Well, I’ll give you a spoiler: they’re ridiculously far away.
Let’s consider for a moment what a light-year actually means. It sounds like a unit of time, but it’s actually the distance that light travels in one Earth year.
Think of it this way: if your name is Bob, and you can travel a certain distance in one year, that distance could be called a Bob-year.
I know it’s strange to think of light traveling at a certain speed. When you flip a light switch, the room immediately brightens. When you shine a flashlight, its beam immediately falls across the nearest surface.
But that just goes to show how insanely fast light travels. If it takes 2 million years for light to get from one object to another…imagine how far apart those objects are?
Well, that’s the case for our home galaxy, the Milky Way, and our nearest galactic neighbor, the Andromeda Galaxy.
By now, I’ve introduced you to a lot of different ways to classify stars.
Months ago, I talked about the different spectral classes—O, B, A, F, G, K, and M. Even before that, I told you about apparent visual magnitude, our ranking system for how bright stars appear to the naked eye.
More recently, we explored absolute visual magnitude and the related absolute bolometric magnitude and luminosity. All these are related to a star’s actual brightness, not just how bright they seem to be from Earth.
And last but not least, we talked about the H-R diagram and how to rank stars by their luminosity classification.
In short, it may seem like sorting stars is a complicated business. But it’s not really. And here, I intend to give you an overview to put all this together.
Stars don’t look small because they’re really the size of pinholes in a blanket. The smallest are the size of Earth. The largest have 128,865,170 times Earth’s diameter.
They look small in the sky because they’re distant. It’s for the same reason you can tell how far away your surroundings are by how small they appear; you know the mountains on the horizon are far away because they look shorter than your house.
The nearest star to our solar system is 4.3 light-years away. But what exactly is a light-year?
Light seems to travel instantaneously from your flashlight to the nearest surface, but it actually has a finite speed. In one second, it travels 299,792 km—fast enough to wrap itself around Earth’s equator 7.5 times.
In one year, light covers 9,460,730,472,580.8 kilometers, enough to wrap around the sun’s equator 2160.5 times. Four times that is the distance to the nearest star.
Find yourself a dark, unpolluted night sky on a clear night free of clouds, and you are very likely to look up into the heavens and see a sight quite like this. It’s what we see of the Milky Way, our galaxy.
When I’m at an astronomy event with a sky like the one above, I find it absolutely incredible. Do you notice how the stars don’t all look the same?
A couple are startlingly bright, there are numerous stars that are somewhat dimmer, and if you look really hard, you notice that even the dark night background is sprinkled with stars so faint they can barely be seen.
But what if I told you that you’re not even seeing the half of it?
Have you ever looked up at the night sky and noticed that while relatively bright stars outline the constellations, there are numerous other stars that are almost too faint to see with the naked eye?
If you ever noticed this, you probably guessed that the brighter stars are literally brighter, and the fainter stars truly are fainter. Or maybe you guessed that they don’t vary in brightness that much, but fainter stars are much farther away.
But that’s not really true…or, at least, it’s not the whole answer.
So what’s the real reason why some stars appear to be brighter than others—and how can we tell how bright they really are?