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OBSERVING BLOG by Alan MacRobert
Comet Holmes Beckons Skygazers Worldwide
|This article will be continuously updated. At the top are the most recent observing reports. Scroll down to see past entries and reader comments. Feel free to add your own observations in the comments section. Also, check out where to spot Comet Holmes with our northern hemisphere finder charts, or make a personalized map with our Interactive Sky Chart.|
In late October amateur astronomers were amazed by the weirdest new object to appear in the sky in memory. And for a couple weeks it was one of the brightest, too. It's still visible with binoculars if you know exactly where to look.
On October 24th, periodic Comet Holmes (17P) brightened by nearly a million times overnight. For no apparent reason, it erupted from a very dim magnitude 17 to about magnitude 2½. Within a day its starlike nucleus had expanded into a perfectly round, bright little disk in binoculars and telescopes. It looked like no comet ever seen.
Its startling outburst, however, has a precedent. The comet was also in a major eruption 115 years ago, in November 1892, when English amateur Edwin Holmes was the first to spot it. It reached 4th or 5th magnitude, faded in the following weeks, and then underwent a second eruption 2½ months after the first. (Gary Kronk provides full details in his online history.)
This time Comet Holmes has outdone itself. I've outlined some of the comet's day-by-day evolution below; add your own impressions in the comments section at the end.
And check out (and add to!) the photos submitted by readers worldwide.
January 4, 2008: S&T's Tony Flanders writes, "I drove last night to my club’s observing site in Westford, near the edge of the Boston suburbs. It’s not a very dark location, and the sky was brighter than usual due to solid snow cover. Nonetheless, Comet Holmes was quite easy to locate naked-eye, appearing only a little more diffuse than the nearby Double Cluster. Through both my 10x30 and 15x70 binoculars, it was a vague ellipse about 60' by 45', slightly brighter toward the major axis, but otherwise featureless.
"I also picked up Comet Tuttle easily with my 10x30 binoculars, and Tuttle appeared quite bold through my 15x70s. It was a nearly circular blob getting continuously brighter toward the center; I could not see the pseudo-nucleus. I managed to see it intermittently without any optical aid, but only because I knew exactly where to look."
December 11th: The bright, glowing blob that many people saw last night and some mistook for a comet was a fuel dump from a US Atlas Centaur rocket that had just launched a spy satellite. Details.
The real Comet Holmes continues to enlarge, which means its surface brightness is decreasing and it's more easily wiped out by moonlight or light pollution. But if you have a dark sky, the comet's total brightness has remained constant at 3rd magnitude since mid-November! Light curve (scroll down).
December 8th: Tony Flanders comments, "The comet looked just like the photographs under a reasonably dark sky through my 15×70 binoculars and 4.5-inch f/4 scope. It has a sharply delineated leading edge and a fuzzy trailing edge, and was overwhemingly bright."
November 27th: Did I say a few days ago that Comet Holmes had become invisible to the naked eye? That was then; now it’s back. The difference isn’t in the comet. It’s that the Moon has just left the early-evening sky, allowing a dark view for the first time in nearly two weeks.
Around 7 p.m., I stepped out my front door in Boston’s moderately light-polluted outer suburbs, looked up, and there it was again a sizeable puffglow just above Alpha Persei (Mirfak). Not only is the moonlight gone, but now the comet is separated enough from Alpha Per that the star’s 2nd-magnitude glare doesn’t get in the way.
Binoculars actually gave a prettier view than my 12.5-inch reflector framing the big, parabola-shaped comet head (a good 0.7° wide, I estimated) next to the gorgeous starry field of the Alpha Persei Association. Inside the head was the familiar largish glow behind the (invisible) nucleus, and the broad “spine” that’s now extending behind the glow. The 12.5-inch at 75× showed these features hardly any better, so big are they now; the whole thing overfilled the telescope’s view.
Meanwhile, Tony Flanders was observing from a badly light-polluted inner suburb of Boston. He says:
“I got my best naked-eye view of Comet Holmes to date. When the comet first exploded in October, it was extremely bright but nearly stellar. Later on, it got tangled with Alpha Persei and the surrounding group of bright stars. And when the full Moon was nearby, I couldn’t see the comet at all without binoculars.
“But now it’s much easier to spot naked-eye than any deep-sky objects except the Pleiades. It’s big enough so that nobody could mistake it for a star, and it appears significantly brighter than the Andromeda Galaxy or the Double Cluster.
“My most detailed view was through my 70-mm refractor at 20×. The outer coma was pretty vague in the suburban skyglow, but I could trace it to a diameter of roughly 45′. Inside that was a brighter ellipse about 10′ by 25′. The southeast end of the ellipse was quite intense, and it faded out gradually to the northwest.”
