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The Sun
—Our Star
Astronomy: The Solar System and Beyond
5th edition
Michael Seeds
The Sun
Chapter 11 —Our Star
All cannot live on the piazza,
but everyone may enjoy the sun.
– Italian proverb
The Sun
—Our Star
• A wit once remarked that astronomers
would know a lot more about the sun if it
were farther away.
– This comment contains a grain of truth.
– Astronomers know plenty about stars in general, and
there are plenty of stars out there for them to observe,
in order to test and refine their theories.
– However, there is only one star nearby—close
enough for astronomers to see swirling currents of
gas and arching bridges of magnetic force that their
present theories seem inadequate to completely
describe.
The Sun
—Our Star
• The more closely the sun is observed, the
more complex it seems, and the less
astronomers seem to know about it.
• It pays to look at this paradox from another
direction.
– The sun is an average star, and there are billions like
it in the sky.
– The more astronomers can learn from the convenient
example of the sun, the more they will know about
other stars, whose distances from Earth render
detailed study impossible.
The Sun
—Our Star
• What do astronomers know about the
sun?
– It is made up of the gases hydrogen and helium,
along with a scattering of heavier elements.
– It is 109 times Earth’s diameter and 333,000 times its
mass.
The Sun
—Our Star
– Nuclear reactions produce a tremendous amount of
energy in the interior of the sun.
– As this energy escapes to space, it stirs the sun’s
layers by generating convection currents.
– The sun has a powerful
and ever-changing
magnetic field that
causes dark sunspots
and eruptions to
appear on its surface.
The Sun
The Solar Atmosphere —Our Star
• Your grand tour of the sun begins with its
atmosphere.
• Its atmosphere is made up of three layers.
– The photosphere is the thinnest, innermost layer.
– The chromosphere lies above the photosphere and is
a few times thicker.
The Sun
The Solar Atmosphere —Our Star
– The corona, the highest layer, is huge and
extends very far from the sun.
The Sun
The Solar Atmosphere —Our Star
• As you study the solar atmosphere, you
will find that many phenomena are driven
by the outward flow of heat energy from
the interior of the sun.
– Like a pot of boiling soup on a hot stove, the sun’s
surface is in constant activity as heat flows up from
below.
The Sun
The Photosphere —Our Star
• The photosphere is the visible surface
of the sun.
– It is the source of most of the sun’s light.
– However, it is very thin—less than 500 km deep.
The Sun
The Photosphere —Our Star
• To put its thinness in perspective,
imagine a model of the sun the size of
a bowling ball.
– The photosphere would be no thicker than a piece of
tissue paper wrapped around the ball.
– Due to the relative thinness of the photosphere,
astronomers often refer to it as a surface, or ‘the
surface of the sun.’
The Sun
The Photosphere —Our Star
• Thin as it is, the photosphere emits a
great deal of energy.
– The temperature at the sun’s surface is about 5,800 K.
– At that temperature, each square centimeter must
radiate more energy than a 6,000-watt lightbulb.
– With all that energy radiating away into space, the sun’s
surface would cool rapidly if energy did not flow up from
the interior to keep the surface hot.
The Sun
The Photosphere —Our Star
• The photosphere is dense enough to emit
plenty of light but not so dense that the
light can’t escape.
– Below the photosphere, the gas is denser and hotter
and therefore radiates plenty of light.
– However, that light cannot escape from the sun
because of the outer layers of gas.
– So, you cannot detect light from these deeper layers.
– Above the photosphere, the gas is less dense and so
is unable to radiate or absorb much light.
The Sun
The Photosphere —Our Star
• Although the photosphere appears to be
substantial, it is really a very low-density
gas.
– Even in the deepest and densest layers visible, the
photosphere is 3,400 times less dense than the air
you breathe.
– To find gases as dense as the air you breathe, you
would have to descend about 7 x 104 km below the
photosphere—about 10 percent of the way to the
sun’s center.
– With a fantastically efficient insulation system, you
could fly a spaceship right through the photosphere.
The Sun
The Photosphere —Our Star
• The spectrum of the sun is an absorption
spectrum.
– That can tell you a great deal about the composition
of the photosphere.
The Sun
The Photosphere —Our Star
• You know from Kirchhoff’s third law that an
absorption spectrum is produced when a
source of a continuous spectrum is viewed
through a gas.
The Sun
The Photosphere —Our Star
• The deeper layers of the photosphere are
dense enough to produce a continuous
spectrum.
• However, atoms in the photosphere
absorb photons of specific wavelengths,
producing absorption lines of hydrogen,
helium, and other
heavier elements.
The Sun
The Photosphere —Our Star
• In good photographs, the photosphere has
a mottled appearance because it is made
up of dark-edged regions.
• The regions are called granules.
• The visual pattern they produce is called
granulation.
The Sun
The Photosphere —Our Star
• A typical granule is about the size of Texas,
and exists for only 10 to 20 minutes before
fading away.
– Faded granules are continuously replaced by
new granules.
The Sun
The Photosphere —Our Star
• Spectra of these granules show that the
centers are a few hundred degrees hotter
than the edges.
• Doppler shifts reveal that the centers are
rising and the edges are sinking at speeds
of about 0.4 km/s.
– From this evidence, astronomers recognize
granulation as the upper surface of a convecting layer
just below the photosphere.
The Sun
The Photosphere —Our Star
• Most of the convection occurs below the
photosphere, but the tops of the cells
extend up into its lower reaches.
• Convection occurs when hot fluid rises
and cool fluid sinks.
– For example, a convection current of hot gas rises
above a candle flame.
The Sun
The Photosphere —Our Star
• You can observe convection in a liquid by
adding a bit of cool nondairy creamer to an
unstirred cup of hot coffee.
– As the coffee in the cup convects, the creamer colors
some of the currents, so that you are able to see
them.
