The actual process of star formation remains shrouded in mystery because
stars form in dense, cold molecular clouds whose dust obscures newly
stars from our view. For reasons which are not fully understood, but
have to do with collisions of molecular clouds, or shockwaves passing
molecular clouds as the clouds pass through spiral structure in
magnetic-gravitational instabilities (or, perhaps all of the above) the
core of a molecular cloud begins to condense under its self-gravity,
fragmenting into stellar mass clouds which continue to condense forming
As the cloud condenses, gravitational potential energy is released -
this released gravitational energy goes into heating the cloud, half is
radiated away as thermal radiation. Because gravity is stronger near
of the cloud (remember Fg ~ 1/distance2) the
condenses more quickly, more energy is released in the center of the
the center becomes hotter than the outer regions. As a means of
stellar life-cycle we follow its path on the Hertzsprung-Russell
initial collapse occurs quickly, over a period of a few years. As the
heats up, pressure builds up following the Perfect Gas Law:
PV = NRT
most importantly P=pressure and T=Temperature. The outward pressure
balances the inward gravitational pull, a condition called hydrostatic
R ~ 50 Rsun
Tcore = 150,000K
Tsurface = 3500K
Energy Source: Gravity
is cool, so its color is red, but it is very large so it has a high
and appears at the upper right in the H-R Diagram.
has been established, the contraction slows down, but the star
radiate energy (light) and thus must continue to contract to provide
gravitational energy to supply the necessary luminosity. The star must
to contract until the temperatures in the core reach high enough values
nuclear fusion reactions begin. Once nuclear reactions begin in the
star readjusts to account for this new energy source Gravity releases
potential energy throughout the star, but due to the very high
dependence of the nuclear fusion reactions, the proton-proton chain is
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
also spewing material outward in strong winds or jets as shown in the HST Photo below.
R ~ 1.33 Rsun
Tcore = 10,000,000K
Tsurface = 4500K
Energy Source: P-P Chain turns
Age Main Sequence
another several million years for the star to settle down on the main
The main sequence is not a line, but a band in the H-R Diagram. Stars
at the lower boundary, called the Zero-Age Main Sequence
the fact that stars in this location have just begun their main
phases. Because the transmutation of Hydrogen into Helium is the most
of the nuclear burning stages, the main sequence phase is the longest
a star's life, about 10 billion yrs for a star with 1 solar mass.
R ~ Rsun
Tcore = 15,000,000K
Tsurface = 6000K
Energy Source: P-P Chain in core.
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:
thermal radiation law, L ~ R2T4, determines the
energy output, which fixes requirement for nuclear energy production.
nuclear reaction rates are very strong functions of the central
temperature; Reaction Rate ~ T4 for the P-P Chain.
inward pull of gravity is balanced by the gas pressure which is
determined by the Ideal Gas Law: PV=NRT.
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
produced by nuclear reactions in the core is not sufficient to match
radiated away at the surface. The star will then lose energy; this can
replenished from the star's supply of gravitational energy, thus the
contract a bit. As the core contracts it heats up a bit, the pressure
increases, and the nuclear energy generation rate increases until it
the energy required by the luminosity.
Similarly, if the star overproduces energy in the core the excess
heat the core, increasing the pressure and allowing the star to do work
gravity. The core will expand and cool a bit and the nuclear energy
rate will decrease until it once again balances the luminosity
4. End of Main Sequence
Energy Source: P-P Chain in shell
Age: About 1 billion
years from Point 4
R ~ 2.6Rsun
Tsurface = 4500K
Energy Source: P-P Chain in
Gravitational contraction of core.
Giant - Helium Flash
Helium core of the star contracts, nuclear reactions continue in a
surrounding the core. Initially the temperature in the core is too low
of helium, but the core-contraction liberates gravitational energy
helium core and surrounding hydrogen-burning shell to increase in
which, in turn, causes an increase in the rate of nuclear reactions in
shell. In this instance, the nuclear reactions are producing more than
energy to satisfy the luminous energy output. This extra energy output
the stellar envelope outward, against the pull of gravity, causing the
atmosphere to grow by as much as a factor of 200. The star is now cool,
very luminous - a Red Giant.
