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Welcome again to our monthly newsletter with features on exciting celestial and earth science events, product reviews, tips & tricks, and a monthly sky calendar. We hope you enjoy it!
We are now only a few weeks away from the winter solstice on December 22. Anyone who lives in northern latitudes must have noticed how late the sun is rising and how early the sun is setting, especially since the end of Daylight Saving Time earlier this month. Most people think of the winter solstice as the shortest day of the year, but stargazers tend to see it as the longest night of the year. The Sun is above the horizon roughly 9 hours, varying with latitude. Subtracting twilight, the night is fully dark for almost 12 hours. One result of this long night, combined with the southern position of the sun, is that a very large part of the night sky can be seen in one night. As evening twilight ends, the “autumn” constellations fill the sky: Cygnus, Pegasus and Aquarius. As morning twilight begins, the “spring” constellations are visible: Hercules, Boötes, and Virgo. About the only constellations not visible at some point during the night are the ones right close to the Sun, such as Ophiuchus, Sagittarius, and Scorpius. During the months between autumn equinox and winter solstice, the lengthening of the night almost keeps pace with the sun’s movement against the stars, so that the constellations visible at the end of evening twilight hardly seem to change. Pegasus hangs high in the southern sky for months on end. At the other end of the night, new constellations appear rapidly out of the dawn. The opposite occurs once winter solstice is past: the morning sky changes hardly at all, while the autumn constellations rapidly disappear into evening twilight. The long December nights allow us to take advantage of these oddities. In particular, the late rising of the sun means that many people need to be up well before sunrise to get to work. If you can get up just a little bit earlier, you can get a jump on the spring constellations. In particular, in the next few weeks between now and solstice, an early riser can observe two “morning stars.” These are not the usual suspects, Mercury and Venus, but rather two old friends only recently emerged from behind the sun: Mars and Saturn. In the accompanying chart, the positions of the planets are shown for tonight, November 30, and the orange trails show how they will move between now and December 22. Mars’ trail is much longer because it is moving much faster because of its relative closeness to Earth. By a happy coincidence, both planets are currently paired with bright stars, Regulus and Spica respectively. Both planets are also on the rise in brightness, but for different reasons. Both planets are getting brighter because they’re getting closer. Mars is moving towards opposition on March 3, while Saturn is moving towards opposition on April 15. At present both planets are just slightly brighter than the stars with which they are paired, but both will get much brighter by the time they reach opposition, especially Mars. Geoff Gaherty
Last month we examined the birthplace of stars—huge clouds of gas and dust found in interstellar space. But what happens to these stars once they begin nuclear fusion and become full-fledged members of the stellar community? It seems that after a short and somewhat turbulent “childhood” the star settles down as it consumes its nuclear fuel. During this “adult” phase, its size and luminosity remain relatively stable.
Note: Alpha Centauri is actually a multiple star system. The reference here is to the brightest member, alpha Centauri A.
It is interesting and instructive to locate a stars on a graph which shows luminosity or absolute magnitude on the vertical axis and surface temperature or spectral class on the horizontal axis. Such a graph is shown below. It is known as a Hertzsprung-Russell diagram. Note that stars are not scattered at random but are gathered in distinct groups. The red line going from top left to bottom right represents the group known as the Main Sequence. Alpha Centauri is a Main Sequence star and remains there during its well-behaved adult life. In contrast, Alnilam or epsilon Orionis, is a supergiant star forty times a massive and 375 000 as luminous as the Sun. It is estimated to be only about four million years old but already near the end of its life! Note its position on the H-R diagram shown below.
What is happening here? It turns out that the rate at which stars consume their nuclear fuel depends on their mass. The more massive the star, the faster it consumes its fuel. But what happens when the fuel runs out? Stay tuned: we’ll look at how stars die in the next issue of Starry Night Times. Further Study The H-R diagram can be found in Starry Night under the Status tab along the left side of the program window. In a dark sky, point to a star and it will show as a red dot in the H-R diagram. Question: are any of the seven stars in the Big Dipper Main Sequence stars? Answer to last month’s question: Herb Koller
Star of Bethlehem — 3 B.C. Press the Step time forward button to step through the close conjunction of Venus with Jupiter in Leo (near Regulus), culminating with a closet approach on August 12, 3 BC. Some scholars believe that this was the first appearance of the "star" which prompted the Wise Men to go to Judea seeking a King, the second being the Jupiter-Venus conjunction 10 months later on June 17, 2 BC. Star of Bethlehem — 2 B.C. Press the Step time forward button to step through the extremely close conjunction of Venus with Jupiter in Leo (near Regulus), culminating with a closest approach on June 17, 2 BC. Some scholars believe that this was the second appearance of the "star" which the Wise Men saw upon reaching Bethlehem, the first being the prior Jupiter-Venus conjunction 10 months earlier. Pedro Braganca
Marking the shoulder of The Bull, M45 The Pleiades is perhaps the most beautiful binocular target in the night sky. A telescope with low magnification will bring out a chain of stars running through the middle like a row of pearls. M1 The Crab Nebula is what remains of a very bright supernova seen from Earth in 1054. It shone so brightly (about four times brighter than Venus) that it was quite visible during daylight hours. NGC 1514 a nice planetary nebula with a dim outer shell and a bright inner shell; the inner shell has a blobbed appearance. Aldebaran is the brightest star in Taurus and distinctly orange-hued. The giant star is in the late stage of its life-cycle: having burnt off its hydrogen, it's currently fusing helium into carbon. Sean O'Dwyer
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DEC 2011
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Let’s examine alpha Centauri, a star not unlike our own Sun. Situated in the southern constellation of Centaurus, it lies about 4.3 light years from our solar system. With a mass and luminosity slightly larger than the Sun, it is also about the same age, roughly 5 billion years. Like the Sun, it can probably look forward to another 5 billion years of blissful existence.
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