Comet in the June Dawn

Comet 2009 R1 (McNaught) on June 10, 2010
"I made this image on June 10 (UT 00.35) at a very dark place in the mountains," Michael Jaeger in Austria posts to "The comet was nearly 20° high. I used an 8-inch f/2.8 astrograph and CCD with a KAF 6303 chip."

The colors are real. Gas molecules of cyanogen (CN) and diatomic carbon (C2) in a comet's coma fluoresce green in sunlight. Ions of carbon monoxide (CO+) and carbon dioxide (CO2+) in the ion tail fluoresce blue. A comet's dust tail, on the other hand, simply reflects sunlight and is basically Sun-colored: pale yellow-white. This is clearly not a very dusty comet. Click image for larger view.

Update June 18: Still 5th magnitude. It seems to have stopped brightening a week ago.

Update June 11: Now that the Moon is out of the way, many people have sighted Comet C/2009 R1 (McNaught). Most report it as very bright, and observers with dark skies have spotted the long, thin ion trail even through small binoculars.

Update June 9: It's brightening! S&T associate editor Tony Flanders viewed Comet C/2009 R1 (McNaught) at 3:30 this morning through 12×60 binoculars from Boston's inner suburbs. The comet was well above the horizon before the sky got perceptibly lighter, and the thin crescent Moon provided no interference.

The comet was very easy to spot, forming an attractive tight triangle with the open cluster Messier 34 below and the star 14 Persei left of M34. All three were roughly equal in brightness. This puts the comet around magnitude 5.5, which means it should be visible naked-eye in a dark sky.

The comet's head was fairly small, no more than 10? across, and quite intense. No tail was visible. However, a very long, thin gas tail is showing up well in deep images — such as these gorgeous ones of the comet passing NGC 891 June 8th taken by Francois Kugel and by Michael Jaeger.

Article: We rarely see a good comet when it's at its best. Most comets are brightest when nearest the Sun — just when they’re most likely to be hidden in the Sun’s glare or below the sunrise or sunset horizon.

That's the situation this spring with Comet C/2009 R1 (McNaught). Even so, observers in the Northern Hemisphere should be able to pick it up with binoculars just before dawn in June, during its runup in brightness.

Comet C/2009 R1 (McNaught) on May 19, 2010
Back on May 10th the comet was much less impressive at 8th magnitude. Michael Jaeger shot this image too. Click image for larger view.

And in fact, the comet is turning out to be 1 or 2 magnitudes brighter that we predicted in the June Sky & Telescope (page 60). Let's hope this behavior keeps up!

Comet Timetable

As of mid-May the comet was about magnitude 8.5 (compared to the 10 we originally predicted), as it rose about an hour before the start of astronomical twilight for mid-northern observers. Throughout this apparition it will be low in the east or northeast when dawn begins to brighten.

May 31st will find McNaught, now hopefully 6th or 7th magnitude, passing 2½° southeast of the 2nd-magnitude star Beta Andromedae. At the beginning of astronomical twilight it’s a respectable 20° up as seen by observers at 40° north latitude. But the waning gibbous Moon will brighten the sky.

The June comet crosses Perseus, low just before dawn, when at its best in June. Click image for larger, printable chart.
Sky & Telescope
On the morning of June 5th the comet skims just north of the large, loose open cluster NGC 752. On June 6th and 7th it’s within about 2° of the 2nd-magnitude double star Gamma Andromedae. The Moon is much thinner then, but also closer to the comet.

Mid-June is when Comet McNaught should be most interesting, offering the best compromise between its increasing brightness and its decreasing altitude at the start of dawn. Moreover, the sky will be free of moonlight.

The helpful conjunctions continue as the comet passes about 1° north of the open cluster M34 in Perseus on the morning of June 10th, and 3° south of 1.8-magnitude Mirfak (Alpha Persei) on the 13th. It’s still about 15° high in the northeast as the sky starts to grow light on June 15th, but it appears roughly 1° lower every day after that. The comet passes zero-magnitude Capella on the 21st, and it’s very low by the 24th, when it passes 2nd-magnitude Beta Aurigae. By now Comet McNaught may be as bright as 4th magnitude, but moonlight is returning.

The comet will be lost to view by June’s end — just before it reaches perihelion on July 2nd, 0.405 astronomical unit from the Sun. It remains far from Earth throughout this apparition, never venturing closer than 1.135 a.u. (in mid-June). After perihelion it will fade rapidly as it heads to the far-southern sky.

The comet is approaching on a hyperbolic orbit, which means that it’s making its first trip in from the Oort Cloud. So its brightness is even less predictable than usual. Will it flare unexpectedly or perhaps fizzle out?

Print out our map, plan where and when to look (find when astronomical twilight begins at your location using our almanac; don't forget to check the Daylight Saving Time box if necessary), and see for yourself.

Check on pictures and light curve (scroll down). Here are recent magnitudes (scroll 40% of the way down).

More pictures.

Beautiful Astronomy of the Day image.

Extraordinary medium-field view June 6th by Michael Jaeger; the bright star is Gamma Andromedae.

Wide-field view by Melsky.

Many McNaughts

This particular Comet McNaught is one of 54 (and counting) named for Robert H. McNaught of Australia’s Siding Spring Observatory. He works in the Siding Spring Survey, funded by NASA to record large swaths of sky to find potentially hazardous near-Earth objects. The survey also turns up many other moving objects. McNaught found this comet (which will never come near Earth) at 17th magnitude on an image taken last September 9th. Pre-discovery images quickly established its orbit.

The most famous of the Comet McNaughts is C/2006 P1, also known as the Great Comet of 2007. It was an easy naked-eye sight when passing near the Sun in mid-January of that year, shining at magnitude –5 or –6, and in the following days it flung a gigantic, multi-banded tail across the Southern Hemisphere’s evening sky.


Greg Bryant is editor of Australian S&T.

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