Atomic electrons have specific allowed energy states. Why?

If atoms occupy only certain allowed levels, how do they get from one allowed level to another?

Why do atoms emit or absorb only certain specific colors of light?

How can you tell that there is calcium in the atmosphere of the star Betelgeuse, which is about 310 light years away from the earth?

How do you determine the temperature of a hot object using the blackbody radiation curve?

How do you determine the temperature of a star?

If an ambulance with a siren passes you, what do you observe about the pitch of the siren before and after it passes you?

What happens to the wavelength of a light source that is approaching you? receding from you?

How do you identify hydrogen in a star that is moving with respect to you?

What is the source of the energy which makes stars shine?