Twelve years ago today, Comet Holmes (17P) brightened from magnitude 16.5 to 2.6, forming a right triangle with Mirfak (α Persei) and δ Persei, opposite to Algol. Here is what I wrote in The Sky This Week at that time.
Comet Holmes Bursts on the Scene!
Who ever said astronomy isn’t exciting? Sure, much of what we observe in the cosmos seems predictable and unchanging—but then something unexpected happens and we are scrambling to get a front-row seat and our lives are thrown into an exhilarating tizzy for a few hours or days. Whether it be an unexpected auroral display, a meteor fireball, a nova, supernova, or comet, the result is the same: it is exciting to be an astronomer, to be attuned to a universe that existed long before we were born and that will be here long after we are gone. That, to me, is comforting.
Very early Wednesday, October 24, a 16th-magnitude short-period comet presently in Perseus by the name of Holmes brightened about 14 magnitudes from 16.5 to 2.6 in little more than 12 hours: a brightness increase of 363,000 times! While such a cometary outburst was unexpected, it is not unprecedented. From time to time, solar heating (greatest when a comet is near perihelion) must cause pressure to build up inside a comet as subsurface ices volatilize. Eventually, the pressure builds up until it explodes through the surface of the comet, spewing gas and dust into space and exposing fresh material to solar radiation. Sometimes, this process is so violent that the comet breaks into multiple fragments.
Comet Holmes (17P) is one of the so-called “short period” comets, meaning it orbits the Sun in less than 200 years or has been observed at more than one perihelion passage. Comet Holmes orbits the Sun every 6.9 years, ranging from just inside the main part of the asteroid belt (2.1 AU) to the orbit of Jupiter (5.2 AU). No doubt Comet Holmes’ original orbit has been substantially altered by the gravitational influence of Jupiter. Comet Holmes is presently 2.5 AU from the Sun (230 million miles) and 1.6 AU from the Earth (150 million miles), having just passed perihelion on May 4, 2007.
Comet Holmes was discovered during its last outburst, which occurred on November 6, 1892 by English amateur astronomer Edwin Holmes (1839-1919). It was observed again in 1906, but was then lost until being recovered in 1964. It has since been observed near perihelion at every return.
The recent outburst of Comet Holmes may be one for the record books. I am not aware of any other comet outburst being recorded where the comet brightened by as much as 14 magnitudes in less than a day! Fortunately, the first two nights after the outburst the sky was beautifully clear here. The first night, October 24, Comet Holmes looked like a star to the unaided eye. In binoculars, it looked like a tiny yellow or orange planetary nebula, only slightly bigger than a star, and of uniform brightness. The following night, October 25, it still looked like a star to the unaided eye, but in binoculars it was larger than the previous night. The total brightness had not diminished. In the telescope, the comet was truly spectacular, made all the more amazing considering how the comet was only 43° away from the closest full moon of the year! The round coma contained a bright off-center fan-shaped wedge with a brilliant tiny pointlike nucleus. There was definitely evidence of concentric, spiraling shells of material opening outward from the center of the coma to the outermost parts of the coma.
You have just got to get out to see this comet! And as often as possible! Here is an ephemeris for Dodgeville for the coming week.
Comet Holmes (17P)
Comet Holmes slowly moves towards Mirfak this week, an impressive binocular and telescopic object in Perseus. It is easily visible to the unaided eye, too, as a small fuzzball on the Capella-side of Perseus.
Sunlight and solar wind particles are hitting the comet on the north-northeast side, and photographs show the comet is sharp edged there. The opposite, south-southwest side is ragged, with ionized gas streamers spreading out in that direction in long-exposure photographs.
Whatever tail the comet has is pretty much hidden behind it, as our viewing angle (known as the phase angle) diminishes from 15° to 13° this week. The phase angle is the Sun – Comet – Earth angle. A phase angle of 0° would mean we are looking directly down the tail (least favorable, maximum foreshortening). A phase angle of 90° would mean we are looking perpendicular to the tail (most favorable, no foreshortening).
Prime time for observing the comet is pretty much all night, with the comet transiting the celestial meridian at 2:05 a.m. CDT at the beginning of the week, and at 12:30 a.m. CST by the end of the week. Look at it every clear night, because surprising changes can and do occur. Don’t miss it! It may be a while until something like this happens again. The last time Comet Holmes went into a major outburst was 115 years ago!