Comet Hale-Boop at its peak

Will we be dazzled or disappointed by this much-heralded cosmic visitor?

The comet will be at its best from late March through mid-April (see last month's issue, page 37). As comets go, Hale-Bopp's nucleus is huge - it's estimated to be 10 to 40 kilometers wide - and extraordinarily active. With an absolute magnitude of -1, the comet is intrinsically the brightest visitor to pass inside the Earth's orbit since the great comet seen by Tycho Brahe in 1577.

However, Hale-Bopp will remain quite far from our home planet - coming no closer to us than 1.315 astronomical units (197 million kilometers) on March 22nd. Its visual appearance may thus present some interesting contrasts with that of Comet Hyakutake, C/1996 B2, one year ago, which put on a spectacular naked-eye display for observers with dark skies because it passed us by only a tenth of the Earth's distance from the Sun. In any case, Comet Hale-Bopp should be a fine sight easily visible to the unaided eye as you read this.

In the first half of March, early morning risers should find the comet fairly high and bright in the east-northeast just before break of dawn. It is also beginning to show itself low in the northwest in evening twilight. By March 20th the best viewing shifts from the morning to the evening sky, with the ever-brightening comet climbing higher in the northwest after dusk. (For observers north of 50' north latitude, Hale-Bopp will be a circumpolar object, remaining above the horizon all night until early April.) Unfortunately, the Moon also grows brighter in the evening during the middle of March.

Observers get a brief respite from moonlight on the night of March 23-24. That night the full Moon undergoes a deep (92 percent) partial eclipse visible throughout most of the Americas and western Europe; see last month's issue, page 82. Comet watchers in wertern North America, especially those in the Pacific Northwest, will get a bonus: the eclipse will cleanse the sky of most lunar glare from about 8:15 to 9:00 p.m. Pacific Standard Time -squarely in the prime comet-wiewing window. Mars, at magnitude -1.2, is also visible 12¡ above or to the upper right of the eclipsed Moon. The folowing evening, Hale Bopp glides less than 5¡ north of the Andromeda Galaxy, M31.

The climax of Comet Hale-Bopp's performance should extend through the moonlight-free period from March 26th to about April 11 th. The comet should then be highest in the north-west after dusk and also its brightesst. Depending on how vigorously water ice sublimates off its nucleus, the comet's total brihgtness could peak anywhere from magnitude 0 to -2.

Hale-Bopp, which is spewing more dust than Hyakutake did, should also display a more luminous tail. The gossamer fan could become 20¡ to 25¡ long, though 10¡ to 15¡ is more likely. The tail will probably grow to more than 1 a.u. across space in April and May, but it may appear to shrink from foreshortening caused by Earth's viewing angle. This could result in a shorter but very bright and spectacular dust tail.

According to solar physicist Patrick S. Mclntosh (HelioSynoptics), "It appears that the sunspot minimum is over and that the rise of the new sunspot cycle may accelerate early in 1997. Disturbances to the interplanetary environment may rise to a significant level by April. If so, observers may see some added attractions in their watch of Comet Hale-Bopp. Comet-tail disconnections are associated with discontinuities in the solar wind."

The comet reaches perihelion at 3:13 Universal Time on April lst (10:13 p.m. Eastern Standard Time on March 31st) at a distance of 0.914 a.u. (137 million km) from the Sun. The comet will then be traveling at a speed of about 44 km (27 miles) per second relative to the Sun.

On April 6-7 Hale-Bopp passes within a degree of the 5th-magnitude open star cluster M34 in Perseus. By April 14th the first-quarter Moon again becomes a problem. Five days later the comet passes between NGC 1499, the large but extremely dim California Nebula in Perseus, and the Pleiades cluster in Taurus. By the end of April it will be fading and getting lower in the westnorthwest, but another moonlight-free viewing window opens from April 24th to about May llth.