– Convection cells form in hot coffee as the surface of
the drink cools: the cooler coffee sinks as warmer
coffee rises from the bottom to replace it.
The Sun
The Photosphere —Our Star
– This process is invisible until you pour in the
creamer.
– When you do, you may see small regions on
the surface of the coffee that mark the tops of
convection currents.
– Viewed from above, these regions look much
like solar granules.
The Sun
The Photosphere —Our Star
• In the sun, the tops of rising currents of hot
gas are brighter than their surroundings.
– As the gas cools slightly, it is pushed aside by rising
gas from below.
– The cooler gas, sinking at the edge of the granule, is
slightly dimmer.
– Consequently, granules have bright centers and
dimmer edges.
The Sun
The Photosphere —Our Star
• Less obvious structures called
supergranules appear to be caused by
larger, deeper convection currents in the
sun’s layers.
– These features can be over twice the size of Earth.
• The presence of granulation, and the
convection that it implies, is clear evidence
that energy is flowing upward through the
photosphere.
The Sun
The Chromosphere —Our Star
• Above the photosphere lies the
chromosphere.
– Solar astronomers define the lower edge of the
chromosphere as lying just above the visible surface of
the sun, its upper regions blending gradually with the
corona.
– You can think of the
chromosphere as
being an irregular
layer with an average
depth of less than
Earth’s diameter.
The Sun
The Chromosphere —Our Star
• As the chromosphere is roughly 1,000 times
fainter than the photosphere, you can see it with
your unaided eyes only during a total solar
eclipse—when the moon covers the brilliant
photosphere.
– Then, the chromosphere
flashes into view as a thin
line of pink just above the
photosphere.
The Sun
The Chromosphere —Our Star
• The pink color is produced by the combined light
of the red, blue, and violet Balmer emission lines
of hydrogen.
– The word chromosphere
comes from the Greek
word chroma,
meaning ‘color.’
The Sun
The Chromosphere —Our Star
• As the chromosphere is roughly 1,000 times
fainter than the photosphere, you can see it with
your unaided eyes only during a total solar
eclipse—when the moon covers the brilliant
photosphere.
– Then, the chromosphere
flashes into view as a thin
line of pink just above the
photosphere.
The Sun
The Chromosphere —Our Star
• Astronomers know a great deal about the
chromosphere from its spectrum.
– As the chromosphere produces an emission
spectrum, Kirchhoff ’s second law tells you the
chromosphere must be an excited, lowdensity gas.
– The density is about 108 times less dense
than the air you breathe.
The Sun
The Chromosphere —Our Star
• Spectra reveal that atoms in the lower
layers of the chromosphere are ionized
and atoms in the higher layers are even
more highly ionized.
– That is, they have lost more electrons.
• From the ionization state of the gas,
astronomers can find the temperature in
different parts of the chromosphere.
The Sun
The Chromosphere —Our Star
• Just above the photosphere, the
temperature falls to a minimum of about
4,500 K and then rises rapidly to the
extremely high temperatures of the
corona.
The Sun
The Chromosphere —Our Star
• This fact is a little surprising.
– You would expect the outer layers of the sun to get
progressively cooler.
– You will learn later what makes the chromosphere and
corona hotter than the photosphere.
The Sun
The Chromosphere —Our Star
• Solar astronomers can take
advantage of some elegant physics to
study the chromosphere.
– The gases of the chromosphere are transparent to
nearly all visible light.
– However, atoms in the gas are very good at
absorbing photons of specific wavelengths.
– This produces certain dark lines in the absorption
spectrum of the photosphere.
The Sun
The Chromosphere —Our Star
• A photon having one of those wavelengths
that is emitted in a deeper layer of the sun
is very unlikely to escape through the
chromosphere without being absorbed.
– If a photon of one of these particular
wavelengths reaches Earth, you can be sure
that it came from higher in the atmosphere.
The Sun
The Chromosphere —Our Star
• A filtergram is a photograph made using
light in one of those dark absorption lines.
– Filtergrams reveal detail in the upper layers of the
chromosphere.
• A filtergram made at the wavelength of the
Hα Balmer line is displayed.
– It reveals complex structure in the
chromosphere.
The Sun
The Chromosphere —Our Star
• Spicules are flamelike jets of gas rising
upward into the corona that last 5 to 15
minutes.
– They appear to be cooler gas
from the lower chromosphere
extending upward into hotter
regions.
The Sun
The Chromosphere —Our Star
• Seen at the limb (edge) of the sun’s disk,
spicules blend together to look like flames
covering a burning prairie.
– Filtergrams of spicules located
at the center of the solar disk
show that they spring up
around the edges of
supergranules.
The Sun
The Chromosphere —Our Star
• Spectroscopic analysis of the
chromosphere reveals that it is a lowdensity gas in constant motion, where the
temperature increases rapidly with height.
– Just above the chromosphere lies even hotter
gas.
The Sun
The Solar Corona —Our Star
• The outermost part of the sun’s
atmosphere is called the corona,
after the Greek word for ‘crown.’
• The corona is so dim that it is not
visible in the daytime sky because of
glare from the brilliant photosphere.
The Sun
The Solar Corona —Our Star
• During a total solar eclipse, the moon
covers the photosphere and the corona
shines with a pearly glow not quite as
bright as the full moon.
– Special telescopes on
earth, and in space, can
block the light from the
photosphere and image
the corona out beyond
20 solar radii, almost
10 percent of the way
to Earth.
The Sun
The Solar Corona —Our Star
• The images show streamers in the
corona that follow the lines of force of
the sun’s magnetic field.
The Sun
The Solar Corona —Our Star
• The spectrum of the corona can tell you a
great deal about the coronal gases and
simultaneously illustrate how astronomers
can analyze a spectrum.
