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Tuesday, November 27, 2007

Global Weather and the Consequences of Interstellar Dust

Interstellar matter is not distributed uniformly throughout our galaxy, but is slightly concentrated in regions behind the density wave that rotates around the galac­tic center and sweeps past the sun at intervals of roughly 100 million years, triggering star formation and the appear­ance of spiral arms. Recently, Raymond J. Talbot, Jr., of Rice University and his associates Michael J. Newman and Dixon M. Butler have examined the effects that would occur if the sun encountered a dense interstellar cloud of atoms, mole­cules, and dust.

Their calculations indicate that a star similar to the sun and lying in the galactic plane would, during its lifetime, likely pass through about 135 clouds having densities greater than the equivalent of 100 hydrogen atoms per cubic centimeter and about 16 clouds that are at least 10 times denser.

One effect of such an encounter on the sun itself would be to modify the flow of the solar wind. For example, at a relative velocity of 20 kilometers per second, a cloud denser than 100 hydrogen atoms per cubic centimeter would compress the solar wind to within the earth's orbit on the side of the sun facing the approaching cloud. This would fully expose the earth to galactic cosmic rays for at least part of the year, possibly resulting in modification of the terrestrial climate.

At higher densities, the cloud could overcome the solar wind altogether, forc­ing the particles back onto the sun and allowing material to fall in after them. During its lifetime, the sun may have accreted about 0.0001 solar mass or more from the interstellar medium in this way. The gravitational energy released during accretion would enhance the sun's lumi­nosity, especially at ultraviolet and X-ray
wavelengths. A sustained increase equiva­lent to one percent of the sun's total radiation would produce a significant change in climate and hazards to life.

The accretion of interstellar matter would also likely enrich the solar surface with heavy elements. Thus, the abun­dance of these elements observed today may be greater than that in the solar interior. If so, the rate of high-energy neutrino emission at the sun's center could be significantly lower than predicted by conventional models, thereby reducing the discrepancy between theory and recent observations (see sky and telescope for November, 1976, page 324).

The planets would also be directly af­fected by a dense interstellar cloud. For example, an encounter with a cloud would considerably enrich the hydrogen in the earth's atmosphere, replenishing about 25 percent of that lost by leakage since the previous encounter. Helium, other ele­ments, and substantial amounts of dust would also be added.

This news note is based largely on two papers in Nature for August 12, 1976, and talks given in January at the American Astronomical Society's Division for Plane­tary Sciences meeting in Honolulu.

(Reprinted without permission. Source: Sky and Telescope, September 1977)

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