The M.A.S. Newsletter

Journal of the Mauritius Astronomical Society

September 2000

The members of the Society who were present at the MRT for last month's visit would like to thank Mr Bhupendar Misra for his kindness and clear explanations.

The Next Meeting:

The next meeting will be due Friday 29^th of September 19:30 at Collège du Saint Esprit. This will be a general observation night in the rich Scorpius - Sagittarius region.

The sky this month:

Both the planets Mercury and Venus are now in the constellation of Virgo. On the 20^th, Mercury and Venus are about 5° from each other above the western horizon and set at 19:36 and 20:01 respectively, while Saturn and Jupiter are still in the constellation of Taurus and rising at 22:32 and 23:19 respectively. Mars and a crescent moon are in conjunction at dawn on the 25^th. There are a few planetary conjunctions this month, find out more at: http://space.jpl.nasa.gov

News:

A large new radio telescope in West Virginia, the Robert C. Byrd Telescope, performed its "first light" observation on August 22, tuned at 403 MHz. The telescope, the largest fully-steerable radio telescope in the world, features a unique design.  Rather than a circular collecting dish, the telescope uses an asymmetric surface that measures 100´ 110 m² and is made of 2004 individually controlled panels. Also, unlike a conventional radio telescope which mounts its receiver on a set of supports in the middle of the telescope, the new Green Bank telescope uses an off-axis receiver, mounted on one side of the dish. This keeps more of the telescope's collecting surface unblocked and reduces reflection and diffraction of radio signals off supports that makes it more difficult to analyse data. Astronomers hope that the new telescope's large collecting area will provide key data needed to tackle a number of key astronomical issues, including studies of the age and expansion rate of the universe, pulsar and black hole observations, examinations of the composition of interstellar gas and dust clouds. The $74.5-million telescope funded by the National Science Foundation, was built to replace a 91-metre telescope at Green Bank which collapsed in 1988.

A newly-released set of HST images is giving astronomers new information about the later stages of the lives of Sun-like stars. The new images, released by NASA and ESA, show the creation of a planetary nebula in the later stages of the life of a Sun-like star, and also reveal how older stars can masquerade as younger versions of themselves. One set of images from Hubble shows CRL 618, a planetary nebula in the making. Despite their name, planetary nebulae are not related to planets themselves but are clouds of material ejected by Sun-like stars near the ends of their lives. They are so named because they appeared like planets to early astronomers. CRL 618 is of particular interest to astronomers as it gives them an opportunity to see a nebula in the making, after the red giant has sloughed off most of its material but before a hot stellar wind emitted by the star heats up the gas and pushes it away. This particular phase of development is thought to last only a few hundred to a thousand years, a tiny fraction of the star's life. Current theories predict that such nebulae should form simple, spherical shells, but observations reveal a wide range of complex but symmetric forms. Astronomers believe a series of jets of material from the star, as seen in Hubble images of CRL 618, influence the nebula's final shape. "We speculate that the protruding jets observed in the Hubble images might be the cause of the more intricate structure seen in more evolved planetary nebulae," said A. Tielens of Kapteyn Institute in the Netherlands. "Maybe the jets are rotating, or precessing about the central star and 'draw' structures in the gas. The star seems to be using these jets to carve out the butterfly-like structures from its cocoon, which are so prominent during the later planetary nebula phase," he said. "If so, we might see pieces of the larger puzzle coming together, greatly helped by Hubble's high resolution." Another object is giving astronomers new insights into the later stages of the lives of stars.   He2-90 was first classified as a young, dust-enshrouded star, emitting jets of materials. Astronomers later reclassified it as a much older planetary nebula. New Hubble images, though, indicate it may in fact be a pair of older stars. "We think that He2-90 is a binary star," said R. Sahai of the JPL.  "Both stars in the system are old, dying stars, one of them being a red giant and the other being a white dwarf." Astronomers believe that the red giant of the pair is shedding material, which then forms an accretion disk around the white dwarf. "We believe that the gravitational interaction of these two stars produces the jets that we see," added Sahai. An object other than a white dwarf may form half of the binary system, though. A survey by the Compton Gamma Ray Observatory turned up a gamma-ray emitter in the vicinity of He2-90, opening the possibility that a neutron star or a black hole, rather than a white dwarf, may coexist with the red giant. Followup observations are planned to pinpoint the gamma-ray source and see if it is associated with He2-90.

Serge Florens, Secretary

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