How a 5M-Year-Old Star Somehow Took a 39M-Year-Long Trip Through Space

Astronomers may have solved the mystery of how some short-lived massive stars travel distances across space that should take many times their lifespans to traverse, suggesting the key lies in how they transform during the journey.

An example of such a misplaced massive star is HD93521, located 3,600 light-years above the disc of the Milky Way, its place of birth. At 17 times the size of the sun, astronomers estimate this massive star should be just 5 million years old. But the motion of the star indicates that the flight time to its present location should have taken 39 million years, almost eight times its predicted lifespan.

The answer arrived at by a Georgia State University team is that HD93521 didn't begin its journey as a single massive star. Instead, they propose that the star, which is around 3,837 light-years from Earth, started out as two lower-mass stars with longer life-spans that went on to merge during their journey.

Georgia State astronomer Douglas Gies said in a press release from the university: "Astronomers are finding massive stars far away from their place of origin, so far, in fact, that it takes longer than the star's lifetime to get there.

"How this could happen is a topic of active debate among scientists."

Gies is the lead author of a paper documenting the team's findings, which has been published in The Astronomical Journal.

Massive stars have incredibly short lifespans in comparison to stars like our sun, 10 million years rather than 10 billion years. This is because these stars are much hotter and thus burn through their hydrogen fuel much more rapidly than smaller stars.

That should mean that massive stars during their main hydrogen-burning phase like HD93521 should never be found too far away from the place of their formation, the dense gas clouds located in the flat disc of galaxies.

Two Stars Merging

One of the main clues that HD93521 started life as two smaller stars is the fact that it's one of the fastest rotating stars in our galaxy. During the stellar merger process two orbiting stars move closer together.

This speeds up their rotation much like how an ice skater spins more quickly when they draw their arms in. This means by the time they collide, what is left spins rapidly just as HD93521 does.

Gies continues: "HD93521 probably began life as a close pair of medium-mass stars that were fated to engulf each other and create the single, fast-spinning star we see today."

Further evidence that mergers could be responsible for massive stars that seem to wander far from home could be delivered by Georgia State graduate student Peter Wysocki.

Wysocki is investigating IT Librae, a massive binary star pairing that seems to be on verge of a merger. The masses of the stars in this binary present a similar problem to that found with HD93521, its location from its birthplace seems much further away than could logically be reached in its lifetime.

This could be because the smaller star in the binary is donating material to the larger star, which began life as a smaller star. As a consequence, as they travel away from the galactic disc, the larger mass-star swells in size and looks younger than it actually is.

In addition to solving the mystery of how massive stars wander great distances, Gies said that these results could also explain how massive stars undergo gravitational collapse to form rapidly rotating black holes.

Milky Way Disk
A file picture of the disk of the Milky Way. Astronomers may have solved the mystery of how some massive stars travel tremendous distances away from their birthplaces in galactic discs. alex-mit/GETTY