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.
advertisement
