Chapter 12 Notes: Olivia Ward

Star Stuff
12.1 Star Birth
How do stars form?

• Star-Forming Clouds
  
  • Stars form in dark clouds of dusty gas in interstellar space.
    • The gas between...
• Gravity vs. Pressure
  
  • Gravity can create stars only if it can overcome the force of thermal pressure in a cloud.
  • Gravity within a contracting gas cloud becomes stronger as the gas becomes denser.
• Mass of a Star-Forming Cloud
  
  • A typical molecular cloud (T ~ 30 K, n ~ 300 particles/cm^3) must contain at least a few hundred solar masses for gravity to overcome pressure.
  • The cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud.
• Fragmentation of a Cloud
  
• Glowing Dust Grains
  
  • As stars begin to form, dust grains that absorb visible light heat up and emit infrared light.
  • Long-wave length infrared light is brightest from regions where many stars are currently forming.
• Solar system formation is a good example of star birth.
  
  • The original cloud is large and diffuse, and its rotation is imperceptibly slow. The cloud begins to collapse.
  • Cloud heats up as gravity causes it to contract due to conservation of energy. Contraction can continue if thermal energy is radiated away.
  • As gravity forces a cloud to become smaller, it begins to spin faster and faster, due to conservation of angular momentum.
• Flattening
  • Collisions between gas particles in a cloud gradually reduce random motions.
  • Collisions between gas particles also reduce up and down motions.
  • The spinning cloud flattens as it shrinks.
• Formation of Jets
  
  • Rotation also causes jets of matter to shoot out along the rotation axis.
  • Jets are observed coming from the center of disks around protostars.
• Protostar to Main Sequence
  
  • A protostar contracts and heats until the core temperature is sufficient for hydrogen fusion.
  • Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface.
  • It takes 30 million years for a star like the Sun (less time for more massive stars) to go from a protostar to a star.
• 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.

How massive are newborn stars?

• A cluster of many stars can form out of a single cloud.
• Very massive stars are rare. Low-mass stars are common.
  
• Upper Limits on a Star's Mass
  
  • Photons exert a slight amount of pressure when they strike matter.
  • Very massive stars are so luminous that the collective pressure of photons drives the matter into space.
  • Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart.
  • Observations have not found stars more massive than about 300 MSun.
• Lower Limits on a Star's Mass
  
  • Fusion will not begin on a contracting cloud if some sort of force stops contracting before the core temperature rises above 10^7 K.
  • Thermal pressure cannot stop contracting because the star is constantly losing thermal energy from its surface through radiation.
• Is there another form of pressure that can stop contraction? Yes.
  
  • Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place.
    • Particles can't be in the same states in the same place.
  • Thermal Pressure
    
    • Depends on heat content
    • The main form of pressure in most stars
• Brown Dwarfs
  • Degeneracy pressure halts the contraction of objects with <0.08 MSun before the core temperature becomes hot enough for fusion.
  • Star-like objects not massive enough to start fusion are brown dwarfs.
• Brown Dwarfs in Union
  
  • Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous.
• Stars more massive than 300 MSun would blow apart.

12.2 Life as a Low-Mass Star
What are the life stages of a low-mass star?

• Main-Sequence Lifetimes and Stellar Masses
  
  • A star remains on the main-sequence as long as...
• Life Track After Main Sequence
  
  • Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over (red giant).
• Broken Thermostat
  
  • As the core contracts H begins fusing to He in a shell around the core.
  • Luminosity increases because the core thermostat is broken - the increasing fusion rate in the shell does not stop the core from contracting.
  • Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion. A larger charge leads to greater repulsion.
  • The fusion of two helium nuclei doesn't work, so helium fusion must combine 3 He nuclei to make carbon.
• Helium Flash
  
  • The thermostat is broken in a low-mass red giant because degeneracy pressure supports the core.
    
    • The core temperature rises rapidly when helium fusion begins.
  • Helium core fusion stars neither shrink nor grow because the core thermostat is temporarily fixed.
• Life Track After Helium Flash
  
  • Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.
  • Observations of star clusters agree with these models.
  • Helium core fusion stars are found in a horizontal branch on the H-R diagram.
  • Combining models of stars of similar age but different mass help us to age-date star clusters.

How does a low-mass star die?

• Double Shell-Fusion
  
  • After core helium fusion stops, He fuses into carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer.
• Planetary Nebulae
  
  • Double shell-fusion ends with a pulse that ejects the H and He into space as a planetary nebula.
  • The core left behind = white dwarf
• End of Fusion
  
  • Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements (some He fuses to C to make oxygen).
  • Degeneracy pressure supports the white dwarf against gravity.

12.3 Life as a High-Mass Star
What are the life stages of a high-mass star?

• CNO Cycle
  
  • High-mass main-sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts.
  • A great core temperature enables He nuclei to overcome greater repulsions.
• Life States of High-Mass Stars
  • Late life stages of high-mass stars are similar to those of low mass stars.

How do high-mass stars make the elements necessary for life?

• The Big Bang made 75% H, 25% He. The stars made everything else.
• Helium fusion can make carbon in low-mass stars.
• The CNO Cycle change C into N and O.
• Helium Capture
  
  • High core temperatures allow helium to fuse with heavier elements.
  • Helium capture C turns into O, Ne, and Mg.
• Advance Nuclear Burning
  
  • Core temperatures in stars with 8 MSun allow fusion of elements as heavy as iron.
  • Advanced reactions in stars make elements such as Si, S, Ca, and Fe.
• Multiple Shell Burning
  
  • Advanced nuclear burning proceeds in a series of nested shells.
  • Iron is a dead end for fusion because nuclear reactions iron do not release energy (Fe has lowest mass...)
  • Evidence for Helium Capture
    
    • Higher abundances of elements with hundreds of protons.

How does a high-mass star die?

• Iron builds up in the core until degeneracy pressure can no longer resist gravity.
  
  • The core suddenly collapses.
• Supernova Explosion
  
  • Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos.
    Neutrons collapse to the center, forming a neutron star.
  • Energy and neutrons released in a supernova explosion enable elements heavier than iron to form, including Au and U.
• Supernova Remnant
  
  • Energy released by the collapse of the core drives outer layers into space.
  • The Crab Nebula is the remnant of the supernova seen in 1054 AD.
• Supernova 1987A
  
  • The closest supernova in the last four centuries was seen in 1987.
  
  

12.4 Summary of Stellar Lives
How does a star's mass determine it's life story?

• Role of Mass
  
  • A star's mass determines its entire life story because it determines its core temperature.
  • High-mass stars have short lives, eventually becoming hot enough to make iron.
• Life Stages of Low-Mass Stars
  
• Reasons for Life Stages
  
  • Core shrinks and heats
• Life Stages of High-Mass Star