***Much of the material in these notes on stellar evolution is discussed in this
- All stars are born from a nebula (cloud) of gas (hydrogen, helium, and a
little bit of everything else) and dust.
- One such nebula, often studied, is the Orion
- If the nebula is dense enough, it will eventually start collapsing under
the influence of its own gravity.
- As the nebula contracts, it heats up and the
rising core temperature and pressure eventually causes
hydrogen to fuse. At that point, a star is born.
- For stars to be able to sustain nuclear fusion, their mass must be at
least 0.08 times our Sun's mass, which is equivalent to about 80 Jupiters.
On the high end, stars cannot be more massive than about 150 times our Sun
because they simply produce too much energy and become unstable at that
point. (Note that several stars more massive than 150 times the Sun have
been discovered, but these are extremely rare.)
- When the gravitational tendency to collapse is balanced by the tendency
to expand due to the heat generated by nuclear fusion, the star is said to
be in hydrostatic equilibrium. At that point,
the star is on the main sequence of the HR diagram.
- The newborn star will have a luminosity and surface temperature now that
will change very little over the course
of its lifetime on the main sequence.
- It is estimated stars are born at a rate of about 1 per year in our
Galaxy, and roughly the same number dies.
video may help you visualize the birth of a star.
Death of low-mass stars
- Low-mass stars are those that end up as white dwarfs. High-mass stars
are those that end their lives in a supernova.
- Our Sun is an example of a low-mass star; Betelgeuse is an example of a
- Stars with masses 0.08M
< M < 10M (which is the
majority of them) live quietly and contently, not changing much, for several
billion years on the main sequence.
- A star begins to die once it converts all the hydrogen in the core into
- As hydrogen is used up in the inner and hottest part of the core, the
relatively inert helium-rich (hydrogen-depleted) core begins to collapse,
and in so doing releases gravitational energy. This energy quickly heats up
the outer hydrogen-rich layers and ignites the fusion of hydrogen in a thin
shell immediately surrounding the hydrogen-depleted core. As this process
continues, the hydrogen-fusing shell migrates outward and heats up the
envelope of the star, which then causes the whole star to expand into a Red Giant.
- Here's roughly what our Sun will look like as
red giant viewed from Earth.
- For the lowest-mass stars, core pressure and temperature are not
sufficiently high to ignite nuclear fusion of helium. The core then cools
while the outer envelope continues to expand. Ultimately, the helium core
forms a hot but cooling corpse known as a white dwarf, surrounded by an
expanding outer envelope of hydrogen and helium known as the
- The Helix Nebula (also known as NGC 7293)
is one of the closest planetary nebulae to Earth (650 light-years away). It
is often referred to as the Eye of God on the Internet. The Helix Nebula is
an example of a planetary nebula created at the end of the life of a
- Somewhat higher-mass stars will fuse helium
into carbon for a while to produce a denser core composed of carbon
"ash" in the center, surrounded by a shell of burning helium, surrounded by
a shell of burning hydrogen, which is surrounded by an envelope of inactive
(nonburning) hydrogen and helium.
- If the star has even more mass and thus a denser and hotter core, carbon
will start to fuse to produce even heavier elements in the center. Another
shell of hydrogen burning will form, and beneath it a shell of helium
Death of high-mass stars
- High-mass stars
have relatively short main-sequence lives. A 15M
star, for example, lives for only about 10 million years before
turning into a Red Giant.
- When the star first runs out of hydrogen to fuse in its core it will
behave similarly to lower mass red giants. It will first begin fusion of
hydrogen in a shell around the core and the core will heat and fuse helium
into carbon. There will also be carbon and helium fusion into oxygen. The
star's envelope also bloats out to very large sizes (Supergiants).
- The supergiant's core will fuse very heavy elements from carbon and
oxygen all the way up to Iron. Elements heavier than iron cannot be
used as a source of energy through fusion.
They can, however, be split into lighter elements to release energy, but
this process (fission) does not occur in stars.
- The star takes on an onion-like structure,
with shells of different elements fusing into heavier elements, in
progressively shorter phases. For 20-Sun star, hydrogen is exhausted in the
core within a few million years and iron develops within about a day (see
- Ultimately, when the star exhausts its supply of elements in the core
lighter than iron, the core collapses in an extremely violent event known as
- The supernova leaves behind either a neutron star or, in the case of the
heaviest stars, a black hole.
|Fusion phases for a 20-Sun star
||10 million years
||1 million years
Density/size comparison of white dwarfs, neutron stars, and black holes
- A white dwarf consists essentially of tightly packed atoms which
constitute the core of a Sun-like star.
