Overview - Stars are born, live out their lives, and die
I want to give you a very brief overview of the rest of the Unit
on Stars. In the lifetime of any star, it is the mass that determines its fate.
The more massive the star, the shorter the lifespan, the more rapid the consumption
of nuclear fuel, and the more exotic and spectacular the death. Large stars
can evolve in their death stages and blow up, leaving behind tiny white dwarfs,
tinier neutron stars, and impossibly tiny black holes. The less massive the
star, the longer the lifespan, the slower the consumption of nuclear fuel, and
the more boring the death. Very lightweight stars, in fact, never evolve into
real stars at all, and simply exist as "Jupiters." We will look at
the average star first, and compare all of the other star types to it. There
is a lot of information packed into this section, and I do not expect anyone
to get all the way through it. However, if you can learn the basics of star
lives, I will be happy, and then if you want to learn more simply to satisfy
your thirst for knowledge, then dig deeper into the links within this unit.
Mass, Pressure and Energy
A star's mass is the most important quantity that determines
its lifetime. Paradoxically, the more massive a star the shorter its lifetime.
Stars are extremely hot at their centers, where the energy is produced by thermonuclear
fusion ... the same process that occurs when a hydrogen bomb explodes. Yet stars
can remain stable. Why? Because the enormous outward pressure, produced by the
thermonuclear fusion, is balanced by the inward force of gravity. It is gravity,
as it were, that keeps the "lid" on the star and prevents it from
This balance is called hydrostatic equilibrium. If the force of gravity becomes
stronger than the gas pressure from fusion, the star will contract; if the pressure
from fusion becomes stronger the star will expand. Since gravity acts as a force
toward the center, and fusion is an expanding force from the center, the stars
assume a spherical shape, and the size of the star is determined by the interplay
between these two forces.
Sooner or later, every star will exhaust the fuel at its center. When this happens
the star will change, because it will no longer be in hydrostatic equilibrium.
The energy produced in the star's core can reach the surface
by: conduction, convection and radiation.
Conduction: energy moves from place to place as heat (motional energy), without
the movement of material.
Convection: energy is transported from one place to another by the movement
Radiation: energy is transported by light. To see this in our Sun,
go to the website.
We cannot actually follow the evolution of a star, because they live far too
long compared to human timescales. Instead we see stars at different stages
of their evolution, as represented on the H-R
diagram. The only known way to follow the evolution of stars is to simulate
the evolution of stars on a computer. A stellar (star) model is a complex calculation
in which scientists try to describe the behavior and properties of stars using
the best current understanding of the principles of physics. The models are
used to make predictions about stellar characteristics that can be measured.
That way we can check the validity of the stellar models.
With these models we can trace the evolution of stars and study how the mass,
temperature, luminosity and chemical composition of stars change with time.
When we plot, as a function of time, the luminosity versus the surface temperature
of a stellar model we get an evolutionary track in the H-R diagram. Much of
our understanding of how stars evolve is based on the study of stellar models
whose predictions have been checked against observations.
Clusters Reveal Much
clues about stellar evolution can also be had by studying star clusters, such
as M55 seen in the image to your left. The basic assumption made is that the
stars in a star cluster were created at about the same time, and began with
the same chemical composition. If one observes variations in the chemical composition
of the stars in a star cluster this must be due to stars being at different
stages of their evolution.
When stars are born as part of a star cluster, about the only variable that
can vary across the cluster are the stellar masses: the cluster will have stars
of differing mass and, therefore, right from the start, stars of differing luminosity.
This means that a cluster that is very young will be composed of ALL of the
star classes of the HR Diagram. Astronomers have looked at enough different
clusters to discover that their appearance matches the prediction. Older clusters
have no O or B class stars. The oldest clusters do not even have A, F, or G
class stars. This is powerful evidence that the hot stars of the upper end of
the main sequence of the HR Diagram die first.
with its B stars, is about 100 million years old; the Hyades,
with no B stars, is closer to a billion years. Open cluster ages range from
near zero (those filled with O stars) to about 10 billion years (those filled
with nothing by G,K, and M stars). Since open clusters make up much of our Galaxy's
disk, we date the disk to be about the same age as the oldest open clusters.
Globular clusters, however, ALL have stars missing above the cooler G subclasses,
and therefore average about 13 billion years in age. They are the oldest known
star system, and since they occupy a position in the Galaxy's halo, that too
must be the age of the halo, and perhaps it means that the galactic halo came
before the galactic disk!
