A&B's Astronomy Lab, Spring 2002, No. 3

Stars - Birth to Death

How do stars form?
Why do they not evolve identically?
When and how do stars die?

Stellar Evolution and HR Diagram

The H-R Diagram is an extremely useful way to follow the changes that take place as a star evolves. Most stars are on the Main Sequence because that is where stars spend most of their lives, burning hydrogen to helium through nuclear reactions. As stars live out their lives, changes in the structure of the star are reflected in changes in stars temperatures, sizes and luminosities, which cause them to move in tracks on the HR Diagram. Stellar Evolution is driven entirely by the never ending battle between Pressure and Gravity. As imbalances are reached, the star is driven to find a new Energy source. Each new stage in stellar evolution is hence marked by a different energy generation mechansism (see the diagram below).

Stellar Evolution I - Solar Type Stars

The actual process of star formation remains shrouded in mystery because stars form in dense, cold molecular clouds whose dust obscures newly formed stars from our view. For reasons which are not fully understood, but which may have to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds pass through spiral structure in galaxies, or magnetic-gravitational instabilities (or, perhaps all of the above) the dense core of a molecular cloud begins to condense under its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars. As the cloud condenses, gravitational potential energy is released - half of this released gravitational energy goes into heating the cloud, half is radiated away as thermal radiation. Because gravity is stronger near the center of the cloud (remember F_g ~ 1/distance²) the center condenses more quickly, more energy is released in the center of the cloud, and the center becomes hotter than the outer regions. As a means of tracking the stellar life-cycle we follow its path on the HR Diagram.

1. Protostar
The initial collapse occurs quickly, over a period of a few years. As the star heats up, pressure builds up following the Perfect Gas Law:
PV = NRT
where, most importantly P=pressure and T=Temperature. The outward pressure nearly balances the inward gravitational pull, a condition called hydrostatic equilibrium. The star is cool, so its color is red, but it is very large so it has a high luminosity and appears at the upper right in the H-R Diagram.
PROTOSTARS
Age: 1 ~ 3 yrs
R ~ 50 R_sun
T_core = 150,000 K
T_surface = 3500 K
Energy Source: Gravity

2. Pre-Main Sequence
Once near-equilibrium has been established, the contraction slows down, but the star continues to radiate energy (light) and thus must continue to contract to provide gravitational energy to supply the necessary luminosity. The star must continue to contract until the temperatures in the core reach high enough values that nuclear fusion reactions begin. Once nuclear reactions begin in the core, the star readjusts to account for this new energy source Gravity releases its potential energy throughout the star, but due to the very high temperature dependence of the nuclear fusion reactions, the proton-proton chain is highly centrally concentrated. During this phase the star lies above the main sequence; such pre-main sequence stars are observed as T-Tauri Stars, which are going through a phase of high activity. Material is still falling inward onto the star, but the star is also spewing material outward in strong winds or jets.
PREMAIN SEQUENCE
Age: 10 million yrs
R ~ 1.33 R_sun
T_core = 10,000,000 K
T_surface = 4500 K
Energy Source: P-P Chain turns on

3. Zero Age Main Sequence
It takes another several million years for the star to settle down on the main sequence. The main sequence is not a line, but a band in the HR Diagram. Stars start out at the lower boundary, called the Zero-Age Main Sequence referring to the fact that stars in this location have just begun their main sequence phases. Because the transmutation of Hydrogen into Helium is the most efficient of the nuclear burning stages, the main sequence phase is the longest phase of a star's life, about 10 billion yrs for a star with 1 solar mass.
ZAMS
Age: 27 million yrs
R ~ R_sun
T_core = 15,000,000 K
T_surface = 6000 K
Energy Source: P-P Chain in core

