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The Sun
Classification
 The sun is a star, and a typical one at that


Its physical properties (mass, radius, brightness, composition) lie in the middle of the
observed ranges of stellar properties
Though mediocre (as far as stars go), life on Earth would not be possible without the
energy supplied by the sun
 Classified as a main sequence star on the Hertzsprung-Russell diagram
Vital Statistics
• The Sun is an ordinary G2 star, one of more than 100 billion
stars in our galaxy.
– Middle-aged star, estimated at ~5 billion years old, likely to die out
by ~10 b.y.
Classification
Evolution
☼
Towards the end of its life cycle, the sun will
transition from a main sequence star to a red
giant star…
•
•
☼
luminosity will increase
radius will increase by about 100 times its
present value (which means the end of the
terrestrial planets!)
And then to a white dwarf star
•
•
•
outer layers will be shed, leaving a planetary
nebula
only the core will be visible
eventually, any remnant energy will be
exhausted and the sun will quietly burn out
Evolution
Vital Statistics
•
Name:
–
•
The Sun is personified in many mythologies: the Greeks called it Helios and the
Romans called it Sol.
Diameter:
– 1,390,000 km
•
Mass:
– 1.989 x 1030 kg.
– the largest object in the solar system.
– contains more than 99.8% of the total mass of the Solar System
• Jupiter contains most of the rest
•
Composition:
– hydrogen (H)(75% by mass) and helium (He)(~25% by mass).
– minor amounts of heavier elements (0.1%), but they do occur with the same
relative abundance as the same elements on Earth and in meteorite
Vital Statistics
•
Rotation Period:
– Prograde rotation direction, the
common west-to-east rotation
(counterclockwise as seen from
above the north poles) of the sun
and planets (except Venus).
– sun has differential rotation –
• ~25 days at equator,
• ~36 days at poles
• warps and distorts the sun's
magnetic field
•
Axial tilt:
– sun's axis inclined 7 degrees from
the ecliptic
Structure

The sun is divided into different layers

Core
 temp = 15,000,000 K
 generation of energy by fusion
 200,000 km radius

Envelope




radiation zone + convection zone
transmission of energy by radiation and
convection
500,000 km total thickness
Photosphere