The four images above show the difference between Open Clusters (M35 and M46
on the left and right atop respectively) and Globular Clusters (M13 and M4 left
and right respectively below). The left and right top images are courtesy of
Alson Wong, while the bottom left comes from Yuugi Kitahara, and the bottom
right from the NOAO/AURA. M35 is guessed to be about 300 millions years old,
M46 at 100 million years, and M4 and M13 at nearly 13 billion.
To see a recent press release where a cluster of sun-like stars is teaching
us about the evolution of stars, go to the NGC
2420 star cluster site.
Energy Generation in a Sun-Like Star
Stars like the Sun, with a core temperature of about 15 million K, generate
energy by fusing four hydrogen nuclei to one helium nucleus. This is called
the proton-proton (PP) reaction. At some point all the hydrogen in the star's
core will be converted into helium. Without outward pressure from thermonuclear
fusion, the core collapses under gravity. The gravitational collapse raises
the internal pressure which, in turn, raises the internal heat.
If the star's internal temperature can reach about 100 million K, another reaction
can happen: the triple-alpha reaction. In this reaction three helium nuclei
fuse to form one carbon nucleus. It is this reaction that has produced the carbon
in our bodies. At this point, a new type of thermonuclear fusion occurs and
the outward pressure pushes against the inward gravitational pressure and the
star may expand. Additionally, at the boundary between the helium-burning core
and the next outer layer of hydrogen, thermonuclear fusion of hydrogen to helium
may occur, creating further outward pressure. The star may expand in size. This
expansion will spread out the photosphere and result in a cooler surface temperature.
According to Wein's Law, the lowered temperature will cause the surface to appear
more red. Voila! You may then see a Red Giant. These words are repeated later
in this text, but serve here to help guide you through a star's life.
Following the fusion of core helium into carbon, the core may collapse further,
thus driving internal core pressures and temperatures to even greater extremes.
If the star's internal temperature can reach about 600 million K the star can
synthesize elements heavier than carbon. At higher and higher temperatures a
star can create heavier and heavier elements. With each successive fusion/exhaustion/collapse/reignition/
sequence, layers of different forms of fusion occur and the star may get larger
and produce more energy. At some point the star produces iron at which point
this nucleosynthesis (creation of nuclei) ceases. Iron is the element in the
periodic table which absorbs energy upon its fusion, and thus represents the
endpoint in a star's life.
is a beautiful photograph of the interior of the Trifid Nebula. This giant cloud
of dust and gas lies in the Sagittarius region of the Milky Way and is easily
visible with a pair of binoculars on a dark summer night ... even in Minnesota.
To me, part of the image looks like the head of a snail with two eye stalks.
Now, I am quite partial to snails since my doctoral thesis was written about
one particular species, so I see what I choose to see in this image while you
may see something completely different. The purpose of this picture is to get
you focused on these "eye" at the tips of the "stalks."
Those are regions of active star formation, and the subject found briefly below.
This incredible NOAO/AURA photo from the southern Gemini telescope in Chile
shows the Eagle Nebula as never before seen at this level of clarity. Earlier
in the course, in the section "Tour of the Universe," you saw the
famous Hubble Space Telescope photo of M16 ... the gaseous nebula at the heart
of the Eagle Nebula where new stars are being formed. Below left is a look at
the larger region of Aquila, and to the right below is a close-up of that same
area. I put these pictures in here because they are so incredible. If you want,
try clicking on the image to your lower left for a larger version of this tremendous
picture, and a few more close-ups of the pillars.
enter the main sequence on the HR Diagram when the nuclear fusion (often called
nuclear burning) supplies enough energy, and therefore pressure, to stop further
gravitational compression. The Sun is currently in its main sequence phase.
Stellar models of the Sun predict that the Sun may have taken about 30 million
years to reach this stage from its protostar stage (refer to image at your left).
Main sequence stars convert hydrogen to helium. As noted above, how a star evolves
depends upon its mass.
It is important for you to grasp from the image above is the realization that
ALL stars begin their lives in the same fashion. A star is "born"
from a giant cloud of gas and dust that somehow begins collapsing under the
inward force of gravity. Funny, but gravity is the cause of the star's birth
and ultimately the cause of the star's death.