During the main sequence phase there is a "feedback" process that regulates the energy production in the core and maintains the star's stability. The basic physical principles are:
A. The thermal radiation law, L ~ R²T⁴, determines the energy output, which fixes requirement for nuclear energy production.
B. The nuclear reaction rates are very strong functions of the central temperature; Reaction Rate ~ T⁴ for the P-P Chain.
C. The inward pull of gravity is balanced by the gas pressure which is determined by the Ideal Gas Law: PV=NRT.
A good way to see the stability of this equilibrium is to consider what happens if we depart in small ways from equilibrium: Suppose that the amount of energy produced by nuclear reactions in the core is not sufficient to match the energy radiated away at the surface. The star will then lose energy; this can only be replenished from the star's supply of gravitational energy, thus the star will contract a bit. As the core contracts it heats up a bit, the pressure increases, and the nuclear energy generation rate increases until it matches the energy required by the luminosity. Similarly, if the star overproduces energy in the core the excess energy will heat the core, increasing the pressure and allowing the star to do work against gravity. The core will expand and cool a bit and the nuclear energy generation rate will decrease until it once again balances the luminosity requirement of the star.

4. End of Main Sequence
Age: 10 billion yrs
Energy Source: P-P Chain in shell around core

5. Post Main Sequence
Age: About 1 billion years from Point 4
R ~ 2.6R_sun
T_surface = 4500 K
Energy Source: P-P Chain in shell,
Gravitational contraction of core

6. Red Giant - Helium Flash
As the Helium core of the star contracts, nuclear reactions continue in a shell surrounding the core. Initially the temperature in the core is too low for fusion of helium, but the core-contraction liberates gravitational energy causing the helium core and surrounding hydrogen-burning shell to increase in temperature, which, in turn, causes an increase in the rate of nuclear reactions in the shell. In this instance, the nuclear reactions are producing more than enough energy to satisfy the luminous energy output. This extra energy output pushes the stellar envelope outward, against the pull of gravity, causing the outer atmosphere to grow by as much as a factor of 200. The star is now cool, but very luminous - a Red Giant.
RED GIANT
Age: 100 million yrs from Point 5
R ~ 200 R_sun
T_core = 200,000,000 K
T_surface = 3500 K
Energy Source:
P-P Chain in shell around core
Ignition of Triple-Alpha Process

The contraction of the core causes the temperature and density to increase such that, by the time the temperature is high enough for Helium nuclei to overcome the repulsive electrical barrier and fuse to form Carbon, the core of the star has reached a state of electron degeneracy. Degeneracy comes about due to the Pauli Exclusion Principle, which prohibits electrons from occupying identical energy states. The core of the Red Giant is so dense that all available lower energy states are filled up. Because only high-energy states are available, the core resists further compression -- there is a pressure due to the electron degeneracy. This pressure has a significant difference from pressure produced by the Ideal Gas Law -- it is independent of temperature. This removes a key element in the feedback-stability mechanism that regulates hydrogen burning on the main sequence.

7. Helium Burning Main Sequence
Once again the core of the star readjusts to allow for a new source of energy, in this case fusion of Helium to form Carbon via the Triple-Alpha Process. The Triple alpha process releases only about 20% as much energy as hydrogen burning, so the lifetime on the Helium Burning Main Sequence is only about 2 billion years. During this phase some Carbon and Helium will fuse ¹²C + ⁴He --> ¹⁶O, resulting in the formation of a Carbon-Oxygen core. When the Helium is exhausted in the core of a star like the sun, no further reactions are possible. Helium burning may occur in a shell surrounding thecore for a brief period, but the lifetime of the star is essentially over.
HELIUM BURNING MS
Age: About 10,000 yrs from point 6
T_surface = 9000 K
T_core = 200,000,000 K
Energy Source:
Triple-alpha process in core;
P-P Chain in shell

