Stars that change

The stars look as if they never change. Most of them hardly do. But some grow brighter and fainter, and a few do it so plainly that you can follow it with nothing but your eyes. Astronomers call them variable stars.

Some of them really are changing. A star of this kind swells and shrinks, over and over, like something breathing. As it does, it grows brighter and fainter. Delta Cephei, which you can see without a telescope, takes 5.366 days over each breath. At its brightest it gives about twice the light it does at its faintest.

The most startling is a red giant called Mira. At its brightest it is easy to see. Then it fades until it cannot be seen at all without a telescope, and about 332 days after it last peaked it is back. David Fabricius was the first to notice it, in August 1596. The name Mira means “the wonderful”.

Other stars only seem to change. Algol looks like one star, but it is two, going round each other. Every 2 days and 21 hours the fainter one passes in front of the brighter one and blocks most of its light. For a few hours Algol drops to about a third of its usual brightness. Then it recovers.

Now and then a star does something nobody expected. Late in 2019 Betelgeuse, the orange star at Orion’s shoulder, began to fade. By February 2020 it was down to about a third of its normal brightness, and anyone who knew Orion could see that something was wrong. By April it was back. The star had thrown out a bubble of gas, which cooled into a cloud of dust and hid part of it.

These stars are useful as well as strange. The breathing kind keep such good time that they can be used to measure the distance to other galaxies. As for the Sun, we are lucky. Its light changes by only about one part in a thousand.