Upon recently taking a state assessment test in the field of science, I was given the questions
“A-Which is hotter, an orange star or a blue star and why?”
and
"B-“Which is brighter, an orange star or a blue star and why?”
I think I understood why the blue star is hotter and explained as best as I could but I wasn’t sure if and why the blue star is Brighter. When I asked my science teacher after the test, he said that there were many ways to measure brightness or something like that.
So which is Brighter the blue star or the red star and why? As an aside, none of the students at my school were ever taught the mechanics of stars, making us think that that area of study has been neglected.
The second question is meaningless. A red giant a few lightyears away will certainly be brighter than a hot new blue star in the next galaxy. And even a nearby blue star is likely to be much smaller than a blue giant and perhaps not as bright. But given a blue star and an orange one of the same size and distance, well naturally the blue one will be brighter.
Here’s a Hertzsprung Russell diagram of absolute brightness vs surface temperature. Absolute brightness or magnitude refers to how bright a star would appear if it were a standard distance of 10 parsecs away.
Along the main sequence hotter (bluer) is always brighter and more massive, but the Giants and Supergiants may be both cool and bright. That makes the second question ambiguous.
Yeah, it is a little off, but it’s not completely screwed up. The majority of blue stars are bright OB stars, and the majority of orange stars are dim low main-sequence stars. But there are exceptions, like orange supergiants and blue “white” dwarfs. If the question had read, “In general, which is brighter, an orange star or a blue star?” then it would have been valid. However, you would have had to have studied some basic Astronomy to know that.
