Stellar Evolution
Protostar
phase – when the materials are collapsing
The process that forms a star is
known as Gravitational collapse. The gravitational force gets stronger when the
particles go up. The gravitational force is proportional to the density. If the
density goes up so does the gravitational force and vice versa. The
gravitational force changes within the cloud. Gravity is pulling all of the
material towards the center so the density of all the particles is increasing. The
temperature (measure of average energy of particle) is also increasing. The
pressure is also going up. The particles are moving faster and there will be
collisions. All we need to remember is that the gravity, density, temperature
and pressure are going up.
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Temperature, density, pressure increase
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For example letting temp increase (adding
energy), if we have water that’s very cold, it’s ice and when it melts it’s a
liquid. Notice how water moves more freely. Then when water boils it turns into
a gas. Once we add energy to the gas, it then becomes plasma; at this point it
is no longer water and cannot turn back into water because all of the molecular
bonds have been broken.
o
So we have a molecular cloud and a high density
region, as everything is pulled together it forms a star. Note that this is
done at a very high temperature
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Plasma is a collection of positively and
negatively charged ions.
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We should think of a proton as also hydrogen. A
proton is hydrogen that has been ionized (because the electron was removed)
because the proton is the nucleus.
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Plasma core forms
Main sequence
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Fusion begins at core
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Hydrogen begins to fuse
Nuclear fusion
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Nuclear
– pertaining to the nucleus
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Fusion
– to combine
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Nuclear fusion is combining particles of the
nucleus
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So how many types of particles are in the
nucleus? Only protons and neutrons, but we will be concentrating more on the
protons
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The reason for this is because the number of
protons lets us know what element we’re working with.
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The protons must be in a higher temperature in
order to fuse. When the two protons collide and fuse together, that’s what
drives a star.
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Stars have a plasma core, which are the protons
of hydrogen
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What is taking place is conservation of energy:
E = mc2 (Energy = mass * speed of light2)
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Energy must be injected in the protons to pull
them apart. As energy goes up, the mass goes up. This only applies on a
molecular level.
o
Nuclear fusion is taking place in the core.
Anytime a reaction takes place, energy is released in the form of photons.
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What is the problem with fusing two protons
together? They will still try to push each other apart which is an instability. The universe does not like
to be unstable. The solution is that one of the protons changes to a neutron
and the instability disappears because we no longer have like charges.
Hydrogen fusion
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Our sun has a constant outward flow of energy.
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Is the sun losing mass? The sun and all other
stars are constantly losing mass, due to the fusion taking place inside them.
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Hydrogen fusion produces helium
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All stars begin on the main sequence
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Hydrogen takes place at a lower temperature
because of the electromagnetic force
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Every second our sun converts 700 million tons
of hydrogen to 695 million tons of helium. The missing 5 million are the
photons that are released.
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Fusion moves farther away from the center, and
this has a huge change on the star. The only change that happens is the
location of the fusion.
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The rate at which the fusion takes place is the
same but the amount of material being fused goes up
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When a star begins to run out of hydrogen at the
core and the fusion moves outward.
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As the star gets bigger the density decreases
and the gravitational force is getting weaker
o
After the star leaves the main sequence, then it
enters the Giant phase
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The longer the star is in the Giant phase, the
more the helium increases (the star is getting thicker). Hydrogen fusion starts
at around 10 million kelvin, helium fusion begins around 100 million kelvin.
***Helium requires higher temperature because they have more mass and are more
positively charged and are hard to run into each other.
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A molecule of helium is heavier than a molecule
of hydrogen
Helium fusion
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Takes place at a higher temperature because it
has more protons
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A change takes place in the star, then the star
begins to shrink
o
When helium fusion begins, the star begins to
shrink
Proton-proton chain, CNO cycle
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If the temperature is below 18 Million Kelvin
it’s the proton-proton chain, if the temperatures above 18 Million Kelvin is
the CNO cycle
Giant phase
As the star gets
bigger the density decreases and the gravitational force is getting weaker.
o
Increased outward pressure
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Star gets bigger, brighter and cooler
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After the star leaves the main sequence, then it
enters the Giant phase
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The longer the star is in the Giant phase, the
more the helium increases (the star is getting thicker). Hydrogen fusion starts
at around 10 million kelvins, helium fusion begins around 100 million kelvins.
***Helium requires higher temperature because they have more mass and are more
positively charged and are hard to run into each other.***
o
A molecule of helium is heavier than a molecule
of hydrogen
Helium flash ends giant phase.
Anything less than 8 Mass is Low
Mass, anything greater than 8 Mass is High Mass. The temperature needed for
hydrogen fusion is 10,000,000 kelvin, for helium fusion 10,000 kelvin is
needed.
Low mass stars
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Planetary nebula
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White dwarf, gas shell
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For a low mass star, when the helium fusion
begins, it becomes unstable.
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This disturbance causes the fluid within the
star to move towards the outer edge which causes problems
Wind
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The particles in the air running into us
(migration of particles).
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In the low mass star, the outer 1/3 of the mass
is ejected. This is known as a massive wind (something like a solar flare).