• Some of the light from the outer corona
produces a spectrum with absorption lines
the same as the photosphere’s spectrum.
– This light is just sunlight reflected from dust
particles in the corona.
The Sun
The Solar Corona —Our Star
• In contrast, some of the light from the
corona produces a continuous spectrum that
lacks absorption lines.
– That happens when sunlight from the photosphere is
scattered off free electrons in the ionized coronal gas.
– As the coronal gas has a temperature over 1 million K,
the electrons travel very fast.
– The reflected photons suffer large, random Doppler
shifts that smear out solar absorption lines to produce a
continuous spectrum.
The Sun
The Solar Corona —Our Star
• Superimposed on the corona’s continuous
spectrum are emission lines of highly
ionized gases.
• In the lower corona, the atoms are not as
highly ionized as at higher altitudes.
– This tells you that the temperature of the
corona rises with altitude.
The Sun
The Solar Corona —Our Star
– Just above the chromosphere, the temperature
is about 500,000 K.
– In the outer corona, it can be as high as 2 million
K or more.
The Sun
The Solar Corona —Our Star
• The corona is very hot, but it is not
very bright.
– Its density is very low, only 106 atoms/cm3 in its lower
regions.
– That is about a trillion times less dense than the air
you breathe.
– In its outer layers, the corona contains only 1 to 10
atoms/cm3, less dense than the best vacuum on
Earth.
– Due to this low density, the hot gas does not emit
much radiation.
The Sun
The Solar Corona —Our Star
• Astronomers have wondered for
years how the corona and
chromosphere can be so hot.
– Heat flows from hot regions to cool regions.
– So, how can the heat from the photosphere—
with a temperature of only 5,800 K—flow out
into the much hotter chromosphere and
corona?
The Sun
The Solar Corona —Our Star
• Observations made by the Solar and
Heliospheric Observatory (SOHO) satellite
have mapped a magnetic carpet of looped
magnetic fields extending up through the
photosphere.
– Unseen turbulence below the surface may be
whipping these fields about and churning the low
density gases of the chromosphere and corona,
heating the gas.
– In this instance, energy appears to flow outward as
the agitation of the magnetic fields.
The Sun
The Solar Corona —Our Star
• Gas follows the magnetic fields pointing
outward and flows away from the sun in a
breeze of ionized atoms called the solar
wind.
– Like an extension of the corona, the low-density
gases of the solar wind blow past Earth at 300 to 800
km/s, with gusts as high as 1,000 km/s.
– Earth is bathed in the corona’s hot breath.
The Sun
The Solar Corona —Our Star
• Due to the solar wind, the sun is slowly
losing mass.
• However, it is a minor loss, amounting to
only about 107 tons per year, only about
10-14 of a solar mass per year.
• Later in life, the sun, like many other stars,
will lose mass rapidly.
The Sun
The Solar Corona —Our Star
• Do other stars have chromospheres,
coronae, and stellar winds like the
sun?
• Ultraviolet and X-ray observations
suggest that the answer is yes.
– The spectra of many stars contain emission lines in
the far ultraviolet that could only have formed in the
low-density, high-temperature gases of a
chromosphere and corona.
The Sun
The Solar Corona —Our Star
– Additionally, when gas is as hot as it is in the
corona, it emits X rays.
– Many stars are sources of X rays, which
means that those stars have coronas too.
– This observational evidence gives
astronomers good reason to believe that the
sun, for all its complexity, is a typical star.
The Sun
Helioseismology —Our Star
• Almost no light emerges from below the
photosphere.
• Thus, you can’t see details in the solar
interior.
• However, solar astronomers can use
vibrations in the sun to explore its depths
in a process called helioseismology.
The Sun
Helioseismology —Our Star
• Random movements in the sun caused by
convection or eruptions constantly
produce vibrations.
– Some of those vibrations resonate like sound waves
in organ pipes.
– These vibrations are just sound waves of very long
period.
– The periods range from about 3 to 20 minutes.
– A vibration with a period of 5 minutes is strongest.
The Sun
Helioseismology —Our Star
• Astronomers can detect these vibrations
by observing Doppler shifts in the solar
surface.
– As a vibrational wave travels down into the sun, the
increasing density and
temperature curve its
path.
– This causes the wave to
return to the surface,
where it makes the
photosphere heave up
and down.
The Sun
Helioseismology —Our Star
• By observing these movements,
astronomers can determine which
vibrations become stronger and which
become weaker.
– The strength of different
wavelengths of vibration
is determined by the
temperature, pressure,
and density of the
layers that the vibrations
travel through.
The Sun
Helioseismology —Our Star
• Just as geologists can study Earth’s
interior by analyzing vibrations from
earthquakes, solar astronomers can use
helioseismology to explore the sun’s
interior.
• To understand the process better, think of
a duck pond.
– If you stood on the shore and looked down at the
water, you would see ripples arriving from all parts of
the pond.
The Sun
Helioseismology —Our Star
– Each duck produces ripples.
– Thus, in principle, you could analyze the
ripples at your feet and draw a map showing
the position and velocity of every duck on the
pond.
– Of course, it would be difficult to untangle all
the ripples.
– So, you would need lots of data and a big
computer.
The Sun
Helioseismology —Our Star
• Solar astronomers need large
amounts of data for helioseismology.
– The Global Oscillation Network Group
(GONG) uses a network of telescopes spread
around the world to observe the sun
continuously as Earth turns.
– Spacecraft such as SOHO can also observe
the sun uninterrupted by the cycle of day and
night.
The Sun
Helioseismology —Our Star
• Analysis of such data has allowed
astronomers to map the temperature,
density, and rate of rotation inside the
sun.
– Helioseismology has been able to detect great
currents of gas flowing below the photosphere and to
detect the formation of a sunspot before it appeared
in the photosphere.