The material in the collapsing cloud condenses as the radius of the cloud
decreases, and the pressure of particles in the interior rises dramatically.
With increased pressure comes increased temperature. Eventually, the pressure
will become so great that temperatures soar to 7 million K, and this is the
ignition temperature for Hydrogen fusion. At the point when fusion first begins
and gamma rays first escape, the star is born, and it occupies a spot someplace
on the main sequence of the HR Diagram, a spot whose location is dependent on
the starting mass.
The two charts below depict the basic life of a star at its most stable stage.
As you will be reminded repeatedly in this course, it is the mass of the star
that determines its fate. The more massive the star, the greater the gravitational
compression, the hotter the interior, and the more outward pressure. The less
massive the star, the less gravitational compression, the cooler the interior,
and the less the outward pressure. Throughout the life of all stars, it is this
ceaseless interplay between gravity's crushing force and thermonuclear fusion's
explosive force that cause the star to be spherical in shape and stable in a
size dictated by the mass. The lower left diagram shows this balancing act in
words called, "Hydrostatic Equilibrium." The HR Diagram to the left
shows where stars on the main sequence are, and demonstrates the difference
in spectral classes that is a direct result of the effect of mass on inward
pressure and outward luminosity. Change the mass or change the thermonuclear
fusion, and the star will change, and you will learn more about this later in
Main Sequence Lifetime Formula
year, the Sun radiates energy equivalent to a mass of about 2 x 10e19 Kg. According
to stellar models of the Sun (called solar models) it will leave the main sequence
when it has used up about 1/10th of its total mass. Therefore, we can estimate
how long a star, like the Sun, will remain as a stable main sequence star if
we know how much fuel it has available (which is related to its mass) and the
rate at which it uses that fuel (which is determined by its luminosity).
Let us assume that a star that is M times heavier than the sun will have M times
as much fuel available. By definition, a star whose luminosity is L times that
of the sun the star is using up its fuel L times faster.
If t is the length of time a star will remain on the main sequence then
t = Fuel Available/Rate of Fuel Consumption
= M x (1/10) x (2 x 10e30 kg) / L x 2 x 10e19 kg/year
= 10e10 x (M/L) years
For the Sun we have M = 1 and L = 1; so t = 10e10 years!
The larger the mass of a star the greater the gravitational compression at its
core and therefore the hotter it is. The hotter the core the greater the rate
of fusion reactions. More fusion reactions means a larger energy release per
second, that is, luminosity. Thus we expect the luminosity to increase with
Indeed, that is what is observed. By measuring the mass and luminosity of a
large number of stars it has been found that L and M (when measured relative
to the luminosity and mass of the Sun) are approximately related as follows
L = M^3.5
Low-Mass and Medium-Mass Stars
Eventually, all the hydrogen in the core of a star will be transformed into
helium. The enormous pressure of the surrounding material causes the helium
core to shrink, thereby increasing the core's temperature and that of the material
immediately surrounding the core.
In fact, the material surrounding the helium core becomes hot enough for hydrogen
to start fusing into helium. This is called shell hydrogen fusion, because the
hydrogen fusion reactions occur in a shell about the core. As more and more
helium sinks into the core the latter continues to shrink and heat up.
hydrogen fusion shell expands away from the core causing the outer layers of
the star to heat up and therefore expand. As the outer layers expand they cool,
thus turning red. A RED GIANT is formed. Betelgeuse, the right shoulder of Orion
is such a Red Giant, and is the first star whose disk has been photographed
When this happens to the Sun it will probably engulf Mercury and perhaps Venus.
The Earth will, for a while, orbit through the Sun's atmosphere at its present
velocity. But gradually, due to the friction between the Earth and the Sun's
atmosphere, the Earth will spiral in towards the Sun and be vaporized.
Degenerate Gas. After about 1 billion years the star shrinks until the temperature
of the core is hot enough (about 100 million kelvin) to cause helium to fuse
to carbon. But to reach such enormous temperatures the gas within the core must
become highly compressed; so compressed in fact that the electrons are forced
to fill all the available energy states up to some maximum level.
When all the lowest available energy states are occupied the gas is said to
be degenerate. The electrons are so compressed that they are unable to change
their speeds: they can't slow down because all the lower energy states are occupied;
they cannot speed up because to get to the higher unoccupied energy levels would
require the input of an enormous amount of energy.