As always, the darker your sky is, the better. Artificial light pollution is the nemesis of comet viewing, so try to get as far away as possible from any glary local lights and give your eyes plenty of time to adapt to the dark. For most people, light pollution will be by far the most important factor determining whether the comet is spectacular or barely detectable.

EARLY WINTER SIGHTINGS

Some of the first observations of ale-Bopp this year - on New Year's ay - were made by Timo Karhula om Virsbo, Sweden, and Bjorn H. Granslo from Malm, Norway. Such high orthern latitudes allowed them to spot he comet's 3rd-magnitude glow deep ithin the twilight.

The Earth passed through the orbital lane of the comet on January 2nd. Nine days later, astrophotographer Bob Yefl observing from the Mojave Desert in California, reports that an obvious Sun ward-pointing glow could be seen or Schmidt-camera images he took before dawn. This almost 2-long antitail ap peared broad and diffuse "It is easily detectable in all of my black. and-white and color negatives, even on a 165-mm telephoto shot!" says Yen. The feature, which was offset slightly fron the main tail, reminded him of the an gled antitails of Comet Bradfield in 1988 and Comet Arend-Roland in 1957.

On January 13th contributing editor Stephen James O'Meara saw Hale-Bopp from Volcano, Hawaii, without opticd aid, this time as a greenish fuzzy star al 6 a.m. local time. "I trained my 4-inch refractor on it and at 74x I saw some nice jet structure and a knot in a north. ward-pointing jet," says O'Meara.

By January 31st estimates of Hale. Bopp's magnitude were running at around 2.5 to 2.7. Contributing editor John E. Bortle notes that through 2Ox8O binoculars the comet's head appeared in. tensely condensed with a starlike pseudo. nucleus and a curved dust tail that ex. tended 2.1' to the northwest. "The ejected material from the comet's nucle. us displayed a striking clockwise spiral pattern, its root rising from the pseudonucleus well ahead of the comet. Sun line," says Bortle. "The extreme tail curvature far exceeds what I have ever seen in a major comet, including Comel Mrkos in 1957. This suggests that near perihelion the naked-eye dust tail will dramatically lag the comet-Sun line, giving Hale-Bopp a rather peculiar look."

AMATEUR PROJECTS

This fine comet should provide many opportunities for useful visual, photographic, and CCD observations. Taking pictures of Hale-Bopp can be as simple as a long-exposure shot with a tripod-mounted camera. Or you might consider a nightly series of images to record the changing appearance of the comet and its tail. Large-scale tail structure and tail-disconnection events can be captured with telephoto lenses or Schmidt cameras.

A careful watch at higher power with large-aperture telescopes is needed to note the appearance of bright hoods, jets, knots, and other features in the coma. Because these phenomena can develop and change within a few hours, nearly continuous monitoring is required. Observers at sparsely populated longitudes are especially needed to fill time gaps.

Particularly desirable are systematic estimates of the comet's total brightness (see below). This can be done with binoculars or with the naked eye when the comet is sufficiently bright. Measure the diameter of the comet's head, or coma, from night to night, and determine the angular length and position angle of the tail by plotting on a star atlas.

Visual observers can send their Hale-Bopp observations to Daniel W E. Green, editor of the International Comet Quarterly (ICQ) at the address given at the end of the article. Published by the Smithsonian Astrophysical Observatory, the ICQ is a journal devoted to corn news and observations and is available b subscription. Before contributing you reports, check the ICQ Web site first fo the proper format and procedure to us when making, recording, and submittin observations. As always, the editors o Sky & Telescope welcome copies of you Hale-Bopp reports and images. You ca send them to P.O. Box 9111, Belmon MA 02178. e-mail: comet@skypub.com.

PROFESSIONAL PROGRAMS

Last year researchers began instituting worldwide Comet Hale-Bopp Days for coordinated observations. They have chosen the period from early March to mid-April as the prime window for observing programs that include astrometry, photometric and spectroscopic monitoring, and imaging. (The Hubble Space Telescope will not he able to view Hale-Bopp until late August, five months after its closest approach to Earth, because the comet will be positioned too near the Sun.) For details visit the Hale-Bopp Observer's Bulletin Board and Archives at http://pdssbn.astro.umd.edu/hbhtml/.

