• Overall Cause: Gravity
  
  2 conditions must be present:
• cloud must be cool enough
• cloud must be dense enough

1. Interstellar cloud
2. Interstellar cloud fragment
3. fragmentation ceases
4. Proto-star/T Tauri star - Hayashi track
5. Proto-star Evolution
6. New Star - H fusion begins in core 10mK
7. Main Sequence (ZAMS)

prestellar object hot enough to emit IR, but not hot enough for fusion.

• Stars Mass #1 factor that determines how a star evolves.
• The more masive the star - the hotter and faster it burns fuel (evolves)

• Proto-stars exhibit bi-polar flow: to collimated jets of gas flowing out from the star from opposite directions.
• as cloud contracts forms disk (pizza dough)
• disk around new stars are important - clumps of matter in disks could form proto-planets.

• High Mass stars use the CNO Cycle.
• CNO cycle requires the presence of C,N,O to occur
• H converted to He
• He converted to C
• C to heavier elements, eventually Fe
• Once Fe - fusion over - star dies
• Carbon has 6 protons in nucleus (to make carbon fuse with H, it takes a lot more heat).

• He nucleus also called Alpha Particle
• 3 He in = 1 C nucleus out (i.e. triple alpha process)

• He flash- core pressure is so great that atoms are compressed together as closely as possible.
• Limit is called Pauli Exclusion Principle - temp rises rapidly, rate of fusion increases - explosive beginning to He flash.
  
  *Our sun will not get hot enough for C fusion to take place. Eventually outparts of star escape leaving behind naked carbon core.
• Escaped gas = Planetary Nebula
• White Dwarf Star
• Pauli Principle prevents further compression - Not a Diamond!
• Chandrasekhar Limit = 1.4 solar mass

Notes

• Large High Mass Stars
• H-He,He-C,C-O,O-Ne,Ne-Fe, star dies.
• Supernova! Spectacular Explosion!!

1. Varible Stars - stars that vary in brightness
   
   
2. Star Clusters - densely packed group of stars

• Cepheid Varibles: class of stars that vary in luminosity (i.e north star)
• **Cepheid varibles become tools for finding distance! (apparent mag./absolute mag for period - calculate distance)
• Period Lumnosity Relationship: Longer period, higher average luminosity.
• RR Lyrae Stars: all have about the same luminosity. *also distance indicators..

• Globular: very old, outside plane of galaxy, 100,000 stars very crowded.
• Open/Galactic: vary in age, occur inside plane of galaxy, 100-1000 of stars.
• Stars in clusters formed from same gas cloud: same age, same chemical composition, same distance away.
• Turn-off point determines clusters age.

Exploding Stars

1. Nova
2. Supernova Type I
3. Supernova Type II

• White Dwarf Binary System
• White Dwarf accretes matter from Companion Star (Red Giant)
• H gets real Hot, H gets very dense
• Fusion begins on surface H & quickly covers star.
  
  Can happen multiple times
  Core Star unharmed

** A white dwarf exceeds ChandraseKhar Limit 1.4 M.

            similar process to forming nova EXCEPT:

• White Dwarf gets so massive that gravity overcomes electron degeneracy pressure (Chandrasekhar limit)
• One humougous fusion C bomb!
• Supernova brighter than all of the stars in the galaxy combined.
• Type I - has no H spectral lines.

*** the core of a massive star collapses

Exploding high mass star

When Fe core collapses (collision triggers explosion)

Pressure is so great that electrons are squeezed into nucleus of atoms. The electrons neutralize p + e = n + v

v fly into space

n squeeze together until they are touching

core becomes one big neutron

leaves behind pulsar/blk hole. Exhibits H spectra lines.