Transcript The Sun: the Solar Atmosphere, Nuclear Fusion

The Sun: Our Star
The Sun is an ordinary star and
shines the same way other stars
of its type do.
The bright part normally seen is
called the photosphere, which is
about 500 km deep.
It is an almost perfect black body
with a temperature of 5800 K.
Deviation from a black body are
due to absorption lines by gas in
the Sun’s atmosphere
Assigned Reading
• Chapter 8.
What is the Sun?
• The Sun is a star, a fairly common and typical star
• Its spectral type is G2, which means Teff~5800 K
• A star is a ball of gas held in equilibrium against its own
self-gravity by the thermal pressure and outflow of energy
from center to the surface
• The Sun radiates away a lot of energy. Every second, the
equivalent of a 60 Watt bulb is being emitted by each and
every 2-mm2 of the Sun surface (there are 3x1024 of them).
• IMPORTANT: it would cool off very quickly if energy
were not replenished. Where does this energy come from?
• The energy comes from nuclear fusion reactions at the
center of the Sun
The Sun’s General Properties
• Average star, very very common
• Spectral type G2, relatively modest luminosity
• It appears so bright because it is so close.
• Absolute visual magnitude = 4.83 (magnitude if it
were at a distance of 32.6 light years)
• 109 times Earth’s diameter
• 333,000 times Earth’s mass
• Consists entirely of gas (av. density = 1.4 g/cm3)
• Central temperature = 15 million 0K
• Surface temperature = 5800 0K
Why is the Sun important
for life?
Provides virtually all of the energy necessary for
life on Earth.
That energy is required for
1)
2)
3)
Producing the primary link of the food chain
(plants)
Making life possible (enabling tis chemical
reactions)
The weather, etc.
Balance in the stars
Thermal
Pressure
Gravitational
Contraction
Pressure and Temperature
of a Gas
How does a
star hold itself?
This balance between weight and
pressure is called hydrostatic
equilibrium.
The Sun's core, for example, has a
temperature of about 16 million K.
It is hotter than the surface (T~5800
K), because it needs lots of pressure to
sustain the immense weight of all the
upper layers.
That is why deeper layers have higher
temperature: they need more pressure
to establish the equilibrium.
What is the Sun made of
It is made of hot gas; there is no solid material in the sun!
The gas is mostly hydrogen (~80%) and helium(~20%), as is in the whole
universe
The gas is a plasma, i.e. its atoms are ionized. This means that they have
lost most or all of their electrons, and that the gas is a mixture of
electrically charged electrons (negative) and nucleii (positive)
A plasma is an ideal medium the realize a black body radiator:
Electrically charged particles can easily be accelerated and decelerated
Why does the Sun have such a sharp edge?
Pretty much an optical illusion!
Structure of the Sun
The Atmosphere of the Sun
Although the sun appears to have sharp edge, its
“surface” actually has complicated structure.
Its atmosphere, too, is complex
“Surface” and atmosphere of the Sun consist of:
"
"
Photosphere: 5,800 K,
continuum + abs.
Chromosphere: 4,500-500,000 K,
emiss.+abs.
Corona:
1-2 106 K, continuum+emiss.+abs.
Each square millimeter of the
Photosphere emits the same
Energy as a ~60 Watt light bulb
Corona (1-2,000,000 K)
The Sun surface (photosphere)
would cool off very quickly if Energy
were not produced continuously from
Below
spicules
It seems to have a sharp edge, because
immediately above it the temperature and
density drop atoms recombine, and there is no
more thermodynamic equilibrium: no B.B. any
more, no continuum emission
(4500-500,000 K)
(5,800 K)
The light we see originates from the thin photosphere
The Solar Atmosphere
Apparent surface
of the sun
Heat Flow
Only visible
during solar
eclipses
Solar interior
Temp.
incr.
inward
What would you expect to see if
you looked at the Sun’s
photosphere now?
• The photosphere: the region where the black body
radiation that we see is made. Only ~500 km thick
• Thin gas: 3,400 times thinner than air
• It is the transition region from a perfect blackbody emitting plasma to a transparent gas
• Its upper layers produces the absorption spectrum
• Limb darkening: proof that the Photosphere has a
thickness and that energy comes from below
• How does the photosphere look like if we close up
on it?