8. Planetary Nebula
When the helium is exhausted in the core of a star like the sun, the C-O core will begin to contract again. Central temperatures will never reach high enough values for Carbon or Oxygen burning, but the Helium and Hydrogen burning shells will conyinue burning for a while. Throughout the star's lifetime it is losing mass via a stellar wind, like the solar wind. This mass loss increases when the star swells up to the size and low gravity of a Red Giant. During Helium Burning, thermal pulses, caused by the extreme temperature sensitivity of the 3-alpha Process, can cause large increases in luminosity with accompanying mass ejection. During Helium Shell Burning, a final thermal pulse produces a giant "hiccough" causing the star to eject as much of 10% of its mass, the entire outer envelope, revealing the hot inner regions with temperatures in excess 100,000 K, shown in this animation of the Helix, below. The resulting Planetary Nebuala is the interaction of the newly ejected shell of gas with the more slowly moving ejecta from previous events and the ultraviolet light from the hot stellar remnant, which heats the gas and causes it to fluoresce. The Ring Nebula in Lyra (Messier Database, Web Nebulae) shown here is the prototypical Planetary Nebula. Rather than a spherical shell as initially believed, the Ring's shape is probably a torus or cylinder of gas, seen nearly pole-on. Its age is estimated to be a few thousand years; the central star has a surface temperature over 100,000 K. The Planetary Nebula phase is relatively short lived, estimated to be about 25,000 years, and there are about 10,000 planetaries in the Milky Way.

9. White Dwarf
As the nebula disperses, the shell nuclear reactions die out leaving the stellar remnant, supported by electron degeneracy, to fade away as it cools down. The white dwarf is small, about the size of the earth, with a density of order 1 million g/cm3, about equivalent to crushing a volkswagen down to a cubic centimeter or a "ton per teaspoonful." A white dwarf star will take billions of years to radiate away its store of thermal energy because of its small surface area. The white dwarf will slowly move down and to the right in the H-R Diagram as it cools until it fades from view as a "black dwarf". To the right is the white dwarf companion to the nearby star Sirius.
WHITE DWARF
R ~ R_earth (a few thousand km)
T_surface = 30000 K - 5000 K
Energy Source: "Cooling Off"

The Russell-Vogt Theorem

It follows that the single most important determinant of the life-history for a star is its mass; this principle is called the Russell-Vogt Theorem. Important mass regimes for stellar evolution:

M_* < 0.01 M_sun - Planet. Jupiter, for example, has a mass of about 0.001 x M. Jupiter's temperature is slightly warmer than would be expected from the amount of solar energy it receives; this is interpreted as due to gravitational potential energy stored as heat from Jupiter's contraction out of the proto-solar nebula. But the energy balance for Jupiter and other planets is largely determined by the energy received from the sun and central temperatures never come close to the 1 million K required for even the simplest nuclear reactions.
0.01 M_sun < M_* < 0.085 M_sun - Brown Dwarf; these objects will never become hot enough in their cores to ignite the P-P Chain. Release gravitational potential energy will cause them to heat up to core temperatures as hot as 3 million K, hot enough for the first stages of nuclear reactions, perhaps, but never hot enough to establish stable hydrogen burning. With atmospheric temperatures Tsurface < 2000 K, brown dwarfs will be very faint, radiating the vast majority of their luminous energy in the infrared, and very hard to detect. A new near-infrared survey of the sky called 2MASS has detected a large number of cool stars, now classified as L-stars which are likely to be brown dwarfs. Here's a press release of another possible brown dwarf detected at Palomar and the Hubble Space Telescope.
0.085 M_sun < M_* < 0.4 M_sun - these stars will be very long lived, but will never reach temperatures hot enough for the Triple-alpha process to occur. They will not have a helium flash in the red giant stage nor a helium-burning main-sequence phase.
0.4 M_sun < M_* < 1.2 M_sun - these stars like the sun will burn hydrogen to helium via the P-P Chain and will burn helium to crabon via the Triple-alpha process following a path through the H-R Diagram essentially like that outlined in the table Stellar Evolution I - Solar Type Stars.
M_* > 1.2 M_sun - these stars will reach high enough core temperatures to burn hydrogen via the CNO cycle.
M_* > 8 M_sun - stars more massive than about 8 solar masses (this number is very uncertain compared with those above) will have a larger number of nuclear burning cycles and their cores will be more massive than the limiting mass of 1.4M, the largest mass that can be supported by electron degeneracy, and thus the largest possible mass for a white dwarf. As we shall see these stars end their lives with a cataclysmic explosion called a supernova.

