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The Sun
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The Sun is a star.
It is 4,500 million years old
It takes 8 minutes for its light
to reach the Earth.
98.6% mass of the Solar
System
Consists of Hydrogen (74%)
and Helium (24%)
• Radius: 695,000 km
• Mass: 333,000 Earths!
• Surface Temperature: 5,500 °C
• Core Temperature: 15,500,000 °C
Star Size Comparison
Internal Structure of the Sun
Radiative Zone
photons travel
out through it
Convective Zone
temperature falls with
increasing distance
towards the surface
Core
Hottest part where
hydrogen fusion
occurs
Photosphere
granular appearance.
EM radiation emitted.
Photosphere
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The Sun’s surface - the layer of the Sun
where most of the visible light comes from.
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The energy released in the Sun's core takes
over 100000 years to reach the photosphere.
• A magnified portion of the solar surface.
• Energy from below the surface is transported by
convection and resulting in granulation.
• The lighter areas reveal where gases are rising from
below, while the darker areas show where cooler
gases are sinking back down.
Chromosphere
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The 2nd of the three main
layers in the Sun's
atmosphere
~2,000 km deep - just
above the photosphere
Normally invisible - can
only be seen during a total
eclipse with special
equipment (the
photosphere is too bright).
The temperature varies
from ~6000K to +35000K
It is not yet fully
understood what
causes the
chromosphere to
increase in
temperature as
you move away
from the Sun
Corona
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The plasma
"atmosphere" of the
Sun
Extends millions of
kilometres into space
Most easily seen during
a total solar eclipse
Much hotter (nearly
200 times) than the
surface of the Sun: the
photosphere's average
temperature is 5800K
compared to the
corona's 1-3 million K.
The solar wind is a stream of protons and electrons which
flow outwards from the coronal holes.
The particles get accelerated up to 200 - 400 km/s.
The solar wind particles flow through-out the solar system
beyond Pluto
• Prominences (aka coronal loops) are large, bright
loop-shaped features on the photosphere that extend
out into the corona
• A prominence takes ~1 day to form and may persist
for several months.
• A typical prominence extends over many 1000s of
kms; the largest recorded was over 800000 kms long
• Coronal loops are found in the lower corona resulting
from the Sun’s magnetic field.
• Their frequency is linked with the solar cycle.
• They are often found with sunspots at their footpoints.
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A solar flare is a sudden
brightening observed over the
Sun's surface
• Involves a large energy release
of ~160 billion megatons of TNT
• The frequency of solar flares varies
from several per day when the Sun is
"active" to less than one per week
when the Sun is "quiet“.
• Filaments are prominences viewed
from above instead of side on.
• As they are cooler than the
photosphere, they appear darker
What are sun spots?
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Temporary “blemishes” on the photosphere
Caused by intense magnetic activity which reduces
convection, forming areas of reduced temperature
(3000–4500 K).
Sunspots expand and contract as they move round the
Sun and can be as large as 80000 km in diameter making
them clearly visible from Earth.
The Sun from the dwarf-planet Sedna
What powers a star?
• A star is ‘powered’ by nuclear fusion reactions in its
core.
• Nuclear fusion involves light atomic nuclei fusing
together to form heavier ones.
• This process releases huge amounts of energy - each
second, the Sun produces 4 x 1026 joules of energy!
• It would take 2000 million nuclear power plants a
whole year to produce the same amount of energy
on Earth.
What’s inside an atom?
Inside an atom, there are three smaller things:
 Protons - positively charged
 Neutrons - neutral (no charge)
 Electrons - negatively charged
 The protons and neutrons form the nucleus in the center of the
atom and this is surrounded by electrons.
Nuclear Fusion
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We can add neutrons to a nucleus without changing the way an
element reacts.
These different variations are called isotopes.
Deuterium and tritium are two- and three- times as heavy as
normal hydrogen.
Nuclear Fusion
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Inside stars, a proton (hydrogen) fuses to a neutron to make
deuterium.
This then fuses with another proton (hydrogen) to make
helium-3.
Nuclear Fusion
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Once we have two helium-3 nuclei, we combine them
to make helium-4 (the most common isotope of
helium).
Lots of energy is released at every stage of this process,
in the form of light, making the Sun shine.
How are elements made?
• Nuclear fusion in stars produces
new atoms.
• In the early stages of a star’s life,
light elements such as helium are
mainly formed.
• When all the hydrogen has been
used, other elements are fused
together to make the heavier
elements of the Periodic Table.
However, not all elements are made in the early stages of a star’s life.
Some of the heavier elements are only made when a star explodes at the
end of its life.