The Ulysses Comet Watch Network established in 1992, is a joint professional-amateur program being carried out in conjunction with studies of the Sun by the Ulysses spacecraft (S&T.. November 1990, page 496). According to Martin A. Snow and John C. Brandt (University of Colorado), the program's goal is to "gather comet images suitable for correlation with Ulysses's on-the-spot measurements of the solar wind, particularly when the spacecraft was at high solar latitudes. If successful, comets could be used as calibrators of high-latitude solar winds."

More than 250 observers worldwide are participating in this endeavor, which is patterned after the International Halley Watch in 1985-86. To join, contact Snow (snowm@lasp.colorado.edu) or Brandt (brandt@orion.colorado.edu), or the network at LASP CB 392, University of Colorado, Boulder, CO 80309; phone: 303-492-3744; World Wide Web: http:// lasp.colorado.edu/ucw/ucw.html.

COMET FEVER

By late May Hale-Bopp becomes too close to our line of sight to the Sun for good viewing. And the great comet show will be over for the Northern Hemisphere. As spring gives way to summer the comet will become a Southern Hemisphere object, fading and shrinking as it rapidly recedes from the Sun and Earth. Telescope users in farsouthern latitudes should still be able to follow the comet throughout 1998.

"Whatever performance Hale-Bopp puts on, enjoy it," reminds Morris. "When bright comets come frequently, we tend to forget what rare events they are. It is possible that our next bright comet may be many years in the future."

Estimating a Comet's Brightness

Just how bright is Comet Hale-Bopp these nights? Comets are notorious for not following predictions, but even judging the magnitude of a bright comet that's right in front of you is not straightforward.

Stellar magnitude estimates made by long-time variablestar observers often agree to within 0.1 or 0.2 magnitude. These observers are comparing stars with stars. But a comet's coma or head may be anywhere from a few arcminutes to a degree or more in size. Because comets appear radically different from the pointlike stars used for brightness comparisons, determining a comet's integrated (total) magnitude is far more difficult.

For centuries the reported magnitudes of naked-eye comets were very ambiguous. Often they seem to refer to the brightness of the intense nuclear condensation - the strong, sometimes starlike feature seen at the heart of the coma. As such, the total brightness of the comet's head was usually underestimated. Not until the turn of the century were satisfactory visual methods developed for determining the brightness of extended objects.

As with variable stars, ascertaining a comet's brightness requires two comparison stars of known magnitude - one slightly brighter than the comet and the other slightly fainter. It helps greatly if they are all in the same field of view and at a similar altitude above the horizon to avoid errors caused by atmospheric extinction.

Listed below are five widely recognized methods used by amateurs to estimate a comet's integrated brightness. Each has its faults, but all (except perhaps the lastone) will give acceptably accurate results if carefully employed. In order of common usage, they are:

Sidgwick's method. This is the most widely used procedure, popularized by J. B. Sidgwick within the British Astronomical Association in the 1950s. Here the observer carefully memorizes the brightness and size of the in-focus comet. The instrument is then racked out of focus until the images of the comparison stars have the same diameter as that of the in-focus coma. The observer then judges the recalled brightness of the comet relative to the defocused stars. Typically, you need several tries before arriving at a definite brightness value. This method works very well for diffuse comets. However, it is difficult to apply to those that appear strongly condensed (concentrated toward the center), as Hale-Bopp probably will be. Defocused stars look "flat," while the brightness of an in-focus comet changes markedly from the pseudonucleus outward.

