Primordial Light: COMETS
Latest: Comet 103P/Hartley on 7 and 8 October, 2010
Exploring our own back yard.
The comets pictured here would look nothing like these photographs if we could observe them from nearby. They would not be neat round balls and they would be very dark, perhaps nearly as black as soot. The late astronomer Fred Whipple famously described comets as “dirty snowballs” as he hypothesized that they consist of a conglomeration of water ice and very dark, apparently fine-grained rocky material. Whipple was proven correct when we finally got a close-up look at a comet from a spacecraft. In fact, what we are seeing when we “see” a comet is the coma, a cloud of gas and dust that has been boiled off the nucleus—the dirty snowball itself—by sunlight, and that surrounds the nucleus before being blown away by the solar wind, the stream of particles emitted by the Sun. If we’re lucky we also see a tail (or two; see bottom photos) formed from the gas and dust. Although the tail seems to be following the comet through space, it will be on the side of the comet opposite the Sun because it is being shaped and moved by the solar wind. Comets are named after their discoverers. Here is a stunning photograph of the spectacular 2007 Comet McNaught made by its discoverer, Dr. Robert H. McNaught, an Australian astrophysicist. Unfortunately for those of us who live north of the Equator, Comet Mcnaught was seen only poorly from northern latitudes.
Comet C/2007 N3 Lulin on the Morning of 30 January, 2009

Many comets (more correctly, their comas and tails) are green due to the presence of cyanogen gas (CN) and C2, or diatomic carbon. Both of these molecules emit green-blue light when subjected to sunlight. CN is a toxic gas, and some scientifically illiterate people (but not you and me) believe that if the Earth were to pass through the tail of a comet we would all be poisoned. Not so. The Earth passes through comet tails very often, yet we’re alive. It is our atmosphere that protects us from comet tails and cosmic rays. C2 is found not only in comet tails; it is the compound that emits blue light in candle flames.

Thanks to my cyberbuddy, retired chemist Perry Holcomb, for reminding me to mention CN and C2 in this caption.

I would like to say that I timed this photo of Comet Lulin so as to capture the distant spiral galaxy NGC 5890, and the asteroid Proserpina, in close proximity—but I did no such thing. The presence of the other objects is pure serendipity and I found them while inspecting the photo. (Place your cursor over the photo to identify these objects and some representative stellar magnitudes.) The broken blue line represents the orientation but not the location of the ecliptic plane. At the time of the photograph both the comet and asteroid Proserpina were moving parallel to the ecliptic. If we were to photograph a sufficient number of asteroids and comets at intervals to plot their movement we would note that most asteroids orbit in the plane of the ecliptic, while comets are less restricted, as their orbits are determined by random events, some of them relatively recent in the history of our Solar System, and not by the gravitational constraints that were in place at the time of the formation of our Solar System and that gave our Solar System the appearance it has today. It is the motion of asteroids against the background stars that leads to their discovery. Comets may also be discovered in this way if they appear star-like with little or no coma. Proserpina was discovered in 1853 by the German astronomer Karl Theodor Robert Luther. The orientation of this photo is naked-eye and we are looking at the constellation Libra.

This is a combination of 10 two-minute exposures made between 1024 GMT and 1045 GMT (0524-0545 EST) on 30 January, 2009. The comet nucleus is elongated because I guided on the stars and the comet's motion over 20 minutes was recorded. I used high-contrast processing to reveal the ghost of a tail and an anti-tail extending from the 2:00 and 8:00 positions.
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Lulin
Another C/2007 N3 Lulin Photo Showing the Blue Ion Tale at Top and the Dusty Anti-Tail at Bottom.
Here the comet nucleus is round and the stars trail because I guided this exposure on the comet rather than on the stars.
Canon 40D, Epsilon 180ED.
Comet 17P/Holmes and the star Mirfak on November 13, 2007
This photo was made through moderately dense ground fog in an otherwise good sky. The fog may be seen around Mirfak. A rotational gradient applied in Photoshop causes stars that are visible through the coma to be portrayed unrealistically, but emphasizes the jets coming from the nucleus. The star field outside the coma is accurately portrayed.
Comet 17P/Holmes was as obscure as a known comet can be until October 24, 2007. Within a few hours on that day Comet Holmes became as much as a million times brighter than its typical 17th magnitude. It brightened to at least magnitude 3, which made it an easy object for the unaided eye, even in many urban areas where little can be seen in the night sky. The presumed cause of the brightening is a sudden release of a large quantity of gas and/or dust that reflects sunlight back to the Earth. No tail was evident in early photographs.
 
A coma-within-a-coma is seen in this photo. The fuzzy spot in the center of this December 17, 2007, image is 17P/Holmes’ pseudo nucleus. It is so-named because the real nucleus—the “dirty snowball”—cannot be seen in a telescope from Earth, but the dense cloud of dust an ice particles surrounding it is visible.
Comet 2006 M4/SWAN on October 26, 2006
Comet 73P Schwassmann-Wachmann (Fragment C—I think) on April 29, 2006
I annotated this image after I performed a bit of astrometry—the measurement of star positions and magnitudes—using the Unix astronomy software XEphem on my Mac because I was curious to know the visual magnitude of the dimmest stars in the image. I captured one star of magnitude 19.19. That is nearly a million times dimmer than the brightest stars that are visible in our night sky. High-contrast processing revealed what may be even dimmer stars, but the magnitude 19.19 star is the dimmest that I can confirm. None of the stars in this image is bright enough to be seen with the naked eye.
 
Comet Hale-Bopp, 1997
  
Comet Hyakutake, March, 1996
One of the most striking features of the comets above is the presence of two tails. Most comets exhibit two tails. One (the blue one in the Hale-Bopp image) is the ion tail —a stream of gas emitted by the comet whose atoms have been stripped of an electron by the solar wind. The other tail is the dust tail.
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