– It can even locate sunspots on the far side of the sun,
sunspots that are not visible from Earth.
The Sun
Building Scientific Arguments —Our Star
• How deeply into the sun can you see?
– Scientific arguments usually involve observations.
– It is always important to know how observations are
made.
• When you look into the layers of the sun,
your sight does not really penetrate into
the sun.
– Rather, your eyes record photons that have escaped
from the sun and traveled outward through the layers
of the sun’s atmosphere.
The Sun
Building Scientific Arguments —Our Star
• If you observe at a wavelength at the
center of a dark absorption line, the
photosphere and lower chromosphere are
opaque.
– Photons can’t escape to your eyes.
– The only photons you can see come from the
upper chromosphere.
The Sun
Building Scientific Arguments —Our Star
• In contrast, if you observe at a wavelength
that is not easily absorbed (a wavelength
between spectral lines), the atmosphere is
more transparent.
– The photons from deep inside the
photosphere can escape to your eyes.
The Sun
Building Scientific Arguments —Our Star
• However, there is a limit.
– At a certain depth, the sun’s atmosphere is opaque for
almost all wavelengths, few photons can escape, and
you can’t see deeper.
• By choosing the proper wavelength,
solar astronomers can observe to
different depths.
– However, the corona is so thin and the gas below the
photosphere so dense that this doesn’t work in these
regions.
The Sun
Building Scientific Arguments —Our Star
• Now, it is time to build a new
argument.
– How can you observe the corona and the
deeper layers of the sun?
The Sun
Nuclear Fusion in the Sun —Our Star
• Like soap bubbles, stars are structures
balanced between opposing forces that
individually would destroy them.
• The sun is a ball of hot gas held together
by its own gravity.
– If it were not for the sun’s gravity, the hot, pressurized
gas in the sun’s interior would cause it to explode.
– Similarly, if the sun were not so hot, its gravity would
compress it into a small, dense body.
– In this section, you will explore the way in which the
sun generates its heat.
The Sun
Nuclear Fusion in the Sun —Our Star
• The sun is powered by nuclear
reactions that occur near its center.
– The energy produced keeps the interior hot and
keeps the gas in the interior totally ionized.
– That is, the electrons are not attached to atomic
nuclei.
– Thus, the gas is an atomic soup of rapidly moving
particles colliding with each other at high velocities.
The Sun
Nuclear Fusion in the Sun —Our Star
• Nuclear reactions inside stars depend
on atomic nuclei, not whole atoms.
– How exactly can the nucleus of an atom yield
energy?
– The answer lies in the force that holds the
nuclei together.
The Sun
Nuclear Binding Energy —Our Star
• The sun generates its energy by
breaking and rearranging the bonds
between the particles inside atomic
nuclei.
– This is quite different from the way you generate
energy by burning wood in a fireplace.
– The process of burning wood extracts energy by
breaking and rearranging chemical bonds between
atoms in the wood.
The Sun
Nuclear Binding Energy —Our Star
• Chemical bonds are formed by the
electrons in atoms.
• You have learnt that the electrons are
bound to the atoms by the electromagnetic
force.
– Thus, the chemical energy released when
bonds are broken originates in the
electromagnetic force.
The Sun
Nuclear Binding Energy —Our Star
• There are only four known forces in
nature: the force of gravity, the
electromagnetic force, the weak force,
and the strong force.
– The weak force is involved in the radioactive decay of
certain kinds of nuclear particles.
– The strong force binds together atomic nuclei.
– Thus, nuclear energy comes from the strong force.
The Sun
Nuclear Binding Energy —Our Star
• There are two ways of generating nuclear
energy from atomic nuclei.
• Nuclear power plants generate energy
through nuclear fission reactions that split
uranium nuclei into less massive
fragments.
– A uranium nucleus contains a total of 235 protons and
neutrons, and it can split into a range of fragment
sizes containing roughly half as many particles.
The Sun
Nuclear Binding Energy —Our Star
• The graph shows
the binding
energy
that holds
different
nuclei together.
The Sun
Nuclear Binding Energy —Our Star
– When uranium splits,
the fragments are
lower in the diagram
than the original
uranium nucleus.
– Any nuclear reaction
that sinks lower in
the diagram
produces energy.
The Sun
Nuclear Binding Energy —Our Star
• Stars make energy in nuclear fusion
reactions that combine light nuclei
into heavier nuclei.
– The most common reaction fuses four
hydrogen nuclei (single protons) into a helium
nucleus (two protons and two neutrons).
The Sun
Nuclear Binding Energy —Our Star
– Because a helium
nucleus is lower in
the diagram than a
hydrogen nucleus,
energy is released.
The Sun
Nuclear Binding Energy —Our Star
• Both fusion and
fission reactions
move downward
in the diagram.
– Thus, both produce
energy by releasing
the binding energy
of atomic nuclei.
The Sun
Hydrogen Fusion —Our Star
• The sun fuses four hydrogen nuclei to
make one helium nucleus.
– As one helium nucleus contains 0.7 percent
less mass than four hydrogen nuclei, it seems
that some mass vanishes in the process.
– That mass is converted to energy, and you
can figure out how much by using Einstein’s
famous equation, E=mc2
The Sun
Hydrogen Fusion —Our Star
• You can symbolize the hydrogen
fusion reaction with a simple
equation.
4 1H → 4He + energy
– 1H represents a proton, the nucleus of the hydrogen
atom.
– 4He represents the nucleus of a helium atom.
– The superscripts indicate the approximate weight of
the nuclei.
The Sun
Hydrogen Fusion —Our Star
• The actual steps in the process are
more complicated.
– Instead of waiting for four hydrogen nuclei to
collide simultaneously, a highly unlikely event,
the process can proceed step by step in a
chain of reactions called the proton–proton
chain.
