Eli5 How do we measure the distance and magnitude of deep sky objects?

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Eli5 How do we measure the distance and magnitude of deep sky objects?

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Answer 1

We can measure distances in several different ways, but a simple one to explain is “parallax measurement”, where we take one picture in June, and another one in January, compare the difference in position of the star, and do some trigonometry to deduce the distance.

Magnitude is easier and harder, because there are two types. Absolute magnitude and relative magnitude. Relative magnitude is incredibly easy to measure, as it is directly what we measure in a telescope. Absolute magnitude is how bright a star is from some fixed distance. This is harder to see, because not all stars are that distance away from us. Most are much farther.

What we have to do is get measurements of distance first, and we can use the distance and the relationship between distance and relative magnitude to extrapolate absolute magnitude.