This process is known as Planetary
Nebula
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If the star is a low mass star, it will most
likely be a Planetary Nebula
Planetary Nebula
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***The star is a (star) white dwarf with a gas
shell around it. This takes place after helium fusion
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All stars produce absorption spectrum. The white
dwarf is giving off energy which is injected in to the gas shell, this causes
the gas shell to grow, which then the gas shell produces an emission spectrum.
o
The emissions spectrum tells us the composition
of the gas shell.
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When the white dwarf runs out of energy and is
no longer glowing, it then becomes a black dwarf.
o
The brown dwarf is when the gravity is pulling
together initial material and there’s not enough to create a star. Brown dwarfs
form at the early stages of stellar evolution. Basically, brown dwarfs are
failed stars.
High Mass stars
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Supernova
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Fusion processes
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High mass stars are more stable
o
A high mass star goes through several additional
fusion products
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The final product of helium fusion is carbon.
Carbon builds up around the core, and then it starts to fuse.
o
Hydrogen fusion starts at the core then moves
away, which produces helium. Helium fusion starts at the core and then moves
away, which produces carbon. Below are the fusions in order…
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Hydrogen
§
Helium
§
Carbon
§
Neon (heavier than oxygen)
§
Oxygen
§
Silicon
§
**Iron fusion does not take place in a star
because, the temperature is too high and iron absorbs energy, and the star
begins to collapse (leads to a super nova).
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As the star gets bigger, the fusion pressure
decreases.
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A low mass star isn’t able to produce multiple
fusion process and generate a high enough temperature to produce carbon.
o
Our sun is a low mass star
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The death of a high mass star is going to be die
when the fusion rate decreases
o
While the star is collapsing, the outward
pressure is decreasing and the inward pressure is increasing; this causes the
star to explode, which causes a Super Nova
Forces and particles
Star forms a plasma core > Fusion takes place >
Hydrogen fuses > The Star is then on the Main Sequence and is stable >
Then the Giant phase takes place
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There are four forces of the universe
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Gravity – attractive force between massive
objects
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EM – force between charged particles
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Strong Nuclear – an attractive force that hold
together protons and neutrons
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Weak Nuclear - W and Z
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Photons transmit information
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When we look through a telescope we look at a
universe the way it was in the past. For example when we look at the sun, we’re
not seeing what the sun looks like right now, we’re seeing how it looked 8.5
minutes ago.
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Force
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Particle
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Strong Nuclear
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Weak Nuclear
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Electromagnetic
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Photon – carries info about the electromagnetic force
across the universe
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Gravity
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Real Particles and Virtual Photons
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Real – can be observed and is a product of a
physical reaction. For example, starlight.
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Virtual – cannot be observed and is a result of
the environment. For example, a clear plastic box with metal shavings move
around when a magnet is waved around it. Notice how you don’t see anything moving
the shavings.
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The photons is the messenger for the
electromagnetic force
The Graviton
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Deals with the gravitational force. We can’t see
the force that brings objects down towards the center
Gluon
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Deals with Strong Nuclear and uses W and Z
particles
Particles
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Real
particle – one that can be detected and is the result of a physical
process. For example, light from a star.
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Virtual
– one that cannot be detected and is the result of the environment.
Matter and Anti-Matter
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Matter
– protons and electrons.
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Anti-Matter
– positrons (opposite electrons [it’s dark twin] but has a positive charge) and
anti-proton (opposite protons [it’s dark twin] and have a negative charge)
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Matter and anti-matter come together all the
time; the particles just annihilate and release energy. Nothing like Science
Fiction movies/novels.
Integer and half-integer Spin
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Spin
– describes how particles interact with the environment
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Spin 0 particle always looks the same to its
environment. It will be a sphere with a single color.
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Spin 1 particle looks the same once when it
turns 360.
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Spin 2 particle looks the same twice when turned
360.
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These are all integer spin particles.
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Spin ½ particle is a fractional spin particle.
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Integer spin particles are called bosons
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Fractional spin particles are called fermions
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The Pauli
Exclusion Principle – two things can’t be at the same place at the same
time. For example everyone is using light to see, with no interference. Photons
(that are fermions) are excluded; however, photons (that are bosons) are not.
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Pauli –
are fermions
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No Pauli –
are Bosons.
Uncertainty principle
It’s impossible
to know the exact position and velocity at the exact same time of individual
particles. We find things by either catching a signal from them or bouncing a
signal off of it. For example, light is bouncing off of it, and that signal you
to see it. Another example is looking for keys in a dark room, and using a
flash light to bounce the light off of the keys. The two main signals (ways) we
use are light and sound. Radio waves the wavelength will be a meter or longer. The
size of a molecule and visible light reflects off of the atmosphere, which
causes the sky to be blue. The blue wave is reflected much more. This same
process makes the sun look red at sunset (on the horizon). The sun looks yellow overhead because the
thickness of the atmosphere filters the light and makes the sun appear yellow. A
small particle is knocked and it’s behavior changes. A signal of a high energy
is used to find particles and this changes its behavior. The uncertainty principle is the observation
changing the behavior of a particle. This apply only to things on the atomic
level (very small in size).
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Trying to measure individual particles
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Observe something you have to bounce a signal
off of. The signal must have a smaller wavelength than what you are looking for
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Position and velocity
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Proton beam
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Once you find the individual proton, you knock
it out of the beam and change it’s behavior.
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