The Fundamentals of Stargazing Sky Tours South
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The Fundamentals of Stargazing Sky Tours South 01 – The March Sky Copyright © 2014-2016 Mintaka Publishing Inc. www.CosmicPursuits.com -2- The Constellation Orion Let’s begin the tours of the deep-southern sky with the most famous and unmistakable constellation in the heavens, Orion, which will serve as a guide for other bright constellations in the southern late-summer sky. Head outdoors around 8 or 9 p.m. on an evening in early March, and turn towards the north. If you can’t find north, you can ask someone else, or get a small inexpensive compass, or use the GPS in your smartphone or tablet. But you need to face at least generally northward before you can proceed. You will also need a good unobstructed view of the sky in the north, so you may need to get away from structures and trees and so on. The bright stars of the constellation Orion (in this map, south is up and east is to the right) And bring a pair of binoculars if you have them, though they are not necessary for this tour. Fundamentals of Stargazing -3- Now that you’re facing north with a good view of a clear sky, make a 1/8th of a turn to your left. Now you are facing northwest, more or less. Turn your gaze upward about halfway to the point directly overhead. Look for three bright stars in a tidy line. They span a patch of sky about as wide as your three middle fingers held at arm’s length. This is the “belt” of the constellation Orion. In the northern hemisphere, where Orion was first conceived by the ancient Greeks and other classical cultures, these stars depict a hunter standing upright adorned with a belt and sword and holding a westward facing shield in his left hand and an upraised club in his right. In the southern hemisphere, the constellation appears upside down as if he is doing a cartwheel. Note: The descriptions in this tour are accurate for 35 degrees south latitude. If you live north of this latitude, Orion will appear slightly farther above the horizon. If you live south of this latitude, the stars appear lower in the sky. For southern-hemisphere observers, well below Orion’s Belt closer to the horizon, lie two brighter stars. The brighter star to the right is clearly orange-red. This is the star Betelgeuse (“BAY-tell-jewz”). It marks one of the shoulders of Orion. The left star is whitish-blue and goes by the name of Bellatrix. Above the belt lie two more bright stars marking Orion’s feet. The brilliant blue star to the west is Rigel (“RYE-jel”). The eastern star is called Saiph (“safe”). Take a moment to marvel at this splendid constellation. Glittering like a gigantic tapestry of celestial jewels, the constellation holds a number of the brightest stars in the sky in close proximity to each other. Many of these stars formed at the same time from the vast and invisible mass of gas and dust in this part of the sky. As you will see shortly, new stars are being formed here even now. The bright red-orange star marking Orion’s shoulder, called Betelgeuse, is a massive “red supergiant” star that’s burned through most of its fuel and is nearing the end of its life. It will explode, sometime in the next million years or so, as a supernova that shines so bright it will cast shadows by night and be visible in our daytime for several weeks. At present, the core of the star has shrunk and grown hotter, which in turn has caused the outer layers of the star to expand. So Betelgeuse is immense: if it were to replace our Sun at the center of our solar system, the visible surface of the star would extend to the orbit of Jupiter! You will learn more about how stars evolve through their lifetimes later in this program. Fundamentals of Stargazing -4- Image of the star Betelgeuse (credit: NASA) Betelgeuse lies about 650 light years from Earth, which means the light you see now has been travelling towards Earth for 650 years. One light year, the distance light travels in a year, works out to a distance of about 6 trillion miles. Rigel (“RYE-jel”), which marks one of the feet of the inverted celestial hunter, is on the other side of the evolutionary scale. This star is massive, like Betelgeuse, but it’s much younger and still burns hydrogen in its core. So its core is cooler, its outer layers more compact, and its surface is hotter (12,000 K) than Betelgeuse (3,500 K). Rigel is also slightly brighter than Betelgeuse. Indeed, only five stars in the entire sky outshine Rigel. Three stars lined up in a tidy row mark Orion’s