Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about fascinating discoveries, scientific developments and more.
An object spotted with the help of citizen scientists was moving so fast through the Milky Way that it escaped the galaxy’s gravity and reached intergalactic space, new research shows.
Probably a faint red star, the object was zooming along at about 1.3 million miles per hour (600 kilometers per second). For comparison, the sun orbits the Milky Way at 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 citizen scientists whose study has been accepted for publication in The Astrophysical Journal Letters.
There are many more low-mass stars than high-mass stars because star formation favors lower-mass objects and more massive stars have shorter lifespans, said study co-author Roman Gerasimov, a postdoctoral researcher in the department of physics and astronomy at the University of Notre Dame. But low-mass stars are harder to detect because they are cooler and less luminous.
Hypervelocity stars, first suggested in 1988 and discovered in 2005, are already extremely rare, making this new discovery “particularly exciting,” he said.
Volunteers participating in a project called Backyard Worlds: Planet 9 first discovered the star, dubbed CWISE J124909.08+362116.0, or J1249+36 for short. Researchers involved in the project are searching for evidence of undiscovered objects or a large hypothetical world, dubbed Planet Nine, in “the solar system’s backyard” beyond Neptune.
Backyard Worlds participants will look for patterns and anomalies in images and data collected by NASA’s Wide-field Infrared Survey Explorer mission, which mapped the sky in infrared light from 2009 to 2011. (In 2013, the space agency renamed the mission Near-Earth Object Wide-field Infrared Survey Explorer to track asteroids and comets near Earth. However, the mission was permanently decommissioned on Aug. 8.)
According to the study authors, J1249+36 caught the attention of citizen scientists reviewing the data a few years ago because the star was moving at about 0.1% of the speed of light.
“I can’t describe the level of excitement,” study co-author Martin Kabatnik, a citizen scientist in Nuremberg, Germany, said in a statement. “When I first saw how fast it was moving, I was convinced it had to have been reported.”
Follow-up observations with multiple telescopes focused on the object and helped confirm the discovery.
“This is where the source became very interesting, because its speed and trajectory showed that it was moving fast enough to potentially escape the Milky Way,” lead researcher Adam Burgasser, a professor of astronomy and astrophysics at the University of California, San Diego, said in a statement.
Solving a cosmic mystery
The star’s low mass initially made it difficult to classify. Astronomers wondered whether it was a low-mass star or a brown dwarf, a body that is neither quite a star nor a planet.
Brown dwarfs are more massive than planets, but not as massive 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 traveling on a trajectory that would take them out of the Milky Way, like the “runaway” hypervelocity stars observed by astronomers 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 University of Hawaii Institute for Astronomy’s Pan-STARRS telescope on Haleakalā volcano in Maui.
Data from the Keck Observatory’s Near-Infrared Echellette Spectrograph suggested the star was an L subdwarf, or a star with a much lower mass and cooler 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 velocity in space, allowing them to predict that the star would eventually leave the Milky Way.
However, questions remain about the true nature of the object.
“I calculated that the mass of this object is about 8% of the mass of the Sun by comparing the observed properties with computer simulations of stellar evolution,” Gerasimov said. “This puts this object right at the lower limit of allowed stellar masses, and it is in fact possible that the mass of the object is slightly below that limit, which would imply that the object is not a star, but a brown dwarf.”
By revealing more details about the object, astronomers can determine whether it is part of a broader population of high-speed, low-mass objects that have undergone extreme accelerations, the study authors said.
By understanding its exact nature, they can also determine when it will leave the Milky Way. Previously, astronomers have spotted the supermassive black hole at the center of the Milky Way giving a quick kick to a star that will leave the Milky Way for good in about 100 million years.
A fast, great kick
The researchers believe there are two possible scenarios that put J1249+36 on its high-speed path.
The research team said it is likely that the star was a companion to a white dwarf star, which is the remaining core of a dead star that has ejected the gases that serve as its nuclear fuel. In these stellar pairs, if the two stars are close together, the white dwarf will suck mass away from its companion and have an outburst called a nova. And when the white dwarf gathers too much mass, it will collapse and explode in a supernova.
“In this kind of supernova, the white dwarf is completely destroyed, so its companion is released and flies away with its original orbital velocity, 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 millions of years ago, have already disappeared, so we have no definitive proof that this is the origin.”
Another possibility is that J1249+36 existed in a globular cluster, or a spherical, tightly bound grouping of stars. Astronomers predict that black holes of different masses exist at the centers of such clusters. The black holes could form binary pairs that could catapult stars that come too close.
“When a star encounters a binary star system with a black hole, the complex dynamics of this three-body interaction can throw that star right out of the globular cluster,” study co-author Kyle Kremer, an associate professor in the department of astronomy and astrophysics at the University of California, San Diego, said in a statement.
Kremer performed simulations and found that three-body interactions can eject a low-mass subdwarf star from a cluster and put it on an orbit similar to that of J1249+36.
“It’s a proof of concept, but we don’t yet know exactly which globular cluster this star came from,” Kremer said.
Gerasimov is particularly fascinated by the idea that the object was ejected from a globular cluster, because such clusters contain stars older than 13 billion years.
“The chemical composition and distribution of stellar masses in globular clusters record 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 higher-mass members, because low-mass stars and brown dwarfs are simply too difficult to observe.”
The James Webb Space Telescope recently allowed astronomers to identify the first brown dwarfs in a globular cluster that have a similar mass to the object. But there are too few examples yet to gain a broader understanding.
“However, the existence of this hypervelocity star, if it is indeed a former member of a globular cluster, opens up a new way to study low-mass cluster members by searching for those that have been ejected and are traveling at high speeds near the Sun,” Gerasimov said. “Since we were able to find one example, many more will likely be discovered in the future.”
If we could follow the path J1249+36 has taken so far in reverse, we could find a busy patch of the night sky where as yet undiscovered star clusters are waiting to be discovered, the researchers say.
Now scientists hope to learn more about the star’s elemental composition, which could explain how it ended up on an orbit that diverges from the Milky Way.
When white dwarfs explode, they create heavy elements that could exist around J1249+36. Similarly, stars in globular clusters in the Milky Way have clear patterns of elements that serve as calling cards to their origins.
“We are essentially looking for a chemical fingerprint that could determine which system this star comes from,” Gerasimov said.
For more CNN news and newsletters, create an account at CNN.com