Citizen scientists discovered a star speeding through the Milky Way. Now, astronomers are trying to track down its origins.

Citizen scientists and professional astronomers have teamed up to spot a rare hypervelocity star racing through our Milky Way Galaxy. At its current speed and trajectory, it’s possible it could one day escape from the galaxy forever.

The discovery was only possible thanks to the legions of volunteers who lend their time to the Backyard Worlds: Planet 9 project. These volunteers assist astronomers by poring over 14 years’ worth of data from NASA's Wide-field Infrared Survey Explorer (WISE) mission, looking for objects that move between images. Astronomers can then follow up on interesting finds to learn more.

Star's position over time
Multiple observations taken over the years reveal the motion of nearby and/or speedy stars. These images show the motion of the L subdwarf star dubbed J1249+36. Once citizen scientists tagged the star, astronomers followed up with additional observations.
Burgasser et al. / AAS 244

At the recent 244th national meeting of the American Astronomical Society in Madison, Wisconsin, Adam Burgasser (University of California, San Diego) announced the discovery of a fast-moving object known as CWISE J124909+362116.0 (J1249+36 for short), about 400 light-years from Earth. He estimates that it's traveling through the Milky Way at about 1 million miles per hour (450 km/s).

Burgasser used the W.M. Keck Observatory on Maunakea, Hawai'i, to obtain the spectrum of J1249+36 in the infrared. The spectrum matched atmospheric models created by Roman Gerasimov (also UC San Diego) of a class of low-mass stars known as L subdwarfs. These cool stars are some of the rarest and oldest in the universe.

Next, Burgasser combined the data gathered from the spectra with images from ground-based telescopes to measure the star’s position and speed. “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,” Burgasser says.

We may know where it is going, but where did this high-speed racer come from?

One option is that it once orbited a white dwarf star, which subsequently exploded as a Type Ia  supernova. “In this kind of supernova, the white dwarf is completely destroyed, so its companion is released and flies off at whatever orbital speed it was originally moving, plus a little bit of a kick from the supernova explosion,” Burgasser says. If this happened, it occurred so long ago that there’s no longer a supernova remnant to hunt down.

The second possibility is that the star started out deep inside a dense group of stars known as a globular cluster. It then encountered a pair of black holes nestled deep within the cluster. “When a star encounters a black hole binary, the complex dynamics of this three-body interaction can toss that star right out of the globular cluster,” says team member Kyle Kremer (also UC San Diego).

Globular cluster origin?
Using the star's current position and velocity, astronomers can project its motion backwards for millions of years. Its trajectory doesn't show a clear origin from a known globular cluster, but this doesn't fully rule out such clusters as an origin.
A. Burgasser et al. / AAS 244

There’s a way that astronomers might be able to decide between these two options. “We’re essentially looking for a chemical fingerprint that would pinpoint what system this star is from,” says Gerasimov. But that would require a more detailed spectrum of J1249+36. Such a spectrum could show that the subdwarf was polluted with elements blasted out by the supernova. On the other hand, it could show a close match with the chemistry of stars in globular clusters; since globulars are so old, their stars contain very few elements beyond hydrogen and helium.

“This type of object is certainly rare and hasn't been previously found,” says Sergey Koposov (University of Edinburgh, UK) who was not involved in the research. “But we do have a lot of objects of much larger masses moving with similar velocities.” Koposov also doesn't think the list of potential backstories for the star is exhaustive, meaning its exact history remains an important puzzle to solve.

One thing is clear, however, Koposov says: “The discovery would not have been made without citizen scientists.”


Image of Anthony Barreiro

Anthony Barreiro

June 18, 2024 at 7:34 pm

From the coordinates, this dim little old star is in Canes Venatici, about 2.25 degrees southwest of Cor Caroli and about 9.25 degrees north of the North Galactic Pole. Which way is it moving on the sky and in the three-dimensional space of the galaxy? From the backward orbit figure it looks like the star is moving from the galactic halo in toward the center, or perhaps on a trajectory to pass above / north of the center of the galaxy. Is that right? I don't have access to the Keck telescope, so I'm trying to create a mental picture. 😉

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Image of Ze De Boni

Ze De Boni

June 22, 2024 at 6:20 pm

Anthony, I always appreciate your observations here, the same about your attempt to locate this star. I'm not an astronomer, but I also imagined that it could be the case of a foreign star speeding through our galaxy. However, as it's written above, that star is at "400 light-years from Earth" (BTW, it should be said "from the Sun"). That seems to be still deep in the galaxy disk, far from its center. So the halo origin can be just a guess, I guess.

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