- The white dwarf of a Sun-like star is about 100
times denser than the Sun.
- A teaspoon of white dwarf material would weigh about 15 tons.
- The typical white dwarf is
roughly the size of the Earth.
- A neutron star is essentially the core of a star
collapsed into a ball of tightly packed nuclei.
- A neutron star is over a thousand times denser than a white dwarf.
- A teaspoon of neutron star material would weigh about 4 billion tons.
- A typical neutron star is
roughly the size of a city.
- A black hole is the collapsed core of a star so densely
packed that it has virtually no size.
- The infinitely small volume into which all the matter in a black
hole is compressed is called the central singularity.
- The imaginary sphere that measures how close to the singularity you
can safely get is called the event horizon. Once you have passed the
event horizon, it becomes impossible to escape: you will be drawn in by
the black hole's gravitational pull and squashed into the singularity.
- The size of the event horizon (called the Schwarzschild radius) is
proportional to the mass of the black hole.
- Astronomers have found black holes with event horizons ranging from
6 miles to the size of our solar system, although event horizons can, in
principle, be bigger or smaller than this. But in principle, black holes
can exist with even smaller or larger horizons.
- Any object compressed sufficiently can be turned into a black hole.
The the Scharzschild radius for this object is directly related to the
mass of the object. For example, the Schwarzschild radius of the Earth
is about the size of a marble (if the Earth could somehow be compressed
to this size).
CORE OF SUN
Final composition of dead stars
Initial mass (in units
helium white dwarf
C-O white dwarf
O-Ne-Mg white dwarf
Comparison of Fusion and Fission
Energy source in stars?
Energy source on Earth?
bombs and power plants
Major source on Earth
Flowchart of Stellar Evolution
- All stars follow the same basic series of steps in the lives.
- Low-mass stars go through a red giant phase which ultimately turns into
a planetary nebula with a white dwarf in the center.
- High-mass stars go through a red supergiant phase which
ultimately results in a supernova, leaving behind either a neutron star or a
- The high-mass stars burn brighter and burn out faster, as depicted in
this mass-luminosity relation.
- The deciding factor in the fate of a star is its mass.
- Stars whose core is less than 1.4 solar masses
(the Chandrasekhar limit) will leave behind a white dwarf, the
size of which is inversely related to
its mass. Note that the initial mass of this star is much greater than
its core, but much of the mass is lost once the planetary nebula
separates from the core.
- Stars whose core is in the range 1.4-3 solar masses will leave
behind a neutron star (much denser than a white dwarf). Note that the
initial mass of this star is much greater than its core, but much of the
mass is lost during the supernova phase.
- Above 3 solar masses (the
Tolman-Oppenheimer-Volkoff limit), a
quark star might be created, although this is currently mostly
- Any stellar core over 5 solar masses will inevitably succumb to
gravitational collapse, producing a
black hole (much denser than a neutron star).
- If a white dwarf has a close binary companion, the
white dwarf may accrete gas from the companion's outer atmosphere. The
gravitational energy released by the captured gas may be sufficiently great
to start a fusion reaction on the surface of the white dwarf. This flare-up
is known as a nova.
- A nova
- can recur many times.
- is about 100,000 solar luminosities.
- fades after a few months (sometimes years).
- is more common than supernovas (2-3/yr observed, about 200 so far).
- Eventually, the white dwarf could explode as a Type Ia supernova if it
absorbs enough mass from its companion to push it over the Chandrasekhar
X-ray burster (or accretion-powered pulsars)
- Like accreting white dwarfs that occasionally flare up into novae, an
accreting neutron star (i.e., a neutron star which is a member of a binary
system) can flare up with enormous luminosity.
- The infalling gas can reach half the speed of light before it impacts
the neutron star surface. So much gravitational potential energy is released
by the infalling gas, that the hotspots can have a peak luminosity nearly a
hundred thousand times that of our Sun. This is the equivalent of detonating
the entire world's nuclear arsenal on every square centimeter of the neutron
star's surface within a minute.
- Temperatures of millions of degrees are produced, which results mostly
in x-ray radiation, hence the label x-ray burster.
