Amber Reed's Notes

Chapter 12: Star stuff

12.1
How do star form?
-stars form in dark clouds of dusty gas in interstellar space
-e gas between the  stars called e interstellar medium 
- gravity can create stars only if it can overcome the force of thermal pressure in a cloud
-gravity within contracting gas cloud becomes stronger as the gas becomes denser
Mass of a star forming cloud :
--a typical molecular cloud must contain at least a few hundred solar masses of gravity to overcome ( T~30k, n~300 particles/ cm^3)
- the cloud can prevent a pressure build up by converting thermal energy into infrared and radio photons that escape the cloud 
fragmentation of cloud:
-ThIs simulation begins with a turbulent cloud counting 50 solar masses of gas
-the random moons of different sections of the cloud cause it to become lumpy 
-Each lump of the cloud in which gravity can overcome pressure can go on to become a star
-a large cloud can make a whole cluster of stars 

Glowing dust grains:
-as star begin to form , dust grains that absorb visible light heat up and emit infrared light 
-long wavelength infrared light is brightest from regions where many stars are currently forming

-Solar system formation is a good example of star birth
-Cloud heats up as gravity causes it to contract due to conservation of energy. Contraction can continue if thermal energy radiated away. 
- as gravity forces a cloud to become smaller , it begins to spin faster and faster due to conservation of angular momentum. 
-as gravity forces a cloud to become smaller it begins to spin faster and faster due to conservation of angular momentum . Gas settles into spinning disk because spin hampers
-flattening : collision between particles in it cause it to flatten .
Formation of jets : 
-rotation also causes jets of matter to shoot out Along the rotation axis
-jets are observed coming from the centers of disks around protostars 
Protostar to main sequence:
-Protostar contracts and heats until the core temp is sufficient for hydrogen fusion
-contraction  ends when energy released by hydrogen fusion balances energy radiated form the surface
-it takes 30 million years for a star like the 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 schedules 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 in a stats 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 their 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 300Msun
Lower limits : 
-fusion will not begin in contracting cloud if some sort of force stops contraction before the core temp rises above 10^7 K
-thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation

Degeneracy pressure :
-Laws if quantum mechanics prohibit two electrons from occupying the same state in the same place 

Thermal pressure: depends on heat content 
-the main form of pressure in most stars

Degeneracy pressure:
-particles can't be in same state in same place
-doesn't depend on heat content 

Brown dwarfs :
- degeneracy pressure halts the contraction of objects with <.08Msun before the core temp becomes hot enough for fusion
-starlike objects not massive enough to start fusion are brown dwarfs
- a brown dwarf emits infrared light because of heat leftover from contraction

Brown dwarfs in Orion:
-infrared observations can reveal recently formed brown dwarfs because that are still relatively warm and luminous 

12.2 Life as Low Mass stars :

What are the life stages of a low mass star?
-a star remains in the main sequence as long as it can fuse hydrogen into helium in its core 

Life track after main sequence :
-observations of stars cluster show that a star becomes larger redder and more luminous after its time on the main sequence is over 

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 temp than Hydrogen fusion larger charge leads to greater repulsion
-The fusion of two helium nuclei doesn't work so helium fusion must combine three He nuclei to make carbon
Helium flash 
-thermostat is Bren in low mass red giant because degeneracy pressure supports the core

-Helium core fusion stars enthused shrink nor grow because the core thermostat is temp fixed
-models show that red giant should shrink and become  less luminous after helium fusion begins in the core
-Combining models of stars of similar age but different mass helps 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
-this double shell fusion stage never reaches equilibrium the fusion trade periodically spikes

-Double shell fusion ends with a pulse that enacts the H and He into space as a planetary nebula 
-the core left behind becomes a white dwarf 

End of fusion :
-fusion progresses no further in low mass star because the core temp never grows hot enough for fusion of heavier elements ( some He fuses to C to make oxygen)
-degeneracy pressure supports the white dwarf
-Life stages of low Mass star such as Sun 


12.3 Life as a high mass star:

What are the life stages of high mass star?
A high mass star lives a much shorter life than a low mass star, fusing hydrogen into helium via the CNO cycle. After exhausting its core hydrogen , high mass star begins hydrogen shell fusion and then goes through a series fusing successively heavier elements. The furious rate of this fusion makes the star swell in size to become  supergiant. 


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


in its final stages of life, a high mass stars core becomes hot enough to fuse carbon and other heavy elements. the variety if different fusion reactions produces a wide range of elements-including all the elements necessary for life- that are then released into space when that stars dies

How dies a high mass star die?

A high mass star dies in a cataclysmic  explosion called supernova, scattering newly produced elements into space and leaving behind a neutron star or black hole. The supernova occurs after fusion begins to pile up iron in the high mass star's core.Because iron fusion cannot release energy, the core cannot hold off the crush of gravity for long. In the instant that gravity overcomes degeneracy pressure, the core collapses and the star explodes. 

How does a star's mass determine the life story?

A star's mass determines how it lives its life. Low-mass stars never get hot enough to fuse carbon or heavier elements in their cores and end their lives by expelling their outer layers and leaving white dwarfs behind. High-mass stars live short but brilliant lives, ultimately dying in supernova explosions.

How are the lives of stars with close companions different?
When one star in a close binary system begins to swell in size at the end of its main-sequence stage, it can begin to transfer mass to its companion. This mass exchange can then change the remaining life histories of both stars. 



Amber Reed