Seeing the stars at night, year after year, generation after generation, it seems that all is quiet up there. The stars appear fixed on that black canopy above. Nothing ever changes.

But stars are born, they live, they die. And it is because stars have a life cycle that we can make these bold statements: The skies we see tonight are our skies alone. Earlier in our history they were different. In the future there will be a whole new menage of constellations.

Explain!

Most of the stars we see in the sky have short lifetimes. Regular readers here will recall that most of the naked eye stars are intrinsically very bright, thus very energetic. This means they burn through their hydrogen fuel in short order, many in just tens of millions of years.

Now the Earth is over four-and-a-half billion years old. It follows then that most of our present-day stars have been up there only recently in our history.

Another factor in our changing sky is that stars are formed in nurseries. Here they interact gravitationally with each other. In doing so, some get tugged and flung out of the nursery, jettisoned into lonely space.

All this movement means that even if stars could live longer, they would not stay in the same place in the skies. They continue to move through it. Some are moving very fast, tens or hundreds of kilometers per second, others not so.

They don't appear to be moving because they are so very, very far away. But they do move. And this is another reason the skies are changing, however slowly.

Not only do stars appear in our skies over time, not only do they move through them, but they also vanish.

The biggest stars exit with the proverbial bang. They exhaust their nuclear fuel and collapse, then explode, into a supernova, vanishing from the sky altogether. Medium-sized stars fade away with a whimper, eventually shedding their outer layers and leaving nothing behind but a miniscule, hot corpse of a core.

On top of all this appearing and disappearing, in the future, because hydrogen gas in our universe is being depleted, the birthrate of stars will continue to decrease. There will be fewer stars to take the place of the dead and dying ones.

Stars are here for just one scene of one act of the Great Play, in really nothing more than a mere cameo appearance. So enjoy Orion and Sagittarius and the rest of the troupe while you can. In a couple millions of years from now, our heavens above will not be recognizable.