StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

AUGUST 1999

AUGUST STAR MAP | INDEX | MOON PHASES

153 AUGUST 1, 1999: Perseid "Rain": Here’s the perfect way to end the worry about the drought. Put together a couple of really dynamite astronomical events, get everyone excited about them, and then watch the skies darken. The sunrise solar eclipse that will be visible on August 11 was discussed in this column during the last two weeks. During the next two nights the Perseid meteor shower will be in full swing. These shooting stars, which can be as numerous as 50 per hour, seem to diverge from near the top of the constellation of Perseus. Perseids can be seen from late July until the 22nd of August, so they are not just a "one night stand." But they become really interesting from about August 11-14. So there will be plenty of opportunities to dampen the celestial fireworks by dumping buckets of precipitation on parched Valley fields. On the "slim" chance that it will be clear and dry on these evenings, here’s how to host your own meteor observing party. You’ll want to get comfortable, so lounge chairs, sleeping bags, air mattresses, pillows, and ground tarps are all part of a meteor observer’s repertoire. A thin plastic painter’s tarp pulled over your gear will help to keep the dew off. Munchies and a warm drink can also help to keep the spirits elevated. Meteors will become more evident after midnight, and be most abundant around dawn. Look northeast about halfway up in the sky until about 2 a.m. Thereafter, focus more upon the zenith. As shooting stars become evident, you’ll notice that most of them will seem to radiate from a particular area in the sky. Those will be the Perseids. Happy observing, but watch out for those clouds! It will probably be a rainy week.

154a AUGUST 8-10, 1999: August 11 Eclipse Wrap: The sunrise partial solar eclipse which will be visible from the coastal mid-Atlantic states and New England on Wednesday must be viewed with protective filters. Mylar Eclipsers can be purchased from Dan’s Camera City for $1.95/per unit, and are well worth the price. You will need a good eastern horizon, free from trees or other obstructions. Sunrise for Allentown is 6:08 a.m. At that time the moon will cover about 36 percent of the sun’s diameter. If it is hazy on Wednesday, and the eclipsed sun rises as a dull red globe, anyone seeing the sun will also see the eclipse. It will look as if a substantial cookie bite has been taken out of the bottom left portion of the sun. In case you’re traveling on Wednesday, here are some selected cities, the time of sunrise, and the percentage of the diameter of the sun covered by the moon: Philadelphia--6:09/32 percent Harrisburg--6:14/26 percent, Pittsburgh--6:25/7 percent, Richmond--6:22/6 percent, Norfolk--6:19/9 percent, Baltimore--6:15/21 percent, Washington, DC--6:19/14 percent, Dover--6:12/25 percent, New York City--6:02/46 percent, Hartford--5:54/61 percent, Providence 5:50/69 percent, Boston--5:48/73 percent, Concord, NH--5:46/77 percent, Portland, ME--5:41/85 percent, Bar Harbor, ME--5:30/91 percent, and Halifax, Nova Scotia--5:33/93 percent. In Bar Harbor and Halifax, maximum coverage occurs shortly after sunrise. The next solar eclipse visible from the Lehigh Valley occurs on December 25, 2000 a real Holiday treat. More information about Wednesday’s eclipse can be found at the web site below.

154b AUGUST 11-13, 1999: Why Meteors Radiate: With the solar eclipse now history, I’d like to add a few more thoughts about the Perseid meteor shower which will be in full swing during the next three nights. You can find a constellation map of Perseus and its environs, a general sky map for August, and a moon phase calendar by simply following the StarWatch links posted at the web site below. The distinguishing feature of any meteor shower is the condition that the shooting stars will seem to radiate away from a common point or localized region in the sky. If that area is between the tip of Perseus and the bottom of Cassiopeia (See the web map), you’re viewing Perseid meteors. Meteors are the result of very tiny particles, the debris from comets, which orbit the sun. Each time a comet comes around, it loses a part of itself. The cometary gases are ionized (become charged) and are blown out of the solar system by the sun’s magnetic field. But the dust remains, pretty much in the orbital path of the comet. If the Earth intersects or passes near the path of a comet, we see meteors, as the cometary dust slams into the atmosphere and makes the air glow. We never see the particle itself, just the path through which it has traversed. Since these meteoroids are essentially moving parallel to each other, it’s like looking down a long straight section of railroad track. The track seems to intersect at some vanishing point in the distance. Meteors do the same thing; but since they are coming towards us, they appear to diverge from the vanishing point or radiant. Since this explanation is really three dimensional in scope, we see meteors diverging from the radiant across the entire sky.

155 AUGUST 15, 1999: Brigtness vs. Distance: Part 1: The dog days of summer are upon us. Nights are often muggy and hazy, leaving few stars to be easily seen. However, there are three stars high overhead in the early summer hours which are almost always visible. In order of brightness, they are Vega, of Lyra, the harp; Altair, of Aquila the Eagle, and Deneb, of Cygnus, the Swan. They are collectively called the Great Summer Triangle. Talking about triangles in the sky may seem like a cop out, because almost any three stars will fit the bill. But this is a "great triangle," and its stars are indeed bright. In fact, on really hazy nights they may be the only three stars visible. Deneb is the 19th brightest star of the heavens, while Altair is the 11th, and Vega ranks as 5th brightest. They also illustrate an important concept with regards to distance and the true brightness of stars. Vega was the star from which Ellie Arroway discovered the first interstellar message in the "must see" movie Contact. It’s distance is about 25 light years, one of the closest stars to us, a mere 145 trillion miles. One might say that Vega is bright because of its closeness, and as a generalized statement, you could consider yourself correct. Taking that assumption a little farther, Altair might be considered to be a little more distant, since it is fainter than Vega, and Deneb, even more distant, but not too far away. SURPRISE! Altair is actually closer at about 17 light years, but Deneb is a staggering 3200 light years distant. This confusion will be resolved in next week’s column. For now, go to the web star map posted with this article to familiarize yourself with these three famous stars. Then go outside and view them in person.

156 AUGUST 22, 1999: Brigtness vs. Distance: Part 2: The details which led to our understanding of the evolution of stars occurred shortly after the first decade of this century. Ejnar Hertzsprung (Danish-1911) and Henry Norris Russell (American-1913) began to reveal what, up to that time, had been the secret lives of stars. The three bright luminaries of the Great Summer Triangle discussed last week: Vega (25 light years), Altair (17 ly), and Deneb (3200 ly) seemed to present no relationship among their distances and their brightnesses. Stars, just like people, really aren’t all that different. In the human experience people may look radically different, but they all function basically the same way. Their lifestyles, however, are very often influenced by their physical characteristics, such as weight. That’s what Hertzsprung and Russell found for the stars. All stars belong to the same family too. They are changing hydrogen into helium via nuclear fusion. Astronomers say that stars "burn" hydrogen, but be careful here. It’s a nuclear reaction, not a chemical one. The intensity of the reaction is a function of the temperature and pressures generated inside of a star. And that is a factor of a star’s mass or the quantity of material which it contains. "Heavier" stars are more luminous because their internal temperatures and pressures are higher, and hydrogen burning can proceed at a faster rate. A rough idea of what is happening inside of a star is revealed by that star’s color. In a star’s life, blue stands for hot, red for cool. So faraway Deneb is a powerhouse of energy with a mass of over 30 times that of our sun. It won’t be around for too long. Burn bright; die young. Hey, it sounds just like us!

157 AUGUST 29, 1999: A Wonderful Eclipse