Stellar Evolution II - Massive Stars

We may crudely distinguish between stars more massive than the sun and solar type stars in their evolutionary characteristics:

1. Massive stars live their lives more rapidly than do solar-type stars -- they "live fast and die young." One can determine relatively straightforwardly from the balance between gravity, pressure and temperature that the luminosity of a star will be approximately proportional to the Mass^3.5 (this is the Mass-Luminosity Relation which applies to all phases of stellar evolution). Since the available fuel is effectively the mass of the star, the lifetime will be approximately proportional to 1/Mass^2.5. A star of 10 solar masses can thus be expected to go through its life cycle about 300 times faster than the sun, with a main sequence lifetime of about 30 million years. (The most massive stars have lifetimes shorter than about a million years, while stars with masses less than about 3/4M_sun have lifetimes longer than the age of the Universe!)

2. As described above massive stars require higher central temperatures to balance the greater pull of gravity. This means that massive stars produce helium from hydrogen via the CNO cycle rather than the P-P Chain.

3. Higher central temperatures and pressure dictate that the stellar core will not become electron degenerate at the onset of helium burning, so there will be no helium flash.

4. Because a massive star will reach higher core temperatures, massive stars will experience more advanced nuclear burning stages producing a wider range of nucleosynthesis products, up to iron.

5. As already mentioned, stars whose core is greater than 1.4 solar masses exceed the "Chandrasekhar limit" to the mass for a white dwarf. They will end their lives with a dramatic explosion, becoming either neutron stars or white dwarfs. Because stars lose considerable mass due to stellar winds in the later stages of evolution and in the planetary nebula phase, it is currently believed that stars with M < 8 M_sun end their lives as white dwarfs.

The Death of Stars

Stellar Mass Nature of collapse Size of Radius (km) Density (g/cm³) End Product

M_star < 1 M_sun Slow gravitational contraction --- --- White Dwarf

1 M_sun to ~ 5 M_sun Mild core collapse 7000 107 White Dwarf

~ 5 M_sun to 15 M_sun Fast core collapse 20 3 x 1014 Neutron Star

M_star < 15 M_sun Very fast core collapse 4 1016 Black Hole

Application - Ages of Stellar Clusters

At an age of 1 million years the most massive stars have contracted to the Main sequence, lived out their hydrogen-burning lifetimes and are evolving off the Main Sequence. Lower mass stars like the sun are still in the Pre-Main Sequence phase. The youngest clusters observed in the Milky Way are estimated to have ages of a few million years.
At 10 million years stars of 1 solar mass are still above the Main Sequence, just beginning nuclear reactions. They will be observed as T-Tauri stars. Stars with M ~ 20 M_sun are just moving off the Main Sequence. Such clusters will still be associated with regions of gas & dust from which they formed.
At 100 million years most stars are on or nearing the Main Sequence, but stars with M > 5 M_sun are now moving off the Main Sequence. The Pleiades cluster is estimated to have an age of about 100 million years.
With an age of a billion years, cluster stars with masses between 2~3 M_sun are moving off the Main Sequence. The Main Sequence location at which stars are just beginning to exhaust the hydrogen fuel in their cores and move toward the Red Giant region is called the Main Sequence Turnoff. The oldest clusters in the Milky Way, the globular clusters, are estimated to have ages of the order of 10-15 billion years and show HR Diagrams like that at the left. Because the globular cluster stars have very low abundances of the elements heavier than helium (C,N,O ...) some corrections need to be made to compare their HR diagrams to younger clusters with higher abundances.

The theoretical HR diagrams can be compared with the schematic HR Diagrams of a selection of clusters shown below. [Figure] Schematic HR Diagrams for star clusters in the Milky Way. The "Main Sequence Turnoff" is used to estimate the cluster age.

