4.6THE SUN

General PropertiesAverage star; G2V (we’ll discuss

in detail later)Only appears so bright because

it’s so closeAbsolute Magnitude 4.83

Magnitude if it were 32.6 light years (10 parsecs)

109 times Earth’s diameter333,000 times Earth’s massConsists entirely of gas (Average

density ~ 1.4 g/cm3)Core Temperature ~ 15,000,000 KSurface Temperature ~ 5800 K

The Solar AtmosphereThe Sun’s atmosphere is made up of 3 layers:

PHOTOSPHERE

CHROMOSPHERE

CORONA

The PhotosphereApparent surface layer of the Sun

Depth ~ 500 kmTemperature ~ 5800 KAtoms in the photosphere absorb photons of specific

wavelengths, producing absorption lines of Hydrogen, Helium, and other elements.

Energy is transferred through convection.

Cool gas sinking down

Bubbles of hot gas

rising up

GranulationIn good photographs, the Sun has a mottled

appearance because it is made up of dark-edged regions called granules size of TX; 10-20 minutes.

The overall pattern is called granulation, providing evidence of convection.

The ChromosphereAbove the photosphere lies the

chromosphere.Visible, UV, and X-ray lines from

highly ionized gases.

Roughly 1000 times fainter than photosphere; visible during a total solar eclipse.

Comes from Greek word, chroma, meaning “color”.The pink color is produced by the

combined light of three bright emission lines red, blue, violet Balmer lines of Hydrogen.

The ChromosphereA filtergram is an image of

the Sun made using light in a dark absorption line of the spectra produced.Reveal fine detail in the

upper layers of the chromosphere.

Spicules are flamelike jets of gas extending upward into the chromosphere and lasting 5-15 minutes.Spectra show they are cooler

gas from lower chromosphere extending upward.

The CoronaOutermost part of Sun’s

atmosphere coming from the Greek word meaning “crown”.Also visible only during

total solar eclipse.Extremely low-density, hot

gas (1-2 million K).Satellites have mapped a

magnetic carpet of looped magnetic fields extending up through the photosphere (magnetic waves create heating of the gas?)

The Solar Atmosphere

Photosphere

Corona

Chromosphere

Sunspot

Regions

The Solar Atmosphere

Solar WindIonized, low-density gas cannot

cross magnetic fields, so where the Sun’s field loops back to the surface, the gas is trapped.

However, some of the magnetic fields lead outward into space, and there the gas flows away from the Sun in a breeze called the solar wind.300-800 km/s with gusts as high

as 1000 km/s.The Sun is constantly losing mass!

107 tons every second. Still only 10-14 of its solar mass per

year.

HelioseismologyThe solar interior is opaque

(absorbs light) out to the photosphere (“surface”) .

The only way to investigate the solar interior is through helioseismology, naturally occurring vibrations can be mapped using the Doppler Effect.

Allowed astronomers to map the temperature, density, and rate of rotation inside the Sun.

Universal ForcesThe four main universal

forces existing in nature include:

GRAVITATIONAL

ELECTROMAGNETIC

STRONG

WEAK

Energy ProductionThere are 2 ways to generate

energy from atomic nuclei:

Nuclear Fission: reactions splitting larger nuclei into less-massive fragments.Power plants on Earth Uranium

nucleus (235 protons) split into a range of possible fragments.

Nuclear Fusion: reactions combining light nuclei into heavier nuclei.Most common reaction inside

stars Hydrogen nuclei (single protons) into Helium nuclei.

Fission

Fusion

Energy ProductionBoth fission and fusion

nuclear reactions move downward in the diagram, meaning the nucleus produced by a reaction is more tightly bound than the nuclei that went into the reaction.

They both produce energy by releasing binding energy of atomic nuclei.

Iron has the most tightly bound nucleus; for elements heavier than Iron energy is gained by fission.

Energy Production in the SunNeed large proton speed ( high

temperature) to overcome Coulomb barrier (electrostatic repulsion

between protons)

4 1H 4He + ENERGY

4 1H protons have 0.048 x 10-27

kg (= 0.7 %) more mass than 4He.

Does mass disappear?

NO …

It is converted into energy!

E = mc2

= (0.048 x 10-27 kg) x (3.0 x 108 m/s)

= 0.43 x 10-11 J per reaction

The Sun completes 1038 reactions per second, transforming 5 million

tons of mass into energy every second … sound crazy? In its entire 10 billion year lifetime,

that’s only 0.07% of its total mass!

Proton-Proton Chain

Series of three nuclear reactions building a

Helium nucleus by adding together protons.

Observing the SunIMPORTANT WARNING

Never look directly at the Sun through a

telescope or binoculars.

Could cause permanent eye damage and even

blindness.

Use a projection technique or a special

Sun viewing filter.

SunspotsDark, cooler regions on the Sun’s

photosphere are called sunspots.Temperature of sunspots ~ 4200 K.Clearly linked to Sun’s magnetic

field.Typical size of a sunspot is about

twice the size of Earth.

Number of spots varies in a cycle with a period of 11 years.Solar Maximum

100+ sunspotsSolar Minimum

very few sunspots

The Sunspot CycleMagnetic fields exist

around sunspot groups, projected to be 1000 times stronger than the magnetic field of Earth.

Surface of the photosphere is cooler where the sunspots are.Heat prevented from

emerging can be detected in UV and infrared imaging devices.

The Maunder MinimumHistorical records show very few sunspots from 1645 – 1715, a

phenomenon known as the Maunder Minimum.Coincides with a period called the “Little Ice Age” unusually cool

weather in Europe and North America.Link b/w solar activity and the amount of solar energy Earth receives?

The Sun’s “Dynamic” Magnetic CycleSun’s magnetic field powered by

energy flowing outward through the moving currents of gas.Very good conductor of electricity.

When an electrical conductor rotates and is stirred by convection, it can convert some of the energy flowing outward into a magnetic field through a process known as the Dynamo Effect.Different levels in the Sun rotate

with different periods and is known as differential rotation.

The Sun’s “Dynamic” Magnetic CycleAlthough the magnetic cycle of the Sun

is not fully understood, the Babcock model explains the magnetic cycle as a progressive tangling of the solar magnetic field.

Complete magnetic cycle = 22 years

Sunspot cycle = 11 years

Solar PhenomenaProminences are composed of ionized gas

trapped in a magnetic arch rising up through the photosphere and chromosphere into the lower

corona.

Can form dark filaments when seen against the Sun’s bright surface (photosphere).

Temperature of the gas in prominences may be 60,000 K – 80,000 K, which is quite cold

compared with the low-density gas in the corona.

Solar PhenomenaSolar Flares are extreme events and can significantly

influence Earth’s magnetic field structure.Particles reach Earth hours, or days later as gusts in the

solar wind.

Solar PhenomenaAurora Borealis

(“Northern Lights”)

~ 5

m

inu

tes

Creates auroras on

Earth northern/ southern

lights.

Solar PhenomenaLarge amounts of ionized

gas blown outward from the corona is known as a coronal mass ejection, otherwise known as a CME.

Much of the solar wind comes from coronal holes, areas where the magnetic field does not loop back into the Sun.

X-ray image showing solar activity

The Solar ConstantThe energy we receive from the Sun is

essential to all life on Earth alike and can be expressed as the:

SOLAR CONSTANT

F = 1360 J/s/m2

F = Energy Flux = the amount of radiation, per unit of time and per unit of surface area.

Energy Flux