The Sun
Hydrogen Fusion —Our Star
• The proton-proton chain is a series of
three nuclear reactions that builds a
helium nucleus by adding protons one
at a time.
The Sun
Hydrogen Fusion —Our Star
• As this process is efficient at temperatures
above 10,000,000 K, it is the mechanism
by which energy is produced in the interior
of the sun, where temperatures reach
15,000,000 K.
The Sun
Hydrogen Fusion —Our Star
• The three reactions in the chain
are as follows.
1H
+ 1H → 2H + e+ + v
2H
3He
+ 1H → 3He + γ
+ 3He → 4He + 1H + 1H
The Sun
Hydrogen Fusion —Our Star
• In the first reaction, two hydrogen nuclei
(two protons) combine to form a heavy
hydrogen nucleus called deuterium,
emitting a particle called a positron (a
positively charged electron, symbolized e+)
and another called a neutrino (ν).
1H
+ 1H → 2H + e+ + ν
The Sun
Hydrogen Fusion —Our Star
• In the second reaction, the heavy
hydrogen nucleus absorbs another proton
and, with the emission of a gamma ray (γ),
becomes a lightweight helium nucleus
(3He).
2H
+ 1H → 3He + γ
The Sun
Hydrogen Fusion —Our Star
• Finally, two light helium nuclei
combine to form a normal helium
nucleus and two hydrogen nuclei.
3He
+ 3He → 4He + 1H + 1H
The Sun
Hydrogen Fusion —Our Star
• As the last reaction needs two 3He
nuclei, the first and second reactions
must each occur twice for each 4He
produced.
The Sun
Hydrogen Fusion —Our Star
• The net result of this chain reaction is the
transformation of four hydrogen nuclei
into one helium nucleus plus energy.
• The energy
appears in the
form of gamma
rays, positrons,
and neutrinos.
The Sun
Hydrogen Fusion —Our Star
• The gamma rays are photons that
are absorbed by the surrounding
gas before they can travel more
than a few centimeters.
– This heats the gas.
The Sun
Hydrogen Fusion —Our Star
• The positrons produced in the
first reaction combine with free
electrons.
– Both particles vanish, converting their mass
into gamma rays, which are also absorbed
and so help keep the center of the star hot.
The Sun
Hydrogen Fusion —Our Star
• The neutrinos, however, are particles
that travel at nearly the speed of light
and almost never interact with other
particles.
– The average neutrino could pass unhindered through
a lead wall 1 light-year thick.
– Consequently, the neutrinos do not help heat the gas
but race out of the star—carrying away roughly 2
percent of the energy produced.
The Sun
Hydrogen Fusion —Our Star
• Creating one helium nucleus makes only a
small amount of energy—hardly enough to
raise a housefly 0.001 inch into the air.
• Only by concentrating many reactions in a
small area can hydrogen fusion produce
significant results.
– For instance, a single kilogram (2.2 lb) of hydrogen,
converted entirely to energy, would produce enough
power to raise an average-sized mountain 10 km (6
miles) into the air.
The Sun
Hydrogen Fusion —Our Star
• The sun has a voracious energy appetite
and needs to produce 1,038 reactions per
second, transforming 5 million tons of
mass into energy every second, just to
balance its own gravity.
– It might sound as if the sun is losing mass at a furious
rate.
– However, during its entire 10-billion-year lifetime, the
sun will convert less than 0.07 percent of its mass into
energy.
The Sun
Hydrogen Fusion —Our Star
• You can see that nuclear fusion is
very powerful, especially if you
calculate that the fusion of a
milligram of hydrogen, roughly the
mass of a match head, produces
as much energy as burning 30
gallons of gasoline.
The Sun
Hydrogen Fusion —Our Star
• However, the nuclear reactions in the sun
are spread through a large volume in its
core.
• Any single gram of matter produces little
energy.
– A person of normal mass who is eating a normal diet
produces about 4,000 times more heat per gram by
burning calories than the matter in the core of the sun.
– The sun produces a lot of energy because it contains a
lot of grams of matter.
The Sun
Hydrogen Fusion —Our Star
• Fusion reactions can occur only when the
nuclei of two atoms get very close to each
other.
• As atomic nuclei carry positive charges,
they repel each other with an electrostatic
force called the Coulomb force.
– Physicists commonly refer to this repulsion
between nuclei as the Coulomb barrier.
The Sun
Hydrogen Fusion —Our Star
• To overcome the Coulomb barrier,
atomic nuclei must collide
violently.
– Violent collisions are rare unless the gas is
very hot.
– In that case, the nuclei move at high speeds
and collide violently.
The Sun
Hydrogen Fusion —Our Star
• So, nuclear reactions in the sun take
place only near the center—where
the gas is hottest and most dense.
– A high temperature ensures that collisions
between nuclei are violent enough to
overcome the Coulomb barrier.
– A high density ensures that there are enough
collisions, and thus enough reactions, to meet
the sun’s energy needs.
The Sun
Energy Transport in the Sun —Our Star
• Now, you are ready to follow the
energy from the core of the sun to the
surface.
– The surface of the sun is relatively cool—only
about 5,800 K—whereas the center is about
15 million K.
– So, energy must flow outward from the core.
The Sun
Energy Transport in the Sun —Our Star
– As the core is so hot, the gas acts as a black
body and emits gamma rays.
– Each time a gamma ray encounters an electron,
it is deflected or scattered in a random direction.
– As the gamma ray bounces around, it slowly
drifts outward toward the surface.
– This process, repeated innumerable times each
second for individual gamma rays, carries energy
outward in the form of radiation.
The Sun
Energy Transport in the Sun —Our Star
• Astronomers refer to the inner
parts of the sun as the radiative
zone.
The Sun
Energy Transport in the Sun —Our Star
• Imagine picking a single gamma ray
and following it to the surface.
