Stars are a fascinating component of our universe. They may seem like permanent
objects in the sky, but technology has allowed us to photograph the heavens, and
now we know more about stars than ever before. They are born, they live, and then
they die. How does this happen?
That’s what this activity is all about! A star’s life is long compared to that of a
human, but we can see the stages of stellar birth, aging, and death in the heavens.
They follow a pattern similar to many of the life cycles we see here on earth. Stars
are born, they “grow up,” exist many years, and then they die, and there’s an
exciting battle between the force of gravity and gas pressure to that makes it
exciting and potentially explosive!
Space: What’s out there?
Space may seem empty, but actually it is
filled with thinly spread gas and dust. This
gas and dust is called interstellar medium.
The atoms of gas are mostly hydrogen
(H2), and the gas atoms are typically about
a centimeter apart. The dust is mostly
microscopic grains and comprises only a
few percent of the matter between stars.
The dust is mostly carbon and silicon. In
some places, this interstellar medium is
collected into a big cloud of dust and gas
known as a nebula. This is the birthplace of
stars because the gas and dust is what
makes up a star. In fact, our sun was probably born in a nebula nearly 5 billion
years ago.
Protostars and the Nebula
A nebula is a cloud of dust and gas, composed primarily of hydrogen (97%)
and helium (3%). Within a nebula, there are varying regions when gravity
causes this dust and gas
to “clump” together. As these
“clumps” gather more atoms
(mass), their gravitational
attraction to other atoms
increases, pulling more atoms into
the “clump.”
Pictures from NASA - Protostar in the Eagle Nebula
We just know that nebulae (plural for nebula)
are the birth place of stars. The Hubble Space
Telescope has increased our knowledge about
this with some great photos from space which
clearly show stars in different stages of
development within a nebula.
What causes these “gravitational
centers” to form in these huge
clouds?
If you knew that, you’d have a Nobel
Prize!
Adding atoms to the center of a
protostar is a process astronomers
call accretion. Because numerous
reactions occur within the mass of
forming star material, a protostar
is not very stable.
In order to achieve life as a star, the protostar will need to achieve and
maintain equilibrium. What is equilibrium? It is a balance, in this case a
balance between gravity pulling atoms toward the center and gas pressure
pushing heat and light away from the center. Achieving and keeping this
balance is tough to do. When a star can no longer maintain equilibrium, it
dies.
Equilibrium: How it Works!
Equilibrium is a battle between gravity and gas pressure. It works like this:
1. Gravity pulls gas and dust inward toward the core.
2. Inside the core, temperature increases as gas atom collisions
increase.
3. Density of the core increases as more atoms try to share the
same space.
4. Gas pressure increases as atomic collisions and density
(atoms/space) increase.
5. The protostar’s gas pressure RESISTS the collapse of the
nebula.
6. When gas pressure = gravity, the protostar has reached
equilibrium and accretion stops
Equilibrium for a protostar occurs when gas pressure equals gravity.
Gravity remains constant, so what changes the gas pressure in a
protostar? Gas pressure depends upon two things to maintain it: a
very hot temperature (keep those atoms colliding!) and density (lots
of atoms in a small space).
There are two options for a protostar at this point:


Option 1: If a critical temperature in the core of a protostar is not
reached, it ends up a brown dwarf. This mass never makes “star
status.”
Option 2: If a critical temperature in the core of a protostar is
reached, then nuclear fusion begins. We identify the birth of a star
as the moment that it begins fusing hydrogen in the core into
helium.
So, what is a star? A star is a really hot ball of gas, with hydrogen fusing
into helium at its core. Stars spend the majority of their lives fusing
hydrogen, and when the hydrogen fuel is gone, stars fuse helium into
carbon. The more massive stars can fuse carbon into even heavier
elements, which is where most of the heavy elements in the universe are
made. Throughout this whole process is that battle between gravity and
gas pressure, known as equilibrium. It’s crucial to keep this battle in your
mind when trying to understand how stars live and die.
The Main Sequence
Stars live out the majority of their lives in a phase termed as the Main
Sequence. Once achieving nuclear fusion, stars radiate (shine) energy into
space. The star slowly contracts over billions of years to compensate for
the heat and light energy lost. As this slow contraction continues, the
star’s temperature, density, and pressure at the core continue to increase.
The temperature at the center of the star slowly rises over time because
the star radiates away energy, but it is also slowly contracting. This battle
between gravity pulling in and gas pressure pushing out will go on over the
entire life span of the star.
A Matter of Mass
What determines how long you will live? You could live a long full life, dying
of old age primarily because your old, tired body has worn out. You could
get a disease, like cancer, and that could impact the length of your life.
You could have a heart attack, be in a car accident, or fall off a cliff on a
hiking excursion. But most people start to see health decline when their
bodies cannot maintain a good balance. Biologists call this homeostasis,
which means balance or equilibrium.
