Download How stars form slide show File

NCEA Physics
Gravitational Fields and Star
Formation
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The birth of stars
Aims:
•To describe how stars form from very large clouds
of hydrogen, helium and dust which collapse under
the influence of gravity so that the core becomes hot
enough for nuclear reactions to begin.
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Protostar formation
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Galaxy arms
Stars often
form in the
spiral arms of
galaxies where
there is a lot of
gas and dust.
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Nebulae, where stars are born
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Nebulae
•Nebulae are huge clouds of gas and dust that are
the birth place of new stars.
•The gas in a nebula is nearly all hydrogen with a
small amount of helium.
•When light from nearby stars hits these atoms they
emit beautiful colours that give nebulae their
distinctive look.
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Nebula picture 1
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Orion nebula (M42)
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Dust particles
•The dust
particles are
very small.
Smoke particles
are about the
same size.
•These dust
particles mix
with the
hydrogen in the
nebula.
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Areas of star formation
Parent
cloud
A
10pc
B
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Star birth clouds
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Elements that form an average star
Element
Abundance
Hydrogen
91 %
Helium
8.3 %
Oxygen
0.06 %
Carbon
0.02 %
Nitrogen
0.01 %
The rest
…
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Close up picture 1
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Close up picture 2
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Collapse of gas into a protostar
A huge cloud of gas has a massive amount of GPE. As it
shrinks some of this energy is transferred to kinetic energy
as the protostar starts to spin. The rest is transferred as
kinetic energy to warm the gas and as radiation that is
emitted from the protostar.
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Clusters of stars
•A large dense cloud of gas and dust will break up into smaller
parts as it shrinks, scientists are still trying to figure out why!
Stars form in clusters
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Protostar Collapse
• The spinning of the protostar collapses clouds into disks
through which material flows down to the central object.
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More than anything
else, mass plays a
critical role in
determining whether
the central object will
become a star.
Layer upon layer of
gas encircling the
centre adds its weight
to the layers beneath.
.
Imagine laying on the floor while layers of sand bags are being piled
on top of you. The more layers of sand bags there are, the more
crushed you feel beneath them and the hotter the temperature rises.
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Maturing protostars
When temperatures reach about 5 million degrees C at the centre,
nuclear reactions begin. This takes about 80 times the mass of
Jupiter (0.08 of the sun's mass). Any less mass and the
temperature does not reach a high enough value to ignite the
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fusion reaction. The gas ball does not become a star
Protostar disks
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Getting hotter
•As the protostar collapses it gets hotter.
•The bigger the initial protostar the quicker it gets hotter.
•Massive protostars evolve to be normal (main sequence)
stars very quickly (10,000 years); less massive stars
evolve more slowly, up to 10 million years.
•Eventually the core of the protostar becomes hot enough
for nuclear fusion to take place, this needs a temperature
of at least 15 million Kelvin / degrees Celsius.
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Nuclear fusion - The simple picture
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Nuclear Fusion
• The star is full of hydrogen moving about very fast. We can
imagine the gas as full of protons and electrons.
• At a million Kelvin the electrical repulsion force between the
protons prevents them becoming close when they move towards
each other.
• As the temperature increases further the protons become closer.
During nuclear fusion two protons become so close that the
‘Strong nuclear force’ grabs them and holds them together, this
releases lots of energy.
•This force only works over very short distances so the protons
have to move very fast to get close, moving fast means being
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very hot!
Fusion diagrams
At low temperatures
the two positively
charged protons repel
each other and cannot
come into contact.
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Fusion diagrams
As the particles become
hotter they move faster.
When they fly towards each
other they become closer
but the force of repulsion is
still too strong to allow
them to come into contact.
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Fusion diagrams
At extremely high
temperatures the protons
are moving so fast that the
force of repulsion cannot
stop the protons from
hitting one another.
When they impact on one
another nuclear fusion
takes place and they stick
together.
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Hydrogen Fusion
As soon as two protons fuse together there will be a radioactive
decay as two protons are not stable. When a proton decays into a
neutron it also emits a positron (beta plus) particle.
p
p
1
1
n
p
p
p
positron
p p He
e+ e-
1
2
2
1
2
2
Within a fraction of
a second the
positron will meet
an electron and
both will be
annihilated to
produce pure EM
radiation.
He H  e
2
0
1
1

