Some of the stars you see in the night sky are already gone. Why is it possible to still see their light?

they are light years away

Explanation:

when you look at the sky you are looking at about 6 million years in the past because time travels differently between space and our atmosphere. That's why astronauts can feel like they have been gone for 3 days but really for 3 months.

Key point:

Light travels differently between space and Earth

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.