The limb is slightly fainter
A real effect: we see less into the depth of
the photosphere at the edge
Granulation of the
Photosphere
Each Granule is about the
size of Texas and lasts for
only 10-20 minutes before
fading away!
Energy Transport near the
Photosphere
Energy generated in the sun’s center must be transported outward.
Near the photosphere, this happens through
Convection:
Cool gas
sinking down
Bubbles of hot
gas rising up
≈ 1000 km
Bubbles last for ≈ 10 – 20 min
Granulation of the Photosphere
Each Granule is about the size of Texas and lasts for only
10-20 minutes before fading away!
Granules: convective
cells
• The granules are just densely packed
convective cells
• Convective cells are very similar to
thunderstorms
Sunspots
Sunspots, about 1000 K
cooler than the rest of
the Sun's photosphere,
appear as dark spots.
Sunspots come and go
with time. Big sunspots
can live for several
weeks.
Chromosphere
Reddish in color, which
is the origin of its name
(chromos meaning
``color'')
2000-3000 km thick
Faint relative to the
photosphere
From 4,500 up to
500,000K: hotter than the
photosphere and much
less dense.
Absorption + Emissionline spectrum
The Chromosphere
• Region of sun’s atmosphere just above the photosphere
• Visible, UV, and X-ray lines
from highly ionized gases
• Temperature increases
gradually from ≈ 4500 oK to
≈ 10,000 oK, then jumps to
≈ 1 million oK
Transition region
Chromospheric
structures visible in Ha
emission (filtergram)
The Chromosphere
Spicules: Filaments
of cooler gas from
the photosphere,
rising up into the
chromosphere
Visible in Ha
emission
Each one lasting
about 5 – 15 min
The Layers of the Solar
Atmosphere
Visible
Sun Spot
Regions
Ultraviolet
Photosphere
Corona
Chromosphere
Coronal activity,
seen in visible
light
UltraViolet (UV) image of the Sun:
looking at areas that are more
energetic than average.
It shows regions of the
Photosphere above Sunspots
are more energetic than
elsewhere.
It also shows magnetic
link between Sunspots
extending into the
Cromosphere and Corona
Corona
Only visible during eclipses
- the outermost layer
- T~1 million K
-made up of very diffuse
(but extremely hot gas)
- coronal emission is
dominated by X-rays
The Magnetic Carpet of the Corona
• Corona contains very low-density, very hot
(1 million oK) gas
• Coronal gas is heated through motions of magnetic fields
anchored in the photosphere below (“magnetic carpet”)
Computer
model of
the
magnetic
carpet
Solar Prominence
Composed of hot gas
trapped in magnetic
fields extending from
one sunspot to another.
Prominence
X-ray emission from the
Sun's corona
The corona is so hot that it emits
X-ray radiation
Rotation period: about a month
The middle rotates faster than the
north or south.
Corona has an emission line spectrum and a continuum spectrum
(dust scattering of photospheric black body light).
Some of the continuum spectrum has no absorption lines,
because of the dilution of the absorption lines by
very hot electrons
The heating of the
corona
The heating of the corona has
puzzled scientists for a long time.
The corona is heated by energy
outflowing from the sun's interior
not as heat but as magnetic energy.
How can heat go from cooler regions to hotter ones?
It doesn’t! Radiant heat penetrates the corona nearly
Undisturbed
Corona heated by something else:
whipping by magnetic force lines
The gas is heated by the motion
through it of long lines of magnetic
force, which act like whips.
The gas is being “whipped”.
Solar Flare
A solar flare is a violent outburst that lasts in an
hour or less. It radiates X-ray, ultraviolet, and
visible radiation, plus streams of high-energy
protons and electrons.
A large flare can be a billion times more energetic
than a large hydrogen bomb.
Solar Wind
Sun loosing mass, but only a small fraction: 10-14 Mo/yr
Mass loss will intensify later, and will much larger (~1/3)
Auroras: the Northern
and Southern Lights
November 24, 2001
• This was a Big One!
• Caused by two fast moving Coronal Mass
Ejections
• Seen as far south as Texas and Arkansas,
New Zealand and Australia also witnessed
them.
May 11, 2002
• Auroras seen as far south as New England
• Caused by a gust of solar wind
• These powerful gusts are guided by Earth’s
magnetic field and can excite gases in the
upper atmosphere, causing the air there to
glow
The key point here is that nearly all “solar
weather” is a result of changes in the magnetic
fields that penetrate the Photosphere.
