The short video shows a hypothetical white dwarf star exploding as a supernova, pushing a second star out of this binary system at a speed of about 1.6 million kilometers per hour. This scenario is one explanation for the discovery of a single star hurtling through our Milky Way galaxy. The fast star is called a hypervelocity star and in this case is designated CWISE J1249. Image via Adam Makarenko/ Video via Keck Observatory.

• Citizen scientists discover a superfast star. It is moving so fast that it will leave our galaxy, the Milky Way. Its speed is about 1.6 million kilometers per hour, in contrast to about 720,000 kilometers per hour for our sun and solar system (!).
• Scientists call such stars hyperspeed stars. This one is called CWISE J1249 and is the smallest hyperfast star to date. It is also the closest to our sun.
• Could this star be a brown dwarf? Maybe. It is small and could be either a low-mass star or a brown dwarf, a mixture of a large planet and a star.

NASA originally published this article on August 15, 2024. Modifications by EarthSky.

A lonely star racing through our galaxy

Most known stars peacefully orbit the center of the Milky Way, but citizen scientists working on NASA’s Backyard Worlds: Planet 9 project have helped discover an object moving so fast that it is escaping the Milky Way’s gravity and shooting into intergalactic space.

This hypervelocity object is the first object of its kind found with the mass of a small star. It is also the closest hypervelocity star to our Sun.

The Backyard Worlds project uses images from NASA’s Wide-field Infrared Explorer (WISE) mission, which mapped the sky in infrared light from 2009 to 2011. It was reactivated in 2013 as Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) and will be retired on August 8, 2024.

Several years ago, longtime Backyard Worlds citizen scientists Martin Kabatnik, Thomas P. Bickle, and Dan Caselden spotted a faint, fast-moving object called CWISE J124909.08+362116.0 marching across their screens in the WISE images. Subsequent observations with several ground-based telescopes helped the scientists confirm the discovery and characterize the object. These citizen scientists are now co-authors of the team’s study of this discovery, which will be published in the Astrophysical Journal Letters (A pre-release version is available here.) Kabatnik, a citizen scientist from Nuremberg, said:

I can’t describe the excitement. When I saw how fast it was moving, I was convinced it must have been reported already.

Low-mass star or brown dwarf?

CWISE J1249 is speeding out of the Milky Way at about 1.6 million km/h. But it is also notable for its low mass, which makes it difficult to classify as a celestial object. It could be a low-mass star. However, if it is not steadily fusing hydrogen at its core, it would be considered a brown dwarf, somewhere between a gas giant and a star.

Ordinary brown dwarfs are not that rare. Volunteers from Backyard Worlds: Planet 9 have discovered more than 4,000 of them! However, the others are not known to leave the galaxy.

This new object has another unique property. Data obtained with the WM Keck Observatory in Maunakea, Hawaii, show that it contains much less iron and other metals than other stars and brown dwarfs. This unusual composition suggests that CWISE J1249 is quite old and probably belongs to one of the first generations of stars in our galaxy.

Why is the star so fast?

Why is this object moving at such a high speed? One theory is that CWISE J1249 originally came from a binary star system with a white dwarf that exploded as a supernova when it stripped too much material from its companion. Another possibility is that it came from a tightly connected star cluster called a globular cluster and flew away through a chance encounter with a pair of black holes.

Kyle Kremer, future assistant professor in the Department of Astronomy and Astrophysics at UC San Diego, said:

When a star collides with a binary black hole system, the complex dynamics of this three-body interaction can eject the star directly from the globular cluster.

Scientists will study the elemental composition of CWISE J1249 more closely to find clues as to which of these scenarios is more likely.

A success for citizen science

This discovery was the result of a team effort on several levels: a collaboration between volunteers, professionals and students. Kabatnik thanks other citizen scientists for their help in the search, including Melina Thévenot. He said that Melina:

…blew me away with her personal blog about searching with the Astronomical Data Query Language.

According to Kabatnik, software written by citizen scientist Frank Kiwy was also crucial to this discovery.

The study is led by Backyard Worlds: Planet 9 science team member Adam Burgasser, a professor at the University of California, San Diego, and includes co-authors Hunter Brooks and Austin Rothermich, astronomy students who both began their astronomy careers as citizen scientists.

Would you like to help discover the next extraordinary object in space? Enter Backyard Worlds: Planet 9 here. Participation is open to anyone in any country in the world.

Conclusion: Citizen scientists have discovered a single star that is moving so fast that it will eventually escape the gravity of the Milky Way and shoot into intergalactic space.

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