chapter 9 the sun - university of floridafreyes/classes/ast1002/ch9.pdf · chapter 9 – the sun...
TRANSCRIPT
Chapter 9 – The Sun •Our sole source of
light and heat in the
solar system •A common star: a glowing
ball of plasma held together by its
own gravity and powered by
nuclear fusion at its center.
Nuclear fusion: Combining of light
nuclei into heavier ones
Example: In the Sun is conversion of
H into He
Plasma: Ionized material composed
of electrons, protons and ions
An image of the Sun and the large sunspot taken on October 22,
2014
The Stellar balance
The outward pressure (from
heat caused by nuclear
reactions) in the core
balances the gravitational
pull toward the Sun’s center.
This balance is called
Hydrostatic equilibrium
This balance leads to a
spherical ball of plasma,
called the Sun.
What would happen if the
nuclear reactions in the
core (“burning”) stopped?
Main Regions of the Sun
• Core
• Radiation Zone
• Convection Zone
• Photosphere
• Chromosphere
• Transition Zone
• Corona
• Solar wind
Radius of the Sun =
696,000 km
(The thickness of
the regions are not
to scale)
Radius = 696,000 km
(100 times Earth’s radius)
Mass = 2 x 1030 kg
(300,000 times Earth’s
mass)
Av. Density = 1410 kg/m3
Rotation Period =
25 days (equator)
36 days (poles)
Surface temp = 5780 K
Solar Properties
The Moon’s orbit around the
Earth (Radius around 385,000
km) would easily fit within the
Sun!
Luminosity of the Sun = LSUN
How do we determine the luminosity of the Sun?
- First, we measure the amount of power received
from the Sun at the Earth per squared meter per
second. This is power in W/m2 It is called the
Solar constant = 1400 W/m2
-Second, we multiply this by the surface of a sphere
of radius d (4d2 ), where d is the distance
between the Earth and Sun (1 AU, ~ 150 million
km). In other words, we “integrate” the power over
the whole sphere
-We assumed here that the Sun emit the same
amount of energy in all directions.
Luminosity: Total light
energy emitted per second
(Power)
LSUN ~ 3.96 x 1026 W
Watt (W) is a unit of power.
Power is energy emitted per
unit of time. Joule is a unit of
energy
1 W = 1 Joule/sec
d
The Standard Solar Model
1 g/cm3 = 1000 kg/m3
The temperature of the core must be least 10 million K in order to be able to convert H into
He. The Sun’s central core temperature is about 15 million K
The temperature of the layer that we see from the Sun (Photosphere) is about 6,000 K
The standard solar model
Energy Transport within the Sun
• Extremely hot core , 10-15 million K. All the matter is completely ionized (plasma)
• Radiation zone The temperature is so high that no electrons are left on the atoms to be
able to capture photons – radiation zone is transparent to light. Energy here is
transported by radiation
• Convection zone Temperature falls further away from the core – at lower temperatures,
more atoms are not completely ionized. The electrons left in the atoms can capture
photons – The gas becomes opaque to light. Energy is transported here by convection
• Farther out, the low density in the photosphere makes it transparent to light - radiation
takes over again
Solar Granulation: Evidence of Convection
Solar Granules are the tops
of convection cells.
Bright regions are where hot
material is upwelling (1000
km across).
Dark regions are where
cooler material is sinking.
Material rises/sinks at a rate
~1 km/sec (2200 mph)
Detected by Doppler effect.
The solar spectrum has
thousands of absorption
lines
(The scale is wavelength in nanometers )
More than 67 different
elements are present!
Hydrogen is the most
abundant element followed
by Helium (1st discovered
in the Sun!)
The Solar Atmosphere
Spectral lines only tell us about the composition of the part of the
Sun that forms them. But these elements are also thought to be
representative of the entire Sun.
The composition of the Sun
The chromosphere and the photosphere The chromosphere can only be seen in a total solar
eclipse when the size of the disk of the moon is
slightly larger than the disk of the Sun so it will
block the light from the photosphere
The layer of the Sun that we see is the
photosphere.
The photosphere has higher temperature
(5,800 K) and higher density .
The chromosphere has lower temperature
(4,500 K) and lower density
• The photosphere forms the
continuous spectrum
• The chromosphere produce the
absorption lines. (Remember Kirchhoff ‘s laws)
Transition Zone and Corona
Transition Zone
& Corona
Why does the Temperature rise further from the hot light source?
