NGSSS Check
6. Summarize how scien-
tists measure the
brightness of a star.

Astronomers define star brightness in terms of apparent magnitude — how bright the star appears from Earth — and absolute magnitude — how bright the star appears at a standard distance of 32.6 light-years, or 10 parsecs. (A light-year is the distance light travels in one year — about 6 trillion miles, or 10 trillion kilometres.) Astronomers also measure luminosity — the amount of energy (light) that a star emits from its surface.

My source:   https://www.space.com/21640-star-luminosity-and-magnitude.html

Hope this helps :)

the reason the two sea urchins don't compete with each other for food, resources, etc. is because the urchins have adapted to the environment but they have also adapted to each other. this is most likely because the genes of the sea urchins.