Rare 'Runaway' Supergiant Star Discovered Speeding Across Neighboring Galaxy At 300,000 Mph

Observations of the yellow supergiant runaway were conducted using the large 6.5-meter Magellan telescope. The Large Magellanic Cloud is visible right above the telescope enclosure. Kathryn Neugent

Astronomers have discovered a rare supergiant "runaway" star hurtling across a neighboring galaxy at speeds of around 300,000 miles per hour.

The star—known as J01020100-7122208 (we'll call it J01 for short)—was spotted by an international team of researchers in a dwarf galaxy near the Milky Way known as the Small Magellanic Cloud, according to a study to be published in the Astronomical Journal (a pre-print can be viewed at arXiv.org).

"This yellow supergiant star is traveling at an abnormally large speed across the Small Magellanic Cloud, earning it the status of a 'runaway,'" Kathryn Neugent, a researcher from the Lowell Observatory in Flagstaff, Arizona and graduate student at the University of Washington, told Newsweek.

The scientists think that the 30-million-year-old star was once part of a binary system—where two stars orbit a common center—until its companion exploded in a powerful supernova, which released vast amounts of energy. This explosion caused J01 to be blasted off into space at tremendous speed.

According to the team's observations, J01 has now evolved into a yellow supergiant—massive stars (greater than around 10 solar masses) in a late stage of stellar evolution that are yellow in color (like Polaris, the North Star, for example).

These stars are very rare because the yellow supergiant phase is relatively short in stellar terms, lasting only tens or hundreds of thousands of years.

Neugent said it's not rare to find runaways that are main sequence stars—stars in the main stage of their life cycles which still undergo the process of fusing hydrogen into helium in their cores. "In fact half of all main sequence stars could be runaways. However, this star is moving much faster than most runaways and it is evolved, meaning it has left the main sequence and is starting to die."

This makes it the first runaway yellow supergiant star ever discovered and only the second evolved runaway star to be found in another galaxy.

It was found using the National Optical Astronomy Observatory's 4 meter Blanco telescope and the Carnegie Observatory's 6.5 meter Magellan telescope, which are both located in northern Chile.

"We were in the process of searching for Yellow Supergiants (YSGs) in the Small Magellanic Clouds when we found this star," Neugent added.

"The way you find YSGs is by taking a spectrum of the star and looking to see how the spectral lines shift to the right and left. This shift gives you the radial velocity, or how fast the star is traveling. Stars in the Small Magellanic Cloud are going to be traveling at very specific radial velocities," Neugent said. "One of the stars, this runaway YSG, was traveling at a much larger radial velocity than we expected and thus we decided to investigate it further."

The team's predictions suggest that J01 will continue travelling through space for another three million years before it eventually becomes a red supergiant—stars that can grow to sizes which would reach from the Sun to beyond the orbits of Mars or Jupiter. After this stage, J01 will likely meet the same fate as its old companion, blowing up in a cataclysmic supernova.

The remnants of this supernova could subsequently lead to the formation of new stars and planets in the outer regions of the Small Magellanic Cloud.