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It may appear that the sun is stationary while the planets in its orbit move, but the sun is actually orbiting the Milky Way galaxy at an impressive speed of about 220 kilometers per second – almost half a million miles per hour. As fast as that may seem, when a faint red star was discovered crossing the sky at a noticeably rapid pace, scientists took notice.

Thanks to the efforts of a citizen science project called Backyard Worlds: Planet 9 and a team of astronomers from around the country, a rare hypervelocity L subdwarf star has been found racing through the Milky Way. More remarkably, this star may be on a trajectory that causes it to leave the Milky Way entirely. The research, led by UC San Diego astronomy and astrophysics professor Adam Burgasser, was presented at a press conference during the 244th National Meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.

The star, charmingly named CWISE J124909+362116.0 (“J1249+36”), was spotted for the first time by some of the more than 80,000 citizen science volunteers involved in the Backyard Worlds: Planet 9 project, which are combing through vast swaths of data collected over the past 14 years of NASA’s Wide-field Infrared Survey Explorer (WISE) mission. This project takes advantage of the keen ability of humans, who are evolutionarily programmed to look for patterns and notice anomalies in a way unmatched by computer technology. Volunteers tag moving objects in data files, and when enough volunteers tag the same object, astronomers investigate.

J1249+36 immediately stood out because of the speed at which it was moving through the sky, initially estimated at about 600 kilometers per second (1.3 million miles per hour). At this speed, the star is fast enough to escape the Milky Way’s gravity, making it a potential “hypervelocity” star.

To better understand the nature of this object, Burgasser turned to the WM Keck Observatory in Mauna Kea, Hawaii, to measure its infrared spectrum. These data revealed that the object is a rare L subdwarf, a class of stars with very low mass and temperature. Subdwarfs represent the oldest stars in the Milky Way.

Insight into the composition of J1249+36 was made possible by a new set of atmospheric models created by UC San Diego graduate student Roman Gerasimov, who worked with UC LEADS scientist Efrain Alvarado III to generate models specifically tuned to study L subdwarfs.

“It was exciting to see that our models were able to match the observed spectrum exactly,” said Alvarado, who is presenting his modeling work at the AAS meeting.

The spectral data, along with imaging data from several ground-based telescopes, allowed the team to precisely measure J1249+36’s position and velocity in space and thereby predict its orbit through the Milky Way.

“This is where the source became very interesting because its velocity and trajectory showed it was moving fast enough to potentially escape the Milky Way,” Burgaser said.

What did this star throw?

The researchers focused on two possible scenarios to explain J1249+36’s unusual trajectory. In the first scenario, J1249+36 was originally a low-mass companion of a white dwarf. White dwarfs are the remnant cores of stars that have exhausted their nuclear fuel and disappeared. When a companion star is in very close orbit with a white dwarf, it can transfer mass, resulting in periodic outbursts called novae. If a white dwarf gathers too much mass, it can collapse and explode as a supernova.

“In this kind of supernova, the white dwarf is completely destroyed, so its companion breaks free and takes off at whatever orbital speed it was traveling at originally, plus a little kick from the supernova explosion,” Burgasser said. “Our calculations show that this scenario works. However, the white dwarf is no longer there, and the remnants of the explosion, which probably happened several million years ago, have already dissipated, so we have no definitive proof that this is its origin.”

In the second scenario, J1249+36 was originally a member of a globular cluster, a closely related cluster of stars immediately recognizable by its distinct spherical shape. The centers of these clusters are predicted to contain black holes with a wide range of masses. These black holes can also form binary systems, and such systems prove to be great catapults for any stars that happen to wander too close to them.

“When a star collides with a binary black hole, the complex dynamics of this three-body interaction can eject that star right out of the globular cluster,” explained Kyle Kremer, a future assistant professor in UC San Diego’s Department of Astronomy and Astrophysics. Kremer ran a series of simulations and found that, in rare cases, these types of interactions can kick a low-mass subdwarf out of a globular cluster and on a trajectory similar to that observed for J1249+36.

“This demonstrates a proof of concept,” Kremer said, “but we don’t really know what globular cluster this star is from.” Tracing J1249+36 back in time places it in a very crowded part of the sky that may be hiding undiscovered clusters.

To determine whether either of these scenarios or some other mechanism could explain J1249+36’s trajectory, Burgasser said the team hopes to take a closer look at its elemental composition. For example, when a white dwarf explodes, it creates heavy elements that could “contaminate” J1249+36’s atmosphere as it escapes. Stars in the Milky Way’s globular clusters and satellite galaxies also have different abundance patterns that may reveal the origin of J1249+36.

“We are essentially looking for a chemical fingerprint that would indicate which system this star is from,” said Gerasimov, whose modeling work allowed him to measure the element abundances of cool stars in several globular bowls, work he is also presenting at a meeting of AAS.

Whether J1249+36’s rapid travel is due to a supernova, a chance encounter with a binary black hole, or some other scenario, its discovery provides a new opportunity for astronomers to learn more about the history and dynamics of the Milky Way.