– As the ray is scattered over and over again by
the hot gas, it drifts outward into cooler layers,
where the gas tends to emit photons of longer
wavelength.
– The ray is eventually absorbed and reemitted
as two X rays.
The Sun
Energy Transport in the Sun —Our Star
• Now, you must follow those two X rays
as they bounce around.
– As they drift outward into even cooler gas,
they in turn become
a number of even
longer-wavelength
photons.
The Sun
Energy Transport in the Sun —Our Star
• The packet of energy that began as a
single gamma ray is broken down into
a large number of lower-energy
photons.
• It eventually emerges from the sun’s
surface as about 1,800 photons of
visible light.
The Sun
Energy Transport in the Sun —Our Star
• However, something else
happens along the way.
– The packet of energy eventually
reaches the outer layers of the sun,
where the gas is so cool that it is not
very transparent to radiation.
The Sun
Energy Transport in the Sun —Our Star
• The energy is backed up like water
behind a dam, causing the gas to churn
in convection.
– Hot blobs of gas rise
and cool blobs sink.
– In this region, known
as the convective
zone, the energy is
carried outward
as circulating gas.
The Sun
Energy Transport in the Sun —Our Star
• The granulation visible on the photosphere
is clear evidence that the sun has a
convective zone just below the
photosphere, carrying energy up to the
surface.
The Sun
Energy Transport in the Sun —Our Star
• Sunlight is nuclear energy produced
in the core of the sun.
– The energy of a single gamma ray can take a
million years to work its way outward—first
through radiation and then through
convection—on its long journey to the
photosphere.
The Sun
Energy Transport in the Sun —Our Star
• It is time to ask the critical question
that lies at the heart of science.
– What is the evidence to support this
theoretical explanation of how the sun makes
its energy?
– The search for that evidence will introduce
you to one of the great problems of modern
astronomy.
The Sun
Neutrinos from the Sun’s Core —Our Star
• The center of a star seems forever hidden.
• The sun, though, is transparent to
neutrinos because these subatomic
particles almost never interact with normal
matter.
– Nuclear reactions in the sun’s core produce
floods of neutrinos that rush off into space.
– If astronomers could detect these neutrinos,
they could probe the sun’s interior.
The Sun
Neutrinos from the Sun’s Core —Our Star
• As neutrinos almost never interact with
atoms, you never feel the flood of over
1012 solar neutrinos that flows through
your body every second.
– Even at night, neutrinos from the sun rush through
Earth as if it weren’t there, up through your bed,
through you, and onward into space.
• You are lucky to be transparent to them.
– It also means that they are extremely hard to detect.
The Sun
Neutrinos from the Sun’s Core —Our Star
• Certain nuclear reactions, however,
can be triggered by a neutrino of the
right energy.
• In the late 1960s, chemist Raymond
Davis, Jr. began using such a
reaction to detect solar neutrinos.
The Sun
Neutrinos from the Sun’s Core —Our Star
• Davis filled a 100,000-gallon tank
with the cleaning fluid
perchloroethylene (C2Cl4).
– Theory predicts that, about once a day, a
solar neutrino will convert a chlorine atom in
the tank into radioactive argon—which can be
detected later by its radioactive decay.
The Sun
Neutrinos from the Sun’s Core —Our Star
• To protect the
detector from cosmic
rays from space,
the tank was buried
nearly a mile deep
in a South Dakota
gold mine.
– Of course, the mile of
rock overhead has no
effect on the neutrinos.
The Sun
Neutrinos from the Sun’s Core —Our Star
• The result of the Davis
experiment startled astronomers.
– The cleaning fluid detected too few
neutrinos—not one neutrino per day as
predicted by models of the sun, but about
one every three days.
The Sun
Neutrinos from the Sun’s Core —Our Star
• The experiment was refined,
tested, and calibrated for over
three decades.
– However, it did not find the missing neutrinos.
• Other detectors were built.
– They too counted very few neutrinos coming
from the sun.
The Sun
Neutrinos from the Sun’s Core —Our Star
• The missing neutrinos were one of
the great mysteries of modern
astronomy.
– Some scientists argued that astronomers
didn’t correctly understand how the sun and
stars make their energy.
– Other scientists wondered if there was
something about neutrinos that could explain
the problem.
The Sun
Neutrinos from the Sun’s Core —Our Star
• Astronomers had great confidence in
their theories of the sun’s interior.
• Helioseismology confirmed those
theories.
– So, they did not abandon their theories
immediately.
The Sun
Neutrinos from the Sun’s Core —Our Star
• As the 21st century began, scientists
were able to solve the mystery.
– Physicists now know of three kinds of
neutrinos, which they call ‘flavors.’
– The Davis experiment could detect, or ‘taste,’
only one flavor: electron-neutrinos.
The Sun
Neutrinos from the Sun’s Core —Our Star
• Theory hinted the electron-neutrinos
produced in the core of the sun
might oscillate among the three
flavors as they rushed out through
the sun, and across space, to Earth.
The Sun
Neutrinos from the Sun’s Core —Our Star
• Observations begun in 2000 confirm
the theory.
– Some of the electron
neutrinos produced in the
sun transform into tau- and
muon-neutrinos, which
most detectors cannot
count.
The Sun
Neutrinos from the Sun’s Core —Our Star
• This solution to the solar neutrino
problem is exciting because neutrinos
can’t oscillate unless they have mass.
– Neutrinos were long thought to be massless.
– However, if they have even a small mass,
they are so common their gravity could affect
the evolution of the universe as a whole—an
idea you will learn about later.
The Sun
Neutrinos from the Sun’s Core —Our Star
• The detection of neutrino
oscillation also excites
astronomers because it confirms
the theories that describe the
interior of the sun and stars.
The Sun
Building Scientific Arguments —Our Star
• Why does nuclear fusion require that the
gas be very hot?
