Wonderful Star

The stars we see don't change during our lifetimes. The stars of the Big Dipper are as bright today as they were when you were born. They have been the same for millions of years. There are a few exceptions: Betelgeuse wasn't always so bright. By and large, though, changes seldom occur, and when they do, they happen very slowly. But the star Mira, "the wonderful," in the sky tonight, changes quite a bit, and very rapidly.

In the far south, midway betweem Orion and the Autumn Square is a circle of dim stars, the head of Cetus the whale. The brightest one of the five is his eye.

Mira is the heart of Cetus, just a few stars down along his body, to the west of his massive head. The first mappers of the skies saw that one month, the star was there, and the next, it wasn't. It was one of the first of the "variable" stars to be discovered.

But it was its range of variability that was truly fantastic, earning it the name "wonderful." At its brightest, it was almost as bright as the stars of the Big Dipper. Then, it would fade into invisibility. Every year, it would repeat this cycle.

The reason for this variability is a problem the sun won't have to face for another four billion years or so: depletion of its hydrogen fuel. Most stars "burn" hydrogen for most of their lives: converting the hydrogen into helium in their gigantic nuclear cores. But, just like the smallest log in a fireplace, eventually the fuel is exhausted. This has happened to Mira, and as the nuclear furnace dies out, the star pulsates.

Imagine that our sun were a star like Mira. One day it would shine as it does now. Then, over the next few months, it would grow thousands of times dimmer. Temperatures would plummet. At last, after almost a year, it would return to its original brightness. What an inhospitable world this would be!

(12/26/07)