Bobrovnikoft's method. This is usually credited to Nicholas T. Bobrovnikoff but apparently was devised decades earlier. 'Ihe comet and comparison stars are defocused simultaneously to such a large extrafocal diameter that they can be compared directly with one another. In the case of a bright naked-eye comet, eyeglass wearers can often simply remove their glasses to create the desired effect. Bobrovnikoff's procedure is certainly the easiest to master. It also works best for highly condensed objects like Hale-Bopp, since it smoothes out the coma's steep brightness gradient. Very diffuse objects, on the other hand, can be significantly underestimated in brightness using this method.

Beyer's method. Devised by Max Beyer, one of the foremost amateur astronomers of the 20th century, this method is similar to Bobrovnikoff's but takes the extrafocal procedure to a more extreme level. To work effectively, the comet's head must be defocused to many times its in-focus diameter. The instrument is racked out of focus until the comet and stars begin to disappear into the sky background. Their order of disappearance is then noted. If a given star disappears before the comet does, it must necessarily be fainter, and vice versa. Estimating disappearance increments between the stars and comet as you turn the focuser allows you to obtain a magnitude value. Beyer's method works best for highly condensed objects of fairly small diameter but is unsuited to very diffuse comets.

Morris's method. Independently formulated by Charles Morris and Stephen James O'Meara in the early 1970s, this procedure was developed to bridge the perceived gap be. tween the Sidgwick and Bobrovnikoff methods when the coma appears moderately condensed. The comet is pu@ slightly out of focus, just enough to "flatten" the brightness profile and make it easier to determine the comet's average surface brightness. This brightness is then memorized, as well as the out-of-focus diameter. The comparison stars are then defocused to the same out-of-focus diameter. Some ob. servers consider this procedure more difficult to master than the others.

The in-focus method was used for centuries. With the un. aided eye one simply attempts to compare the brightness of the comet with the surrounding stars, all in focus. As noted earlier, unless the coma is extremely compact and starlike, this will underestimate its brightness. However, if done in conjunction with one of the other, more appropriate meth. ods, it will provide a magnitude value roughly comparable to those obtained for pre-20th-century comets. Ibus it helps calibrate the true brightnesses of earlier, historically interesting objects.

As readers can imagine, the visual measurement oi cometary brightness has a highly convoluted history. For i most interesting and detailed account of its development. see the article by Green in the October 1996 Internationd Comet Quarterly. An explanation on how to correct magni. tude estimates for atmospheric extinction and how to esti. mate the coma's degree of condensation are given in the July 1992 and July 1995 ICQ, respectively.

For Hale-Bopp's latest developments, check Sky & Telescope's Weekly News Bulletin and Comet Page in SKY Online at http://wwwskypub. corn, or listen to the Skyline voice recording at 617-497-4168. Other sources of information on Hale-Bopp and comets in general are:

* Intemational Comet Quarterl ' v and Comet Handbook. Contact Daniel W E. Green, Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138; c mail: green@ cfa.harvard.edu; World Wide Web: http://cfa-www.harvard.edu/cfa/ps/ icq.html.

* IAU Circulars. Contact the Central Bureau for Astronomical Telegrams, 60 Garden St., Cambridge, MA 02138; iausubs@cfa.harvard.edu; http:// c f a - w w w. h a r v a r d. e d u / c f a / p s services/CBATServices.html.

* Comet Observation Home Page, Charles S. Morris, Jet Propulsion Laboratory; csm@encke.jpl.nasa.gov; http://encke.jpl.nasa.gov/.

* ESO Hale-Bopp Home Page, Richard M. West, European Southern Observatory; rwest@eso.org; http:// www.eso.org/comet-halc-bopp/comet hale-bopp.html.

* Comet Comments, Don Machholz, PO. Box 1716, Colfax, CA 95713; DonM353259@aol.com; http://wwwlpl. arizona.edu/-rhill/alpo/cometnws.html.

* The Shallow Sky Bulletin. Contact Stephen M. Smith, Comet Rapid Announcement Service, PO. Box 110282, Cleveland, OH 44111; 72527.364@ compuserve.com.