November 23rd: Through full moonlight late this evening, Holmes was very large and diffuse through a 10x50 finderscope: just a vague glow in the edge of the Alpha Persei Association. Through the 12.5-inch scope at 75x it was actually harder to see, it was so big. What I was seeing in both cases was the condensation and "spine" that has been showing up in images. No sign of the nucleus.
Enough of moonlight! On Monday the 26th a window of dark sky opens up again between the end of twilight and moonrise (depending on the latitude where you live). This dark window lasts only about a half hour on the 26th (if you're near latitude 40° north) but grows about an hour longer each night after that.
November 22nd: The comet continues to enlarge and fade. It's now moving away from Alpha Persei night by night. The Moon is full on the nights of the 23rd and 24th; this is when the moonlight interference will be worst.
November 18th: No question about it, the comet has lost a lot of its surface brightness in the last few days as it continues to enlarge. In addition it's passing very close to 2nd-magnitude Alpha Persei this week, and the star is bright enough to interfere with the big puffball's visibility. Nor does the increasing moonlight help.
This evening, for the first time, I couldn't see it with the naked eye from my suburban site even with averted vision. Even with binoculars (10x50's) it took a moment to recognize it, as a large, diffuse glow next to Alpha Per and the lovely star-scattering of the Alpha Persei Association.
November 14th: The comet is still as bright as ever, and it has now grown to at least 30 arcminutes as wide as the Sun or the full Moon. The surface brightness is decreasing as the light spreads out over a larger area, but the comet is still visible through binoculars and to the naked eye from typical suburban locations. For the best possible views, try looking after the Moon has set. That's easy to do, as the comet is well above the horizon all night long at mid-northern latitudes.
Comet Holmes is getting rapidly closer to Mirfak (Alpha Persei). At its closest approach, on November 19th, this star should actually appear to be inside the comet's glow.
November 5th: Still the comet remains as bright as ever to the naked eye, though its average surface brightness continues to drop as it enlarges. As of last night it was 14 arcminutes wide, or nearly ¼° half the apparent diameter of the Moon. During this dark-of-the-Moon period (which will end around November 15th), seize whatever chance the weather allows to show family, friends, and strangers something memorable. This is how new amateur astronomers are born.
Deep exposures now show lots of blue gas streamers forming a wide fan of a tail, as in this extraordinary image by Michael Jaeger and Gerald Rhemann. The tail is even being seen visually in large amateur scopes. At last, this thing is starting to look like a comet rather than some kind of lens flare, or a planetary nebula whose evolution was speeded up several million times.
Friday Nov. 2: Many imagers have been recording fine dust trails in the coma, brought out by image processing to heighten contrast and emphasize detail. From Pic du Midi Observatory in the French Pyrenees, Francois Colas writes: "Here is an animation on two days showing the evolution of dust stream in the inner coma. Here you can see details moving, so there are real. It is clear that all the trails are coming from pieces of the comet ejected at the begining the the bigger event on October 24th."
Wednesday, Oct. 31: If anything, the comet appeared a bit brighter last night: magnitude 2.6 or 2.7 by my naked-eye estimate. As readers have noted, when comparing a hazy comet to pinpoint stars your magnitude estimate will depend on the darkness of your sky. I'm looking through suburban light pollution, which hides some of the light from the outer coma but doesn't affect stars as much. Some people with good dark skies are reporting Holmes at magnitudes 2.1 to 2.4.
Both the round disk of the coma and the bright inner patch offset to the southwest continue to enlarge. But some telesope users are reporting that the nucleus is looking smaller again, perhaps as material clears out from around it.
The coma diameter last night was 10 arcminutes. Its southwest edge continues to be a little vaguer than the northeast edge, and the nucleus remains a little offset from the center toward the northeast.
Wednesday, Oct. 31: No change in brightness last night I still get magnitude 2.8 though magnitude estimates are getting harder now because the comet is much larger and fuzzier than Alpha and Delta Persei. (Though in my case, I can blur them to equal sizes just by taking off my glasses.) In binoculars and telescopes the comet is losing its original sharp-edged disk appearance. It's starting to look more like how a small comet is supposed to look, with a bright inner coma and a dimmer outer coma the traditional clump-of-cotton appearance. The inner coma is still offset to the southwest, getting larger and also more prominent by comparison to the rest. The nucleus near center is getting fuzzier. All still yellow.
Last night Clay Sherrod measured the coma as 545 arcseconds (9.1 arcminutes) wide.
