by John Hlynialuk
With the current heat wave holding Ontario in its grip, I am reluctant to talk about particles burning up in our atmosphere and adding more heat to the situation. But, rest assured, the extra energy is only going to make for an interesting night of viewing and not raise our local temperature one iota.
I am of course, talking about the annual Perseid meteor shower. These meteors (the more correct name for shooting stars), appear to stream from the head of the constellation Perseus for the same reason that falling snow appears to come from a point in the road ahead of your moving car. Every August, the Earth passes into a stream of particles (most snow-flake size and smaller) left behind by a comet called Swift-Tuttle.
The shower, already underway because the comet debris is rather wide, can be seen from July 17 to Aug 24, but the main peak of activity falls on the nights around Aug 11 to Aug 13. During the darkest hours after the moon sets (12:36 am Aug 11, 1:12 am Aug 12 and 1:52 am Aug 13) you can see shooting stars fall out of the sky at a rate of 90 per hour or more. This number includes all visible meteors over the entire celestial hemisphere so it is not likely that a single observer will see this many, but expect one per minute on average on those nights. Observing in a group of 4 or so, each covering one-quarter of the sky, is a fun way to get close to the maximum number. In addition, some recent predictions indicate that there may be a burst of higher activity during the night that may raise the number to twice that many.
Over a dozen bright Perseids appear on this composite image taken Aug 12, 2013 by John H.
Perseid meteors hit the atmosphere at 60 km/s, or over 200,000 km per hour and are among the fastest travelling meteoroids, -the proper name before they enter the atmosphere. Most other meteor showers’ particles move much slower (if 30 km/s is slow!). In any case, the energy carried by the particles (remember kinetic energy from physics class?) is mostly converted into light when they encounter the thin upper atmosphere which even at 80 to 120 km is still thick enough to act like a brick wall to meteoroids. The rapid deceleration creates a shock wave that heats the particle to incandescence, producing the glowing trail of light we see from the ground. Some Perseids are a bit more massive than average (maybe the size/mass of a grape) and those meteors may leave a glowing train that persists for several seconds. Some of these trains have been seen with binoculars to persist for several minutes. So bring binos if you have a pair.
The diehard meteor watchers of BAS will be watching in the wee hours after midnight from Aug 11 to 13 from the Fox Observatory and the public is welcome to join us. But, any dark site is suitable even if only a part of the sky is visible. Lying out on a lawn chair watching celestial fireworks is a great way to spend time with family and friends and get a break from the daytime heat.
Shooting stars really are “cool”.