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Stars and the Sun
Chapter 18.2
• Describe the basic structure and properties of
• Explain how the composition and surface
temperatures of stars are measured
• Recognize that all normal stars are powered
by fusion reactions that form elements
• Discuss the evolution of stars
Red star
White dwarf
Neutron star
Black hole
Our sun is a
Typical Star
Objective 1: The Structure of the Sun
• Stars are huge balls of mostly hydrogen and
• Held together by gravity
• Inside the core there is enormous pressure
• Stars are driven by Fusion
• Hydrogen is fused into helium, which releases
The sun’s external temperature
• 5500 Celcius
• Gives off WHITE light
• It appears yellow in our sky because our
atmosphere scatters the violet and blue light,
leaving yellow
• The scattered light gives us our blue sky
• When the sun is low, at sunset, more blue is
scattered and the sun can look orange
Heavy Metals
• Our solar system contains many heavy metals
• Since these form when stars explode – our
solar system must have been formed from the
star dust of exploding stars
1 Core
2 Radiative zone
3 Convective
4 Photosphere
5 Chromosphere
6 Corona
7 Sunspot
8 Granules
9 Prominence
The Core
• The sun is Plasma not gas
• Where most Fusion takes place
• All of the sun’s energy comes from the core
Radiation and Convection Zones
• Remember how heat moves?
• Radiation the energy is absorbed and re radiated
out from the next particles
• In the Radiation zone – Energy moves slowly
• Between the Radiation zone and the Convection
zone is the Tachocline
– a shear zone, the convection zone has fast moving
– may be responsible for the formation of the magnetic
Convection Zone
• In Convection there are currents that help the
energy flow outward
• Each current flows heat up, cools and sinks
back down
• Forms magnetic regions all over the sun
• The Granules are visible turbulence from
these flows
• It takes 10,000 to 170,000 years for energy to
travel from the core to the surface (scientific
• The visible surface of the sun
• Temperature at the surface is 6000 Celcius
• It is not as dense as air on earth
Everything above
the photosphere
is the atmosphere
of the sun
There are 5 zones
• Temperature minimum
• Chromosphere
• Transition
• Corona
• heliosphere
Temperature Minimum
• Coolest place in the sun is between the
Photosphere and the Chromosphere
• 4000 Celcius
• It is unknown why it is cooler
• Named from the
visible colors
• Appears as a bright
flash before an eclipse
• Temperature increases
to20,000 C
• Below transition gravity forms sun
• Above transition gravity is less dominant
• Temperature rises – Helium is ionized and
holds heat
• During a solar
eclipse, the corona
can be seen with
the naked eye
Can extend past earth
Temperatures can reach 1 to 3 million Celcius
Scientists don’t know why
One reason may be magnetic reconnection – a
process that releases energy stored in
magnetic fields
• Extends thru the solar system
• Includes the solar wind and the suns magnetic
• In 2004 the Voyager space probe encountered
a shock wave believed to be part of the
it was 50 AU away from the sun
• Areas of the sun that appear darker because
they are cooler
• They are cooler because they have strong
magnetic fields – magnetic fields decrease
surrounding pressure and don’t let hot gases
• Most sunspots are the size of the earth
• Can be seen with filters
Effect of sun spots on earth
• More sun spots – correlate to higher earth
• Fewer sunspots – cooler temperatures
• In 1645-1715 sunspots disappeared and there
was a mini ice age
• Sunspots increase activity of Northern Lights
Solar Cycle
• Sunspots increase and decrease every 11
• Last peak May 2000
• We are heading back into a peak of sunspot
activity in 2011 – It’s time to take a trip to
• It’s kind of like looking at the surface of a
boiling pot of spaghetti sauce – the bubbling
• Top of convection columns in the sun, they
surface in the photosphere
solar flares
What’s the difference
• A prominence is
something sticking
out – they are often
shaped like loops
• They follow the
magnetic field lines
• Can be stable for
days or months
• Solar Flares are
explosions under the
surface of the sun –
the blow clouds of
gas out into space
• Much larger events
• Prominance
• Sun quake caused
by solar flare
Looking at the sun
• Brief periods don’t cause damage – more
likely to for young people or at high elevation
• UV Exposure believed to contribute to
Cataracts (not from looking directly at sun)
• Looking thru binoculars can damage eyes
• Looking at Partial eclipses without filters is
dangerous – the eye’s pupil doesn’t contract
as much as it should
Objective 2: Explain how composition
and surface temperatures of stars are
• Otherwise known as how do we know all this?
• Cameras, telescopes, filters to detect…
• visible light, radio waves, electromagnetic
• Spectroscopy
• Can determine temperature, age, rotation,
magnetic field and movement towards or
away from us
• Study light
• Split light into wavelengths
• Each element has a spectral pattern
it’s like a fingerprint for Hydrogen and
• The red shift that shows us the Universe is
expanding is seen because the fingerprint isn’t
exactly where it is supposed to be
Young Stars contain more elements
• Stars are mostly hydrogen and helium
• Each element contains a spectral pattern
• Our Sun is middle aged
Temperature seen with color
Again – we look at the spectral patterns
Blue suns are hotter – 220,000 C
Red suns are cooler – 17,600 C
Our Yellow sun – 55,000 C
• Rotation is seen in smudges in the spectral
The boxes show redshifted Galaxies
Where are they in time?
Objective 3: Fusion
• The core of stars is extremely hot and dense –
gravity squishes it together
• In these conditions - Stars convert hydrogen to
• As stars age, the run out of hydrogen and
begin making carbon, oxygen, nitrogen…
• Older stars can produce elements as heavy as
Objective 4: Evolution of Stars
• Stars are born from massive gas clouds that
coalesce from gravity
• When the pressure and temperature get high
enough – fusion starts and the star starts to
How long
will our
sun live?
What happens when the hydrogen
runs out?
• When stars run out of hydrogen, the core
collapses while the outer edge expands (it
• The collapse lets the sun start fusing helium
into heavier elements
• When our sun becomes a red giant it will
swallow up earth – it’s diameter will reach
Planetary Nebula
• When the red giant has nothing left to fuse it
releases the outer layers into space
• What’s left becomes a White Dwarf
• White Dwarfs are as small as a planet but
very dense
• They eventually Burn out
What happens to bigger stars?
Up to 40 times our sun
• When the star burns
it’s hydrogen it
collapses very quickly
• Produces a shock
wave and very bright
• A Supernova
• The final collapse of a
supernova creates a
Neutron Star
Even Bigger
• You get the same
• The final collapse
creates a Black Hole
Supernova Explosion
path will
our sun
path will
Interaction of
White Dwarf and
Black Hole
• This is a computer
simulation –
scientists would like
to find this
happening out
What happens after stars die?
• New stars form from Stardust
• We are stardust
• (better described as NUCLEAR WASTE)