- Gas is accreted from the stellar companion is channeled by the neutron
star's magnetic field on to the magnetic poles producing two or more
localized X-ray hot spots similar to the two auroral zones on the Earth but
far hotter. As the neutron star rotates, pulses of X-rays are observed as
the hotspots move in and out of view if the magnetic axis is tilted with
respect to the spin axis.
- The X-ray periods range from as little as a fraction of a second to as
much as several minutes.
- There are two very different types of supernova:
Type I and Type II. These two types differ by their
light curves and by their composition.
- Type II (core-collapse) supernova:
- A massive star evolves and eventually runs of fuel. The star then
collapses in a massive supernova explosion.
- The expansion and cooling of the star's outer envelope as it is
blown into space by the shock wave sweeping up from below produce the
characteristic "plateau" in the light curve for a few months after the
- The expanding material consists mainly of unburned gas—hydrogen and
helium—so it is not surprising that those elements are strongly
represented in the supernova's observed spectrum.
- Betelgeuse is an example of a Type II supernova.
- If Betelgeuse (which is about 640 light-years away from us) went
supernova, it would be over 10 times as bright as the full moon in
our sky and that we would be able to see it even during the daytime
for a few weeks.
- Type I (carbon-detonation) supernova:
- A carbon-oxygen white dwarf accretes enough mass from its companion
to exceed the Chandrasekhar mass (1.4 solar masses) and collapses in a
- During the collapse, the internal temperature rapidly rises to the
point at which carbon can fuse into heavier elements. This fusion
provides the energy for the so-called carbon-detonation supernova.
- In an alternative scenario, two white
dwarfs in a binary system may collide and merge to form a massive,
unstable star. The end result is the same—a carbon-detonation supernova.
- Because the hydrogen-helium envelope has already been blown off in
the planetary nebula, very little signal is detected from these two
|Type I (carbon-detonation) supernova
||Type II (core-collapse) supernova
|results from a white dwarf in a binary system
||results from any supermassive star
|weak hydrogen emission lines
||strong hydrogen emission lines
|leaves no core remnant behind
||leaves a neutron star or black hole behind
|light curve similar to
that of a nova
||light curve usually has characteristic
|luminosity relatively constant
||luminosity has wide range
|used as standard candle (for distance measurement)
||does not help with distance
|several times brighter than Type II supernova
||about 1 billion solar luminosities
- Interesting because:
- Our solar system may have started with a nearby supernova.
- Supernovae enrich the universe with heavy elements.
- Radiation from nearby (with a couple of hundred light-years) supernovae helps to drive evolution.
- Type I supernovae are used as standard candles.
- Occurs roughly once every 50 years in a galaxy the size of the Milky Way
- No supernova has been seen in the Milky Way since 1604.
- For a typical supernova to be as bright to the human eye as the Sun, it
would have to occur at a distance of about one light-year from Earth. If
Betelgeuse, which is about 500 light-years away, were to go supernova, it
would rival the Moon in brightness.
- Until 1987, only three supernovae had been positively identified in
recorded history, the best known of which is the one that occurred in ad
1054 and left behind a remnant known as the Crab Nebula.
- The nearest supernova witnessed in 4 centuries was Supernova 1987A, in
the Large Magellanic Cloud (galaxy). This supernova was particularly useful
to study because:
- Its progenitor had been observed previously.
- Its distance from us (168,000 ly) and its location in the Large
Magellanic Cloud was already known.
- It was the first opportunity for modern astronomers to see a
supernova up close.
- It was the closest observed supernova since SN 1604, which occurred
in the Milky Way itself.
- In fact, it was close enough to be seen with the naked eye (peak
apparent magnitude 3) from southern hemisphere.
- It occurred after new telescopes, such as Hubble, could observe it
- It was the first opportunity for modern theories of supernova formation to be
tested against observations.
- It was the first time neutrinos emitted from a supernova had been
- The Crab Nebula (M1) is the remnant of a
supernova observed in A.D. 1054.
- About 6000 light-years away, in the constellation Taurus.
- Recorded as a "guest star" by Chinese astronomers.
- It was visible with the naked eye for 23 days in daylight (about
magnitude -6 or about 4 times brighter than Venus at its brightest) and 653
nights before fading from view.
- Pulsar radiates energy from the center.
- Rediscovered in 1758 by French astronomer Charles Messier (1730-1817),
when he was looking for Comet Halley on its first predicted return. At
first, Messier thought that it was a comet but recognized that it had no
apparent proper motion. It was the discovery of this object which led
Messier to compile his famous catalogue of nebulae and star clusters so that
others would not confuse them with comets--beginning with the Crab Nebula as
Messier object "Number 1" or M1.
