ScienceWeek

ASTRONOMY: ON STARS AND MAGNETIC FIELDS

The following points are made by Gibor Basri (Science 2006 311:618):

1) Magnetic fields are pervasive throughout the cosmos. Most of the matter in the universe is a plasma (a gas of charged particles), and thus influenced by electric currents that can give rise to magnetic fields. Such fields are responsible for phenomena as diverse as Earth's aurorae, the solar corona, spectacular bipolar jets of material shooting from newly forming stars or accreting black holes, and the magnetization that suffuses whole galaxies. Angular momentum is also pervasive in the cosmos, and combined with a moving conducting fluid or plasma it can power a magnetic dynamo. For instance, Earth's core contains one example of a self-generating magnetic dynamo, and our Sun's envelope has another. Indeed, most stars manage to generate magnetic fields, because they are rotating, convecting, conducting bodies. Nonetheless, stellar magnetic fields are notoriously difficult to study directly. New work[1] reports an extension of a subtle technique for mapping surface magnetic fields to a very important class of stars.

2) It is often said that we live around an average star. This is not really true. Our Sun is about three times as massive as the average star, nearly twice as hot at its surface, and about 100 times as bright. These average stars ("M stars" in astronomers' parlance) are more than five times as numerous as stars like our Sun, and so constitute most of our stellar neighbors. Despite their plenitude, they have received less attention from astronomers than other stars, because until recently they were too faint to be detected by many of the diagnostic techniques applied to stars (for instance, you cannot see any of them with your naked eye even though the closest star to us is an M star).

3) Convection in stars arises when it is more efficient to transfer energy by mechanical motions rather than simply radiating it outward through stable plasma. The conditions that favor convection arise when the resistance (opacity) of the material to radiation is too high. This tends to happen in cooler material, where there are many more sources of opacity than in fully ionized plasma. Thus, in stars cooler than the Sun, the convection zone deepens to larger percentages of the volume.

4) The magnetic dynamo created by this kind of convection in our Sun reverses every 11 years, giving rise to the well-known solar cycle. It is thought to arise predominantly at the bottom of the solar convective zone (about 30% of the way to the core), where there is a shear layer between the convective envelope and radiative core. A star with mass about a third of our Sun's will be sufficiently cool that its entire interior is convective. Obviously, the magnetic dynamo must change if there is no radiative core. The expectation is that only a turbulent dynamo will remain, and such a dynamo might only generate small-scale fields (more like what is seen at the minimum of the solar cycle).[2,3]

References (abridged):

1. J.-F. Donati et al., Science 311, 633 (2006).

2. Stellar Surface Structure, IAU Symposium 176, K. G. Strassmeier, J. L. Linsky, Eds. (Kluwer, Dordrecht, Netherlands, 1996)

3. http://sohowww.nascom.nasa.gov/gallery/EIT/

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