It’s a moonless week, and from a dark location, the night sky is filled with millions of stars. If you’re a city dweller like me, it’s worth a trip to the southern reaches of town or even a quarter mile into the country just to spend ten minutes in the dark inhaling the view. At casual glance, all those stars they may look alike except for brightness, but in reality they’re big and small, hot and cool and young and old. They also come in a wide variety of colours, something not that easy to see because our eyes’ colour receptors do not work well in the dark.
As unchanging as the sky may look, stars change with age, as a ball of burning gas can’t last forever. Our Sun is a medium sized, middle-aged yellow star. In a few billion years when it uses up most of its hydrogen and helium, it will cool down to a nice reddish colour and expand into a red giant the size of the Earth’s orbit. Hopefully, we will have figured out how to be somewhere else by then.
Straight south this week is Orion with its distinctive three-star belt, and to its lower left, the sky’s brightest star, Sirius, gleams against a black background. Betelgeuse, the bright red star representing Orion’s left shoulder, is much older than the Earth, and has already entered that red giant phase in a big way. Were it at the Earth’s location, Mercury, Venus, Earth and Mars would lie below its surface. In a few million years, it will explode as a supernova, clearing a portion of space the size of our solar system. But, because it’s 640 light years away, all we’ll get here is a really great light show.
Youthful Sirius is a hot, bluish-white giant less than five percent of the Sun’s age. Blue giants are extremely hot young stars; although Sirius is only twice the size of the Sun, it’s about 25 times as bright. Young blue giants party hard and die young. Life spans are measured in millions, rather than billions, of years. Sirius lies only 8.6 light years away.
Half Sirius’s distance, and smaller still, the nearest star to us is tiny Proxima Centauri, a red dwarf one-seventh the size of the Sun (Proxima lies below the horizon and is never visible from Canada). Born small and cool, the opposite of Sirius, Proxima can expect to live some 4 trillion years (400 times longer than the Sun) before quietly warming up to retire as a white dwarf.
The next time you’re out and glance up, be it from the back yard or back country, appreciate that stars have life cycles too. In that universe spread out above you, nothing is at all as it seems
