A Giant Among Stars

The Sun is a pretty ordinary star. It is blinding to us only because it is so close: a mere hundred million miles away. If it were as far away as most of the stars we can see in the sky, we would barely notice it. Most stars are like it, but not most of the stars we can see. What we see, mostly, are the giants.

Two stars in the sky tonight show this difference very dramatically. They are Sirius, low in the eastern sky, and Rigel, to its west. Sirius is the brightest star in the nighttime sky, Rigel almost as bright. But two more different stars could hardly be found.

Sirius is an ordinary star. It shines so brightly mostly because it is so close to us: only eight and a half light years. The light we see tonight left Sirius in the summer of 1999. It is one of the closest stars.

Rigel, on the other hand, is a hundred times further away: nine hundred light years. In reality, Rigel is ten thousand times brighter than Sirius. If Rigel were as close to us as is Sirius, it would be a blazing point of light as bright as a crescent moon. If it were as close to us as the Sun, we would be instantly incinerated.

Rigel is a supergiant as big as the orbit of Mercury. Sixty thousand times brighter than the Sun, it is one of the brightest stars in the entire galaxy.

Is Rigel at all like the Sun? It is fifty times as massive, and fifty times its diameter: don't those balance out? Well, no. Rigel is also fifty times as high, and fifty times as thick. Its volume is fifty times fifty times fifty - over a hundred thousand times that of the Sun. Its mass is spread over this enormous region.

Rigel is so insubstantial that our atmosphere would be a brick wall by comparison. Like other giant stars, Rigel is a big ball of almost nothing.

Sirius is more like our Sun: a little more massive, a little brighter, a little bigger, but really a lot like it. Rigel is utterly different, a giant among stars. Reality is often so different from appearance.

(01/16/08)