____ 13. The size of a white dwarf is typically like that ofA) the SunB) the EarthC) a red dwarf starD) Jupiter
____ 14. What are the main features that make clusters of stars useful to astronomers?
____ 15. The final remnant of the evolution of a Sun-like star is a
____ 16. The source of a protostar's heat is
____ 17. What event occurs at the end of the protostar stage of a star's life?A) Nuclear reactions begin in its core, converting hydrogen into helium and generating energy, thus raising internal pressure that balances infall due to gravity.B) It gives off shock waves and induces further star formation.C) It begins a long period of contraction, in which gravitational energy is converted into heat.D) Gas is spun off from its equator, forming planets.