Protostars and the Nebula
Name ________________________________________________
This animation shows how a star begins to form out of a 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.”
What is your hypothesis about causes these “gravitational centers” to form in these huge clouds? _________________________
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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.
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 __________________________________________________________
2. Inside the core, _______________________________________________________________
3. Density of the core ________________________________________________________________
4. Gas pressure ___________________________________________________________________
5. The protostar’s gas pressure ______________ the collapse of the nebula.
6. When gas pressure = gravity, the protostar __________________________________________
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: ___________________________________________________________________________________________________
Option 2: ___________________________________________________________________________________________________
Stars
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 _______________________________________ 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.
Interactive Lab
Please complete this activity that shows what happens to different size stars at the beginning of their life cycles. Indicate the
steps/final results for the different mass beginning “stars”
Small ________________________________________________________________________________________________
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Low ________________________________________________________________________________________________
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Medium _____________________________________________________________________________________________
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Massive ______________________________________________________________________________________________
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A Matter of Mass: 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.
To find out if you are correct, read the explanation carefully
Equilibrium: Life Goal of a Star: Look at the diagram on the website. 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.
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Try the interactive lab on equilibrium and then the practice quiz. 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.” Let’s see if
you understand the relationships between gas pressure, temperature, and gravity as it relates to equilibrium. Consider it a practice
quiz so you are ready for the one your teacher will undoubtedly give to you.
Practice quiz answers
1.
2.
3.
4.
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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:
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Test your knowledge!
So what happens when the star is out of hydrogen fuel? Without fuel, there is no burning (nuclear fusion). This fusion is what allows
a star to maintain equilibrium. Try to predict what will happen to the star at this point: Star Quiz (part 1) Record your answers here:
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: ____________________________
Try the Interactive Lab: Helium Burning Process. Summarize what you learned ___________________________________________
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For the most part, ___________ 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.
Test Your Knowledge!: Star Quiz (part 2) Record your answers here:
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 _______ __________. The red giant is the first step in old age.
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: 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, _________ _______. With this in mind, we will consider the death of stars and group them into three categories according to
mass:
1.
2.
3.
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Low-mass stars:A low mass star becomes a white dwarf
Low mass stars (0.08-5 SM during main sequence) will go the ____________________________. 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. L earn more about how white dwarves are formed. Summarize ______________________________________
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Medium-mass stars become neutron stars
A higher mass core (between 1.4-3 SM) shrinks to ________________. 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. Learn more about how neutron stars are formed.
Summarize _______________________________________________________________________________________________
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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 ________________________ Learn more about the Carbon Burning Process.
Summarize_______________________________________________________________________________________________
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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 _____________________:
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! Complete the activity that shows what happens to different size stars at the end of their
life cycles.
Summarize _______________________________________________________________________________________________
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