Magnetic star attracts astronomers.
The star HD 37776, located in the Orion constellation, has several unusual properties. For instance, it is one of a few dozen "helium-strong," stars whose atmosphere contain equal amounts of helium and hydrogen. The vast majority of stars, including the sun, have atmospheres of 90 percent hydrogen and about 10 percent helium. After analyzing light from the hot, 10-solar-mass object, two astronomers have now added to the star's list of intriguing oddities.
David A. Bohlender and John D. Landstreet of the University of Western Ontario discovered that HD 37776 may possess the largest surface magnetic field and the largest variation in that field of any star other than a white dwarf or neutron star. In addition, they say it is the only object ever observed to have a predominantly quadrupole magnetic field. Evidence for the findings, which may influence how astronomers think about the origin of stellar magnetic fields, came from several starlight measurements, including detections of telltale changes in the spectra emitted from certain of the star's atomic elements, most notably silicon and helium.
In or near their cores, most stars have a magnetic field shaped like that of a dipole, or bar magnet. The more complicated quadrupole field would be created by two bar magnets laid end to end with their respective north or south poles touching. Whether dipole or quadrupole, the field alters the motion of ions in the stellar atmosphere, redistributing the particles into patches according to their atomic weight. In addition, it broadens and intensifies certain spectral lines, adding another fingerprint for identifying its strength and type. Moreover, as the star spins at a rapid 100 kilometers per second, the patchy pattern of ions and the brightened spectral lines move with it, creating periodic variations in the brightness and location of spectra as detected from Earth.
These variations, as well as measurements of light polarization -- the orientation of a light wave's electric and magnetic fields -- signaled to Bohlender and Landstreet that HD 3776 has a dipole field of 3,500 gauss and a whopping quadrupole field of 53,000 gauss. The star's total magnetic field strength varies from about 20,000 gauss at one magnetic pole to 60,000 gauss at the other -- both thousands of times stronger than the sun's mostly dipole field. Bohlender notes that stellar dipole magnetic fields may originate in fields that predate stars, compressing and strengthening under the force of gravity as star formation begins. But that model cannot explain the large and more complex quadrupole field the researchers observed. Instead, says Bohlender, the new finding may support the notion that some magnetic fields arise after a star forms, presumably from energetic material in the stellar interior.
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|Date:||Jan 27, 1990|
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