Tuesday, Oct. 30: Still no fading last night. Which makes sense; the dust isn't going to disappear, and even if no more is being produced, what's there now will continue to reflect the same total amount of sunlight even as its surface brightness dims as it spreads. Only when the surface brightness of the coma (or parts of it) becomes comparable to the surface brightness of the sky will the comet become harder to see. And we're about to enter two weeks of moonless dark evenings.
As for now, the naked-eye disk appearance is only making the comet more obvious and easier to spot. It's not like any old star any more! Why not set up your scope in front of the house on Halloween evening for an impromptu star party?
Tony Hoffman of New York City posted to the CometChasing mailing list, "The past two nights I've had the pleasure of observing Comet Holmes witih the naked eye from my Queens neighborhood (one of only 3 comets I've observed with the naked eye from NYC in close to 30 years of living here, the other two being Hale-Bopp and Hyakutake)."
Last night the bright coma was about 8 arcminutes wide, according to a measurement by Clay Sherrod. Its diameter has been growing by 1.3 arcminutes per day. At Paris Observatory, F. Colas and J. Lecacheux that the bright coma has been expanding at a constant rate of 575 meters per second from the nucleus.
Monday, Oct. 29: Last night the comet was very plainly a little disk to the naked eye rather than a star. Definitely a more interesting naked-eye view now! No change in brightness in the last 24 hours. In fact, the average of all good magnitude estimates has basically stayed flat since the outburst. (Light curve.)
Finally, the bright round disk is starting to show some asymmetry. In the 12.5-inch scope at 75x, the disk's northeast edge the side away from the bright fan near the nucleus is looking more compressed and sharp-edged than the southwest edge, which is a bit vaguer. The slightly darker 'moat' inside the disk's edge is better defined on the northeast side too.
The fan southwest of the nucleus seems more diffuse now. The nucleus itself is fuzzier too, no longer so starlike. This was easy to judge tonight because an actual orange star, roughly as bright as the nucleus, was shining right through the disk!
Even in the waning-gibbous moonlight, the dim, diffuse, outermost glow of gas was a little more than twice the diameter of the bright disk of dust. All this was around 12:25 a.m. EDT (4:25 UT) October 29th, right after the Sox won the World Series. I'm sure I wasn't the only one running back and forth between the telescope and the baseball game just like the last time the Sox won the Series, in 2004 in the middle of a total lunar eclipse.
Sunday, Oct. 28: By last night the comet had enlarged to 6.3 arcminutes across, as measured by Sherrod on his CCD images. "It's beginning to look a bit nonstellar to the naked eye," writes Alan MacRobert. "Not as crisp and sparkly as Alpha and Delta Persei."
Within a few more days the nonstellar appearance ought to be obvious, making this object more of an attraction for the non-astronomical public. Hold a star party, and alert your local news media!
"When I first saw it, I thought it looked like a frosted incandescent light
bulb on a dimmer switch," commented Mike Foreman of Carrollton, Texas, on the Comets Mailing List.
Using Alpha and Delta Per as comparisons (magnitudes 1.9 and 3.0), I estimated the comet to be magnitude 2.8, just a trace fainter (more similar to Delta) than two nights ago. In stabilized 10x50 binoculars it looked just the same as before only bigger, with all its details easier to see.
And is Comet Holmes starting to grow a tail? Several visual observers and imagers report the subtlest traces of one. For instance, at right is a deep image by S&T's Sean Walker on Sunday night.
Saturday, Oct. 27: Last night the comet was as bright as ever; various people's estimates averaged magnitude 2.5 early in the night (time zones of the Americas). Its overall form in a telescope has not changed, just enlarged a lot (animation by Wah). Writes P. Clay Sherrod, who took the image at right: "Even more incredible each night. The comet now measures 255 arcseconds across (via CCD
direct measure). On October 25, similar measures of the coma revealed diameter of approximately 121 arcseconds." View the comparison.
Friday, Oct. 26: Last night I wrote in my logbook: "Omigod. Through thin clouds lit by the full Moon I had to guess where Perseus was, but I swept around with 10x50 binoculars from my front step, and wham, there was the comet! It's sure isn't starlike now, at least not in the 10x binocs (with homemade image stabilization). It's a very sizable bright fuzz spot, perfectly round, with a large, brilliant, hazy nucleus and a very sharp edge to the circular coma. It's yellow with just a hint of green.
"When the clouds finally cleared and I could see it with the naked eye, it was still starlike to my vision. Magnitude 2.7, based on Alpha Persei being mag. 1.9 and Delta Persei mag. 3.0. You just look up and there it is. It's the brightest 'star' in Perseus after Alpha Per and Algol."