visible “surface”
layer from which energy escapes to
space
less than 500 km thick
temp = 5780 K
 Chromosphere
 lower atmosphere
 Corona
 outer atmosphere
 temp = 1,000,000 K
 thin, grades into solar wind at high altitudes
Interior of the Sun
• Toward the core,
– increases in
• pressure
• density
• temperature
–
–
5800°K (surface),
15,600,000°K (core),
– All the energy of the sun is created in the
core
• flows outward through the sun,
• heating the surface which radiates that
energy as light and heat.
• Energy flow from hot regions to cold
regions occurs by three mechanisms:
– conduction,
– radiation,
– convection
Energy
• Energy is transmitted from the core to
shallower depths in two ways
– Radiation – transfer of energy only, in the form of
a wave
• Ex: electromagnetic radiation
– Convection – transfer of energy and solar gas;
constant upwelling of warm fluid and the
simultaneous downward flow of cooler material to
take its place
• Convective motion is apparent in the granulation of the
photosphere (surface)
•
Interior of the Sun
Core
– Very high density
• 150 g/cc - 150 times denser than water and 10 times denser than lead)
– hot (15 million K)
– Core extends outward 25% of distance from Sun's center.
•
Radiation zone
– zone of radiative heat transfer composed of ionized gases.
– Extends to ~70% of distance from center.
•
Convection zone
– zone of active circulation of gases and heat transfer to the sun's surface.
– Extends from 70% of distance from center to the surface.
– The outer shell of the sun that we see from Earth is the top of the turbulent convective zone that
appears to be boiling
•
Photosphere
–
–
–
–
–
Sun's Atmosphere
visible surface of the sun
yellow that we see from Earth.
only ~500 km deep
relatively very thin
gases are of low enough density to
allow light to escape
• (0.1% of air at sealevel on Earth)
– 5500 K gas
Sun’s Atmosphere
• Photosphere
– surface mottled by a pattern of bright cells called granulation
• each cell about 1000 km across (size of Texas) separated by a dark boundary.
• each granule lasts about 8-10 minutes
• likely the surface expression of underlying convection cells
Sun’s Atmosphere
• Chromosphere
– nearly transparent layer of gas
about 1000 times fainter than
the photosphere
– very low density gas with
red/pink color
– The characteristic red color of
the chromosphere is due to
hydrogen alpha (6563Å)
emission
– rises to heights near 10,000
km,
• temps of 1,000,000 K
– only visible during a total
solar eclipse
Sun’s Atmosphere
• Chromosphere
– spicules are small, flamelike
projections from the chromosphere
• 100-1000 km across,
• extending up to 12,000 km from
the top of the photosphere
• lasting 5-15 minutes
• cooler regions (10,000 K)
extending up into base of much
hotter corona (500,000 K)
• spring up from edges of
"supergranules“
• charged particles that follow
magnetic field lines.
•
Sun’s
Atmosphere
Corona
– faint outer atmosphere, composed of very low
density ionized gases
• mostly free electrons and protons
– density at base of corona is 100 billion times
thinner than the air we breathe
– milky white glow only visible during total
eclipses or with a special telescope outfitted
with a disk to cover the photosphere
– "streamers" from the corona extends out to
10-30 "solar radii" (~7.6 - 23 x 106 km)
– temps rise rapidly outward in corona
• from 500,00 K low to 3,000,000 K in outer
corona)
Energy
☼
All of the sun’s energy is created in the core by nuclear fusion
–
–
fusion is a chemical reaction in which light nuclei are combined (fused) into heavier ones
for a main sequence star, this reaction involves the fusion of hydrogen into helium
– 4H → He + energy
–
☼
the release of energy is most important!
Core-hydrogen burning keeps the sun in hydrostatic equilibrium
–
state of balance in which pressure’s outward push is exactly counteracted by gravity’s inward pull
Proton Proton Chain
•
•
•
•
Two mass-1 isotopes of hydrogen undergo a simultaneous fusion and beta decay to produce a positron, a neutrino, and a mass-2
isotope of hydrogen (deuterium).
The deuterium reacts with another mass-1 isotope of hydrogen to produce Helium-3 and a gamma-ray.
Two helium-3 isotopes produced in separate implementations of steps (1) and (2) fuse to form a Helium-4 nucleus plus two protons.
The net effect is to convert hydrogen to helium, with the energy released going into the particles and gamma-rays produced at each
step of the sequence.
•
Rates for the PP Chain
•
The average time required for a nucleus to undergo each step of this sequence in a typical stellar interior is indicated in the figure
shown above. Thus, for example, a hydrogen nucleus waits on the average 1 billion years before it undergoes an interaction with
another hydrogen nucleus to initiate the sequence! Since all other steps require much less time than this, it is this initial step that
controls the rate of the reaction.
This incredibly small rate nevertheless accounts for the luminosities of normal stars because there are so many hydrogen atoms in
the core of a star that at any one instant many are undergoing the reactions of the PP chain.
•
1H
+ 1H  2H + e+ + e + 0.42 MeV
The positron immediately annihilates with one of the hydrogen's
electrons, and their mass energy is carried off by two gamma
ray photons.
e+ + e- 2 + 1.02 MeV
After this the deuterium produced in the first stage can fuse with
