October 31st, 2007


Comet Holmes

All the rage in lay astronomy for the past week has been Comet Holmes. This tiny short-period comet has been known for over a century but is usually extraordinarily dim (around mag 14-18), a challenging object to find even with a telescope. Last Wednesday, however, it dramatically brightened, shooting from mag 17 to mag 2 in a matter of hours. Clearly some kind of explosion or violent outgassing had occurred very suddenly. Apparent magnitude is a log scale, with 5 magnitudes representing a factor of 100 in brightness. So a jump from 17 to 2 represents a brightening of a factor of about a million!

Mag 2 represents an easy naked-eye object, so while walking Ajay last night I casually looked up at the sky to see if I could find the comet in the constellation Perseus, which is where it appears in the sky. Much to my astonishment, it was absolutely visible even in a neighborhood with street lights, and quite obvious as a non-stellar object. Unlike the sharp points of the nearby stars, Holmes appears a muddy yellowish color, and has an obvious round smear or blur shape.

When I got home I went out again with a pair of field glasses to get a better look. These binoculars have pretty decent magnification but unfortunately only modest light amplification, so while I got to see the comet a little larger, little additional detail was visible. It looked like a round fuzzy brown ball, with a brighter area at the center. There is no visible cometary tail—since the comet is nearly opposite the sun with respect to the earth, any tail will be pointing almost directly away from us and won't be very visible.

It's still an amazing sight, though, and I've been encouraging people to go outside to have a look. Around 9pm, Perseus is about halfway up the eastern sky. Look for the bright guide star Capella in the constellation Auriga, and go just about straight up toward the zenith a bit until you see the comet. Alternatively, you can locate the really easy to spot Pleaides cluster and then turn to the north a little ways. I'm not sure how long it will stay this bright; no doubt whatever it released to intensify in brightness so dramatically is going to dissipate over time, but it should stay a naked-eye object for at least several days.

EDIT: There's a pretty good finder chart at this Sky and Telescope article. Perseus isn't a constellation a lot of people know, but the Pleaides and Cassiopeia are more familiar and make very good guides to locating the comet.