Sun May Be Galactic Hitchhiker

Astronomers have long believed that most stars are homebodies which stick close to their birthplaces. But a new simulation supports the suggestion that our sun might have once hitchhiked through the galaxy.

Where our sun or another star ends up migrating seems to depend on its position in a spinning spiral galaxy. Stars tagging along behind a massive spiral arm can get a gravitational speed boost that sends them into a bigger orbit around the galactic center. The stars leading in front of a spiral arm may end up getting slowed by the arm's gravitational pull and fall back into a smaller orbit.

"Our simulation is still like a toy model, but a step more complex than the ones observing migrations before," said Rok Roskar, an astronomer at the University of Washington in Seattle.

Previous simulations followed star migration in a preset galaxy. The new computer model goes back 9 billion years to track a galaxy's evolution from a giant cloud of gas. It also follows individual stars from before birth until they die in a fiery supernova.

Stars could travel from deep within the interior of a galaxy to its outer edges, and might even get passed around in a "spiral arm relay," Roskar told SPACE.com. Our sun currently has an orbit near the outer edge of the galaxy.

New simulation runs confirmed earlier work that showed how star orbits can remain circular despite expanding or shrinking. That counters assumptions that the gravitational tug-of-war would push and pull orbits into wilder elliptical shapes.

The older research also undermines the assumption that a more elliptical or disturbed orbit reflects the older age of a star.

"You can have a circular orbit for an old star that has been whacked around by many spiral arms," Roskar said.

Such simulations may help explain observations made by NASA's Hubble Space Telescope, which had raised another puzzle for astronomers. Conventional knowledge states that stars get younger and younger further out in a galactic disk, but Hubble turned up evidence that stars are actually much older beyond a certain point in the outer galaxy.

Roskar and other researchers think that the older stars lurking in the galactic fringe represent wandering wayfarers rather than local residents.

"Older stars have had time to get out there," Roskar noted.

Wandering stars could also shake up the notion of galactic habitable zones. Such safe areas would not have the chaos and radiation of star nurseries, but would have enough heavy metals from previous stars to support planets conducive to life. Yet such zones could not be stationary as the galaxy itself changes, or if our sun has migrated across many different parts of the galaxy.

"It seems impossible to talk about a galactic habitable zone without context," Roskar said. "Our sun has a very circular orbit, so some assumed it's always been where it is. To me, it's not so clear that's true."

The new simulation is detailed in the Sept. 10 edition of the Astrophysical Journal Letters.

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