You’d see some of the brighter stars but none of the dust and gas in the nebula, including the horse head shape. Why?

Three Things a Telescope Does

To better understand what’s going on, it helps to know what a telescope does. Just as a pair of binoculars can make the upper-level seats in an arena almost as good as courtside, a telescope can make a distant object appear much closer. But a telescope does more than this. It doesn’t just magnify an object; it also amplifies it. It makes something faint appear much brighter.

Some people think the reason why a telescope can see objects our eyes can’t is because it magnifies something that is too small for us to see. And this is often true. But the Horsehead Nebula is actually not that small. The fields of view of these images of the Horsehead are about twice the size of the full Moon on the sky. You can’t see it because it is too faint, not because it is too small.

So why couldn’t you see the Horsehead Nebula even if you were much closer? For objects that appear to be larger than a point of light (e.g., galaxies and nebulae, but not stars), how bright it appears has little to do with how far away it is. Moving closer to it will make it bigger, but not brighter. This may seem counterintuitive, but you can try it at home. Walk towards a wall. As you approach you’ll notice that the wall is getting bigger but otherwise is the same brightness. The same is true of the Horsehead Nebula. If you can’t see it with your eyes while standing on Earth, you still won’t see it from your spaceship.

Why then can a telescope see it? A telescope offers several advantages over our eyes. As marvelous as the human eye is, it’s not that well suited for nighttime observing. First, our eyes are tiny. The opening that allows light to enter, known as the pupil (the black area at the center of the eye), is only about one-quarter of an inch wide when fully open.