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).