- The undead star: The temperamental Eta Carinae
exploded about 160 years ago and may be ready to go supernova.
- 8,000 light-years away
- 100 times more massive than Sol
- 5 million times more luminous than Sol
- could damage satellites with gamma blast
- The study of typical light curves from Type I
supernovae indicates that radioactive nuclei form as a result of the
explosion. These curves have two distinct features. After the initial peak,
the luminosity first declines rapidly, then decreases at a slower rate. This
change in the luminosity decay invariably occurs about 2 months after the
explosion, regardless of the intensity of the outburst. Depending on the
initial mass of the exploded star, the luminosity takes from several months
to many years to decrease to its original value, but the shape of the
decay curve is nearly the same for all exploded stars.
- We can explain the
two-stage decline of the luminosity curve in terms of the radioactive decay
of unstable nuclei, notably nickel-56 and cobalt-56,
produced in abundance during the early moments of the supernova explosion.
From theoretical models of the explosion we can calculate the amounts of
these elements expected to form, and we know their half-lives from
laboratory experiments. Because each radioactive decay produces a known
amount of visible light, we can then determine how the light emitted by
these unstable elements should vary in time. More direct evidence for the
presence of these unstable nuclei was first obtained in the 1970s, when a
gamma-ray spectral feature of decaying cobalt-56 was identified in a
supernova observed in a distant galaxy.
- The early Universe was hot and dense (like the core of a star) and
nucleons could fuse together to make some helium. Aside from a tiny amount
of lithium, nucleosynthesis beyond helium could not take because the
Universe expanded and cooled too quickly. (The heavier the elements to be
fused, the greater the required temperature.) Once all nuclear reactions
ceased, 75% of the mass of the Universe was in the form of hydrogen and 25%
in the form of helium. Today, after many cycles of stellar evolution, the
Universe is still mostly hydrogen and helium, in roughly the same
proportions, although 1% now fills out the rest of the periodic table. Your
body is made mostly of these 'trace' elements.
- Stellar nucleosynthesis occurs at many different stages of stellar
evolution, from main-sequence stars all the way to supernovae.
- In perhaps the simplest nucleosynthesis reaction in the stellar core,
hydrogen is produced from helium. This is accomplished by two known sets of
fusion reactions: the proton-proton chain reaction and the
- In the proton-proton chain reaction, 4
hydrogen nuclei fuse to form helium. This
reaction is followed by the fusion of 3 helium
nuclei to form carbon-12. This conversion of three helium nuclei (alpha
particles) to carbon-12 is called the triple-alpha
- The element produced in the fusion of 2
helium nuclei is an unstable isotope of beryllium (Be-8). Once formed,
most of the Be-8 rapidly decays back into two helium nuclei. A tiny
fraction, however, survives long enough to capture another alpha particle to
produce carbon-12, which is quite stable. (Because the triple-alpha process is unlikely, it requires a
long period of time to produce carbon. One consequence of this is that no
carbon was produced in the Big Bang because within minutes after the Big
Bang, the temperature fell below that necessary for nuclear fusion.)
- The second of two known sets of fusion reactions by which
helium is produced from hydrogen is the CNO cycle. In
this cycle, there is still a net production of helium from hydrogen, but
carbon, nitrogen, and oxygen isotopes act as catalysts (i.e., are
regenerated in each cycle) to facilitate the reaction.
- In stars less massive than the Sun, the proton-proton chain is more
dominant. In stars more massive than the Sun, the CNO cycle is dominant.
In general, the higher the temperature, the more dominant the CNO cycle
is relative to the simple proton-proton chain reaction.
- Elements heavier than helium but lighter than iron are built
predominantly through helium capture. These
elements have masses which are divisible by 4, the mass of a helium nucleus,
and are found in the peaks of the chart of cosmic abundances.
- Although many different nuclear reactions undoubtedly occur in the cores
of evolving stars, the efficiency of fusion decreases with increasing size
of the reacting nuclei because of increased internuclear repulsion. For
example, the repulsive force between two carbon nuclei is three times
greater than the repulsive force between a nucleus of carbon and one of
helium. Thus, at a given temperature, carbon-helium fusion should occur more efficiently than
carbon-carbon fusion. Heavier elements therefore tend to form predominantly
through helium-capture rather than through
fusion of heavier-than-helium nuclei,
though these reactions occur as well. Consistent with theory and
observation, elements with atomic masses divisible by 4 (i.e., which are built up through helium-capture)--oxygen-16,
neon-20, magnesium-24, silicon-28, etc.--stand out as
prominent peaks in the chart of cosmic abundances.