A degenerate gas is one in which the heating of the gas does not raise the pressure
of the gas. The pressure cannot increase because of the difficulty in speeding
up the electrons.
Because the pressure does not rise, even as the core is heated, no expansion
occurs. Within a matter of seconds the core grows hotter and hotter and fuses
helium to create heavier elements. This explosive destruction of helium is called
the Helium Flash.
The star becomes a variable star as the outer layers are periodically re-heated
and cooled. This causes the outer layers to pulsate. The star pulsates with
a period of between 200 and 600 days and loses matter from its surface in the
form of powerful winds that seed space with the newly formed carbon and other
elements, thereby setting the stage for the birth of the next generation of
stars and planets. See the Ring and Dumbbell Nebulae in the summer sky to witness
the results of this form of star death.
After the helium flash the star becomes again a red giant and then cools to
a WHITE DWARF. Such a white dwarf occupies the center of the Ring Nebula (M51)
which is visible with a small telescope in the constellation Lyra, or visible
above these lines in this stunning Hubble Space Telescope image. The expanding
gas from the dead star is seen in this beautiful image to your left.
High Mass Stars
Stars whose mass is greater than the theoretical Chandrasekhar Limit of 1.44
Suns end their lives in a different manner than White Dwarf types. When the
hydrogen in the core of a more massive star is used up the helium core shrinks,
the temperature of the core increases and shell hydrogen fusion occurs. The
luminosity increases, the star expands thereby cooling its outer layers. The
star becomes a red giant. (The shell of fusing hydrogen is called a a shell
source.) Meanwhile, the core contracts, but usually does not become degenerate.
Because the star is so massive helium, in the core, begins to fuse with only
a little extra compression, to form carbon. If the core does, however, become
degenerate before carbon fusion a runaway increase in temperature can occur,
similar to what happens in the helium flash of low to medium mass stars, causing
an explosive destruction of carbon in what one could call a "carbon flash",
but which is usually referred to as a carbon detonation. This is a much more
violent explosion than that of the helium flash. Carbon fusion creates oxygen
core burns ever more furiously creating heavier and heavier elements (magnesium,
sulfur, silicon) eventually reaching iron (Fe) created by silicon fusion. When
that stage is reached the massive star is doomed. The nuclear reactions that
produce iron are endothermic, that is, the reactions absorb heat and thus cool
the core. The cooling reduces the pressure in the core, which pressure is insufficient
to hold off the crushing force of gravity. In a split second the core collapses
and triggers the most powerful explosions in the universe, called supernovae.
Such a supernova event was witnessed and recorded by the Chinese in 1054. Today
that object looks like the image to your left ... the Crab Nebula. Within these
explosions all the elements heavier than iron are created. The shock waves from
these explosions can trigger star formation as the shock compresses nearby molecular
clouds. This is really cool! Imagine my wedding ring. It is made of gold. You
have just learned that stars cannot manufacture elements heavier than iron in
their cores because fusion of iron is endothermic. Yet there is gold in them
hills, which was mined and molded into my ring. Gold, silver, platinum, copper,
lead, uranium are all heavier elements than iron, yet are not made in a star's
lifetime. ONLY when a massive star goes supernova and the energy of the explosion
reaches trillions of Kelvins are the heavier elements forged, despite the endothermic
nature of iron fusion. Shortly thereafter, the supernova's expansion cools and
all which remains is debris. If that remaining debris cloud recollapses into
a new star, and if some of the material happens to collapse into local bodies
of planet size, then some of those heavy metals will be locked in the rocks
of those newly formed planets. Gold, silver, platinum, copper, uranium and others
are all so rare because the forces of nature which create them are short-lived,
and supernovae are not all that common. Indeed, my gold ring may be made of
elements forged in a massive star which exploded a very long time ago, but whose
remains recollapsed to form our sun and this planet. STAR LIVES
From this reading, you should now be aware that stars lives their lives according
to the physical laws governed by their masses. The more massive a star, the
hotter the nuclear burning and the shorter the lifespan. The less massive a
star, the cooler and also slower the nuclear burning and the longer the lifespan.
You can take a break and return to Introduction
to the Stars, or to the Spectral
Classes of Stars. Or you can move forward to a discussion
on Main Sequence
Dwarf Stars. Just do not go back to the Syllabus
as a time like this.
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