April 3, 1997
The light curve of the comet through perihelion reveals various conflicting tendencies in estimates. This comet has what must be the best light curve coverage ever for a comet around perihelion and, with the vast archive of data since discovery, is going to provide enormous quantities of information for future research. Here is a quick summary of the situation as of yesterday:
How bright did Hale-Bopp get?
The dispersion in magnitudes at perihelion was very large. The comet was large and extremely condensed, making estimation very tough. My own peak estimate was only -0.4, but that was in very poor conditions in England (limiting magnitude around 4.5). Good observers in very good sites estimated much brighter magnitudes on the whole. The estimate will depend a lot on how the comparison was made by individual observers (I ended up using Mars and Capella as my own comparison stars). This has a complication in that the comet, which is strongly blue, was a very different colour to the best comparison stars and this can exagerate the peak magnitude which was estimated (red stars appear rather fainter to the eye, so comparing with red stars makes the comet seem brighter than it really is).
The brightest individual estimate that I have seen was -1.6, but this is an isolated point. From good sites the peak magnitude seems to have been about magnitude -1.0+/-0.1 with the "average" peak magnitude being around -0.7.
How much brighter than Comet Hyakutake was Comet Hale-Bopp?
Another good question for the reasons given above. Hale-Bopp was definitely much brighter and will remain brighter for some days than Hyakutake was at maximum. Most observers estimated a peak magnitude of Hyakutake around +0.5 last year (which lasted only very briefly), hence Hale-Bopp appears to have been about 1.5 magnitudes (a factor of 4) brighter.
What about the comparison with Comet West?
Comet West reached about magnitude -3.5 very briefly around perihelion. However, it was not visible to the naked eye at this time (rather like Comet Kohoutek which also reached about magnitude -3, but was only observable to the naked-eye for the Skylab astronauts in Earth orbit). Comet West was only a negative magnitude object for a few days, while Hale-Bopp will remain negative magnitude for as much as 5 or 6 weeks in total. In this sense, Comet Hale-Bopp is a much brighter comet. The tail however was much more spectacular and better developed in Comet West.
How did the light curve develop in the end?
It seems that the linear brightening stopped about 3 weeks before perihelion. However, many of the observations very close to perihelion are consistent with an almost unchanged rate of brightening right through. The point where the light curve most obviously flattens out conicides with the Full Moon in late March, so those estimates are not necessarily the full brightness of the comet anyway. Even the faintest estimates (generally the ones by observers who estimated the magnitude in poorer skies), show the comet still increasing in brightness steadily THROUGH perihelion. This is a major surprise because, in theory, the comet should have peaked in late March and then started to decline noticeably by perihelion. At the moment it is a little hard to tell what happened to the light curve at perihelion and after because some of the observations are consistent with both a small outburst at perihelion, followed by a fade and others with a slow increase in brightness even after perihelion.
How does Hale-Bopp compare intrinsically with other comets of the past?
Very favourably indeed. If Comet Hale-Bopp had passed as close as Comet Hyakutake did last year it would have been around magnitude -7 and perhaps the second or third brightest comet since the 17th century! Assuming a maximum magnitude of -1.0, the absolute magnitude of Comet Hale-Bopp was -1.3. These are the ten very brightest recorded values:
1729 Comet Sarabat -3.0 1577 Comet Tycho -1.8 1997 Comet Hale-Bopp -1.3 1747 Comet De Cheseaux -0.5 1811 Comet Flaugergues 0.0 1744 Comet De Cheseaux +0.5 1882 Comet Cruls +0.8 1914 Comet Delavan +1.1 1433 Great Comet +1.2 1962 Comet Humason +1.35
For comparison, Comet Halley has an absolute magnitude of +4.0 and an "average" new comet is around +6.5. Comet Hale-Bopp is thus about 130 times brighter than Comet Halley at the same distance and 1300 times brighter than an "average" comet would have been.
How long is the tail?
The longest estimates at perihelion, from dark sites were 20 degrees. This works out at exactly 1.0AU, or 150 million miles. This is a much longer tail length than for Comet Hyakutake and about 3 times the true length of Comet Halley's tail. We can expect the tail to grow even longer over the next couple of weeks.
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