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
P-P Chain

H1

H1
He3
109 years

o
1 sec
H1
H1
He4
106 years
H1
Other nuclear reactions can take place at these temperatures
so that hydrogen eventually turns to helium. The fuel of the
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Sun is being used up to make new elements.
Stability at last
At each radius
Fgrav=Fpressure
Nuclear fusion
now creates
enough pressure
to stop the star
from collapsing
any further. A
star will spend
most of its life
in this hydrogen
to helium
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phase.
The Sun as a power station
•Near the centre of the core of the Sun nuclear fusion
is proceeding in generating tremendous amounts of
energy. Using E = m c2 we know that 4.7 million
tons of material is being converted each second.
•The Sun has been a main sequence star for
approximately 4500 million years.
•Small stars like our Sun are called yellow dwarfs.
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• The sun converts
4.3 million tons
(4.3x109 Kg) of
mass into energy
every second ...
equivalent to the
mass of 1million
elephants.
• It has been
doing this for
the past 4.6
billion years.
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• The sun is constantly consuming its own mass to generate the
energy it radiates into space. Over its 4.6 billion year lifetime it
has lost some 600 trillion trillion (6x1026) Kg.
• Relax ... this is only a few hundredths of one percent of the sun's
total mass of 2x1030 kg. The sun has barely noticed the weight
loss.
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• The sun is slowly changing - in the solar core, 37% of the
available hydrogen has already been fused to form helium.
For every 600 million tons of hydrogen fused to helium, 5
million tons are converted into energy and eventually
released to space. At this rate, the sun will continue to burn
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hydrogen for the next 5 billion years.
The Sun: Our nearest star
Property
Surface T
Central T
Value
5500 K
15  106 K
Luminosity
2  1033
ergs
Mass
4  1030 kg
Lifetime
(ms)
10 billion
years
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Solar corona
•The Sun’s temperature reaches a minimum in the
photosphere, then increases in the chromosphere and
increases again to millions of degrees in the beautiful corona
region. The corona can only be seen during an eclipse. 36
Corona and sun spot
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More corona
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Sun spots
•The sun suffers from
acne. Spots of magnetic
distortion appear on its
surface.
•Solar flares made of hot
charged particles (mostly
p+ and e-) evaporate from
the Sun and stream into
the Solar System
•About 1 million
tonnes/second stream
away at 1 million mph.
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These solar flares can
stream upwards by
distances biggerSmall
than the
size of the Earth.
solar flares
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Huge solar flares
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Solar Wind
•The solar wind increases
when there are more flares
from sun spots.
•Solar wind `storms’ can have
powerful effects on the Earth,
even overloading electrical
cables.
•Satellites in space are not
protected by the Earth’s
magnetic field and can have
their electrical circuits
destroyed by powerful solar
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winds.
The Earth’s protection
•We are protected by the dangerous solar winds by our
atmosphere. At the poles a small amount of wind enters
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the atmosphere causing a beautiful effect.
Northern lights
•The solar wind creates
light in the same way as
a neon strip light in our
classroom.
•The magnetic field at
the poles of the Earth is
straight downwards so
that some particles can
travel until they hit a
particle in the
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atmosphere.
N lights
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N lights
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Summary – The birth of stars
•Huge amounts of gas and dust from nebulae are slowly
drawn together over millions of years by gravity.
•As the gas compresses it changes its gravitational
potential energy changes into kinetic energy.
•The KE changes into heat and the protostar becomes
hotter and hotter as it becomes smaller.
•Eventually the protostar becomes hot enough for nuclear
fusion to start (10 million degrees).
•The star is now in its main sequence, converting
hydrogen to helium.
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