The Solar Wind
Constant flow of particles from the sun
Velocity ≈ 300 – 800 km/s
The sun is constantly losing mass:
107 tons/year
(≈ 10-14 of its mass per year)
Helioseismology
The solar interior is opaque
(i.e. it absorbs light) out to
the photosphere.
The only way to investigate
solar interior is through
Helioseismology.
= analysis of vibration
patterns visible on the
solar surface:
Approx. 10 million
wave patterns!
The Sun continuously emits a lot of energy.
It must replenish it, i.e. generate the energy it produces and emits or
else it would cool off quickly (100,100 yr).
How, Where?
The Core is where all the action is:
THE PRODUCTION OF THE SUN’S ENERGY
• The core is the only place in the Sun where the
temperature (10 million K) and density are high
enough to support nuclear fusion
(hydrogen bomb needs atomic bomb for ignition!)
• What is nuclear fusion? Every second, about 600
million tons of Hydrogen are transformed (fused ,
really) into 596 million tons of helium.
• The remaining mass (4 million tons) is converted
into energy in line with Einstein’s formula
E=
2
mc
The Key to understand energy
production by NUCLEAR FUSION
• Take 4 free (=not part of a nucleus) protons. Their total
weight is 4xWp
• NOTE: protons do not want to come close to each other:
same electrical charge repel each other
• But if, using sufficient force, one can bring them close enough
that the nuclear force makes them stick together, they become
nuclearly bound
• In this situation, the total weight of the 4 protons is no longer
4xWp, but 4xWp– “some missing mass”
• This “some missing mass” has been converted into energy via
E = mc2
41H --> 4He + energy ( E = mc2 )
mass(4He) = 0.993 x {mass(H)+mass(H)+mass(H)+mass(H)}
mass loss is 0.007 x {mass(4H)} = 5 x 10-29 kg
E=mc2=(5 x 10-29)(3 x 108)2 = 4 x 10-12 joules
How many fusions per second?
Solar Luminosity = 4 x 1026 joules/sec
4 x 1026 joules/sec
----------------------- = 1038 fusions/sec , which is 200 million tons/sec!!!
4 x 10-12 joules
Actually, about 500 million tons/sec are needed!
Nuclear Fusion in the Sun
• Hydrogen is constantly
being transformed into
helium in the Sun’s core.
Energy Generation in the Sun: The
Proton-Proton Chain
Basic reaction:
4 1H  4He + energy
4 protons have
0.048*10-27 kg (= 0.7 %)
more mass than 4He.
Need large proton speed ( high
temperature) to overcome
Coulomb barrier (electrostatic
repulsion between protons)
T ≥ 107 0K =
10 million 0K
 Energy gain = Dm*c2
= 0.43*10-11 J
per reaction
Sun needs 1038 reactions, transforming 5 million tons of
mass into energy every second, to resist its own gravity.
Energy Production
Energy generation in the sun
(and all other stars):
Nuclear Fusion
= fusing together 2 or more
lighter nuclei to produce
heavier ones.
Nuclear fusion can
produce energy up to
the production of iron;
For elements heavier than
iron, energy is gained by
nuclear fission.
Binding energy
due to strong
force = on short
range, strongest
of the 4 known
forces:
electromagnetic,
weak, strong,
gravitational
When nucleons are packed
together (bound), their
energy status changes
relative to when they are
not bound.
If becoming bound puts
them in more energetically
advantageous state, then
they can be fused together.
The quantity to look at is
the binding energy of each
individual nucleon
(note that nucleons can
transform each other from
p to n and n to p as they
need)
The importance of Temperature
•To make the fusion reaction possible we have to bring the protons close
enough that they feel each other’s Strong Force (nuclear force)
•Strong Force acts on a short-range:
•it is zero at distances larger than a critical distance;
•it is different from zero at distances shorter than a critical distance
•At distances shorter than the critical distance, the strong force is
ATTRACTIVE, and it is very, very, VERY STRONG. Much stronger than the
repulsive electromagnetic force.
•To get the protons close enough to give the Strong Force a chance to
work, we have to smash them with sufficiently high speed: we need very
high Temperature
•The higher the temperature, the larger the number of fusion reactions
each second.
•Even small Temperature variations cause large variations in the
number of fusion reactions
Counting Solar Neutrinos
The solar interior can not be
observed directly because it
is highly opaque to radiation.
