The Death of Stars
Question: Are the stars that I look at in the night sky already dead?
Answer: That’s a great question! I get this question often, and what I love about it is the person asking can feel the immensity of the Universe. The question is rooted in the feeling that the stars are at such incredible distances that in the time it takes the light to get to our eyes, the star could have already died. A question like that packs in curiosity for, and awe of, the natural world.
The answer, well, it depends on how you look at it. If we stick with the question at face value as written above, the answer is pretty simple: no. But the INTENT of the question is spot on, because the universe is huge, stars are far, and we are looking back in time when we observe them. Let me explain.
We need to establish a couple of things first. Number one, light travels at a finite speed, denoted “c” by astronomers, and that speed is 300,000,000 m/s, or just over a BILLION km/h. Yeah… it’s fast! A finite speed means it takes time for light to get places, and the further something is away, the longer it takes to get there. Makes sense, right?
But how long does it take to get places? Let’s start close and move further out. It takes light about 1.3 seconds to get to the Moon, therefore, you see the Moon as it was 1.3 seconds ago. To get from the Sun to Earth, light takes about eight minutes, and to get to Pluto, light takes about five hours. We call this “light travel time,” the time it takes for light to travel from one place to another. We can also write this in terms of a light distance. For example, eight minutes of light travel time from the Sun to the Earth is eight light-minutes of distance. Thus, Pluto is five light-hours away.
Now let’s jump to the stars. The next closest star system to Earth, Alpha Centauri, is about four light-years of distance away. That means it takes light about four years to travel the distance from Alpha Centauri to us here on Earth, and thus, we see the Alpha Centauri system as it was four years ago… trippy. But Alpha Centauri is only visible in the Southern Hemisphere (constellation Centaur), let’s pick one the northern hemisphere. The star Sirius, for instance, is the brightest star in our night sky, and is about eight light-years away (aka eight years of light travel time, I think you get the pattern now)*. What about the furthest star we can see? Well, that again depends on how you look at it, but let’s augment the question to be, “What’s the furthest star I can see with just my eyes?” It turns out the answer is the star V762 Cas (in the constellation Cassiopeiae), which is 16,308 light-years away. Crazy!
Now that we know how long it takes the light to travel to us, the question is slightly different; now we have to ask ourselves if, in the last 16,308 years, have any of the stars our eyes can see in night sky died?
To answer that, we need to establish a second thing: how long do stars typically live? The answer to that question is a few hundred million years on the short side, and multiple billions of years on the longer side.
Now with the understanding we have from above we can ask the best form of the question: if stars live millions to billions of years, what’s the likelihood that one has died in the last 16,308 years, and thus are currently dead, but their light is still travelling to us?
If you picked a star at random, it’s unlikely that you picked it in the last 0.02%** of its lifetime. Thus, when you look up at night to the big beautiful sky full of twinkling stars, you are indeed looking back in time. You’re seeing the stars as they were thousands of years ago, but it is extremely unlikely that any of them are dead.
* Side note: isn’t it interesting that the brightest star in our sky isn’t the closest star? What does that tell you about how bright they actually are?
** The shortest star lifetimes are (roughly) 100,000,000 years: 16,308/100000000 = 0.02% (rounded)