Comet Lulin Comet LulinComets are small bodies consisting of clumps of rock, ice and dust left over from the formation of the solar system, which are thought to originate from orbits far beyond even Pluto's in a region known as the Oort Cloud. Occasionally, interaction with a passing star, or perhaps between the "proto-comets" themselves, will cause one of them to have its orbit perturbed sufficiently to start it travelling towards the inner solar system. It may then be further perturbed by the planets (particularly Jupiter) and so end up in a much closer-in orbit, rather like an asteroid, and become a periodic comet. Comets' orbits can be much more extreme than those of asteroids, however, so they can come very near to the sun while also swinging out into the realms of the giant planets. Indeed, it is often the case that a comet will acquire too much speed for it to have a closed (i.e. elliptical) orbit at all. After rounding the sun it will escape the confines of the solar system entirely and disappear into deep space, never to be seen again.
The greatest difference between comets and asteroids is the fact that, due to their construction, as a comet approaches the sun solar heating will cause part of its outer layers to melt and evaporate, thus releasing large quantities of dust and gas which form a halo round the solid part of the comet. This is the reason that comets, though inherently very small, can often be seen easily from earth: the halo reflects much more light than the cometary nucleus alone. The dust trails behind the comet, forming its distinctive visual tail, and the gas is partly ionised by the sun's radiation to form a more tenuous "ion tail". The ion tail is always directed away from the sun due to the pressure of the "solar wind", so can appear to point in the opposite direction from the dust tail when the comet is viewed from the perspective of an observer on earth: it is then referred to as the "anti-tail".
Everyone will have heard of Halley's comet (which was last near earth in 1986) and in recent years Hyukatake (1996) and particularly Hale-Bopp (1997) were dramatic objects in the night sky. Very few comets are that bright though, or visible for as long: most are naked-eye objects just when close to the sun and thus only observable at dawn or dusk, when they are very near the horizon. The spectacular images published when a comet is around are always long time-exposures with large telescopes - even through binoculars, very few are actually more than a smudge of light when viewed by eye!
Very occasionally, a comet may suddenly get brighter for no apparent reason and this may bring it to visibility when it is well-placed for viewing. At the end of October 2007 comet Holmes (no connection, unfortunately!) did just this, brightening by a factor of about half a million! Telescopic observation showed a rapidly expanding dust cloud around it, presumably caused by some sort of explosion. It seems this isn't the first time it's behaved like this: it was also unusually bright when it was discovered in 1892. I didn't get to hear about it until after the initial outburst but, given the name of this comet, I was very keen to image it - this section documents my results.
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| Comet Holmes (top left of centre) is here seen against the stars of the constellation of Perseus in early November 2007. The brightest star (near the top, just over 1/2 way across) is Alpha Perseii, often known as Mirfak. | An increase in magnification begins to show us the fuzzy nature of the comet, particularly noticeable compared to the points of light that are stars. The bright star towards bottom right is Delta Perseii, that somewhat above it is Psi Perseii. |
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| As we zoom in further, it becomes obvious that the comet has a dense bright core surrounded by a less bright halo. Note that the orientation here is slightly different from the images above (compare the pair of stars to the comet's right) | A final increase in magnification shows the structure well. The halo has been increasing in size since the original outburst and is about half the diameter of the full moon in this image, taken on 5th November - the day of closest approach to the earth. No cometary tail had developed at this time. |
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| Three days (left) and six days (right) later, it is clear that the comet has moved considerably. These images have the same orientation as those in the first row above but are centred slightly further up on each occasion, so Mirfak has re-appeared at the top of the second image. | |
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| This composite of images [matched for brightness] from 5th, 8th & 11th November, plus one from 13th, shows the comet's movement well. It also shows that the "core" of the comet becomes fainter as time goes on while the halo gets slightly larger (see below). | |
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| A highly expanded composite image of two shots of the comet on 5th November (converted to black&white) showing the bright core and less bright "halo". | A composite from 8th November - it is clear that the core has dimmed while the halo has expanded. I'm glad to say this is a real effect, supported by professional pictures, not just an artefact of my digital processing! |
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| A further composite, this time from 11th November shows the core getting ever dimmer and the halo continuing to grow. | By the time we get to 13th November the halo has expanded to fill the frame. Note the two stars shining through the halo. |
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| A less-expanded, and enhanced, view from 11th November (left-hand image) shows some (slight!) evidence of a cometary tail - note the 'fish-tail' asymmetry of the halo to the right. The tail would be difficult to see, however, as (very unusually for a naked-eye comet) the earth was almost exactly between the sun and the comet - any tail there might be would thus be pointing directly away from us and hence be masked by the comet itself. However, the long-exposure image on the right (13th November) clearly has a less well-defined right margin than left indicating that this is definitely the direction any tail would emanate from. | |
