Jul 23 1998
From The Space Library
NASA announced that the Hubble Space Telescope (HST) had found the youngest cluster of stars ever observed in a galaxy near the Milky Way. The baby stars were "enveloped in a cloud of luminous gases," in the galaxy known as the Small Magellanic Cloud, about 200,000 light-years from Earth. The HST had discovered the 50 new stars, each 300,000 times as bright as the Sun, "concentrated in a region only 10 light-years across in a cloud known as N81." Before the capture of the HST's high-resolution images, astronomers had referred to the area as The Blob.
NASA announced that engineers had repaired the malfunctioning data transmission systems aboard Galileo. Although both subsystems were "working redundantly, as they had been designed to do," it would be up to a week before the probe could resume transmitting scientific data to Earth. Project engineers believed that debris had short-circuited a signal line, causing computer resets.
Two competing teams using NASA's Rossi X-ray Timing Explorer (RXTE) discovered the first known accretion-powered millisecond pulsar. The scientists believed the pulsar to be the missing link between two known types of stars: 1) "old, accreting neutron stars, which are powerful sources of [x]-rays generated from the material they are gobbling up from their companions," and 2) even older "radiowave emitting pulsars that are rotating very rapidly and slowing down gradually." Because the new star, designated SAX J1808.4-3658, was both emitting x-rays and spinning rapidly, scientists believed it was the link between the accreting and the radiowave-emitting neutron stars. Michiel van der Klis and Rudy Wijnands of the University of Amsterdam found the new pulsar-called the "Holy Grail of X-ray astronomy"-and measured the time between its rapid x-ray pulses, to derive its rotation rate. Van der Klis explained that, based on the fact of accretion, astrophysicists had long theorized that millisecond pulsars existed, but this was the first time that one had been "caught in the act." Accretion, the process of drawing gas from a nearby "companion" star, causes pulsars to heat up, emitting x-rays. After accretion ends, high-velocity beams of subatomic particles continue to "blow material off the companion," eventually causing the companion to vanish. Deepto Chakrabarty and Edward H. Morgan of the Massachusetts Institute of Technology led the team that found the pulsar's 2-hour orbital period, measured the orbit, and inferred the presence of the companion star. Tod E. Strohmayer, a member of the RXTE team at NASA's Goddard Space Flight Center, suggested, "[x]-ray and particle beam ablation may explain why millisecond pulsars are often found alone, despite the fact that they required a companion star to speed up." Pulsars "vaporize" their companions, thus hiding the evidence, which is, as Strohmayer described it, the "stellar version of the perfect crime."
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