by Jeff Wilkerson
In my youth, there was a sign, both annual and unmistakable, that summer's end was approaching. This omen foretold the demise of summer vacation and the beginning of the new school year. Let me pause to say that I didn't actually dislike school but I had a certain fondness for summer vacation. Many of my friends actually preferred the school year because they got to see everyone else and were bored over the summer. I was never all that anxious to see other people and rarely found myself bored as long as there were fish uncaught and baseball games yet unplayed. Anyway, I believe I was talking about that one day each year when I realized my summer freedom was about to disappear. It was the day set aside to buy new clothes for the school year. Surely, never a crueler day has been known. I would have been blithely churning my way through a beautiful summer, only vaguely aware that it must end, when my mother would yank me away for a day of shopping. I'd suddenly realize that I had to squeeze every drop of life out of the waning days of summer, so that each day became urgent. That the first of these days was wasted in stores was nearly unbearable. An unwritten rule (oh, it probably is written in some guidebook) stated that I had to be in attendance while my mother picked out what new stuff I would wear to school for the next year. I believe there was some hinted at notion that I was the one doing the selecting but I found I had little say in the matter. I would merely nod vigorously at each item hoping to speed the process along. Inevitably, one purchase was the back to school blue jeans. These torture devices were "ruggedized" and came with a guarantee that no matter how hard I tried I'd outgrow them before wearing them out. And I never did wear through any of them, probably due to the fact that for three months they were too stiff to allow any movement that was not Frankensteinesque. These tight, unyielding garments were the perfect apparel items for those back to school August afternoons in unairconditioned midwestern classrooms.
Nowadays, I've lost most such time markers, but I still hear the end of summer approaching when it's Perseid meteor shower time. In the July/August 1993 CfPA Newsletter I talked about the meteor shower in detail. That year we were hoping that we might have a meteor storm instead of a mere shower. No storm appeared and last year we again hoped that a storm might show but it didn't. This year we're not looking for as much activity, so we'll likely get it. The last few years of the shower have brought a new peak that lasts only a couple hours and appears well before the traditional peak. If that trend continues, we can look for a burst of activity just before dawn on August 12th. The evening of the 12th or the morning of the 13th should bring the traditional peak. These meteors are always fairly bright and plentiful, even in down years. This year's observing will be severely hampered by the moon which passes through full on the 10th. The bright moon will wash out most of the fainter meteors but it's still a good shower and worth the effort of observing.
Whenever we watch this meteor shower we can't help but be drawn to the Double Cluster and on towards the bright Cassiopeia, the Queen. (Click here to see figure of Cassiopeia.) I'll stop to remind you what a fine target the Double Cluster makes and that it has appeared in several previous columns (see for example, October 1990). It works as a naked eye object, a binocular target or in a telescope using low magnifications. The two clusters point the way to Cassiopeia who circles the pole. On September nights you can watch as the constellation turns from a great "M" to a great "W" and back to a great "M." In the September 1991 CfPA Newsletter I told you a little about Cassiopeia, the mythological queen of Ethiopia, and how she got into trouble with a bunch of sea nymphs. (Haven't we all had trouble with sea nymphs at one time or another? That's why I'm in favor of raising the bag limit on them from 2 to 5.) You might have noticed how effectively I've been referencing my previous work. I'll let you decide if that means I'm too lazy to write something new or I'm trying to pad number of citations. Note also that the January 1992 column had nothing to do with Cassiopeia. (Ding! Citations = 4. Wow, I must do good work. Everybody's talking about it.)
Cassiopeia is a bright, well known constellation that has been watched since antiquity. In Star Names Their Meaning and Lore, Richard Allen points out that the constellation represented the queen bound in her chair to keep her from falling out when upside down over the pole. He also adds that before the constellation became the Queen it was known in Greece as the Laconian Key due to its key-like appearance. Early Arabs knew these stars as a large henna-stained hand with the bright stars as the fingers. Even earlier, the stars represented one of a pair of dogs with the stars of Cepheus as the other. In early Chinese astronomy the stars represented a porch way and later became Wang Liang, a charioteer circa 470 B.C.