Belt. These stars are, from east to west (or right to left) Alnitak, Alnilam, and Mintaka. Like Rigel, these stars are also young, massive, blue- white stars. They will also end their lives, like most of the bright stars of Orion, in brilliant supernovae explosions in the distant future. These stars are a chance alignment: each lies at a different distance. Alnitak, Alnilam, and Mintaka are 800, 1340, and 915 light years from Earth, respectively. If you have binoculars handy, turn them to Orion’s Belt to see many fine arrangements of bright blue-white stars, including a winding S-shaped group between Mintaka and Alnilam. Arab astronomers once called Orion’s Belt the “String of Pearls”, but in binoculars it looks more like a web of diamonds. Halfway between and a little north (or below) the shoulders of Orion, look for the small patch of fainter stars that mark the hunter’s relatively small head. To your unaided eye, the patch may appear cloud-like and unresolved. If your eyes are keen, you may detect three stars here. These stars are called phi-1 Orionis, phi-2 Orionis, and lambda Orionis (also known as Meissa). If you have your binoculars handy, look at these three stars. Your optics will reveal a pleasant surprise: some 20-30 faint stars spread among the three brighter stars. Many are part of the same cluster of newborn stars. Finally, look closely at the group of three dim stars that appear to extend upward from the belt. This is the so-called Sword of Orion. In binoculars or a small telescope, the middle star appears fuzzy and indistinct, because it’s entangled in a mass of glowing gas and dust called the Orion Fundamentals of Stargazing -5- Nebula where a cluster of brand-new stars are in the process of formation. You will learn more about this nebula and other objects in Orion’s sword later in this month’s program. As a constellation, Orion has been known since antiquity. The Sumerians depicted these stars as their legendary hero Gilgamesh. The starry hero was seen as fighting a celestial bull, the V- shaped congregation of stars now known as Taurus, which we will visit next. Despite the prominence of these stars, the Greeks assigned them a less noble namesake. Orion was a mighty hunter, to be sure, but he was a bit of a dim-witted brute. One legend tells of the hunter madly killing the Earth’s animals until too few remained. The goddess Artemis put an end to Orion’s greed by sending the fearsome scorpion Scorpius to sting the hunter, killing him instantly. Another version of the legend has Orion pursuing Artemis with romantic inclinations before the virginal goddess unleashed the Scorpion. In both cases, Artemis regretted the death of the hunter and asked Zeus to place Orion and Scorpius in the heavens at opposite ends of the sky. Orion’s Belt points the way to the constellation Taurus Fundamentals of Stargazing -6- The Constellation Taurus Using Orion as a base of operations, you can now navigate to other prominent constellations hovering over the northern horizon. Extend a line from Orion’s Belt toward the northwest (the lower left as seen from the southern hemisphere in the mid-evening hours in March). You will arrive at a bright orange star just above the northwestern horizon. This is Aldebaran, the brightest star in the constellation Taurus, the Bull. Like Orion, Taurus was one of the 48 original constellations included in the maps of the ancient astronomer Ptolemy in the 1st and 2nd century A.D. The bright star Aldebaran (“all-DEB-a-run”) is a swollen giant star some 45x the diameter of our own Sun. It’s far smaller and intrinsically fainter than Betelgeuse in Orion, but still appears bright because it lies just 65 light years away, about ten times closer than Betelgeuse. A much smaller group than Orion, Taurus is still a lovely constellation. Along with Gemini, it is one of the northernmost constellations of the zodiac so in the southern hemisphere Taurus never gets very far above the northern horizon. This ancient constellation holds two open star clusters, the Hyades and the Pleiades, both of which are a magnificent sight with the unaided eye or with binoculars. The Hyades cluster is the little V-shaped group of stars to one side of Aldebaran. It is a large and nearby star cluster and one of the prettiest and easiest to observe with binoculars. This star cluster lies about 135 light years away, which means Aldebaran is not a member, but merely a foreground star. The constellation Taurus; the Pleiades, the brightest star of which is Alcyone, is at lower left.