With the arrival of Comet C/2011 L4 PANSTARRS and Comet C/2012 S1 ISON about to occur during the year 2013, observers around the world are anxious to learn "how bright" this solar system pair may eventually become. While astronomers can only make estimates for any given date at this time, now is a great opportunity to learn how to judge a comet's brightness - also called magnitude - for yourself!
A comet is made up of four distinct sections: the nucleus, the coma, the dust tail and the ion tail. Not all of these parts may be visible at any given observing session, yet all four play an important role in how the comet can be observed. The brighter the comet becomes - it requires less optical equipment to be observed. When it is very distant, only the coma of a comet is visible? the nucleus is far too small to be seen and the whole structure isn't close enough to the Sun yet to have developed a tail. During this time comets are barely visible and usually only seen in CCD images. However, if you continue to follow a comet, changes will occur!
As a comet nears Earth the nucleus will reveal itself and the coma around it will become larger. It can be seen with telescopic aid and will appear much like an unresolved, fuzzy globular cluster or a small elliptical galaxy. The nucleus may be sharp and bright - or just a concentration. As a comet approaches the Sun and begins to sublimate, the tail will also appear - rocky debris creates the dust tail and frozen gases comprise the ion tail. When you can see a comet with ease, you can start to determine its relative brightness. For formal measurements, try using the Harvard Minor Planet Center information or a planetarium program. However, comets are notoriously fickle creatures and an estimated magnitude on a program could be subject to a real life major change in just a matter of hours! Here's where a little learning and fun come into play...
Once you have located and identified your comet target, try heading off to a nearby star for which you have a known magnitude. You can find this information on a printed star chart as a graduated circular scale which appears along with the given magnitudes - or the information may be located on a planetarium program. Once you have selected a sample star, go to the eyepiece, identify it and defocus it. While an "out of focus" star won't be as soft in appearance as the comet's coma, it will still give you a great suggestion of brightness! Use this same method to judge the magnitude of the comet's tail section(s) as well. However, when it comes to the comet's nucleus, it is best to leave your test star in focus to compare.
While these estimations won't replace genuine scientific measurements, it is great fun to experiment and a great way to teach yourself better observing techniques!