But, neutrinos can penetrate
huge amounts of material
without being absorbed.
Early solar neutrino
experiments detected a much
lower flux of neutrinos than
expected ( the “solar
neutrino problem”).
Recent results have proven
that neutrinos change
(“oscillate”) between different
types (“flavors”), thus solving
the solar neutrino problem.
Davis solar neutrino
experiment
An aside about nuclear fusion
as a viable energy source
A few key points about the P-P Chain fusion reaction:
• It is a “clean” form of energy production.
• Put 4 protons (Hydrogen) in and get one Helium and some
energy back out.
• Given fuel (in the form of Hydrogen) it is a selfsustaining reaction.
• It requires *very* high temperatures (107 K)
• Where to put the reacting gas?
• We are *very* hard at work trying to build a controllable
fusion reactor here on Earth to produce unlimited, clean,
dirt-cheap energy.
Balance in the stars
Thermal
Pressure
Gravitational
Contraction
Pressure and Temperature of a Gas
How does a
star hold itself?
This balance between
weight and pressure is
called hydrostatic
equilibrium.
The Sun's core, for
example, has a
temperature of about 16
million K.
The House’s Thermostat
Air temperature inside drops below
the set point of the thermostat
Electrical signal triggers
ignition of furnace
Furnace shuts off
Furnace burns oil/gas
and generates heat
Air warms thermostat until
electrical signal is shut off
Radiators transfer energy
from water to air
Heat is absorbed by water
running near furnace
causing water temperature
to rise
Hot water is circulated
throughout house
The Solar Thermostat
Outward thermal pressure of core
is larger than inward gravitational
pressure
Core expands
Nuclear fusion rate
rises dramatically
Contracting core heats up
Core contracts
Expanding core cools
Nuclear fusion rate
drops dramatically
Outward thermal pressure
of core drops (and becomes
smaller than inward grav. pressure)
Balance
• If the star contracts at all, the temperature
goes up, the rate of fusion increases, the
pressure increases, and the star re-expands.
• If the star expands too much, the
temperature drops, the rate of fusion drops,
the pressure drops, and the star contracts.
How is the energy transported
from the center to the surface?
• Radiative transfer
• Photons are continually absorbed and re-emitted
by particles like electron and nuclei (reprocessing
of radiation), and when they are re-emitted from
more external layers, which are colder, the loose
energy (but total energy is conserved, because
more photons are emitted)
• Convection
• Large scale motions of gas. Hot gas moves
upward and carries energy there. Cold gas move
downward to restore balance
Radiative Transport
• Photons are absorbed and re-emitted in
random walk motion.
• Each photon mean-free path is very, very
short
• It takes each γ photon produced by nuclear
fusion (gamma ray, very energetic) million
years to reach the surface, and when it does,
it has been reprocessed into ~1,800 cooler
visible-light photons (energy is conserved!!)
Convection Activity
Energy Transport in the Sun
g-rays
Radiative
energy
transport
The energy emitted by the Sun is
produced
• in a small region at the very center of the Sun.
• uniformly throughout the entire Sun.
• throughout the entire Sun but more in the
center than at the surface.
• from radioactive elements created in the Big
Bang.
The bulk of the violent surface activity on the
Sun is due to “seasonal” variations in the
Sun’s _________.
1) energy output
2) radius
3) electric fields
4) magnetic fields
What is burning in stars?
•
•
•
•
Gasoline
Nuclear fission
Nuclear fusion
Natural gas
The Solar Constant
• The S.C tells us the amount of energy the Sun produces:
• It is 1360 Joules per second per square meter…
• …or about the luminosity (=ENERGY PER SECOND)
of 60 W bulb every 2 square millimeters.
• A stable change of ~1% in the S.C. would change
Earth’s temperature by 1-2 degrees
• Random variation ~0.1% over days or weeks
• Long-term, cyclical variation 0.018% per year (period
longer than the 22 years of the sunspots)
• Small random fluctuations do not affect Earth’s climate
The Solar Constant
The energy we receive from the sun is
essential for all life on Earth.
The amount of energy we receive from the
sun can be expressed as the Solar Constant:
Energy Flux
F = 1360 J/m2/s
F = Energy Flux =
= Energy received in the form of radiation, per unit time
and per unit surface area [J/s/m2]
A variation of F of only ~1% would make the Earth’s
temperature change by 1-2 C