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We are now at "midwinter," as this time of year is known to many people. The nights are longest, the days are shortest, and, when we have clear skies, the starry hosts are beautiful. There are mighty Orion and his dog. Over there are Taurus the Bull and the Seven Sisters. The whole majesty of the winter heavens complements beautifully the other riches of the season.
But, alas, it will not last. The very celestial winter artwork that we love so much is destined to leave these skies relatively soon. But how?
Wobble is to blame. Yes, wobble. The earth is wobbling and very soon, on an astronomical time scale, our wintery constellations will be replaced with those from summer. Here is how it works.
We all know our planet rotates to give us day and night. And it revolves around the sun to give us our year. But it also wobbles. Have you ever spun a top on a table and observed it wobbling around as it spun down? The Earth does that, too. Only it takes a long time to wobble, or "precess" in astrospeak. In fact, it takes about 26,000 years to wobble around just one time!
How does this affect the constellations? Well, right now we are at the point of our orbit where we in the north are tilted most away from our sun. Hence, winter. And the stars in that night sky - when we are tilted most away - are the stars we see now.
Ok, now I need you to use your imagination. In your mind's eye, stop the Earth from going around the sun. Allow it to wobble half way round. Now we in the north are tilted towards the sun, right? It is our summer, but the constellations of Orion and Taurus and the rest of the gang are still in that future Earth's night sky. But they are now summer constellations, gone from our holiday skies!
This half wobble will take half of the 26,000 years, or 13,000 years. Then our present summer constellations will be up in winter; our winter constellations will be up in summer.
Of course we humans can reshape our malleable calendar so that the tilt-away time will always be in December and the tilt-towards time will always be in June, but there is no such manipulation of the stars. They are where they are.
So enjoy our winter constellations while you can! Over the next thousands of years they will be slowly and stealthily sneaking through other seasons thanks to our precessing planet. And before you know it, Scorpius and Sagittarius will be headlining our holiday heavens! Yikes!
The best and most meaningful holiday season to you all!
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There is a famous constellation in the winter sky called Orion, the Hunter. It has some real tourist sights for the backyard astronomer. There you'll find the well-known Orion Nebula, filled with brand new baby stars. There is bright Rigel, a blue supergiant star many times bigger and more luminous than our sun. Orion's Belt is made of three extremely bright and lethal stars which are, thankfully, very far away. The famous Horsehead Nebula is located at Alnitak, the lowest of the belt stars.
But one site that stands out prominently in Orion is the great star, Betelgeuse, the bright, pinkish star located farthest to the "left" in the evening at this time of the year. Why is it that color? What's going on? And why the funny name?
The reason for the funny name is probably harder to nail down than why it is that color. Back in Medieval days, the Arabs played a huge role in naming the stars. Many of their star names are with us even today. Most all stars, for example, that start with Al- are arabic in origin, like Aldebaran, Alcor, and Alnilam.
Believe it or not, the jury is still out on the origin of the name Betelgeuse. The common myth, which may be true, is that the name derives from the arabic for something like "armpit of the giant." But since the name travelled through Europe during the Renaissance, some believe it was tweaked so often over time that the present name is corrupted beyond recognition. That may be so, but Armpit of the Giant fits Betelgeuse perfectly, in my humble opinion.
Betelgeuse is a star in the last throes of life. It is a red supergiant. What is that?
When a large star finishes its life, when it finally runs out of fuel in its core, it starts going through some funky elemental gymnastics down at its core. To make a very complicated story very short, the core of the great star gets really, really hot which expands the outer layers of the star very, very far.
How far? In the case of Betelgeuse it is believed that the star has expanded to more than 1000 times the size of our sun. If it were put in our sun's place it would swallow all the planets out to Jupiter.
When a gas expands that much its surface cools, and "cool" to a physicist means reddish in color. Hence, Betelgeuse is classified as a red supergiant. You can detect its redness in its obvious pinkish hue.
Just a little heads up: Betelgeuse is scheduled to supernova - explode in the worst way - in the next million years or so. It's far enough away that it won't do us much harm. But when it does, it will outshine all the other sites Orion has to offer - by far.
Until next time, clear skies!