We might as well know Cassiopeia as the land of open star clusters. The beautiful Double Cluster lies at her back while two Messier clusters, M52 and M103, fall within the constellation's boundaries. M52 offers the more pleasant observing experience of the two and lies near the Cepheus border across Cassiopeia from the Double Cluster. M52 is attractive in binoculars and holds up well in a telescope. It's hard to be as enthusiastic about M103. This group is distant enough and tightly enough packed to stay together in a telescope view. However, the group only contains 30 or 40 fairly dim stars. The result is a decent but unspectacular observing experience. If you are using a telescope, NGC 7789 and NGC 457 might be the best targets in the constellation. NGC 457 is a dense patch containing about 100 stars and NGC 7789 holds more than 1000 stars. At a distance of nearly 10,000 light years, NGC 7789 remains compact enough to fit all the stars into one field of view. In fact, you need something larger than my old 50 mm refractor to do a good job of resolving the cluster into stars, making it one of the few relatively bright open clusters that you might mistake for a globular in binoculars or a small scope.
Cassiopeia is also home to two of the sky's more interesting supernova remnants--Cas A and Tycho's remnant. These objects have absolutely nothing to offer the amateur astronomer or casual observer but are intensely studied in the scientific community. Tycho's supernova is so called because Tycho Brahe studied it in November of 1572. It is one of only four supernovae in our galaxy known to have been observed. A supernova is the explosion that marks the end of a large star's life. Sort of like the shopping trip marked the end of summer, but a little more violent. Tycho's star got as bright as Venus. At the relatively small distance of 10,000 to 15,000 light years and such a young age, the Tycho supernova remnant remains a virtual laboratory for scientific study. You might recall what a stir supernova 1987A caused. It provided a wealth of information but was about 20 times further away than Tycho's supernova, making it harder to study.
Remarkably, Cassiopeia contains another supernova remnant that is as close as Tycho's and even younger. Cas A is the youngest known supernova remnant in the Milky Way. This supernova was not seen when it occurred and the remnant was discovered as a source of radio waves in 1958. We can tell that the explosion must have occurred around the year 1620. It is somewhat surprising that the event was not recorded when it happened although the remnant lies in a dusty part of the galaxy that would have obscured the supernova's light somewhat. It has long been argued that this dimming would have made the supernova not readily noticed. Interestingly, recent gamma-ray observations led to the idea that maybe the supernova should have been bright enough to see. Supernova explosions are expected to produce radioactive titanium which will decay, giving a gamma-ray signature. This gamma-ray signal decreases with time as the titanium decays away. The younger and closer the supernova remnant is, the bigger the expected gamma ray signal. Astronomers have searched Cas A diligently for this signal, since the remnant is so young and nearby. In 1994 results from the Comptel instrument aboard the Compton Gamma Ray Observatory showed the first detection of these titanium gamma rays from Cas A (Iyadun et al., Astronomy and Astrophysics, V. 284 L1). The detection was somewhat marginal (4s if you care). Anyway, the size of this titanium signal should be related to how bright the supernova was, if we understand supernovae. The Comptel detection implied that the supernova probably should have been seen unless (1) people had a harder time seeing in 1620; (2) there is much more dust obscuration than we believed toward the supernova; (3) we don't know quite as much as we thought about supernovae; (4) the observation is in error. There might be other reasons. To make matters interesting, the OSSE instrument aboard the same satellite as Comptel also observed Cas A. In May 1995 OSSE reported seeing no evidence of the gamma rays at a slightly lower threshold than Comptel (L.-S. The et al., Astrophysical Journal, V444 N1:244). That is to say OSSE put an upper limit on the amount of titanium that is slightly lower than the amount Comptel claims to have detected. Stay tuned.
I'd love to tell you more about what's going on in the sky right now but I've gotta get outta these new jeans. They're killing me.