From the corona we see
emission lines from highly
ionized elements (Fe+5 –
Fe+13) which indicates that
the temperature here is
very HOT
The Corona has
very low density
but high
temperature
T ~ 106 K
magnetic “activity” - spicules and other more energetic phenomena
(more about this later…)
Corona (seen only during total Solar eclipse)
Because the
coronal plasma
has high
temperature
(1,000,000 K),
it escapes the
gravitational
attraction of
the Sun
Solar wind
Solar Wind
Solar Wind
The radiation (light or electromagnetic waves) emitted by the Sun
travel at the speed of light and take about 8 minutes to reach Earth.
The plasma (electrons, protons and ions) ejected from the Sun travel
slower, ~500 km/s and take a few days (~ 3 days) to reach the Earth
Solar coronal plasma has enough temperature (kinetic energy) to
escape the Sun’s gravity.
This stream of particles ejected from the Sun is called the solar wind
Radiation and fast moving particles (electron and protons)
continuously leave the Sun .
The Sun is evaporating via this “wind”
The Sun loses about 1 million tons of matter each second!
However, over the Sun’s lifetime, it has lost only ~0.1% of its total
mass.
Hot coronal plasma (~1,000,000 K) emits mostly in X-rays.
Coronal holes
are sources of
the solar wind (lower density regions)
Coronal holes are
related to the Sun’s
magnetic field.
Open magnetic field
line generate the
coronal holes
CME: Coronal
Mass Ejection Ejection of plasma
through the coronal
holes
An example of a coronal hole showing
the magnetic field lines structure
Coronal
hole
An example of a CME The animation was recorded by the SOHO (Solar Heliospheric
Observatory) spacecraft
Sunspots
Granulation around sunspot Umbra: dark center of sunspot
Penumbra: grayish area around the umbra
Sunspots
• Size typically about 10,000 km
across
• At any time, the Sun may have
hundreds (around solar sunspot
maximum) or none (around a solar
sunspot minimum)
• Dark color because they are
cooler than photospheric plasma
(4,500 K in darkest parts,
compared to 5, 800 K in the
photosphere.)
• Each spot can last from a few days to a
few weeks or a month
• Galileo observed these spots and
realized the Sun is rotating differentially
(faster at the equator, slower at the poles)
Rotation of the Sun: An animation
Sunspots &
Magnetic Fields
•The magnetic field in a sunspot is
1000 times strongest than the
surrounding area
•Sunspots are almost always in pairs
at the same latitude with each
member having opposite polarity
•All sunspots in the same
hemisphere have the same magnetic
configuration.
They have opposite polarity in north
and south hemisphere
Why the sunspots have lower temperature?
• The charged particles in the plasma (electrons, protons and ions) from the solar atmosphere
interact with the magnetic field and prevent plasma to reach the sunspot zone. A charged particle
in a magnetic field will follow helical trajectories.
• The plasma in and around the sunspot radiates energy and cool off.
• The temperature of a sunspot is around 4,500 K. The temperature of the photosphere is around
5,800 K
The ratio of the flux F between the
photosphere (Fph) and the sunspot
(Fss) can be calculated by the
Stefan’s Law formula:
Fph/Fss = (Tph/Tss)^4
Fph/Fss = (5800/4500)^4
Fsp/Fss =2.76
The photosphere emit 2.76 times
more flux than the sunspots
Why the sunspots look
darker?
The Sun’s differential rotation distorts the magnetic field lines
The plasma is rotating and drags the magnetic filed lines. The twisted and
tangled field lines occasionally get kinked, causing the field strength to
increase
A “tube” of lines bursts through atmosphere creating sunspot pair
Minimum of
sunspot cycle
Maximum of
sunspot cycle
Sunspot Cycle and Solar Cycle
The Solar Cycle is 22 years long. The direction of the magnetic field polarity
of the sunspots flips every 11 years (back to original orientation every 22 years)
During a solar maximum there is an increase of
solar radiation, ejection of solar material, sunspots
numbers and flares
Solar maximum is reached every ~11 years
~ 11 years
The Sunspot cycle last for about 11 years
The sunspot number last on average about 11 years but occasionally
sunspots may disappear (sunspot number drop to low or zero value over
several years) as it happened between 1645 and 1715. This is called the
Maunder minimum
This period of 70 years of minimum sunspot activity coincided with a
period of cold temperatures called the Little Ice Age
A recent plot of the sunspot numbers including data
until 2018 Some sunspot cycle have two maximum.
Charged particles (mostly
protons and electrons) follow
helical path and are accelerated
along magnetic field “lines” above
sunspots.
This type of activity, not light
energy, heats the corona.
Heating of the Corona
Charged particles follow magnetic fields between sunspots:
Solar Prominences
Sunspots are cool,
but the gas above
them is hot!