Later that night, I checked it out again: "Used the 12.5-inch reflector at 75x, 110x, and 180x. A brilliant, starlike, white nucleus is dead center in the perfectly round coma. What looked like the nucleus in the binoculars is an inner coma or broad fan offset from the nucleus toward the southwest. At these magnifications the bright round disk is no longer perfectly sharp-edged, but still pretty nearly so. It also has a slight but definite ring appearance, as if some of the light is coming from a hollow, spherical, glowing shell. Farther out beyond this is a much dimmer round glow with about twice the diameter of the bright disk. Only out this far does the brilliant skylight of the perigean full Moon in this part of the sky begin to matter. This was at 1 a.m. EDT (5:00 UT) October 26th with the comet (and Moon) near the zenith. Still magnitude 2.7 naked-eye."
Thursday, Oct. 25: "This object is amazing!" posted Brian Cudnik of Houston, Texas, on the Yahoo CometChasing group after coming in from his telescope on the evening of the 24th. "I have just observed it with an 8-inch f/10 Cassegrain, boosting the power up to 163x then to 508x.... The bright inner coma seems displaced off-center toward position angle 315°. The inner coma opens up into a fan toward position angle 300°, and I have noticed one ripple, akin to the hoods/ripples seen in Comet Hale-Bopp ten years ago. The coma is uniform in brightness, aside from this fan-shape material emanating from the central condensation, and has a well-defined edge." He measured the coma to be 69 arcseconds wide using using the drift method. "The entire object has a nice yellow-white color; no sign of any tail. The apparent magnitude is +2.8 (estimated using Alpha Per at +1.9 and the other two bright stars adjacent to it at +3.0 each) and has remained rather steady all evening."
Posted Dan Laszlo of Fort Collins, Colorado: "In an 18-inch Newtonian at 90x, the yellow orb is like a bright spherical planetary nebula. Diameter of the bright portion is about equal to the lunar crater Tycho, so magnification helps. I can detect a very faint spherical outer envelope, about equal in radius to the diameter of the bright portion, tough with the moonlight."
From Florian Boyd, Palm Springs, California: "I think this is about the most amazing thing I've ever seen in the sky!"
The Outburst. The first person to notice something happening, according to IAU Circular 8886 (issued October 24th by the Central Bureau for Astronomical Telegrams) was A. Henriquez Santana at Tenerife, Canary Islands, shortly after midnight on the morning of the 24th local time. The comet was then about 8th magnitude, but within minutes Ramon Naves and colleagues in Barcelona, Spain, caught it at magnitude 7.3.
Internet discussion groups came alive with the news. "To my amazement, 17P had brightened to naked-eye visibility," exclaimed Bob King when he spotted Comet Holmes shortly before dawn in Duluth, Minnesota. "What a sight!" he posted to the Comets Mailing List. Alan Hale of Cloudcroft, New Mexico, concurred. To Hale (well-known codiscoverer of Comet Hale-Bopp) it appeared essentially starlike in a telescope until he switched to high power.
Things only got better. As Earth continued to turn, nightfall arrived in Japan. "It is visible with naked eyes in a large city!" posted Seiichi Yoshida, who observed the comet from beside Tsurumi River in Yokohama. By 17:15 Universal Time he was describing Comet Holmes as magnitude 2.8.
Since then the comet has remained constant at just about that total brightness shining as the third-brightest "star" in Perseus while enlarging substantially. It's still a round disk with a bright, offset core in binoculars and telescopes.
Comet expert Gary Kronk expects Holmes to remain bright and continue enlarging in the coming days, as it makes its way slowly westward across Perseus. Its position on October 30th was 3h 48m, +50.4°; by November 11th it will have moved only a little, to 3h 34m, +50.6° (0hUT). (Ephemeris of its future motion.) On November 1920 the comet will pass closely by Alpha Persei (by 1/3°). For skywatchers in the Northern Hemisphere, Perseus is visible all night around this time of year. The comet will stay in Perseus all the way into next March.
Future prospects. The comet is likely to stay visible to the naked eye until at least mid-November, when evening moonlight returns. The yellow color is dust reflecting sunlight, as confirmed by spectra. Dust is what keeps a comet bright, and it hangs around as opposed to gas (comet gas is green and blue), which blows away more quickly in the solar wind.
The gas tail will probably remain short and wide, due to our perspective on it. The tail is pointing nearly away from us in space we're looking down its length since the comet is nearly on the opposite side of Earth from the Sun. From the comet's viewpoint, the Earth and Sun are only 15° apart, and this "phase angle" will stay small for many months. Which means we'll keep looking down the tail.
Posted by Alan MacRobert, October 27, 2007