of California at San Diego, was presented at the 244th national meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.
The star J1249+36 was identified by some of the more than 80 thousand volunteers of the “Backyard Worlds: Planet 9” project, who analyze data collected by NASA’s Wide-field Infrared Survey Explorer (WISE) mission over the last 14 years. Volunteers report moving objects in the data, and when enough people report the same object, astronomers step in to investigate further.
The initially estimated speed of J1249+36, around 600 km/s, is such as to allow it to escape the gravity of the Milky Way, classifying it as a potential hyperfast star. To better understand the nature of this object, Burgasser used the W. M. Keck Observatory on Maunakea, Hawaii, to measure its infrared spectrum. The data revealed that it is an L-type subdwarf, a class of star with very low mass and temperature, representing some of the oldest stars in the galaxy.
Origin of the hypervelocity star
Scientists explored two possible scenarios to explain J1249+36’s unusual trajectory. In the first scenario, the star may have been the low-mass companion of a white dwarf, ejected at great speed following the white dwarf’s explosion as a supernova. In this case, the white dwarf would have been destroyed and the companion released, flying away at its original orbital speed plus an additional boost.
In the second scenario, J1249+36 could come from a globular cluster, a gravitationally bound group of stars. In these clusters, black holes can form binary systems and, through complex interactions, eject stars at great speeds. Kyle Kremer, an assistant professor at UC San Diego, ran simulations showing how these dynamics could eject a low-mass subdwarf on a trajectory similar to that observed for J1249+36.
To determine which of these scenarios or what other mechanism could explain J1249+36’s trajectory, scientists hope to look more closely at its elemental composition. If the star had been “polluted” by a supernova, it would show a distinctive chemical signature. Similarly, stars in the Milky Way’s globular clusters and satellite galaxies have specific abundance patterns that could reveal the origin of J1249+36.
The discovery of J1249+36 offers new opportunities to better understand the history and dynamics of the Milky Way. Whether its journey was caused by a supernova, an encounter with a binary system of black holes, or another scenario, the hypervelocity star represents a fascinating phenomenon for astronomers.
