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The Unfixed Stars

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Ken Tapping, August 31, 2005

In the sky this week...

> Mars rises in the late evening, and Saturn can be seen in the dawn twilight.
>The Moon will be New on the 3rd.

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A few months ago, close to the end of winter, I was looking at Sirius, the brightest star in Canis Major, the Bigger Dog, and picking my way around the other, much fainter stars in that constellation. I thought back to when I did it the first time, as a schoolboy astronomer. It was reassuring to see it there in the sky, apparently the same as when I first saw it.

The immutability of the heavens was noted by early astronomers, and became a basic principle. The idea was even incorporated into various religions. Stars were divided into two categories: "fixed stars", which made up the unchanging constellations, and "wandering stars" or planets, which travel to and fro along a band of sky containing the twelve (really thirteen) constellations of the zodiac.

In some ways it is comforting to think there is something in the universe that is more or less permanent. However, we now know that the constellations might be long-lived compared with the human lifetime but are actually slowly changing as the stars move around. A good example is shown in Patrick Moore's book Guide to the Stars", which I read on the bus taking me to school. It shows three drawings of the group of stars known as the Big Dipper in North America and The Plough, or Charles' Wain, in England. The first shows the Dipper as it was 200,000 years ago, the second as it is now, and the third as it will be 200,000 years in the future. The first and last drawings do not look much like the Dipper at all. It is clear that in a hundred thousand years or so, many of our familiar constellations will have become unrecognizable. Actually, if one thinks about it for a few moments, one will find the idea of all the stars being nailed rigidly to the sky as rather hard to accept, especially with what we now know about stars and the basic principles of astronomy.

We live in a huge disc-shaped system of some 200 billion stars, together with numerous dust clouds and other things in a spiral galaxy we call the Milky Way. Everything in the Milky Way is moving in orbit around our galaxy's centre. Stars form when a dust cloud becomes unstable and collapses, usually into a number of stars. At the beginning, these stars continue moving in the direction in which the birth cloud was moving in its orbit around the centre of the galaxy, and at the same speed. Then, as the radiation from the new-borns burns away the cloud, the young stars gradually disperse. The result is that the Sun and other stars have unique velocities.

Measurements of the precise positions of stars taken over a few decades show most stars to be changing position. This movement is called "proper motion". The sky's greyhound is Barnard's Star, which lies about six light-years away, and is scooting across the sky at a rate of one diameter of the Full Moon every 200 years! Measuring these proper motions tell us what stars are siblings and also about the dynamics of our small corner of the galaxy. On a clear night, enjoy those constellations while you can. Like everything else, they won't be there forever.


Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca