STELLAR EVOLUTION OVERVIEW
(For low and high mass stellar evolutionary track diagrams, click here.)
Link to a shockwave app that lets you graphically evolve different mass stars:   Watch stars evolve across the HR diagram 

I.  Protostar Stage        (Note:  color blocks = color of stage on color-coded evol. tracks)

A.  Jean's Theory:  gravity over pressure; at the critical density, it predicts mass of initial collapsing gas cloud as a function of temperature

1.  critical density = density of cloud at which the "self-gravity" overcomes its internal pressure and a region in the cloud begins to collapse together
2.  total mass of collapsing region depends on the temperature, for molecular clouds at ~10 K the mass is sufficient to produce several thousand stars, consistent with observed galactic star clusters
3.   subfragmentation = internal turbulence in the collapsing region breaks the cloud into smaller collapsing clouds

B.  Molecular Clouds reach critical density

1.  supernova shock waves or other perturbations initiate collapse of regions within a molecular cloud
2.  free fall collapse of star cluster size mass which then subfragments several times resulting in several thousand collapsing clouds with individual stellar masses (from 0.1 to 100 solar masses)

C.  protostar stage defined: "free-fall" collapse of an individual stellar mass

1.  stage begins with "free-fall" collapse of individual stellar mass size clouds
2.  stage ends when the "collapse" slows to a "contraction" as pressure builds up in the core of what is now a self-gravitating ball of gas
3.  this discussion has neglected certain details such as the dissociation of the hydrogen molecules and the effects of magnetic fields on the collapsing material

II. Pre-Main Sequence Stage (T Tauri Star)     

A.  collapse slows (gravitational energy to heat -> ionized gas absorbs the resulting radiation from the heat and the pressure builds)

1.  central core of gas cloud reaches roughly stellar dimensions
2.  conservation of angular momentum (L) causes the remaining infalling material to form a proto-planetary disk ("proplyd") around the pre-MS star (material falling perpendicular to the axis of rotation of the system "spins-up" into orbit around the pre-MS star while the material falling down the axis of rotation reaches the star more directly or settles into the disk)
3.  polar outflow (bipolar jetting) begins; the high spin rate and almost totally convective nature of the central pre-MS star produces strong magneto-hydrodynamical (MHD) effects on the material spiraling in from the disk, ejecting it out along the axis of rotation of the star and allowing the star to transfer some of its initial angular momentum to the disk
4.  the jet material sometimes radiates light as it plows into the surrounding interstellar medium, these glowing blobs of gas were called Herbig-Haro objects after the astronomers who first catalogued them

B. Brown Dwarfs, substellar mass (< 0.1Msun = 80 Mjupiter)

1.  brown dwarfs are considered to have a range of masses from 2 Mjupiter < 80 Mjupiter ; however, some extra-solar planets that have been discovered have masses in excess of 2 Mjupiter and are not considered brown dwarfs; while the difference between brown dwarf and super-planet is "fuzzy", perhaps we could say that brown dwarfs form at the center of a proplyd and a super-planet forms in the disk itself around a central body
2. in any case, the brown dwarf never initiates sustained nuclear fusion in its core

C. stellar disruption? (> 100 Msun)

1. a pre-MS star with mass greater than ~100 solar masses is though to initiate an extremely rapid, runaway fusion ignition of hydrogen in core as it reaches the main-sequence stage
2. this hydrogen ignition is the equivalent of a gigantic hydrogen bomb explosion which is thought to completely disrupt the star leaving nothing behind (note:  this is NOT a supernova explosion)
3.  stellar disruption has never been observed and may be an extremely rare or even non-existent event, it may be that unknown mechanisms could limit the maximum size of collapsing gas clouds such that individual collapsing gas clouds of the required size may never occur
You Tube video showing the range of star (and planet) sizes.  Brown Dwarfs could actually be smaller in diameter than Jupiter and White Dwarfs are about the size of the Earth.  Neutron Stars are only the size of a medium sized asteroid (~10 km diam) and of course Black Holes are singularities (a "point" - zero diameter) of infinite density, but the event horizon surrounding it would be approximately 10 km in diameter or less.

