Stellar size and magnitude

March 14th, 2019 | by sbieger |

A while ago, I got a great question from one of my students in the Emory class - what does a star’s size got to do with its magnitude? And as she later explained, the words “size” and “magnitude” mean almost the same thing. They are similar, in our common usage, however, we talking specifically about their use in astronomy. It was clear this was a question more about semantics.

So I decided to dig further to come up with a better distinction between these two terms. But I have to start first with a third word, at the risk of further tangling this whole response. That word is mass. When we talk about stars, we start with mass. Stuff. What stars are made of, mostly hydrogen as it turns out.

The terms magnitude and size differ from the term mass. They are interrelated to some degree but in a straightforward linear way. But we always start with mass when considering a star’s properties and then go from there.

Astronomical magnitude, in the sense of what we see, actually refers to apparent magnitude. It is a measurement of the “apparent” brightness of the star as seen from Earth.In the days of the famous Greek astronomer, magnitude meant “bigness”. However, by the mid-nineteenth century it was determined that stars were so far away that no accurate measurement could be made of any starts size.

Sometimes the word mass is interchanged with size. But mass doesn’t always translate directly to the physical size of a star. Our sun, for example, is slowly loosing mass over it’s lifetime but once it nears the end of its life and becomes a red giant, it will be much larger in terms of its physical size, namely its diameter.

There is far too much to say about stars in this blog from the point of view of astrophysics, but if you wish indulge try any of these web pages below for further reading.

List of stellar angular diameters

Universetoday: Size-of-Stars

ClassZone: A Stars Size

Stellar Mass

Astronomical Magnitude

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