Night Watch

Ever looked up at the night sky and noticed the colours of the stars? What colour are Sirius and Canopus? What about Betelgeuse and Rigel? Look carefully around the sky and you'll see stars from blue and white to yellow, orange and red.

The different colours indicate different surface temperatures, but not in the way you might think. We often consider blue "cool" (like the colour of the sea), and red "hot" (like a roaring fire). But with stars, it's the other way around. Blue stars are the hottest and red stars are the coolest.

Our own star, the Sun, is yellow, so its surface temperature is between these extremes. Today our Sun is steadily fusing hydrogen into helium at its core. Billions of years from now, though, it'll use up its hydrogen. The sun will expand, engulfing the planet Mercury, and possibly Venus and even Earth, and its surface layers will cool and redden. In other words, the Sun will become a red giant star. But that won't happen for five billion years or so.

For now, the brightest red giant star in the sky is Betelgeuse, in the constellation of Orion. You can find it high in the northern sky. Betelgeuse hasn't always been this bright. And it might not always have looked orange, as it does now.

Studies have revealed that until about 10,000 years ago, the star probably was blue-white. Then it turned white, then yellow and finally orange.

For most of its 10 million-year life, Betelgeuse "fused" hydrogen in its core to make the next-heavier element, helium. The same thing is happening now inside the Sun.

But Betelgeuse is about 15 times more massive than the sun, which is a critical difference. The cores of heavy stars are extremely hot, so the stars "burn up" their hydrogen much faster.

As Betelgeuse exhausted the hydrogen in its core, it started to burn hydrogen in a thin shell around the core. The star's interior got hotter, so its outer layers expanded. This cooled the surface, which is why Betelgeuse looks orange. With more surface area, Betelgeuse got a little brighter, too.

In all stars, there's an ongoing tug-of-war between gravity, which pulls its gas inward, and radiation, which pushes the gas outward. The two sides are evenly matched in the sun, so it stays pretty stable. In Betelgeuse, though, the sides pull back and forth. Gravity gains the advantage for a while, so the star shrinks. But as it shrinks, it gets hotter, so it produces more radiation. That makes it expand again.

Each time the process repeats itself, over a period of about five years, Betelgeuse expands or shrinks by tens of millions of kilometres. Today, Betelgeuse is probably beginning to burn the helium in its core to make carbon and other elements; a process that'll last no more than a few hundred thousand years. Eventually, Betelgeuse will stop producing energy in its core. The core will collapse and the star's outer layers will blast outward. For a few weeks, Betelgeuse will shine brighter than an entire galaxy of stars.

As we zoom out from Betelgeuse, we find that the summer Milky Way is visible directly overhead, running north-south across the sky. The evening sky is resplendent with many brilliant stars. From Capella in the north to the Pointers in the south, the sky is a sheer delight to explore.

High in the sky, the brilliant star Sirius marks the constellation of Canis Major, the big dog.

Look to the northeast and, below Sirius, you'll find Procyon, in Canis Minor, the little dog. Procyon is one of the brightest stars in the sky, in part because it's only 11 light years away. It's twice as big as the sun, a good bit hotter and about seven times as bright.

The Moon is at first quarter on January 31, full on February 8, at last quarter on February 15, and new on February 22.