 

III.  MS Stage (Main Sequence)     

A.  low mass stars (cooler than ~F0)

1.  proton-proton chain (H to He)
2.  radiative core, convective surface
a.  hydrogen depleted in the core from the center out, produces a "smooth" transition to shell "burning" of hydrogen at the end of the MS stage
b.  hot corona due to surface convection and resulting strong magnetic "heat-pump" effect

B. high mass stars (hotter than ~F0)

1. CNO cycle (H to He); hydrogen fusion reaction which uses carbon, nitrogen & oxygen as catalysis
2. convective core, radiative surface
a. H depleted evenly over entire core region, produces a "discontinuous" transition to shell "burning" of hydrogen at the end of the MS stage; entire core ceases hydrogen fusion before the star restructures itself and a thin shell of hydrogen fusion begins
b. cool corona heated only by radiation from surface, no surface magnetic effects to heat the corona

IV.  Red Giant Stage (1st RG Stage)     

A.  H to He fusion moves to shell around H depleted core (He core)

1. low mass:  smooth transition to thick H burning shell due to radiative core of low mass stars; core H is depleted from the very center outward so that H to He burning ceases first in the very center of the star and the H depleted core (He core) slowly grows from the center leaving a thick H to He burning shell
a.  apparently larger core causes outer envelope to expand increasing the surface area (less power per unit area at surface), surface temperature cools and the star reddens
b.  thick H to He shell quickly begins to produce as much power as core H to He stage so star moves towards the right (cooler surface temperatures) on the HR diagram without changing its luminosity by much
c.  He core evolution becomes somewhat independent of outer layers and the He core begins gravitational contraction, increasing the temperature of the core and shell which increases the fusion rate in the shell, increasing total power output; surface expands as total power output increases causing the power per unit area to remain roughly constant (little change in surface temperature) while the star moves dramatically upwards (higher luminosity) on the HR diagram
2.  high mass:  discontinuous transition to thin H burning shell due to convective core of high mass stars; core H burning ceases because the convective nature of the core allows He to build up through out the entire core (not just primarily in the center of the core), when the H is depleted in the entire core the H to He reactions cease and the star readjusts its structure (pressure verses gravity) until a thin shell of H to He burning begins just outside the He core
a.  apparently larger core causes outer envelope to expand increasing the surface area (less power per unit area at surface), surface temperature dramatically cools and the star reddens
b.  thin H to He shell at first  produces slightly less power than the core H to He stage so star moves towards the right (cooler surface temperatures) and slightly lower (less luminosity) on the HR diagram
c.  He core evolution becomes somewhat independent of outer layers and the He core begins gravitational contraction, increasing the temperature of the core and shell which increases the fusion rate in the shell, increasing total power output; surface expands as total power output increases causing the power per unit area to remain roughly constant (little change in surface temperature) while the star moves upwards (higher luminosity) on the HR diagram

B.  He core ignition (triple alpha process - He to Carbon)

1.  low mass: He flash - He burning begins simultaneously throughout entire core due to the isothermal nature of the degenerate electron gas
a.  before He ignition, the core continues to contract until electrons in the plasma are approximately as densely packed as electrons in an atom, at this point a quantum mechanical effect called degeneracy pressure becomes a major component of the pressure in the star's core
b.  electron degeneracy pressure:  quantum mechanical effect caused by the Pauli Exclusion Principle which states that no two electrons can occupy exactly the same state:  see page 457, The Cosmic Perspective, Bennet, Donahue, Schneider, Voit (intro. astronomy textbook)  [When the "electron gas" in the core of a low mass star compresses to the same "number density" of electrons as is found in an atomic nucleus - the Pauli Exclusion Principle predicts (and of course we observe this to be true) that the electrons will resist getting closer together and a quantum mechanical pressure results called "electron degeneracy pressure".]  Note:  the Pauli Exclusion Principle pertains to certain other particles such as neutrons; such that there can be a "neutron degeneracy pressure" when neutrons are compacted to the same density as in an atomic nucleus - see neutron stars below.
c.  selected characteristics of degeneracy pressure:  does not depend on temperature of the gas as does regular "thermodynamic" pressure, gas takes on characteristics of a "metal" such as incompressibility (takes enormous pressures to compress the degenerate gas, the core behaves somewhat like a hard sphere made of metal) and high thermal conduction (temperature is "isothermal" throughout the degenerate gas)
2.  high mass: smooth transition, radiation pressure prevents degenerate electron gas in the core and He ignition begins at the very center of the core and slowly spreads outward

