Newest Star of the Night

Every night, astronomers learn more about Hyakutake (hya-koo-TAH-kay), this year's stellar sensation. Here are the latest facts - and estimates - about what experts officially call Comet C/1996 B2.

Size: 5 to 10 miles in diameter. It rotates about 6 times per hour.

Tail: 62,000 miles long.

Speed: 93,000 miles per hour. It will accelerate to 175,000 m.p.h. as it loops around the sun.

Orbit: Once around the sun every 9,000 to 17,000 years.

Composition: Comets are popularly called ''dirty snowballs.'' Compounds detected so far in this comet include carbon monoxide, formaldehyde, hydroxyl ions, and hydrogen cyanide.

Closest encounter to Earth: 2 a.m. EST, March 25, at 9.3 million miles, or 40 times the distance from Earth to moon.

Closest encounter to sun: May 1 at a distance of about 21.4 million miles. From there it swings back toward the frigid fringes of the solar system.

Outstanding features: Brightest comet since comet West in 1976. Biggest comet to come this close to Earth since the Great Comet of 1556.

Discovered: Jan. 30 by an amateur Japanese astronomer, Yuji Hyakutake.

Updates: Harvard-Smithsonian Center for Astrophysics recording at (617) 496-STAR. Sky and Telescope magazine message at (617) 497-4168.

Precautionary measures: Russian specialists are taking steps to make sure the Mir space station isn't drawn off course by the passing comet.

What's next? The comet is expected to gradually fade until the first week of April as it moves away from Earth. But it may brighten again in early to mid-April as it nears the sun.

And after that? Though Hyakutake has delighted stargazers, next year's arrival of the larger Hale-Bopp comet could be even more amazing.

One Internet comment on the comet: ''WOW! WOW! WOW! WOW! WOW!''