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Planets appear on horizon, stars in the south this month

For night lights, look low

Posted: Sunday, January 07, 2007

 

 

The view is toward southeast between 7 and 10 p.m. throughout January. Because of Earth’s rotation, this part of the sky moves through the south into the southwest each night all night long. Three constellations are dominating the sky: Orion the hunter, Gemini the twins, and Taurus the bull.

This part of the sky is probably the most interesting and brilliant since it contains a number of easily seen deep sky objects as well as eight of the 20 brightest stars. Starting with Sirius, the brightest one of all, on the horizon, we go clockwise to Procyon, then the twin stars Castor and Pollux, then yellow Capella (just outside the diagram on top left), then red Aldebaran in Taurus’ arrowhead, and finally close the circle with blue Rigel and red Betelgeuse in Orion.

What makes a star (or any other object, like a planet or our moon) appear bright or faint depends on how bright the star really is and how close it is. Sirius and Procyon are actually the least luminous of these stars but are the closest at 9 and 11 light years. In contrast, Betelgeuse is at a distance of 430 light years but almost makes up for it by being 400 times brighter than Sirius.

Rigel, a blue supergiant at Orion’s foot, and Capella, a yellow normal giant high above Gemini, make for a good comparison, as they are of virtually the same apparent brightness. Capella is 42 light years away while Rigel is 770 light years away —18 times farther. But Rigel also is 315 times more luminous. Doing the math and knowing that brightness diminishes with the square of distance, they appear virtually equally bright as seen from our solar system.

Their difference in luminosity is originally based on their mass, 2.7 solar masses for Capella and 17 solar masses for Rigel. For a star to be in a balance between gravitation (due to its mass) and gas pressure (due to its high internal temperature), it has to be of a certain size and surface temperature. Capella has a diameter of 9 million miles (10 times that of our sun) and a surface temperature of 10,000 F (making it yellow; virtually the same as our sun).

Rigel has a diameter of 61 million miles (roughly the orbit of Mercury) and a surface temperature of 20,000 F (making it blue). Those two properties explain why Rigel is so much more luminous than Capella.

Aside from stars, there is of course the Orion nebula, a birthplace for stars, making up the sword which can be glimpsed with binoculars. We find two prominent star clusters in Taurus, the Pleiades (just outside the diagram on the top right) and the Hyades (the arrowhead surrounding Aldebaran) which can easily be seen with the naked eye.

Stretching from Gemini up to Capella are four star clusters which need some prodding when using binoculars. Since Gemini and Taurus also are Zodiac constellations, planets regularly move through them, as Mars will do next winter.

Other prominent constellations and stars in the sky are the Little Dipper high in the north, the Big Dipper in the northeast, Cassiopeia’s W almost in the zenith (overhead), Pegasus’s square shape low in the west, and Cygnus’ cross with bright Deneb and next to it bright Vega, both low in the northwest.

As for planets, Saturn rises between 8 and 10 p.m. due east, together with bright Regulus in Leo. Jupiter can be seen very low in the south around 8 a.m. Mars, Mercury and Neptune are within our sun’s glare during dusk and dawn.

Venus should become visible just after sunset low in the southwest. Neptune draws near Jan. 19 and may be glimpsed with binoculars, since it’ll appear close to Venus. Uranus is potentially visible after dusk but it’s best to check online for finder charts.

Good sources for sky maps are Astronomy and Sky & Telescope magazines, available at Fred Meyer and Safeway, and online monthly sky charts at skymaps.com and at skytonight.com.

Andy Veh is the physics and astronomy instructor at Kenai Peninsula College. He can be reached at aveh@uaa.alaska. edu.



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