C.  post He core ignition, surface temperature increases and star "moves" toward the blue side of the HR diagram

1.  low mass:  rapid evolution toward blue side of HR diagram with decreased luminosity; post He flash evolution is difficult to model due to the rapid changes brought on by the He flash, therefore the resulting luminosity and surface temperature is uncertain... stars with small heavy element abundance's are thought to end up on the horizontal branch - stars on the HB have a large range of colors, but an average luminosity about 50 times greater than the Sun
2.  high mass:  rapid evolution toward blue side of HR diagram with a slight decrease in luminosity begins a "blue loop" in the evolutionary tracks of high mass stars

D.  blue edge of the HB or "loop":  for both low and high mass stars, the core He fusion increases after He ignition and the H shell fusion decreases - this results in a slow increase in luminosity and a relatively stable surface temperature defining the blue edge of the HB (low mass) or "blue loop" (high mass)

V. Variable Stage (Pulsating Variable Stars)     

A.  during the core He burning HR Diagram "loop" the star passes through the "instability strip"

1.  pulsation is driven by a "feed-back mechanism" in the He envelope where ionization of the He envelope increases the opacity in that region making the He envelope act like a "shutter" which traps the outgoing energy from the core (increasing pressure) causing the star to expand, as the star expands outward the temperature in the He envelope drops allowing the gas to become neutral or "un-ionized" again, releasing the energy (lowering the pressure) and allowing the star to contract; this mechanism "pumps" the star once per pulsation cycle allowing the amplitude of the pulsation to build up until dissipative forces limit the amplitude
2.  the "instability strip" is the region in the HR Diagram where the He envelope is at the correct depth in the structure of a He burning  star to drive stellar pulsation; for a given luminosity there is a range of surface temperatures where the He envelope is at the correct depth to drive stellar pulsation by the above mechanism, mapped out on the HR Diagram the region is a narrow strip running roughly vertically through the region of the diagram where core He burning is taking place in stars
a.  for low mass stars where the Horizontal Branch runs through the "instability strip" the pulsating stars are called: RR Lyrae stars
b.  for high mass stars where the Blue Loop runs through the "instability strip" the pulsating stars are called: Cepheid variable stars

C. during the AGB stage (see VI & VII below) unstable shell sources may also drive periodic, semi-periodic, or irregular pulsation

VI. Asymptotic Giant Branch (2nd RG Stage)           (this stage is mostly defined for low mass stars and to a lesser extent those high mass stars which start off on the MS with less than 8 solar masses and end their lives as white dwarfs)

A. He to C fusion moves to shell around He depleted core (low & high mass stars)

1.  apparently larger core causes outer envelope to expand increasing the surface area (less power per unit area at surface), surface temperature cools and the star reddens moving the star out of the "instability strip"
2.  as in the first shell source, the (in this case C) core evolution becomes somewhat independent of outer layers and the C core begins gravitational contraction, increasing the temperature of the core and shell which increases the fusion rate in the shell, increasing total power output; surface expands as total power output increases causing the power per unit area to remain roughly constant (little change in surface temperature) while the star moves dramatically upwards (higher luminosity) on the HR diagram
3.  luminosity becomes greater than 1st RG stage, star expands to tremendous radius (~1.5 - 10 AU) (note: a 10 AU radius star's surface would extend out to Saturn's orbit if it where located in our solar system)
a.  most of stellar mass of these expanded stars are still concentrated near their core, which means their surface gravity is very weak and results in a very low escape velocity at their surface
b.  due to the low surface gravity these stars lose mass readily in intense stellar winds which cause significant mass loss (especially for high mass stars); this causes some higher mass stars to follow the same latter stages of evolution as a low mass star, becoming PN stars and then terminating as white dwarfs rather than exploding as a supernova (stars on the MS with less than 8 solar masses)
c.  the "slow" neutron capture process (s process - reactions that take place slower than beta decay) results in small amounts of elements heavier than iron and lighter than uranium being produced during a star's AGB stage, these elements can be "dredged up" by convection and blown out into the interstellar medium by the intense stellar winds described above

