Jessica Horn: Chapter 12 Notes

Chapter 12: Star Stuff
12.1 Star Birth

• We are "star stuff" because the elements necessary for life were made in stars
• Stars are born in molecular clouds consisting mostly of hydrogen molecules
• Stars form in places where gravity can overcome thermal pressure in a cloud
• Cloud heats up as gravity causes it to contract
• Star-forming clouds emit infrared light because of the heat generated as stars form
• As gravity forces a cloud to become smaller, it begins to spin faster and faster
• Gas settles into a spinning disk because spin hampers collapse perpendicular to spin axis
• Angular momentum leads to rotation of protostar disk formation and sometimes jets from protostar fragmentation into binary 
• Protostar contracts and heats until core temp is sufficient for hydrogen fusion
• Contraction ends when energy released by hydrogen fusion balances energy radiated from surface
•  Takes 50 million years for star like Sun (less time for more massive stars)
• Summary of star birth: gravity causes gas cloud to shrink and fragment, core of shrinking cloud heats up, when core gets hot enough, fusion begins and stops the shrinking, new star achieves long-lasting state of balance
• A cluster of many stars can form out of a single cloud 
• Very massive stars are rare
• Low-mass stars are common

12.2 Life as a Low-Mass Star

• A star remains on the main sequence as long as it can fuse hydrogen into helium in its core
• Helium fusion requires higher temps than hydrogen fusion because larger charge leads to greater repulsion 
• Fusion of two helium nuclei doesn't work, so helium fusion must combine three He nuclei to make carbon
• Helium burning stars neither shrink nor grow because thermostat is temporarily fixed 
• A star like our Sun dies by puffiing off its outer layers, creating a planetary nebula 
• Only a white dwarf is left behind

12.3 Life as a High-Mass Star 

• Early stages are similar to those of low-mass stars 
• Main sequence: H fuses to He in core
• Red Supergiant: H fuses to He in shell around inert He core
• Helium Core Burning: He fuses to C in core (no flash)
• High mass stars become supergiants after core H runs out
• Luminosity doesn't change much but radius gets far larger
• Helium fusion can make carbon in low-mass stars 
• Advanced reactions make heavier elements
• Advanced nuclear burning occurs in multiple shells
• Iron builds up in core until degeneracy pressure can no longer resist gravity 
• Core then suddenly collapses, creating supernova explosion 

12.4 Summary of Stellar Lives

• Core shrinks and heats until it's hot enough for fusion 
• Nuclei with larger charge require higher temperature for fusion
• Core thermostat is broken while core is not hot enough for fusion
• Core fusion can't happen if degeneracy pressure keeps core from shrinking
• Stars in Algol are close enough that matter can flow from subgiant onto main-sequence stars
• Star that is now a subgiant was originally more massive 
• As it reached the end of its life and started to grow, it began to transfer mass to its companion 
• Now the companion star is more massive