Posted: September 16, 2008
Habitable zones are the Goldilocks “just right” locations which are more conducive to supporting life than other areas of a solar system. For our Solar System this zone extends from a distance of 0.95 Astronomical Units (AU) to 1.37 AU, with Venus, Earth and Mars residing at 0.72, 1 and 1.52 AU respectively. Earth, unsurprisingly, is tucked nicely into the habitable zone.
"Our view of the extent of the habitable zone is based in part on the idea that certain chemical elements necessary for life are available in some parts of a galaxy's disc but not others," says Rok Roskar, a doctoral student in astronomy at the University of Washington and lead author of a paper describing the findings published in this week’s edition of Astrophysical Journal Letters. "If stars migrate, then that zone can't be a stationary place. If the idea of habitable zone doesn't hold up, it would change scientists' understanding of just where, and how, life could evolve in a galaxy.”
This image is from a computer simulation showing the development and evolution of the disc of a galaxy such as our own Milky Way Galaxy. Image. R. Roskar.
The team of scientists used more than 100,000 hours of computer time to run a simulation of the formation and evolution of a galaxy disc. They set the basic parameters to mimic the development of the Milky Way up to the point at which material for the disc had collected together, but the actual disc formation had not yet begun. Then they let the simulated galaxy evolve on its own.
Previously, scientists believed that if a star is intercepted by a spiral arm of the galaxy during its orbit around the galactic centre, the star's orbit would become more erratic. But the new simulations show that although the orbits of some stars might get larger or smaller, they still remain very circular after such an interaction. Our Sun has a nearly circular orbit, so the result of the simulation means that when it formed 4.59 billion years ago – about 50 million years before the Earth – it could have been either nearer to or farther from the centre of the galaxy, rather than halfway toward the outer edge where it is now.
“Our simulated galaxy is very idealised in the formation of the disc, but we believe it is indicative of the formation of a Milky Way-type of galaxy," says Roskar. "In a way, studying the Milky Way is the hardest thing to do because we're inside it and we can't see it all. We can't say for sure that the Sun had this type of migration."
Migrating stars also help explain a long-standing problem in the chemical mix of stars in the neighborhood of our Solar System, which is more mixed and diluted than would be expected if stars spent their entire lives where they were born. By bringing in stars from very different starting locations, the Sun's neighborhood can immediately become much more diverse in chemical composition.
Although the research team are not the first to suggest that stars might be able to migrate great distances across galaxies, they are the first to demonstrate the effects of such migrations in a simulation of a growing galactic disc. And although these findings are based on just a few simulations, it is expected additional runs using the same parameters and physical properties would produce largely the same results.
"When you swirl cream into a cup of coffee, it will rarely look exactly the same twice, but the general process, and the resulting taste, is always the same," says James Wadsley, team member from McMaster University in Canada.
The next stage of the research program will see the scientists running a range of simulations with varying physical properties to generate different kinds of galactic discs, to determine whether stars show similar ability to migrate large distances within different types of disc galaxies.
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