26. accretion disk birth line Bok globule brown dwarf Cepheid variable contact binary core helium fusion dense core detached binary electron degeneracy pressure emission nebula evolutionary track giant molecular cloud globular cluster H II regions helium flash horizontal branch star hydrogen shell fusion instability strip interstellar extinction interstellar medium interstellar reddening Jeans instability nebulae (singular nebula) OB association open cluster overcontact binary Pauli exclusion principle period-luminosity relation Population I star Population II star pre–main-sequence star protostar red dwarf Roche lobe RR Lyrae variable semidetached binary stellar evolution supernova remnant T Tauri stars Type I Cepheid Type II Cepheid variable stars zero-age main sequence (ZAMS), Key Terms