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| By 13th November the overall brightness of the comet had decreased to such an extent that images taken at the previous exposure did not show much structure (see left-hand image). I thus changed to taking very long exposures (30sec, at 100ASA equivalent) which caused the stars to "streak" but captured good detail in the comet (though probably somewhat affected by the long exposure). The right-hand image is a composite of views taken on 13th to 16th inclusive and shows the decrease in brightness well, together with the continuing increase in overall size. | |
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 | After several dismal days of cloud and rain I finally got my next glimpses on 21st, 23rd and 26th November (top left, top right and bottom left images respectively). The comet had dimmed and expanded so much it was hard to see even in binoculars - these images are enhanced. Even by the 21st the halo had grown to over 35minutes of arc - wider than the full moon, in fact - while the core became less and less noticeable. Conversely, by the 26th there was definite evidence of a tail (to lower right). These images also show why the comet became known as "The Jellyfish". |
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| Following yet more bad weather, these are the final images I captured, on 3rd and 5th December. By this time the comet was huge but so diffuse it was effectively invisible to the digital camera even at its maximum exposure: these images are enormously enhanced (as can be seen by their poor quality, especially the last one). The tail had continued to grow, however, with the halo itself reaching 45minutes of arc (11/2 times the diameter of the full moon). After this, I occasionally observed the comet with the naked eye and in binoculars but for all practical purposes for me the fun was over. Just another 115yrs to go before the next outburst!! | |
This section deals with a much more "normal" comet - Lulin. While comet Holmes is periodic, Lulin is not and so its appearance in 2009 is the only time it will be seen. It was considerably less bright than Holmes (no explosions!) so imaging it was much more difficult. Weeks of cloudy skies didn't help, either! I did manage to see it in binoculars several times and image it on a couple of consecutive days though, so was pleased not to have missed it completely.
Although I described Lulin as "normal" above, it was unusual in one respect in that it moved across the sky extremely rapidly. A number of factors came together to cause this. Firstly, Lulin was relatively close to the earth - just 38 million miles on 24th Feb. 2009. Secondly, it was near to the earth only just after its point of closest approach to the sun, when it would have been moving most quickly in its orbit. Thirdly, its speed was inherently slightly greater than that of a periodic comet at a similar distance because its orbit is hyperbolic not elliptical. Fourthly, the time of closest approach was quite near the date when the earth was also closest to the sun, and thus moving quickly itself. Finally (and most crucially) Lulin's orbit is in the opposite direction to that of the earth: their relative speed is thus the sum of two speeds which are themselves almost maximised at this time. The result was a motion of almost 5 degrees per day at closest approach. Contrast this with comet Holmes, which when I observed it was four times as far away; five months after closest approach to the sun, and orbiting in the same direction as the earth.
 | This image is a composite of the two wide-angle shots I took to ensure I was looking in the right place, with rectangles overlaid to indicate the area of the actual comet images shown below (I've also added in grey blobs to represent the comet - it wasn't visible at this scale or exposure). The stars are all in the constellation of Leo (The Lion) and form the lion's left-front paw. The brightest star in the constellation (Regulus - the lion's heart) is out of the frame, a short distance away to bottom left. There is only 1 day between the two images but in this time the comet has moved about 41/2 degrees - nine times the width of the full moon! |
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| And here we have the comet itself, on 28th February and 1st March. Each image is a composite of several individual frames, some at different exposure durations (thus accounting for the odd shape of the star trail in the first image!). The brightest stars are 23 Leonis (magnitude 6.43) and 7 Leonis (magnitude 6.31) respectively i.e. nominally less bright than the comet itself (magnitude 6.1). However, because of the diffuse nature of the comet and the very small size of its "core", it appears much dimmer than them. The colour-speckled backgrounds are simply due to camera noise at the sensitivity and exposure necessary to capture the comet, accentuated by the considerable contrast enhancement needed to see it in the image at all. The greenish colour of the comet "halo" is a real effect, however, caused by the presence of certain chemical compounds which glow this colour when excited by radiation from the sun. | |
| I said above that Lulin moved very rapidly across the sky. I didn't realise just how rapidly until I started to "stack" the various frames to construct the composite for 1st March above. While some frames aligned nicely, others didn't: even after allowing for shifts in position due to the movement of the stars caused by the rotation of the earth, the comet simply wasn't in the "right" place. I eventually realised that 4.5 degrees per day equates to about 5.6 minutes of arc per half hour: quite significant if your field-of-view is only 60 minutes of arc! (i.e. 1 degree). The image to the right is a composite of images taken half-an-hour apart (a stack of two in each case), showing that the comet did indeed move really rather a lot! Calibrated against the distance between the two fainter stars in the image (just over 6 arc-minutes), the shift is exactly as expected from the above calculation. A veritable celestial greyhound! |  |
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