another hydrogen to produce a light isotope of helium, 3He:
2H
+ 1H  3He + + 5.49 MeV
Finally, after millions of years, two of the helium nuclei 3He produced
can fuse together to make the common helium isotope 4He, releasing
two hydrogen nuclei to start the reaction again through three different
paths called PP1, PP2 and PP3:
PP1: (69% of the time)
3He
+3He  4He + 1H + 1H + 12.86 MeV
The complete PP1 chain reaction releases a net energy of
26.7 MeV. PP1 chain is dominant in temperatures of 10-14
million Kelvin. Below 10 million Kelvin, the PP chain
does not produce much 4He.
PP2: (31% of the time)
+ 4He 7Be + 
7Be + e- 7Li + 
e
7Li + 1H 4He + 4He
3He
PP2 chain is dominant in temperatures of 14-23 million
Kelvin.
PP3: (0.3% of the time)
+ 4He 7Be + 
7Be + 1H 8B + 
8B 8Be + e+ + 
e
8Be  4He + 4He
3He
PP3 chain is dominant if the temperatures
exceeds 23 million Kelvin.
Sun’s Magnetic Field
• Sun's magnetosphere (its shell of magnetic field lines produced by a
dynamo effect in the sun's core) extends well beyond Pluto
• The Sun’s differential; rotation wraps and distorts the magnetic field
lines
Sun’s Magnetic Field
• Magnetic lines of
force leave the sun and
wrap through space
Sunspots
•
cool dark spots on the solar surface (photosphere) that contain intense magnetic
fields
–
–
–
–
–
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dark because they are cooler than surrounding photosphere
center of sunspot (umbra) is about 4000 K
outer border (penumbra) is about 6000 K
average size is about 2 Earth diameters
tend to form in groups of up to 100 individual spots,
last as long as 2 months or more, most about a week
Sunspots
• why are there sunspots?
– magnetic field within sunspots 1000 times greater than background solar
average
– perhaps rising currents of hot gas just under the photosphere are inhibited
by the intense magnetic fields in sunspots,
• cause a decrease in temp and a dark spot
• higher than average temps have been measured in the immediate margin of
sunspots,
• as if the light and heat can't escape through the intense magnetic field, so it is
deflected around the edges
Sunspots
• Sunspots tend to occur in pairs.
– one spot will be a magnetic
north pole
• where magnetic field lines
exit the sun
– the other spot will be a
magnetic south pole
• where magnetic field lines
enter the sun
• may relate to interaction
between magnetic field lines
and differential rotation of the
sun
– field lines become tangled and
pop through the surface with a
"north pole" and a "south pole"
Sunspots
• Sunspot cycle
–
–
–
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sunspots vary over an 11 year average cycle
from maximums of up to 100 visible spots to minimums of only a few spots
tend to first form in latitudes of 20 degrees then progressively more towards the equator.
for a cycle of 11 years,
•
•
•
•
leading sunspots in the northern hemisphere will have the same polarity,
those in the southern hemisphere will have the opposite polarity,
then reverses for the next 11 tear cycle.
SO, the cycle is actually 22 years.
– last maximum in 2001
• Prominences
The Active Sun
– red protrusions of ionized gases from the chromosphere
– often arch-shaped, controlled by magnetic fields
– seem to represent twisted magnetic fields emanating from the active regions around
sunspots
• Solar flares
The Active Sun
– chromosphere eruption of
• ionized gases
• x-rays,
• uv light
• visible light
– rises rapidly in a few minutes then decays over
an hour or less
– may release the energy equivalent of 2 billion
megatons of TNT
– clearly linked to magnetic field, because occur
of sunspot groups
– significant effects on Earth - most flares
become "gusts" in the solar wind and reach
Earth after a few hours or days
– creates a magnetic storm on Earth that affects
communications and increased auroral activity
near the poles
The Active Sun
• Changing Solar
Cornea
– Varies with sunspot
activity
SOHO
• composite image
combines Extreme
Ultraviolet Imaging
Telescope (EIT)images
from three
wavelengths(171, 195
and 284 angstrom) into
one that reveals solar
features unique to each
wavelength.
• SOHO web site
Solar Wind
•
•
the rapidly moving outward extension of the corona.
composed of ionized hydrogen (protons with free
electrons) called plasma
–
•
•
•
Extends into interplanetary space
travels at measured rates of 300-770 km/sec past the
Earth
Recent data from the spacecraft Ulysses show that the
solar wind emanating from the polar regions flows at
nearly double the rate, 750 kilometers per second, that
it does at lower latitudes.
–
•
•
Carries over 1 million tons of solar matter per second
The composition of the solar wind also appears to differ in
the polar regions
density measured at 50 particles per cc (not g/cc)
"gusts" of the solar wind cause magnetic disturbances
on Earth, which is mostly protected by its
magnetosphere
Solar Wind
• Affects entire solar system
Interaction of Solar
Wind and Earth’s
Magnetic Field
Solar Wind
•
•
Interaction of the Solar
wind with the Interstellar
wind
Heliopause
– gradual boundary
between the heliosphere
and the interstellar gas
outside our solar
system.
•
Heliosphere
– The area in space that
contains our solar
system, solar wind, and
the entire solar
magnetic field.
– It extends well beyond
the orbit of Pluto, out to
the heliopause.
It’s all
over
but the
fireworks
Which is your
homework.