For example, some biologists believe that all individuals die a cellular
death. If your cells are starved of oxygen, for whatever reason, they die.
This happens relatively quickly too.
Five minutes or less without
oxygen will cause brain death in
a human. Without oxygen, the
cells of the body (including your
blood) become more acidic, until
eventually all of the enzymes
that cause your body to work
are “fried” by the acid levels.
Once your enzymes are fried,
your prognosis for recovery is
slim. Your body did not maintain
its proper pH, or acid balance.
Too far out of balance, and your
body shuts down.
copyright 1994 STScI
This is an image of one of the smallest stars
scientists have observed.
A star needs to maintain a balance too – but this balance is between gas
pressure and gravity. What do you think determines the length of life of a
star? Well, your hint is that it’s a matter of mass. What has mass got to
do with it?
Well, here’s some logic to help you figure it out. If a star has a small mass,
it has fewer atoms to maintain at equilibrium. If a star has a large mass, it
has more atoms to keep at equilibrium. Do you think being bigger is better
when it comes to how long a star lives?
Choose from the following hypotheses regarding length of star life:
1) The bigger a star is, the longer it will live.
2) The smaller a star is, the longer it will live.
Now, for whichever hypothesis you chose type a 1-3 sentence explanation
for why you think this is so.
Larger stars have more fuel, but they have to burn (fuse) it faster in order to maintain
equilibrium. Because thermonuclear fusion occurs at a faster rate in massive stars, large
stars use all of their fuel in a shorter length of time. This means that bigger is not better
with respect to how long a star will live. A smaller star has less fuel, but its rate of fusion
is not as fast. Therefore, smaller stars live longer than larger stars because their rate of
fuel consumption is not as rapid.
Equilibrium: Life Goal of a Star
Look at the diagram on the right. There are essentially two
sections of a star: the core (where fusion occurs), and an
outer gaseous shell. The core serves as the gravitational
“center” of the star. It is very hot and very dense. The
outer shell is made of hydrogen and helium gas. This shell
helps move heat from the core of the star to the surface of the star where energy
in the form of light and heat is released into space.
The star’s main goal in life is to achieve stability, or equilibrium. The term
equilibrium does not mean that there isn’t any change in the star. It just means
that there is not a net overall change in the star. In a stable star, the gas
pressure pushing out from the center is equal with the gravity pulling atoms inward
to the center – when these forces are equal, the star is at equilibrium.
Once a star reaches equilibrium for the first time, it will start burning (fusing)
hydrogen into helium.
This 5-step process works like this:
1.
2.
3.
4.
5.
Nuclear fusion. Gravity = gas pressure (equilibrium)
Out of fuel.
Fusion stops, temperature drops.
Core contracts (gravity pulling atoms in).
Increased temperature (more atoms, more collisions) and density in the core
reinitiates nuclear fusion, equilibrium is achieved, and the cycle begins again
at step 1.
Because interstellar medium is 97% hydrogen and 3% helium, with trace amounts of
dust, etc., a star primarily burns hydrogen during its lifetime. A medium-size star
will live in the hydrogen phase, called the main sequence phase, for about 50 million
years. Once hydrogen fuel is gone, the star has entered “old age.”
After Main Sequence
What happens to a star after the main sequence phase? Old age and death! How
long it takes for a star to die depends upon its initial mass. A lower-mass star like
the sun can survive for billions of years, but after the hydrogen and helium fuel is
gone it cannot get hot enough to fuse carbon.
This type of star dies by puffing off its outer
layers to produce expanding planetary nebulae.
These nebulae, which are the remains of dying
stars, serve as the birthplace for future
protostars.
In contrast with our sun, which is really a main
sequence star, massive stars live very short lives,
perhaps only millions of years, before they develop
dead iron cores and explode as a supernova. The core of a dying massive star may
form a neutron star or black hole, but the outermost parts of the exploded star
return to the interstellar medium from which they came.
Let’s look at the relationship between initial mass and length of star life. How long
do most stars survive? Millions to billions of years, depending upon the star’s
“birth-mass.” Is bigger always better? Not with stars. The more mass a star has at
birth, the harder it is to keep that fusion reaction going. It may have more atoms,
but the fusion reaction goes faster and therefore burns the star out faster than
smaller stars. Bigger is not better in this case! Keep in mind that fusion is what
allows a star to maintain equilibrium. If a star cannot achieve a hot enough
temperature to initiate fusion, then it’s dying already. Fusion reactions need a fuel,
and there are three main fuels that a star uses for fusion: hydrogen, helium, and
carbon.