B. massive stars may undergo several "blue loops" as core fusion of successively heavier elements occurs

1.  the main fusion reactions advance through the process of "alpha capture" (since an alpha particle is the same thing as a helium nucleus, this is sometimes referred to as "helium capture"):   stellar nucleosynthesis produces all of the elements heavier than He
2.  for very massive stars several "blue loops" can occur as core fusion of successively heavier elements left over from previous stages occurs:  carbon (C) to oxygen (O) & neon (Ne), and so on all the way up to iron (Fe)
a.  the final internal structure of a massive star has several fusion shells:  H -> He, He -> C, C -> O, O -> Ne, Ne -> Si, Si -> Fe, with an Fe core; all these layers are near the core where most of the mass of the star resides, outside the H -> He shell source is an extended envelope of H of tremendous radius; this layered structure is known as the "onion skin" model
b.  the Fe in the core takes more energy to fuse than the fusion reaction gives off, becoming an energy sink (absorbs energy) and allowing the pressure to drop causing the core of the star to collapse which initiates a supernova explosion (see transition stage below)

VII.  Transition Stage

A.  low mass stars (core mass less than 1.4 solar masses    

1.  near the end of the AGB stage, shell sources "flicker" and die as they gradually burn outward into less dense and lower temperature regions, when the shell source "goes out" the star readjusts (contracts) and the shell source can reignite re-establishing the pressure and causing a pulsation of the star
a.  these pulsation (sometimes called "thermal pulses") can gently (non-explosively) eject large portions or even the entire outer envelope of the star at the end of its life
b.  at this point, energy production ends in the mostly carbon core which is now supported by electron degeneracy pressure (no nuclear reactions to produce pressure means the core contracts until the electrons are close enough together for electron degeneracy pressure to arise); note:  the core must be less than 1.4 solar masses for electron degenerate pressure to support the core against collapse; 1.4 solar masses is the mass of a self-gravitating ball of electron degenerate gas at which the radius of the ball of gas goes to zero, ie:  electron degeneracy pressure fails to support the gas against gravitational collapse, 1.4 solar masses is called the Chandrasekhar limit for the astrophysicist who calculated this limit
c.  extremely hot surface of core revealed as outer layers expand away and the star rapidly evolves toward the blue side of the HR Diagram
2.  Planetary Nebulae stage
a.  hot core (now called the planetary nebula central star) emits UV photons which ionize the expanding shell of gas which becomes luminous and is called a planetary nebula
b.  this expanding shell of gas is "pinched off" at the waist by pre-existing material in a disk around the star (some of this material results from previous intense stellar winds from the star); this results in an "hour-glass" or "dumb-bell" shape for the nebula; if seen from the side PNs have this bi-lobed structure, but if viewed down the long axis they look like a ring or helix of glowing gas
3. Type Ia supernova (binary star, explosion powered by nuclear energy)
a.  carbon-oxygen white dwarf (see VIII below) with red giant companion
b.  mass transfer to white dwarf initiates catastrophic carbon "burning" as the carbon core reaches 1.4 solar masses and collapses (ie: stellar size fusion bomb) - total disruption of star, no neutron star left behind

