Explain it like I’m Five: How do astronomers measure things so far away?

Discovery of Close Stars

When you shift your body side to side, the things closest to you look like they are moving the most. The things further away don’t look like they are moving much.

We compare how much this star shifted with closer and other known stars.

To figure out how far away stars are, we do something similar, but instead of moving our body we have to wait for the earth to move in its orbit. The fancy term for shifting your body from side to side or for waiting for the earth to orbit is “parallax”. Using parallax and a p

rocess which is of similar to how our GPS works, we find the location and distance of these new stars.Discovery of Far Stars:

For stars really far away we can’t move our body far enough side to side. Therefore we have to use a different method based on our observations of color and brightness. Now I’m sure that you’ve observed that fire of different temperatures is a different color. The same goes for stars, so we can figure out how hot a star is by its color. Now the hotter the star the brighter it is, but if a really hot star based on color doesn’t seem very bright in our observations we can assume it is very far away.

Now to Discover Planets:

Since we now know how to find stars, we use those stars to find far-away planets. We look at the stars and see if they wobble. If they do, they have planets.

Why might they wobble? Well, imagine a father spinning around while swinging his daughter through the air. His daughter is “orbiting” around him but because of the forces at work he’ll wobble a bit as his daughter’s mass pulls him in different directions. Things work in a similar way with planets orbiting stars. The gravity from the mass of the star swings the planet around but the mass of the planet creates a slight wobble on the star because of its mass pulling in different directions as it orbits. This wobble is very slight because stars are hundreds of thousands of times more massive than planets but our astronomy equipment keeps improving so that we can detect these slight differences (we will discover the most massive planets first because they create more of a wobble).

Another way to find planets is by checking the brightness of the closet star (kind of like what we said earlier). When scientists are looking at a star, sometimes it will look slightly dimmer than it’s supposed to. This tells us that there is something blocking our view of the star and that usually that means a planet is in orbit.

Our friend at XKCD drew this awesome picture of all the planets we have discovered by using these methods.