Stellar Evolution I - Solar Type Stars
The actual process of star formation remains shrouded in mystery because stars form in dense, cold molecular clouds whose dust obscures newly formed stars from our view. For reasons which are not fully understood, but which may have to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds pass through spiral structure in galaxies, or magnetic-gravitational instabilities (or, perhaps all of the above) the dense core of a molecular cloud begins to condense under its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars. As the cloud condenses, gravitational potential energy is released - half of this released gravitational energy goes into heating the cloud, half is radiated away as thermal radiation. Because gravity is stronger near the center of the cloud (remember F_g ~ 1/distance²) the center condenses more quickly, more energy is released in the center of the cloud, and the center becomes hotter than the outer regions. As a means of tracking the stellar life-cycle we follow its path on the HR Diagram.
1. Protostar The initial collapse occurs quickly, over a period of a few years. As the star heats up, pressure builds up following the Perfect Gas Law: PV = NRT where, most importantly P=pressure and T=Temperature. The outward pressure nearly balances the inward gravitational pull, a condition called hydrostatic equilibrium. The star is cool, so its color is red, but it is very large so it has a high luminosity and appears at the upper right in the H-R Diagram.	PROTOSTARS Age: 1 ~ 3 yrs R ~ 50 R_sun T_core = 150,000 K T_surface = 3500 K Energy Source: Gravity
2. Pre-Main Sequence Once near-equilibrium has been established, the contraction slows down, but the star continues to radiate energy (light) and thus must continue to contract to provide gravitational energy to supply the necessary luminosity. The star must continue to contract until the temperatures in the core reach high enough values that nuclear fusion reactions begin. Once nuclear reactions begin in the core, the star readjusts to account for this new energy source Gravity releases its potential energy throughout the star, but due to the very high temperature dependence of the nuclear fusion reactions, the proton-proton chain is highly centrally concentrated. During this phase the star lies above the main sequence; such pre-main sequence stars are observed as T-Tauri Stars, which are going through a phase of high activity. Material is still falling inward onto the star, but the star is also spewing material outward in strong winds or jets.	PREMAIN SEQUENCE Age: 10 million yrs R ~ 1.33 R_sun T_core = 10,000,000 K T_surface = 4500 K Energy Source: P-P Chain turns on
3. Zero Age Main Sequence It takes another several million years for the star to settle down on the main sequence. The main sequence is not a line, but a band in the HR Diagram. Stars start out at the lower boundary, called the Zero-Age Main Sequence referring to the fact that stars in this location have just begun their main sequence phases. Because the transmutation of Hydrogen into Helium is the most efficient of the nuclear burning stages, the main sequence phase is the longest phase of a star's life, about 10 billion yrs for a star with 1 solar mass.	ZAMS Age: 27 million yrs R ~ R_sun T_core = 15,000,000 K T_surface = 6000 K Energy Source: P-P Chain in core
During the main sequence phase there is a "feedback" process that regulates the energy production in the core and maintains the star's stability. The basic physical principles are: A. The thermal radiation law, L ~ R²T⁴, determines the energy output, which fixes requirement for nuclear energy production. B. The nuclear reaction rates are very strong functions of the central temperature; Reaction Rate ~ T⁴ for the P-P Chain. C. The inward pull of gravity is balanced by the gas pressure which is determined by the Ideal Gas Law: PV=NRT. A good way to see the stability of this equilibrium is to consider what happens if we depart in small ways from equilibrium: Suppose that the amount of energy produced by nuclear reactions in the core is not sufficient to match the energy radiated away at the surface. The star will then lose energy; this can only be replenished from the star's supply of gravitational energy, thus the star will contract a bit. As the core contracts it heats up a bit, the pressure increases, and the nuclear energy generation rate increases until it matches the energy required by the luminosity. Similarly, if the star overproduces energy in the core the excess energy will heat the core, increasing the pressure and allowing the star to do work against gravity. The core will expand and cool a bit and the nuclear energy generation rate will decrease until it once again balances the luminosity requirement of the star.