B.  high mass stars (core mass greater than 1.4 solar masses = Chandrasekhar limit    

1. Type II supernova (single star, explosion powered by gravitational collapse)
a.  due to the endothermic nature of  Fe fusion (takes more energy than the fusion releases) and photodisintegration of Fe (100 MeV gamma ray photons are absorbed by Fe nuclei which then disintegrate into many He nuclei and neutrons - a process which also is very endothermic) no pressure is generated in the core and it collapses from the weight of the overlying layers, slowed only briefly by electron degeneracy pressure that is quickly overcome by the collapsing weight
b.  as the electrons and nuclei are compressed even further, the electrons react with the protons to produce neutrons and neutrinos in a process called inverse beta decay; as the neutrons get squeezed closer together, they (like electrons) obey the Pauli Exclusion Principle and a quantum mechanical pressure called neutron degeneracy pressure begins which halts the collapse of the core
c.  the neutron degenerate core behaves like a "perfect metal" and cannot be compressed further by the infalling layers of the star which creates an intense shock wave which propagates back outward through the infalling layers of the star; this is known as the "core bounce"
d.  the neutrinos from inverse beta decay are able to deposit vast amounts of energy into the extremely dense material below the shock wave, heating the material and causing intense convection which sustains the shock wave allowing it to eject the outer layers from the star; the shock wave heats and compress these layers as it passes through them causing "rapid" neutron capture processes (r process - reactions that take place faster than beta decay) producing those rare elements heavier than uranium
2.  Type Ib supernova (single star, powered by gravitation)
a.  outer H envelope lost by intense stellar wind (Wolf-Rayet star)
b.  same mechanism as Type II SN, but no H in spectra

VIII.  Terminal State

A.  Low Mass Stars     

1.  as the Planetary Nebula dissipates, the PN central star which is the left-over hot, but very small core held up by electron degeneracy pressure (no nuclear reactions) ends up on the lower left region of the HR Diagram:  faint but hot; this terminal state of a low mass star is called a White Dwarf star; note:  a White Dwarf star contains about a solar mass of matter compressed into a ball the size of the Earth, a million times more dense than a regular star
2.  Black Dwarf? (white dwarf after much cooling, takes longer than the present age of the universe)
3.  Type Ia SN result in total disruption of the star

B.  High Mass Stars     

1.  after a type II supernova explosion the surviving core of neutrons held up by neutron degeneracy pressure is called a Neutron Star; this ball of neutrons is as dense as an atomic nucleus and in fact can be described as a giant nucleus made up entirely of neutrons, typically there is more than 1.4 solar masses compressed into the size of an asteroid (~10 km in diameter) approximately 2 billion times more dense than a White Dwarf star
a.  the neutron star has an intense magnetic field caused by "compression" of any pre-existing stellar magnetic field
b.  the neutron star is expected to have a very high rotation rate caused by conservation of angular momentum as the mass of the rotating precursor star is compressed into the neutron star
c.  the high rotation rate and the intense magnetic field combine to produce beams of electromagnetic energy which rotate around with the spinning neutron star like light-house beams; if radio telescopes on Earth can detect the radio pulses as the beam sweeps over the Earth, the spinning neutron star is called a pulsar
2.  Black Hole?
a.  a collapsing core mass greater than 2.5 - 3.0 solar masses may overcome even the neutron degeneracy pressure?
b.  as far as we know, nothing can halt the collapse after it overcomes the neutron degeneracy pressure and the core collapses to a point of infinite density called a singularity
c.  event horizon: radius from singularity where escape velocity = c  (c = speed of light = 3 x 108 m/s ); by Einstein's Theory of Relativity, nothing can exceed the speed of light in a vacuum, so we have no information about the condition inside the event horizon; note:  many movies and TV programs depict a black hole as "sucking" everything near it down into its singularity, however, while there are relativistic effects close to the event horizon, most objects outside the event horizon would orbit or continue the same trajectory they would have around any object of the black hole mass whether it was a black hole or not, it is only if an object approaches closer than the event horizon that it would never return; Comment:  Dr. Jones actually knows someone who has fallen into a black hole, click here for details!
d.  stellar size black holes can be detected by the gravitational effects on an accretion disk if the black hole is accompanied by a companion star; eg:  bipolar jets, high energy radiation as material is heated by tidal forces as it approaches the event horizon, etc