A solitary star speeding across the Milky Way

The short video shows a hypothetical white dwarf star, exploding as a supernova and pushing a 2nd star out of this binary system at a speed of some 1 million miles per hour. This scenario is one explanation for the discovery of a solitary star speeding across our Milky Way galaxy. The fast star is called a hypervelocity star, in this case labeled CWISE J1249. Image via Adam Makarenko/ video via Keck Observatory.

• Citizen scientists discovered a super-speedy star. It’s moving so fast that it’ll leave our galaxy, the Milky Way. Its speed is about 1 million miles per hour, in contrast to about 450 thousand miles per hour for our sun and solar system.
• Scientists call such stars hypervelocity stars. This one is labeled CWISE J1249, and it’s the smallest-mass hypervelocity star found so far. It’s also the closest one to our sun.
• Could this star be a brown dwarf? Maybe. It’s small and might be either a low-mass star or a brown dwarf, which is like a mix between a big planet and a star.

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

A solitary star speeding across our galaxy

Most familiar 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 will escape the Milky Way’s gravity and shoot into intergalactic space.

This hypervelocity object is the first such object found with the mass of a small star. It’s also the closest hypervelocity star to our sun.

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

A few 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. Follow-up observations with several ground-based telescopes helped scientists confirm the discovery and characterize the object. These citizen scientists are now co-authors on the team’s study about this discovery, to be published in the Astrophysical Journal Letters (a pre-print version is available here). Kabatnik, a citizen scientist from Nuremberg, Germany, said:

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

Low-mass star or brown dwarf?

CWISE J1249 is zooming out of the Milky Way at about 1 million miles per hour (1.6 million kph). But it also stands out for its low mass, which makes it difficult to classify as a celestial object. It could be a low-mass star. But if it doesn’t steadily fuse hydrogen in its core, it would be considered a brown dwarf, somewhere between a gas giant planet and a star.

Ordinary brown dwarfs are not that rare. Backyard Worlds: Planet 9 volunteers have discovered more than 4,000 of them! But none of the others are known to be on their way out of the galaxy.

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

Why is the star so fast?

Why does this object move at such high speed? One theory is that CWISE J1249 originally came from a binary system with a white dwarf, which exploded as a supernova when it pulled off too much material from its companion. Another possibility is that it came from a tightly bound cluster of stars called a globular cluster, and a chance meeting with a pair of black holes sent it soaring away.

Kyle Kremer, incoming assistant professor in UC San Diego’s Department of Astronomy and Astrophysics, said:

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.

Scientists will look more closely at the elemental composition of CWISE J1249 for clues about which of these scenarios is more likely.

A success for citizen science

This discovery has been a team effort on multiple levels: a collaboration involving volunteers, professionals, and students. Kabatnik credits other citizen scientists with helping him search, including Melina Thévenot. He said that Melina:

… blew my mind with her personal blog about doing searches using Astronomical Data Query Language.

Software written by citizen scientist Frank Kiwy was also instrumental in this finding, Kabatnik said.

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.

Want to help discover the next extraordinary space object? Join Backyard Worlds: Planet 9 here. Participation is open to anyone in any country worldwide.

Bottom line: Citizen scientists have discovered a solitary star moving so fast that it will eventually escape the Milky Way’s gravity and shoot into intergalactic space.

Via NASA

Via Keck Observatory