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  Night Sky

Night Sky

The full night sky on December 15th at 8:00 p.m.

December 2009 Night Sky

—Charlie Christmann, Signpost

Changing Stars

Slowly, over the course of time, the night sky we know, changes. The Moon changes shape and moves noticeably each night. Planets move quickly around the sky from year to year. We take these changes for granted and would be stunned if they did not occur. But, over the millennia, the sky changes so slowly we don’t really see much difference in a human lifetime.

Short term changes can happen unexpectedly. The Crab Nebula was created by a supernova that occurred in 1054 AD. The blast was so bright, that when the star exploded 6300 light-years away from Earth, it was visible even in daylight. Estimates are that its maximum brightness may have reached an apparent magnitude −7, brighter than everything in the night sky except the Moon. This new “star” came into existence in a day and faded from sight over two years.

The apparent movement of the North Star is an example of long-term change. I have written about how the North Star, thousands of years ago, was not the same star we see today. It has changed over time and will change in the future as Earth’s axis precesses over twenty-six thousand years. Today, the pole star is Polaris. In 3000 BC, the faint star Thuban, in the constellation Draco, was the North Star. Alrai, in Cephei, will be become the northern celestial pole around 3000 AD.

Then there is Menkalinan, the second brightest start in the constellation Auriga. This star is actually three stars when viewed through a large telescope. The two main stars in the triplet are super hot blue-white stars. The third is a small red dwarf, invisible to the naked eye. While Menkalinan is ninety-two light-years away today, in a million years it will pass by our solar system at more than forty thousand miles per hour, missing us by only twenty-nine light-years. At that time, it will be the brightest star in the night sky.

Even the constellations are changing. In about a hundred thousand years, the Big Dipper will not look as it does today because the stars are moving apart. In the future, we, on earth, will probably see an elongated inverted dipper.

Perhaps one of the most noticeable changes to the night sky will be to one of the best-known constellations, Orion. Betelgeuse is a big puffed-up ball in the red giant phase of stellar evolution. Astronomers can already see significant changes to this star in their telescopes. Betelgeuse is shrinking. The star has burped out giant gas bubbles, each throwing the equivalent mass of our sun into interstellar space. With an age of only a few million years, Betelgeuse is nearing the end of its life and is soon doomed to explode as a supernova. When it does, the supernova should be seen easily from Earth, even in broad daylight. When will it go nova? It could be any time between now and a million years from now.

And also in Orion is Rigel, the bright blue star opposite Betelgeuse. It too will be changing. Rigel is the most luminous star in our local region of the Milky Way. At an estimated ten million years old, today Rigel is a very hot, fast-burning star. It will soon run out of hydrogen fuel and eventually expand to become a red supergiant very much like Betelgeuse is today. By that time, it will be fusing helium into carbon and beyond in preparation for its eventual explosion as a supernova. Orion will someday be missing a shoulder and a foot.

So, next time you look up at the night sky, enjoy what you see today. Future generations of humans, like the past generations, will see something different.

The Planets and Moon

Mercury will be elusive this month. On the 17th, use binoculars to look low in the southwestern sky forty-five minutes after sunset. The one-day-old moon will help you find Mercury four degrees to the upper left of the Moon. On the 18th, look for Mercury forty-five minutes after sunset seven degrees to the lower left of the Moon. The 18th brings Mercury’s greatest elongation.

If you want to find Venus, you’ll need to look just before sunrise low in the south-southeast.

Mars rises in the north-northeast about 10:00 p.m. and sets about 4:45 a.m. the first of the month (times are earlier as the month progresses). There will be a Moon-Mars conjunction on the 6th.

After sunset, Jupiter will be in the southwest. Look on the 20th with good binoculars or a small telescope for a Jupiter-Neptune conjunction. Neptune will be 1/2 degree above Jupiter. On the 21st, there will be a Moon-Jupiter conjunction.

Saturn rises at 1:30 a.m. the first of the month, rising earlier as the month goes on.

The Moon is full at 12:30 a.m. on the 2nd and new at 5:02 a.m. on the 16th.

Winter begins at 10:47 a.m. on the 21st.

The 25th will fill the night with delight as Santa and his reindeer flit across the night sky and Rudolph’s nose flashes, lighting the way to homes across the world. My wish this holiday season is for a Merry Christmas and for peace, prosperity, and health to all.





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