An object discovered with the help of citizen scientists was moving through the Milky Way so fast that it could escape the galaxy’s gravity and reach intergalactic space, new research shows.

The object, probably a faint red star, was traveling at a speed of about 1.3 million miles per hour (600 kilometers per second). For comparison, the Sun orbits the Milky Way at a speed of 450,000 miles per hour (200 kilometers per second).

If confirmed, the object would be the first known very low-mass “hypervelocity” star, according to a team of astronomers and amateur scientists whose study has been accepted for publication in the Astrophysical Journal Letters.

There are many more low-mass stars than high-mass ones because star formation favors lower-mass objects, and stars with more mass have shorter lifetimes, said study co-author Roman Gerasimov, a postdoctoral fellow in the Department of Physics and Astronomy at the University of Notre Dame. However, low-mass stars are harder to detect because they are cooler and less luminous.

Hyperfast stars, whose existence was first suspected in 1988 and discovered in 2005, are already extremely rare, which makes this new discovery “particularly exciting,” he said.

Volunteers participating in a project called Backyard Worlds: Planet 9 first discovered the star, designated CWISE J124909.08+362116.0, or J1249+36 for short. Researchers involved in the project are looking for evidence of undiscovered objects or a large hypothetical world called Planet Nine in the “backyard of the solar system” beyond Neptune.

Backyard Worlds participants will look for patterns and anomalies in images and data from NASA’s Wide-field Infrared Survey Explorer mission, which mapped the sky in infrared light from 2009 to 2011. (The space agency renamed the mission the Near-Earth Object Wide-field Infrared Survey Explorer in 2013 to monitor near-Earth asteroids and comets before permanently retiring it on August 8.)

According to the study’s authors, amateur scientists noticed J1249+36 when reviewing the data a few years ago because the star was moving at about 0.1 percent of the speed of light.

“I can’t describe the excitement,” study co-author Martin Kabatnik, a citizen scientist from Nuremberg, said in a statement. “When I first saw how fast it was moving, I was convinced it must have been reported already.”

Subsequent observations with multiple telescopes focused on the object and helped confirm the discovery.

“This is where the source became very interesting, as its speed and trajectory showed that it was moving fast enough to potentially escape the Milky Way,” lead study author Adam Burgasser, a professor of astronomy and astrophysics at the University of California San Diego, said in a statement.

Solving a cosmic mystery

Because of its low mass, the star was initially difficult to classify, which raised the question among astronomers whether it was a low-mass star or a brown dwarf, a celestial object that is not quite a star or a planet.

Brown dwarfs have more mass than planets, but not quite as much mass as stars. Citizen scientists working on the Backyard Worlds project have discovered more than 4,000 of them.

But none of these brown dwarfs were racing along on a trajectory that would carry them out of the galaxy, as was the case with the “runaway” high-speed stars that astronomers have observed over the past two decades.

Astronomers observed J1249+36 with ground-based telescopes, including the WM Keck Observatory on Mauna Kea in Hawaii and the Pan-STARRS telescope of the University of Hawaii Institute for Astronomy on the Haleakalā volcano on Maui.

Data from the Keck Observatory’s Near-Infrared Echellette Spectrograph suggest that the star is an L-type subdwarf, a star with much less mass and lower temperature than the Sun. Cool subdwarfs are the oldest stars in the galaxy.

The telescope data showed that the potential star had a lower concentration of metals such as iron than other stars or brown dwarfs.

By combining data from multiple telescopes, astronomers were able to determine the star’s position and speed in space, allowing them to predict that it will eventually leave the Milky Way.

However, questions remain about the true nature of the object.

“I calculated the mass of this object to be about 8 percent of the mass of the Sun by comparing its observed properties with computer simulations of stellar evolution,” Gerasimov said. “This puts this object right at the lower limit of allowable stellar masses, and it is actually possible that the object’s mass is slightly below this limit, which would mean that the object is not a star but a brown dwarf.”

Discovering more details about the object could help astronomers determine whether it is part of a larger population of high-speed, low-mass objects that have experienced extreme accelerations, the study authors say.

Understanding its exact nature could also help them determine when it will leave the galaxy. Previously, astronomers discovered the supermassive black hole at the center of the Milky Way, which is giving a powerful shove to a star that will finally leave the galaxy in about 100 million years.

A fast, outstanding kick

The researchers believe there are two possible scenarios that put J1249+36 on its rapid trajectory.

The study team said it was likely that the star was a companion to a white dwarf star, the remaining core of a dead star that has ejected the gases that serve as its nuclear fuel. In these star pairings, when the two stars are close together, the white dwarf will pull mass from its companion and experience an outburst called a nova. And if the white dwarf accumulates too much mass, it will collapse and explode in a supernova.

“In this type of supernova, the white dwarf is completely destroyed, so its companion is released and flies away at the original orbital speed, plus a small boost from the supernova explosion,” said Burgasser. “Our calculations show that this scenario works. However, the white dwarf is no longer there and the remnants of the explosion, which probably occurred several million years ago, have already dissipated, so we have no definitive proof that this is its origin.”

Another possibility is that J1249+36 existed in a globular cluster, a spherical, closely spaced collection of stars. Astronomers predict that at the center of such clusters there are black holes with different masses. The black holes can form binary star pairs that can catapult out any stars that come too close to them.

“When a star encounters a binary black hole system, the complex dynamics of this three-body interaction can eject the star directly from the globular cluster,” said study co-author Kyle Kremer, a future assistant professor in the Department of Astronomy and Astrophysics at the University of California San Diego, in a statement.

Kremer conducted simulations and discovered that three-body interactions can eject a low-mass subdwarf star from a star cluster and put it on a trajectory similar to that of J1249+36.

“It represents a proof of concept, but we don’t really know which globular cluster this star came from,” Kremer said.

Gerasimov is particularly intrigued by the idea that the object was ejected from a globular cluster, since such clusters contain stars that are more than 13 billion years old.

“The chemical composition and distribution of stellar masses in globular clusters capture the earliest steps in the formation and evolution of our galaxy,” he said. “Yet virtually everything we know about globular clusters comes from studies of their more massive members, because low-mass stars and brown dwarfs are simply too difficult to observe.”

Using the James Webb Space Telescope, astronomers recently identified the first brown dwarfs in a globular cluster that have a mass similar to the object. However, there are too few examples to gain a more comprehensive understanding.

“However, the existence of this hypervelocity star, if it is indeed a former member of a globular cluster, opens up a new opportunity to study low-mass members of globular clusters by looking for those that have been ejected and are traveling through the solar neighborhood at high speed,” Gerasimov said. “Since we were able to find one example, many more are likely to be discovered in the future.”

Tracing J1249+36’s path in reverse could lead to a crowded part of the night sky where undiscovered star clusters are waiting to be discovered, the researchers say.

Scientists now hope to gain further clues from the star’s elemental composition that could explain how it came to be on a trajectory that takes it away from the Milky Way.

When white dwarfs explode, they produce heavy elements that could exist around J1249+36. Similarly, stars in globular clusters throughout the Milky Way have different element patterns that serve as a calling card of their origin.

“We are essentially looking for a chemical fingerprint that could determine exactly which system this star came from,” Gerasimov said.

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