For the first time, astronomers have traced young orphan stars that exist above the main disk of the Milky Way, far beyond where they should be found, back to their galaxy of origin. 

A new “stellar paternity test” has been made possible by the European Space Agency (ESA) Gaia Space Telescope mission, which is currently building an accurate 3D map of the Milky Way and its two billion stars. 

A common picture of a star’s existence within the Milky Way is likely influenced by our own solar system: a star surrounded by orbiting planets in a sedate cosmic waltz. However, observations have revealed that many stars have more turbulent upbringings, ejected from their place of birth and becoming stellar orphans condemned to wander the galaxy alone.

One place stars could be ejected from is one of the thousand or so open clusters in the Milky Way. These are loosely bound collections of stars containing hundreds or even thousands of stellar bodies formed from the same collapsing cloud of gas and dust. 

This ejection would happen in chaotic open star clusters that contain thousands of stars, as a result of gravitational interactions that can launch individual stars away from their siblings. Sometimes, these stars aren’t just ejected from their home cluster but from the galaxy that cluster sits within altogether.

“Through tracing them back in time to see where they originated, we are able to match 15 of them to the star clusters where they were born,” said M. Virginia (Ginny) McSwain, team member and associate professor of physics at Lehigh University. “If we can say with high confidence where some of these stars came from, we will know more about the history of the star clusters in the Milky Way.”

McSwain and the rest of the Lehigh University presented their results at the 243rd meeting of the American Astronomical Society (AAS) in New Orleans earlier this month. 

Meet the stellar orphans that live with eight billion-year-old stars

The vast majority of star formation in the Milky Way occurs within its thin main disk, and hot, young, massive stars, called “B-type stars”, are rarely found beyond this region. 

However, for at least two decades astronomers have been aware that a relatively small number (between 10 million and 100 million) have been discovered at high altitudes above the disk, getting there after being ejected from their host clusters. This stands out because these ejected young stars exist among stars that are estimated to be around 8 billion years old, far more ancient than these cosmic orphans or our 4.6 billion-year-old Sun.

This makes the ejected stars highly conspicuous among their older counterparts, meaning they were easily distinguished from this population of ancient stellar bodies by the light output and the fact they are still burning hydrogen in their cores–something stars do in their youth during what astronomers call the main sequence. 

 Before the Gaia mission, it wasn’t possible to obtain data precise enough to recreate the journeys that had brought these bright young stars to their high-altitude location over the Milky Way or to trace them back to  their home clusters.

“Hot stars don’t often venture out of the disk, so when they do, they are noticeably out of place,” said  Brandon Schweers sai, a team member and a Lehigh University undergraduate student.

They found that one star they studied looked as if it had ejected from its star cluster in the main disk of the Milky Way with immense energy and at particularly high speeds. Schweers thinks the cause of this high velocity ejection was a “kick” from a supernova that erupted in a close binary system. 

Other stars in the population appeared to have journeyed through space like cosmic boomerangs, whipping away from the star clusters and then swinging back in, only to be hurled out again.

“The ‘parent’ clusters probably ejected most of these B-type stars when close, three- or four-body gravitational interactions flung out a member of the cluster, sending them running away from the plane of the Milky Way,” Schweers continued.

How Gaia tells the orphans’ stories

The Lehigh University team traced the journey of these ejected stars high above the Milky Way using data released from Gaia in 2022. 

This data allowed them to match 95 high-latitude B stars to 1,400 open clusters, factoring in galactic dynamics and the gravitational field of the Milky Way to look for moments in their histories when they could have intersected and when an ejection may have happened. 

“Using their 3D positions and 3D velocities through space, we were able to calculate the trajectories of each cluster and high-latitude star over the past 30 million years,” McSwain said.

Once these matches were made, the team looked at the color and brightness of the ejected stars, determining their ages and comparing them to stars that still resided in the open cluster from which they appeared to have originated. 

This is a hint at the origin of these stars because stellar bodies in open clusters are believed to have formed at around the same time from the same collapsing cloud of dense gas and dust, which means they should have similar ages and characteristics. This narrowed down the possible star/open cluster matches. 

The team further whittled matches down by examining the densities of these potential parental clusters. They reasoned that the clusters with higher densities would be more likely to have experienced gravitational perturbations powerful enough to have ejected stars in the past.

This resulted in positive paternity tests for 15 orphaned stars, which the researchers estimated were ejected between 5 and 30 million years ago at speeds ranging from 67,000 to 490,000 miles per hour. 

Other orphan stars couldn’t be traced back to the main disk of our galaxy. The team suggests that this may indicate unusual formation methods, such as the formation of stars in dense clouds of cold gas, called molecular clouds, that exist high over the Milky Way. The lack of paternal sources for some of these hot young stars could even mean that they are the remains of dwarf galaxies that once surrounded our galaxy and were cannibalized by the Milky Way to facilitate its growth. 

Schweers said the whole process reminded him of the American daytime talk show, The Maury Povich Show. “I think everyone has heard the saying, ‘You are not the father’ that came from that show,” he said. “For many of these clusters, I was essentially telling them they are not the parent of these orphaned stars, so I came up with the name ‘Stellar Paternity Tests’.”

Feature image: Artist’s impression of Gaia mapping the stars of the Milky Way. Credit: ESA/ATG medialab; background: ESO/S. Brunier