eli5: How does brightness determine distance if stars come in different sizes and colors?

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eli5: How does brightness determine distance if stars come in different sizes and colors?

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Astronomers normally don’t use brightness to determine the distance to a star. They use a concept called parallax.

For an earthbound example, imagine there’s a mountain in the distance. Mountains come in all sorts of sizes, so it’s tough to guess how far away it is.

But we can look at that mountain and measure the angle between due North and the mountain. Now we can move due East a few miles and observe the angle between due North and the mountain from there.

The three points (the mountain, our first position and our second position) create a triangle. We know the angles of two of the corners of that triangle. Basic trigonometry gives us the third angle. Since we also know the distance we travelled from the first position to the second position, this allows us to calculate the distance to the mountain.

The same occurs with stars. If I measure the angle to a star on June first and then I measure it 6 months later, I’m getting two points of my triangle. I know how big the Earth’s orbit around the Sun is, so I can simply plug in all the numbers and calculate the distance to the star.

Once I know that distance, brightness is useful for determining other details about the star.

Stars do come in different sizes and colors, but they don’t come in *infinite* sizes and colors. [There are classes of stars](https://en.wikipedia.org/wiki/Stellar_classification) and their brightness and “colors” they emit (as a result of their composition and the types of nuclear reaction that they’re undergoing) are more or less understood. You can see that the wikipedia article includes “luminosity” for each class of stars.

So it’s more like “Ok based on the spectral colors, that’s an O-class giant, it should be this bright but we’re only seeing it as this bright, therefore [quick math] it’s this far away.”

There are a special class of stars called Cepheid Variables. They are special because they change their intensities in a predictable manner and intensities.

So a Cepheid Variable at its brightest can be used to determine distance.