Stars near sun are wild and wayward

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A new survey of stars near the sun reveals a wild and crazy past in which wanderers arrived from all directions under the gravitational influences of black holes, clouds of gas and invading galaxies.

European astronomers spent 15 years making 1001 nights of observations to detail the motions of more than 14,000 stars that are currently in the solar neighborhood, a tiny corner of the Milky Way. Tracing the movements back in time with a computer, the researchers found that most of the stars were once much farther from the sun than they are today.

While astronomers expected the early history of the Milky Way was quite chaotic, most had believed "that it since had been rather calm," said study leader Birgitta Nordstrom of the Niels Bohr Institute for Astronomy, Physics, and Geophysics in Copenhagen. "But this turns out not to be true. Stars have been perturbed all the time throughout the Milky Way history."

The study was released today.

"Now that we know the orbits of all the stars, we can see that many nearby stars come from far away and are just transiting near us now," said Johannes Andersen, also of the Niels Bohr Institute.

Surprise move
The Milky Way galaxy is a gargantuan spiral spanning some 100,000 light-years. The sun orbits the center of the galaxy on an outer portion of one spiral arm, about 26,000 light-years from the center.

All stars in the Milky Way are gravitationally bound to the dense galactic center, around which they all orbit. Most are born in circular orbits, but when they encounter other objects, they can change course and speed up.

Andersen told that older stars in particular move more rapidly than was known. "Something … has stirred up their motions more than we knew," he said.

But what did the stirring, giving each star a different trajectory and speed? Several things were probably at work giving gravitational boosts, according to a preliminary analysis of the data:

  • The Milky Way's own spiral arms, which are denser than the relative voids in between
  • Smaller galaxies falling into the Milky Way and being consumed
  • Giant clouds of gas falling through the galaxy
  • Black holes

Figuring out where a star came from is no easy task, but the new data provide hints. A star passed frantically close to a massive black hole, for example, will have a higher velocity than a star that lazily meandered around a less dense cloud of gas.

Scientists have long known that the Milky Way was not a simple structure, and they suspect it evolved dramatically over the life of the universe, which is 13.7 billion years old. Newer models have the galaxy building its bulk over billions of years by merging with other galaxies and consuming smaller ones. In recent years, observed trails of stars moving at offbeat angles and speeds have provided evidence of this chaotic history.

Along the way, about 4.6 billion years ago, the sun was born from a cloud of gas. Some theorists believe the sun formed within a dense cluster of stars and was later gravitationally kicked out to its current, less dense surroundings. Most stars are born in such clusters.

Rewind and reveal
The survey does not reveal the sun's birthplace, but it offers some clues to the past. An animation shows the stars' movements in relation to the sun projected back through the past 250 million years, or about the time it took the sun to make its most recent trip around the center of the galaxy.

A lot has changed. The stars begin the animation spread broadly across about a quarter of the galaxy, then gather to a small, seemingly insignificant huddle at the end.

And that's nothing compared to the much longer history of the sun.

"The sun has made some 20 laps around the galactic center since it was born, and its 'sisters and brothers' have dispersed long ago," Andersen said of any stars that might have been born near the sun.

The observations were made with the Danish 1.5-meter telescope at the European Southern Observatory in Chile and the Swiss 1-meter telescope of the Observatoire de Haute-Provence in France. Michel Mayor of the Geneva Observatory also participated in the work.

Additional observations were made at the U.S. Harvard-Smithsonian Center for Astrophysics. The research will be detailed in the journal Astronomy & Astrophysics.

This census, which involved only sun-like stars, also catalogued each star's age and chemical makeup. In addition, about one-third of the stars were found to have one or more stellar companions, a ratio that does not surprise astronomers. The research team expects the new catalogue to be a rich data source for future study by many teams.