Earth
Solar Prominence Typical size is 100,000 km
May persist for days or weeks
Very large solar
prominence (1/2
million km across
base, i.e. 39 Earth
diameters) taken
from Skylab in UV
light.
When seen against the bright solar
surface, prominences appear as dark
filaments.
Solar Flares
Eruptions on the Solar surface resulting from stresses applied to the
magnetic field lines, usually near sunspots.
Flares such as these emit enormous amounts of X-ray and ultraviolet radiation as
well as high energy particles both of which have important effects on the Earth.
Those high energy particles produce intense auroral emission
They also compress the magnetic field of the Earth. The compression induces a
voltage (and current) in power lines. This may activate the power lines protections
disconnecting the power from the transmission lines and may create a black out.
The high energy particles can damage satellites which can disrupt communications,
and TV transmissions
Astronauts in interplanetary space are subject to this high energy particles and the
radiation originated in a solar flare
Emission
of X-rays
in a solar
flare
Solar Flares – violent magnetic instabilities
5 hours
The particles ejected in a flare are so energetic (High speed), the magnetic
field cannot keep them trapped close to the Sun – they escape Sun’s gravity
Solar Flare (September 10, 2014)
X-Ray
emission
An
animation
of the
flare
The Sun on
Sept. 10,
2014
Coronal
activity
increases
with the
number of
sunspots.
Nuclear fusion: combining light nuclei into heavier
ones
An example: In the core of the Sun, the conversion
of H into He. Four H nuclei are combined to produce
one nucleus of He
Atomic nuclei are positively charged and repel one
another via the electromagnetic force.
Merging nuclei (protons in Hydrogen) require
high speeds. How it is possible to get high speed
protons?
Nuclear fusion requires temperatures of at least
107 K (10 million K) – why?
Higher temperature – faster motion
At very close range, a force called strong nuclear
force takes over, binding protons and neutrons
together (FUSION).
Neutrinos are one byproduct.
What makes the Sun shine? NuclearFusion!
neutron
proton
4 H
He
More on Nuclear Fusion
The conversion that take place on the Sun: The proton-proton chain Proton: nucleus of an H atom, positive charge
Deuteron: nucleus of a deuterium (one proton, one neutron), an isotope of H
Positron: antiparticle, same mass of an electron but has positive charge
Neutrino: elementary particle with virtually no mass or charge. It hardly interact with mass
Helium-3: Isotope of Helium (Two protons and one neutron)
Helium-4: Stable nucleus of helium (Two protons and two neutrons)
Gamma rays carry the energy produce by fusion
Note: fusion is conversion of a light element into a heavier element. There is another process called nuclear fission in which heavier
nucleus split into lighter nuclei releasing energy. This process is used to generate energy and power nuclear reactors
Proton
Proton
Mass “lost” is converted to Energy:
Mass of 4 H Atoms = 6.6943 10-27 kg
Mass of 1 He Atom = 6.6466 10-27 kg
Difference = 0.0477 10-27 kg
(% of original mass converted to E) = (0.71%)
E = m c 2
(c = speed of light)
But where
does the
Energy
come
from!? The total mass decreases during a fusion reaction.
The Sun has enough mass to fuel its current energy output for another 5 billion years
Relativity!
c2 is a very
large number!
A little mass
equals a LOT of
energy.
The production of energy is an example of the law of conservation of mass and energy
Neutrinos are almost
non-interacting with
matter… So they stream
out freely.
Neutrinos provide important tests of nuclear energy generation.
The energy output from the core of the Sun is in the form of gamma
rays.
These are transformed into visible and IR light by the time they reach
the surface (after interactions with particles in the Sun).
Gamma rays
Visible and IR
Solar Neutrino Problem: Neutrino detectors
found only 30 - 50% of the predicted number
that were expected from the Sun!
A discrepancy between theory and experiments
could mean either
1) standard solar model incorrect or
2) standard particle theory incorrect.
This discrepancy appears to have been resolved
In 2002, Sudbury Neutrino Observatory in
Canada showed that neutrinos oscillate into
different “flavors” during their trip to Earth
from the Sun. Previous neutrino experiments
only detected one type of neutrino. The fluid
used by this detector is “heavy” water. The
hydrogen in the water molecule is replaced by
deuterium)
If all types of neutrinos are accounted for, the
total number of neutrinos agrees well with the
standard solar model prediction.
Detecting Solar Neutrinos – These
light detectors measure photons emitted
by rare electron-neutrino reactions in the
fluid (Fluid is purified water).