– This argument has to include some basic physics of
atoms and thermal energy.
• Inside a star, the gas is so hot it is
ionized—which means the electrons have
been stripped off the atoms and the nuclei
are bare and have a positive charge.
The Sun
Building Scientific Arguments —Our Star
• For hydrogen fusion, the nuclei are
single protons.
– These atomic nuclei repel each other because
of their positive charges.
– So, they must collide with each other at high
velocity to overcome that repulsion and get
close enough together to fuse.
The Sun
Building Scientific Arguments —Our Star
• If the atoms in a gas are moving rapidly,
then it must have a high temperature.
• So, nuclear fusion requires that the gas
have a very high temperature.
– If the gas is cooler than about 10 million K, hydrogen
can’t fuse because the protons don’t collide violently
enough to overcome the repulsion of their positive
charges.
The Sun
Building Scientific Arguments —Our Star
• It is easy to understand why nuclear
fusion in the sun requires high
temperature.
• Now, expand your argument.
– Why does it require high density?
The Sun
Solar Activity —Our Star
• Solar activity refers to short-lived
features on the sun that change over
minutes, days, or years.
– As you use spectroscopic analysis to explore
this aspect of solar astronomy, you will
discover that solar activity is shaped by
magnetic fields and driven by the powerful
flow of energy rising from the sun’s interior.
The Sun
Observing the Sun —Our Star
• Solar activity is often visible with even
a small telescope.
• However, you should exercise great
caution in observing the sun.
– Sunlight is very intense and, when it enters
your eye, it is absorbed and converted into
heat.
The Sun
Observing the Sun —Our Star
• Equally dangerous is the infrared
radiation in sunlight.
– Your eyes can’t detect the infrared.
– However, it is converted to heat in your
eyes and can burn and scar the retina.
The Sun
Observing the Sun —Our Star
• It is unsafe to look directly at the sun.
• It is even more dangerous to look at it
through any optical instrument such
as a telescope, binoculars, or even
the viewfinder of a camera.
– The light-gathering power of such an optical
system concentrates the sunlight and can
cause severe injury.
The Sun
Observing the Sun —Our Star
• You should never look
at the sun with any
optical instrument
unless you are certain
it is safe.
– The figure illustrates
a safe way to observe
the sun with a small
telescope.
The Sun
Observing the Sun —Our Star
• In the early 17th century,
Galileo observed the sun
and saw spots on its
surface.
– Day by day, he saw the spots
moving across the sun’s disk.
– These are sunspots.
– He rightly concluded that
the sun was rotating.
The Sun
Sunspots —Our Star
• The dark spots that you see
at visible wavelengths only
hint at the complex
processes that go on in the
sun’s atmosphere.
– To explore those processes,
you must turn to the analysis of
spectra at a wide range of
wavelengths.
The Sun
Sunspots —Our Star
• There are various points to note about
sunspots.
• They are cool spots on the sun’s surface
caused by
strong
magnetic
fields.
The Sun
Sunspots —Our Star
• The Zeeman effect gives astronomers a
way to measure the strength of magnetic
fields on the sun.
The Sun
Sunspots —Our Star
• Also, they follow
an 11-year cycle
not only in the
number of spots
visible but in their
location on the sun.
The Sun
Sunspots —Our Star
• The cycle can vary over centuries and
appears to affect Earth’s climate.
The Sun
Sunspots —Our Star
• Finally, there is clear evidence that they
are part of a larger magnetic process that
involves all layers of the sun’s
atmosphere.
The Sun
Sunspots —Our Star
• The sunspot groups are merely the
visible traces of active regions.
– What causes this magnetic activity?
– The answer appears to be linked to the
waxing and waning of the sun’s magnetic
field.
The Sun
The Sun’s Magnetic Cycle —Our Star
• Sunspots are magnetic phenomena.
• So, the 11-year cycle of sunspots
must be caused by cyclical changes in
the sun’s magnetic field.
– To explore this idea, you need to begin with
the sun’s rotation.
The Sun
The Sun’s Magnetic Cycle —Our Star
• The sun does not rotate as a rigid
body.
– It is a gas from its outermost layers down to
its center.
– Some parts of the sun rotate faster than other
parts.
The Sun
The Sun’s Magnetic Cycle —Our Star
– The equatorial region of the photosphere rotates
faster than do regions at higher latitudes.
– It rotates once every 25 days.
– At a latitude of 45°,
one rotation takes
27.8 days.
The Sun
The Sun’s Magnetic Cycle —Our Star
• Helioseismology shows that deeper layers
of gas rotate at different speeds.
• Different parts of an object rotating at
different rates is called differential rotation.
– The magnetic cycle in the sun is linked to its differential
rotation.
The Sun
The Sun’s Magnetic Cycle —Our Star
• The sun’s magnetic field appears to be
powered by the energy flowing outward
through the moving currents of gas.
– The gas is highly ionized, so it is a very good
conductor of electricity.
– The dynamo effect occurs when a rapidly
rotating conductor is stirred by convection to
produce a magnetic field.
The Sun
The Sun’s Magnetic Cycle —Our Star
• This process is believed to produce
Earth’s magnetic field.
• Helioseismology provides evidence that
the process also produces the sun’s
magnetic field at the bottom of the
convection currents that lie below the
sun’s photosphere.
– The details of the process are still poorly understood.
– However, the sun’s magnetic cycle is clearly related to
the creation of its magnetic field.
The Sun
The Sun’s Magnetic Cycle —Our Star
• The magnetic behavior of sunspots
provides an insight into how the
magnetic cycle works.
– Sunspots tend to occur in groups or pairs.
– The magnetic field around such a pair
resembles that created by a magnet.
– That is, one end of the field is magnetic north
and one end is magnetic south.