Helium, of course, stands out as well since it was produced in abundance in
the Big Bang and continues to be produced in the cores of evolving stars.
- Other reactions for pre-iron nucleosynthesis are possible but are also
less common than helium capture. For example, protons and neutrons are freed
from some nuclei and are absorbed by others, resulting in new nuclei with
masses intermediate between those formed by helium capture. Such
intermediate elements (e.g., fluorine-19, sodium-23, phosphorus-31, and many
others) have masses which are not divisible by 4, the mass of a helium
nucleus, and are found in the troughs of the chart of
- Around the time silicon-28 appears in the core of a star, the star's
core temperature has reached an incredible 3 billion K, and the gamma rays
associated with this temperature have enough energy to break a nucleus apart
(photodisintegration). This leads to a
competitive struggle between the continued capture of helium to produce even
heavier nuclei and the tendency of more complex nuclei to break down into
simpler ones. This is the same process of
photodisintegration will ultimately accelerate the collapse of the
star's iron core toward a Type II supernova.
- The process of photodisintegration provides
raw material that allows the helium-capture process to proceed to greater
masses. The process continues, with some heavy nuclei being destroyed and
others increasing in mass. In succession the star forms sulfur-32, argon-36,
calcium-40, titanium-44, chromium-48, iron-52, and nickel-56. This two-step
process—photodisintegration followed by the direct capture of some or all of
the resulting helium-4 nuclei (or alpha particles)—is often called the
- Because nickel-56 is unstable, it decays rapidly,
first into cobalt-56, then into a stable iron-56 nucleus. Any unstable
nucleus will continue to decay until stability is achieved, and iron-56 is
the most stable of all nuclei. Thus, the alpha process leads inevitably to
the buildup of iron in the stellar core.
- Elements heavier than iron cannot be produced through helium capture
because they are very unstable and decay back to iron, the most stable
element, as quickly as they are formed. Iron's 26 protons and 30 neutrons
are bound together more strongly than the particles in any other nucleus.
Iron is said to have the greatest nuclear binding
energy per nucleon of any element and is thus the most stable element.
This enhanced stability of iron explains why nuclei tend to "accumulate"
near iron as stars evolve.
- Elements beyond iron form by neutron capture and radioactive decay. The
heaviest nuclei of all are formed by an extremely intense, but brief, period
of neutron capture during a type II (core-collapse) supernova explosion.
Because the time available for synthesizing these heaviest nuclei is so
brief, they never become very abundant. Consequently, elements heavier than
iron are collectively about a billion times less
abundant than hydrogen and helium.
- Two types of neutron capture are recognized:
the s-process and the r-process.
- The s-process or slow-neutron-capture-process refers to a series of
nuclear reactions which occur at relatively low neutron density. The s-process produces
approximately half of the isotopes of the elements heavier than iron, up
to the element Bismuth-209, which is the heaviest-known nonradioactive
nucleus. Nuclei more massive than that of Bismuth-209 cannot be built up
through neutron capture in the low-neutron-density environment of the
s-process; these nuclei are too unstable and decay back to Bismuth as
quickly as they are formed. The s-process is responsible for the
synthesis of copper, lead, silver, and gold, among others, as well as
some of the lighter elements intermediate between those formed by helium
- In the r-process or rapid-neutron-capture-process refers to a series
of nuclear reactions which occur in the high-neutron-density environment
of a Type II supernova. The neutron-capture rate in this environment is
so great that even the most unstable heavy nuclei can capture many neutrons
before decaying. Virtually all the super-heavy elements in the Universe,
including radium, thorium, uranium and
plutonium, are produced via the r-process.
- Evidence for nucleosynthesis in the stars includes:
- Observed abundances of various elements and
their isotopes in the cosmos are in strong agreement with theoretical
predictions based on known properties of nuclear reactions.
- The observed presence of relatively short-lived technetium-99 in the
spectra of many red-giant stars was one of the first strong pieces of evidence that heavy elements
really do form in the cores of dying stars. Because technetium is
radioactive, with a half-life (0.2 million years) much less than the age of
the star, its abundance must reflect its creation within that star during
- The shape of light curves of supernovae is consistent with theoretical
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