6. Red Giant - Helium Flash As the Helium core of the star contracts, nuclear reactions continue in a shell surrounding the core. Initially the temperature in the core is too low for fusion of helium, but the core-contraction liberates gravitational energy causing the helium core and surrounding hydrogen-burning shell to increase in temperature, which, in turn, causes an increase in the rate of nuclear reactions in the shell. In this instance, the nuclear reactions are producing more than enough energy to satisfy the luminous energy output. This extra energy output pushes the stellar envelope outward, against the pull of gravity, causing the outer atmosphere to grow by as much as a factor of 200. The star is now cool, but very luminous - a Red Giant.	RED GIANT Age: 100 million yrs from Point 5 R ~ 200 R_sun T_core = 200,000,000 K T_surface = 3500 K Energy Source: P-P Chain in shell around core Ignition of Triple-Alpha Process
The contraction of the core causes the temperature and density to increase such that, by the time the temperature is high enough for Helium nuclei to overcome the repulsive electrical barrier and fuse to form Carbon, the core of the star has reached a state of electron degeneracy. Degeneracy comes about due to the Pauli Exclusion Principle, which prohibits electrons from occupying identical energy states. The core of the Red Giant is so dense that all available lower energy states are filled up. Because only high-energy states are available, the core resists further compression -- there is a pressure due to the electron degeneracy. This pressure has a significant difference from pressure produced by the Ideal Gas Law -- it is independent of temperature. This removes a key element in the feedback-stability mechanism that regulates hydrogen burning on the main sequence.
7. Helium Burning Main Sequence Once again the core of the star readjusts to allow for a new source of energy, in this case fusion of Helium to form Carbon via the Triple-Alpha Process. The Triple alpha process releases only about 20% as much energy as hydrogen burning, so the lifetime on the Helium Burning Main Sequence is only about 2 billion years. During this phase some Carbon and Helium will fuse ¹²C + ⁴He --> ¹⁶O, resulting in the formation of a Carbon-Oxygen core. When the Helium is exhausted in the core of a star like the sun, no further reactions are possible. Helium burning may occur in a shell surrounding thecore for a brief period, but the lifetime of the star is essentially over.	HELIUM BURNING MS Age: About 10,000 yrs from point 6 T_surface = 9000 K T_core = 200,000,000 K Energy Source: Triple-alpha process in core; P-P Chain in shell
8. Planetary Nebula When the helium is exhausted in the core of a star like the sun, the C-O core will begin to contract again. Central temperatures will never reach high enough values for Carbon or Oxygen burning, but the Helium and Hydrogen burning shells will conyinue burning for a while. Throughout the star's lifetime it is losing mass via a stellar wind, like the solar wind. This mass loss increases when the star swells up to the size and low gravity of a Red Giant. During Helium Burning, thermal pulses, caused by the extreme temperature sensitivity of the 3-alpha Process, can cause large increases in luminosity with accompanying mass ejection. During Helium Shell Burning, a final thermal pulse produces a giant "hiccough" causing the star to eject as much of 10% of its mass, the entire outer envelope, revealing the hot inner regions with temperatures in excess 100,000 K, shown in this animation of the Helix, below. The resulting Planetary Nebuala is the interaction of the newly ejected shell of gas with the more slowly moving ejecta from previous events and the ultraviolet light from the hot stellar remnant, which heats the gas and causes it to fluoresce. The Ring Nebula in Lyra (Messier Database, Web Nebulae) shown here is the prototypical Planetary Nebula. Rather than a spherical shell as initially believed, the Ring's shape is probably a torus or cylinder of gas, seen nearly pole-on. Its age is estimated to be a few thousand years; the central star has a surface temperature over 100,000 K. The Planetary Nebula phase is relatively short lived, estimated to be about 25,000 years, and there are about 10,000 planetaries in the Milky Way.
9. White Dwarf As the nebula disperses, the shell nuclear reactions die out leaving the stellar remnant, supported by electron degeneracy, to fade away as it cools down. The white dwarf is small, about the size of the earth, with a density of order 1 million g/cm3, about equivalent to crushing a volkswagen down to a cubic centimeter or a "ton per teaspoonful." A white dwarf star will take billions of years to radiate away its store of thermal energy because of its small surface area. The white dwarf will slowly move down and to the right in the H-R Diagram as it cools until it fades from view as a "black dwarf". To the right is the white dwarf companion to the nearby star Sirius.	WHITE DWARF R ~ R_earth (a few thousand km) T_surface = 30000 K - 5000 K Energy Source: "Cooling Off"