The Sun
The Sun’s Magnetic Cycle —Our Star
– At any one time, sunspot pairs south of the sun’s
equator have reversed polarity compared with
those north of the sun’s equator.
– At the beginning of the
next 11-year sunspot
cycle, the new spots
appear with reversed
magnetic polarity.
The Sun
The Sun’s Magnetic Cycle —Our Star
• This magnetic cycle is not fully
understood.
• The Babcock model, named for its
inventor, explains it as a progressive
tangling of the solar magnetic field.
– As the electrons in an ionized gas are free to move,
the gas is a very good conductor of electricity, and any
magnetic field in the gas is ‘frozen’ into the gas.
– If the gas moves, the field must move with it.
The Sun
The Sun’s Magnetic Cycle —Our Star
• Thus, the sun’s magnetic field is
frozen into its gases.
The Sun
The Sun’s Magnetic Cycle —Our Star
• The differential
rotation wraps the
field around the sun
—like a long string
caught on a hubcap.
• Rising and sinking
gas currents twist
the field into
ropelike tubes,
which tend to rise.
The Sun
The Sun’s Magnetic Cycle —Our Star
• Where these magnetic tubes burst
through the sun’s surface, sunspot
pairs occur.
The Sun
The Sun’s Magnetic Cycle —Our Star
• The Babcock model explains the
reversal of the sun’s magnetic field
from cycle to cycle.
– As the magnetic field becomes tangled,
adjacent regions of the sun’s surface are
dominated by magnetic fields that point in
different directions.
The Sun
The Sun’s Magnetic Cycle —Our Star
• After about 11 years of tangling, the field
eventually becomes so complex that
adjacent regions of the solar surface
begin changing their magnetic fields to
agree with neighboring regions.
– Quickly, the entire field rearranges itself into a
simpler pattern.
– Differential rotation begins winding it up to start a
new cycle.
The Sun
The Sun’s Magnetic Cycle —Our Star
• However, the newly organized field is
reversed.
– The next sunspot cycle begins with magnetic
north replaced by magnetic south.
• Thus, the complete magnetic cycle is 22
years long, and the sunspot cycle is 11
years long.
The Sun
The Sun’s Magnetic Cycle —Our Star
• The Babcock model may in fact be
incorrect in some details, but it gives you a
framework on which to organize all the
complex solar activity.
• This is the power of a scientific model.
– Even though the models of the sky and the atom,
which you have learned about earlier, were only
partially correct, they served as organizing themes to
guide your thinking.
The Sun
The Sun’s Magnetic Cycle —Our Star
• Similarly, although the details of the solar
magnetic cycle are not yet understood, the
Babcock model gives you a general
picture of the behavior of the sun’s
magnetic field.
– Further observations will help astronomers refine
the model and make it a better description of
what really happens in the sun.
The Sun
The Sun’s Magnetic Cycle —Our Star
• If the sun is truly a representative star, you
might expect to find similar magnetic
cycles on other stars.
– However, they are too distant for spots to be
directly visible.
• Some stars, though, vary in brightness
over a period of days in a way that reveals
they are marked with dark spots believed
to resemble sunspots.
The Sun
The Sun’s Magnetic Cycle —Our Star
• Other stars have spectral features that
vary over periods of years, suggesting that
they are subject to magnetic cycles much
like the sun’s.
• In fact, some stars display sudden flares
that resemble solar eruptions.
– Once again, the evidence tells you that the
sun is a normal star.
The Sun
Chromospheric and Coronal Activity —Our Star
• The solar magnetic fields extend high
into the chromosphere and corona,
where they produce beautiful and
powerful phenomena.
The Sun
Chromospheric and Coronal Activity —Our Star
• There are three important points to note
about these
phenomena.
• One, all solar
activity is magnetic.
The Sun
Chromospheric and Coronal Activity —Our Star
• There are no such events on Earth
because Earth’s magnetic field is weak.
• Also, Earth’s atmosphere is not ionized
and is thus free to move independently of
the magnetic field.
– On the sun, however,
the weather is a
magnetic
phenomenon.
The Sun
Chromospheric and Coronal Activity —Our Star
• Two, tremendous energy can be
stored in arches of magnetic field.
– These are visible near the limb of the sun as
prominences and, seen from above, as filaments.
The Sun
Chromospheric and Coronal Activity —Our Star
• When that stored energy is
released, it can trigger powerful
eruptions.
– Although these eruptions occur far from
Earth, they can affect us in dramatic
ways.
The Sun
Chromospheric and Coronal Activity —Our Star
• Finally, in some regions of the solar
surface, the magnetic field does not
loop back.
– High-energy gas
from these regions
flows outward and
produces much of
the solar wind.
The Sun
Building Scientific Arguments —Our Star
• What kind of activity would the sun
have if it didn’t rotate differentially?
– This is a really difficult question because only
one star is visible close up.
– Nevertheless, you can construct a scientific
argument by thinking about the Babcock
model.
The Sun
Building Scientific Arguments —Our Star
• If the sun didn’t rotate differentially, then
the magnetic field might not get twisted
up, and there might not be a solar cycle.
– Twisted tubes of magnetic field might not form
and rise through the photosphere to produce
prominences and flares, although convection
might tangle the magnetic field and
produce
some
activity.
The Sun
Building Scientific Arguments —Our Star
• Is the magnetic activity that heats the
chromosphere and corona driven by
differential rotation or by convection?
– It is hard to guess but, without differential
rotation, the sun might not
have a strong magnetic
field and high-temperature
gas above its photosphere.
The Sun
Building Scientific Arguments —Our Star
• This is very speculative.
• However, sometimes, in the critical
analysis of ideas, it helps to imagine a
change in a single important factor and try
to understand what might happen.
– For example, redo the argument.
– What do you think the sun would be like if it had no
convection inside?