HYDROGEN BURNING (Stable Star
Life):
93% of interstellar matter is hydrogen
gas. 3% of interstellar matter is helium
gas. When a star forms, it has the same
composition since it’s made of the dust and
gasses in a nebula. Hydrogen gas (H2) is
split into single hydrogen atoms (H+). The basic hydrogen fusion reaction is as
follows:
H2 -> 2H+ + 2e- (UV photons)
4H+ -> He + energy
HELIUM BURNING: The Beginning of the End
For stars that live most of their lives in
the main sequence, helium burning is the
beginning of the end. The overall
thermonuclear reaction for helium burning
is as follows:
3 He -> 1 C + energy released
For the most part, hydrogen in the core is
gone. If the star wants to maintain
equilibrium between gravity and gas
pressure, it needs increased temperatures
in the core to re-ignite fusion. The star is
forced to burn helium in an effort to
maintain stability. It takes a temperature of 10×107 °K to initiate helium burning,
whereas it only takes a temperature of 2×107 °K to initiate hydrogen burning.
Remember, to remain stable the
star must balance the gas
pressure pushing out and the
gravitational force pulling in.
Gravity will cause the core to
contract. Helium burns inside the
core, but a rapid hydrogen
reaction occurs faster in the shell
of the star. As the temperature in
the shell of the star increases,
the outer layers of the star
expand.
Helium in the core of the star is
still burning hot. Gravity keeps
contracting the core to maintain
equilibrium, and as the core
contracts the atoms are packed
together even tighter than before.
The outer shell has expanded in an
effort to help heat from the core
escape into space. At this point,
the star is often termed a red
giant. The red giant is the first
step in old age.
copyright 1997 STScI
Mira is a Red Giant star, as is it's companion star
pictured in these images.
Fusion is releasing more energy during helium burning than at the main sequence
stage, so the star is bigger, but less stable. Eventually, the core will run out of
helium fuel, and in order to maintain equilibrium, the core will contract again to
initiate the last type of fusion – carbon burning.
CARBON BURNING: Death
Up to this point, most of the events of stellar evolution are well documented. What
happens to a star after the red-giant phase is not certain. We do know that a star,
regardless of its size, must eventually run out of fuel and collapse. In theory,
GRAVITY WINS. With this in mind, we will consider the death of stars and group
them into three categories according to mass:
1. Low-Mass Stars (0.5 solar mass or less)
2. Medium-Mass Stars (0.5 solar mass to 3.0 solar mass)
3. Massive Stars (3.0 solar masses or larger)
Low-mass stars
A low mass star becomes a white dwarf
Low mass stars (0.08-5 SM during main sequence)
will go the planetary nebula route. A low mass core
(,1.4 SM) shrinks to white dwarf. Electrons prevent
further collapse. The size of the white dwarf is
close to that of earth, and the outer layers are
planetary nebula.
Medium-mass stars become
neutron stars
A higher mass core (between 1.4-3 SM) shrinks to neutron
star. Supernova happens when a neutron star is created.
Neutrons prevent further collapse. The size of a neutron
star is about that of a large city.
More Massive Stars
These stars are so massive (10-20 solar masses) that the
hydrogen burning and helium burning phases occur relatively
quickly when
compared with
smaller stars. These
stars utilize carbon
burning.
This supernova was
first observed in 1987
by the Hubble
Telescope (NASA)
The overall reactions that occur for
carbon burning occur so rapidly and with so
much energy that the star blows apart in
an explosion called a supernova. The outer layers of the star blast into space, and
the core is crushed to immense densities. Carbon burning occurs when the helium in
the core is gone. The core needs to maintain temperature to keep the gas pressure
up; otherwise the star cannot resist gravity.
When carbon burning does occur, iron is formed. Iron is the most stable of all
nuclei, and ends the nuclear fusion process within a star. When these heavier
elements form in the core, they take away energy rather than release it. With the
decrease in fuel for fusion, the temperature decreases and the rate of collapse
increases. Just before the star totally collapses, there is a sudden increase in
temperature, density, and pressure. The pressure and energy compact the core
further, squeezing it like “Charmin.” The compact core becomes a rapidly whirling
ball of neutrons, and that’s why now this star is termed a neutron star.
The largest mass stars may become black holes
The highest mass star has a core that shrinks to a point. On the way to total
collapse it may momentarily create a neutron star and the resulting supernova
rebound explosion. Gravity finally wins. Nothing holds it up. Space so warped around
the object that it effectively leaves our space – black hole!
Now, here is what you need to do:
1. Grab a piece of black butcher-block paper.
2. Get colored pencils: White, Yellow, Light Blue, Orange, and Red
3. On your piece of paper, draw out the life cycle of a star using the colors
appropriately.
4. Label each step in the cycle and write a 3 sentence summary of each step.
http://sunshine.chpc.utah.edu/Labs/StarLife/starlife_main.html