Stellar Evolution II - Massive Stars
We may crudely distinguish between stars more massive than the sun and solar type stars in their evolutionary characteristics:
1.	Massive stars live their lives more rapidly than do solar-type stars -- they "live fast and die young." One can determine relatively straightforwardly from the balance between gravity, pressure and temperature that the luminosity of a star will be approximately proportional to the Mass^3.5 (this is the Mass-Luminosity Relation which applies to all phases of stellar evolution). Since the available fuel is effectively the mass of the star, the lifetime will be approximately proportional to 1/Mass^2.5. A star of 10 solar masses can thus be expected to go through its life cycle about 300 times faster than the sun, with a main sequence lifetime of about 30 million years. (The most massive stars have lifetimes shorter than about a million years, while stars with masses less than about 3/4M_sun have lifetimes longer than the age of the Universe!)
2.	As described above massive stars require higher central temperatures to balance the greater pull of gravity. This means that massive stars produce helium from hydrogen via the CNO cycle rather than the P-P Chain.
3.	Higher central temperatures and pressure dictate that the stellar core will not become electron degenerate at the onset of helium burning, so there will be no helium flash.
4.	Because a massive star will reach higher core temperatures, massive stars will experience more advanced nuclear burning stages producing a wider range of nucleosynthesis products, up to iron.
5.	As already mentioned, stars whose core is greater than 1.4 solar masses exceed the "Chandrasekhar limit" to the mass for a white dwarf. They will end their lives with a dramatic explosion, becoming either neutron stars or white dwarfs. Because stars lose considerable mass due to stellar winds in the later stages of evolution and in the planetary nebula phase, it is currently believed that stars with M < 8 M_sun end their lives as white dwarfs.

The Death of Stars
Stellar Mass	Nature of collapse	Size of Radius (km)	Density (g/cm³)	End Product
M_star < 1 M_sun	Slow gravitational contraction	---	---	White Dwarf
1 M_sun to ~ 5 M_sun	Mild core collapse	7000	107	White Dwarf
~ 5 M_sun to 15 M_sun	Fast core collapse	20	3 x 1014	Neutron Star
M_star < 15 M_sun	Very fast core collapse	4	1016	Black Hole

Application - Ages of Stellar Clusters
	At an age of 1 million years the most massive stars have contracted to the Main sequence, lived out their hydrogen-burning lifetimes and are evolving off the Main Sequence. Lower mass stars like the sun are still in the Pre-Main Sequence phase. The youngest clusters observed in the Milky Way are estimated to have ages of a few million years. At 10 million years stars of 1 solar mass are still above the Main Sequence, just beginning nuclear reactions. They will be observed as T-Tauri stars. Stars with M ~ 20 M_sun are just moving off the Main Sequence. Such clusters will still be associated with regions of gas & dust from which they formed. At 100 million years most stars are on or nearing the Main Sequence, but stars with M > 5 M_sun are now moving off the Main Sequence. The Pleiades cluster is estimated to have an age of about 100 million years. With an age of a billion years, cluster stars with masses between 2~3 M_sun are moving off the Main Sequence. The Main Sequence location at which stars are just beginning to exhaust the hydrogen fuel in their cores and move toward the Red Giant region is called the Main Sequence Turnoff. The oldest clusters in the Milky Way, the globular clusters, are estimated to have ages of the order of 10-15 billion years and show HR Diagrams like that at the left. Because the globular cluster stars have very low abundances of the elements heavier than helium (C,N,O ...) some corrections need to be made to compare their HR diagrams to younger clusters with higher abundances.
The theoretical HR diagrams can be compared with the schematic HR Diagrams of a selection of clusters shown below. [Figure] Schematic HR Diagrams for star clusters in the Milky Way. The "Main Sequence Turnoff" is used to estimate the cluster age.