Key definitions to memorise

Question 1

how far until stellar parallax doesn’t work anymore

Answer 1

up to a few hundred Parsecs (circa 1000 Lyrs).

Question 2

what do all main sequence stars fuse Hydrogen to?

Answer 2

All Main Sequence stars fuse H into He. in the core

Question 3

why is a main sequence star is stable?

Answer 3

radiation pressure balances gravitational pressure. Once (12%) of the sun’s hydrogen has been used up (12% is the Schonberg-Chandrasekhar limit – not required) the star leaves main sequence.