When you go outside at night and gaze upon the sky, it seems eternal and unchanging.
But that’s our own human limitation coloring our minds. We live on a much shorter time scale than the stars. Stars are much like us, in fact: They are born, they live for an amount of time, and they die. Some fade away, some explode, but in the end, like us, they are mortal.
When looking at stars, you’re actually looking into the past. Many of the stars we see at night might have already died.
It’s actually not too hard to understand. At 300,000 kilometers per second , it takes light more than eight minutes to get from the sun which is 150 million km away which is the closest one.You can think of it as seeing the Sun as it was eight minutes ago. The nearest known star to the Sun is the Alpha Centauri triple-star system, and light takes more than four years to get from there to here.
There are about 6,000 or so stars that are visible with the naked eye, and the vast majority of them are within about 1,000 light-years of the Sun. Stars dim quickly with distance. Only the most luminous of stars can be seen from greater distance, stars like Deneb (probably 1,500–2,500 light-years away), Eta Carinae (7,500 light-years).
So when you look up at the night you are seeing even the most distant stars in the sky as they were less than 10,000 years ago.
But stars live much, much longer than that. Even the most luminous stars, which use up their core fuel far more quickly, can live for 1 million years or more. So the chance of stars to die while its light is on its way to Earth are very small; in terms of the star’s lifetime, a few thousand years is the blink of an eye. A star would have to be very, very near its own death for this to happen after a very, very long life.
Eta Carinae It’s on the edge of exploding; in 1840s it underwent a massive supernova event. Its appearance might not go off for another 50,000 years. But maybe it’s already gone and we just don’t know it yet.
But that’s the exception, with the vast majority of stars still merrily fusing away, lighting up the galaxy.
the Milky Way is 100,000 light-years across, and only a few stars in it have a shorter lifespan . On average, very roughly, only two or three stars are expected to go supernova in the galaxy per century, so the light from a few thousand of such explosions is already on its way here. That may sound like a lot, but the Milky Way has something like 200 billion stars in it. So really, the number already dead but still shining in our sky is very small*.
Also, not every star explodes. Some swell into red giants, and Lower-mass stars just fade over time, lasting hundreds of billions of years. Most of those tiny cool red dwarfs will be around a long, long time. But for this they don’t even matter: Because they are so intrinsically dim, not a single red dwarf is visible to the naked eye—even the closest one, Proxima Centauri, is far too faint to see without a telescope.
So when you look at the sky, feel confident in the fact that the stars you see are still there and will be for some time.
