PHYSICS 20B Lecture Notes - Lecture 10: Main Sequence, Stellar Evolution, Star Formation
Lecture 10:
Star Formation, Evolution, and Death (Part One)
● Main Sequence
○ When stars reach an equilibrium state, they join the main sequence, in which is
spends its most of their life in this state, until it cannot fuse anymore, then it
jumps off the main sequence and begins to die.
○ There is a strong correlation between a star’s temperature and luminosity at this
state
○ Hotter = more luminous, shorter life, and more massive
● Is there a lower limit to the mass of a star?
○ In order for stars to begin nuclear reactions, they need to have enough
gravitational energy to create a high velocity of atoms, thus high-pressure cores
where nuclear fusion can occur
○ The threshold is at least 8% mass of the sun, if it has less, nuclear fusion can not
occur, the star will grow dimmer for the lack of high velocity atoms, thus lack of
high pressure cores, thus lack of high temperature which affects the glow. The
star will grow dimmer as it grows, and if this occurs, sometimes the star may be
called a failed star for failing to produce the necessary high velocity, high
pressure due to its mass
■ Very similar to how premature babies are affected with their life
expectancy
○ Thus the Luminosity of a Main Sequence star is determined by its mass
● How long do stars live?
○ Low-mass star
■ Like a small fuel tank vehicle, it can have great mileage
○ High-mass star:
■ Like a huge fuel tank, it can burn through its fuel very quickly, affecting its
mileage
● Stellar Evolution
○ When a star runs out of Hydrogens to fuse with, it moves the Main sequence,
and begins its path in becoming a “red giant”
○ Eventually, a star’s core temperature will reach new levels of heat and thus will
allow the star to star fusing with Helium, which can be fused to make Carbon,
and similar to how the star ran out of hydrogen, it runs out of helium, and then so
on and so on
● The evolution of massive stars
○ Low mass stars stop fusing once they get to carbon and oxygen
○ High mass stars continue to fuse past carbon and oxygen because of the higher
gravitational force, thus higher atom velocity, thus higher pressure, thus hotter
temperature that allow more possible fusions
■ However, once the high mass star gets an iron core, it can not fuse
anymore, breaking off the equilibrium, and gravitational contraction takes
over, contracting the star upon itself
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Document Summary
There is a strong correlation between a star"s temperature and luminosity at this state. Hotter = more luminous, shorter life, and more massive. In order for stars to begin nuclear reactions, they need to have enough gravitational energy to create a high velocity of atoms, thus high-pressure cores where nuclear fusion can occur. The star will grow dimmer as it grows, and if this occurs, sometimes the star may be called a failed star for failing to produce the necessary high velocity, high pressure due to its mass. Very similar to how premature babies are affected with their life expectancy. Thus the luminosity of a main sequence star is determined by its mass. When a star runs out of hydrogens to fuse with, it moves the main sequence, and begins its path in becoming a red giant . Low mass stars stop fusing once they get to carbon and oxygen.
