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Sky & Telescope News Bulletin - April 5, 1996



Now that the Moon is waning, views of Comet Hyakutake (C/1996 B2) should improve this week. The comet is still a decent naked-eye sight in the northwest after evening twilight. Observers report a total magnitude near 3, with a slightly broadened tail several degrees long. The comet has lost much of the distinct blue cast it had in late March, as abundant dust has made its way into the coma and tail. This week it will pass near the well-known variable star Algol in the constellation Perseus.

Meanwhile, professional astronomers have already reaped a scientific bonanza from this comet. On March 24th and 25th JPL radio astronomer Steven Ostro used NASA's 70-meter Goldstone tracking antenna to bounce 480-kilowatt radio signals off Hyakutake. From the resulting echo, Ostro's team derived a diameter of only 1 to 3 kilometers for the comet's nucleus. In comparison, the nucleus of Comet Halley measures 15 by 8 km. On March 26th and 27th the orbiting Rosat observatory created a sensation when it recorded a strong signature of X-rays coming from the sunward side of Comet Hyakutake's coma. No radiation of this type has ever been detected from a comet before, and astronomers are rather at a loss to explain it. One theory suggests that water molecules in the coma are somehow able to absorb and reradiate solar X-rays. Another idea is that the radiation results from the violent collision of the cometary gases with the solar wind.


* COMET HYAKUTAKE THIS WEEK. Although the comet has faded a bit -- temporarily, we hope -- it should still be visible to the naked eye if you know where to look, especially if you have a dark sky free of light pollution.

This week, look west as soon as twilight fades out and spot brilliant Venus. Sighting past your fist at arm's length, look for the comet roughly two fist-widths to Venus's right. The 2nd-magnitude star near the comet this week is Algol. Binoculars give the best view!

Comet Hyakutake is flying away from Earth toward the Sun. Next week it should start brightening again as it draws nearer to the Sun's intense light and heat. By the end of the month the comet may rebrighten to put on another fine show, very low in the northwest right at the end of twilight. Then it swings around behind the Sun (perihelion is on May 1st) and finally heads far south, out of view for Northern Hemisphere observers.


* Some basic constellation lore: Look east around 9 or 10 p.m. this week for the bright star Arcturus, pale yellow-orange, the brightest in the area. High at its upper left is the Big Dipper, tilting on its handle. The Dipper's handle points in an "arc to Arcturus"... a scene many comet watchers will always associate with their views of Hyakutake there in late March.

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