(You do the arithmetic:
200 x 700,000km = ?; where will the outer radius of the sun be?)
Age: 100 million yrs from
R ~ 200Rsun
Tcore = 200,000,000K
Tsurface = 3500K
Energy Source: P-P Chain in shell
Ignition of Triple-Alpha Process.
contraction of the core causes the temperature and density to increase
that, by the time the temperature is high enough for Helium nuclei to
the repulsive electrical barrier and fuse to form Carbon, the core of
has reached a state of electron degeneracy. Degeneracy comes
to the Pauli Exclusion Principle, which prohibits electrons
occupying identical energy states. The core of the Red Giant is so
all available lower energy states are filled up. Because only
states are available, the core resists further compression -- there is
pressure due to the electron degeneracy. This pressure has a
difference from pressure produced by the Ideal Gas Law -- it is
temperature. This removes a key element in the feedback-stability
that regulates hydrogen burning on the main sequence.
Diagram from Helium Burning to White Dwarf.
Helium Burning Main Sequence
again the core of the star readjusts to allow for a new source of
this case fusion of Helium to form Carbon via the Triple-Alpha Process.
Triple alpha process releases only about 20% as much energy as hydrogen
burning, so the lifetime on the Helium Burning Main Sequence is only
Age: About 10,000 yrs
from point 6.
Tsurface = 9000K
Tcore = 200,000,000K
Energy Source: Triple-alpha process in core;
P-P Chain in shell
this phase some Carbon and Helium will fuse
+ 4He --> 16O
resulting in the
formation of a Carbon-Oxygen core. When the Helium is exhausted in the
a star like the sun, no further reactions are possible. Helium burning
occur in a shell surrounding thecore for a brief period, but the
the star is essentially over.
helium is exhausted in the core of a star like the sun, the C-O core
to contract again. Central temperatures will never reach high enough
Carbon or Oxygen burning, but the Helium and Hydrogen burning shells
conyinue burning for a while. Throughout the star's lifetime it is
via a stellar wind, like the solar wind. This mass loss increases when
swells up to the size and low gravity of a Red Giant. During Helium
thermal pulses, caused by the extreme temperature sensitivity of the
Process, can cause large increases in luminosity with accompanying mass
ejection. During Helium Shell Burning, a final thermal pulse produces a
"hiccough" causing the star to eject as much of 10% of its mass, the
entire outer envelope, revealing the hot inner regions with
excess 100,000K, shown in this animation of the
below. The resulting Planetary Nebuala is the interaction
of the newly ejected shell of gas with the more slowly moving ejecta
previous events and the ultraviolet light from the hot stellar remnant,
heats the gas and causes it to fluoresce. The Ring Nebula in Lyra (Messier Database, Web Nebulae) shown below is
HST images of Planetary
Nebulae: The Ring Nebula and the young Planetary Nebula
known as MyCn18, the Hourglass Nebula.
· More about
from George Jacoby's (b& w) Planetary Nebula Gallery.
· Planetary Nebulae at the SED's Messier
· The Planetary Nebula Observer's HomePage includes more
links to Planetary
· Univ. of Calgary Planetray Nebula Homepage with theoretical models of PN emission structure.
· Bruce Balick's HST Images of Planetary Nebulae.
nebula disperses, the shell nuclear reactions die out leaving the
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
million g/cm3, about equivalent to crushing a volkswagen
down to a
cubic centimeter or a "ton per teaspoonful."
R ~ Rearth (a
few thousand km)
Tsurface = 30000K - 5000K
Energy Source: "Cooling Off".
A white dwarf star will
take billions of years to radiate away its store of thermal energy
its small surface area. The white dwarf will slowly move down and to
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
Picures of the Day of White Dwarfs and Planetary Nebulae