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Comet Hale-Bopp's Light Curve


Comet Hale-Bopp's Light Curve

Mark Kidger

Comet Hale-Bopp's light curve suggests that things may be happening as the comet approaches perihelion:

Updated light curves of Comet Hale-Bopp are being placed in the IAC's Comet Hale-Bopp Web pages ( effectively on a daily basis. These light curves can be found at

within the amateur contributions menu.

The light curves are being produced using data from two sources: observations from "The Astronomer" supplied by Guy Hurst ( and observations from the ICQ/IAU Recent Comet Magnitudes page in the Web ( As of February 12th the plots have over 400 observations in them since March 1996. Guy Hurst's group is gearing-up to collaborate with the big European monitoring effort on Comet Hale-Bopp, making their observations available on a daily basis.

The observations show that the comet's behaviour has been very steady indeed since October, obeying a brightening law similar to the average for dynamically fairly new comets (ie: ones, like Hale-Bopp, which have orbited the Sun a few times, but still have periods of thousands of years). Over the last 10 days, however, there seems to have been a sudden increase in brightness of the comet, just at the time when many comets actually show a significant decrease in their rate of brightening. For some comets the drop in the rate of brightening at this distance can be very large: even for Comet Halley it was more than a factor of 2 and for Comet Austin it was about a factor of 4, explaining why the comet was eventually so disappointing. Comet Hyakutake did something similar, but at a rather closer distance to the Sun, hence it was spectaucular when it passed by the Earth, but a disappointing object near perihelion.

Where the observations have previously neatly straddled the line fitted to the light curve, which would make Hale-Bopp a slightly negative magnitude object around perihelion, the latest data is now well above the line. This may be a random fluctuation (such small deviations have been seen previously), or may indicate that the comet will actually get significantly brighter than expected. It is still possible that the comet will "slow down" as others have done, or we might just be lucky and see a very bright object indeed.

Over the next few weeks the rate of brightening of the comet should drop off as its distance from the Earth stabilises and the comet approaches perihelion slowly (from here to the end of March the comet will only get about 20% closer to the Sun). Within two weeks we will see which of the different possible scenarios turns out to be the correct one.

Mark Kidger, February 12th 1997 comethome.gif Comet Hale-Bopp Home Page