The Sun

Classification Classification The sun is a star, and a typical one at that

Its physical properties (mass, radius, brightness, composition) lie in the middle of the observed ranges of stellar properties

Though mediocre (as far as stars go), life on Earth would not be possible without the energy supplied by the sun

Classified as a main sequence star on the Hertzsprung-Russell diagram

Vital Statistics• The Sun is an ordinary G2 star, one of more than 100 billion

stars in our galaxy. – Middle-aged star, estimated at ~5 billion years old, likely to die out

by ~10 b.y.

Classification Classification

EvolutionEvolution☼ Towards the end of its life cycle, the sun will

transition from a main sequence star to a red giant star…

• luminosity will increase• radius will increase by about 100 times its

present value (which means the end of the terrestrial planets!)

☼ And then to a white dwarf star• outer layers will be shed, leaving a planetary

nebula• only the core will be visible• eventually, any remnant energy will be

exhausted and the sun will quietly burn out

EvolutionEvolution

Vital Statistics• Name:

– The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

• Diameter: – 1,390,000 km

• Mass: – 1.989 x 1030 kg. – the largest object in the solar system. – contains more than 99.8% of the total mass of the Solar System

• Jupiter contains most of the rest

• Composition: – hydrogen (H)(75% by mass) and helium (He)(~25% by mass).– minor amounts of heavier elements (0.1%), but they do occur with the same relative

abundance as the same elements on Earth and in meteorite

Vital Statistics

• Rotation Period: – Prograde rotation direction, the

common west-to-east rotation (counterclockwise as seen from above the north poles) of the sun and planets (except Venus).

– sun has differential rotation –• ~25 days at equator,

• ~36 days at poles

• warps and distorts the sun's magnetic field

• Axial tilt: – sun's axis inclined 7 degrees from

the ecliptic

StructureStructure The sun is divided into different layers

Core temp = 15,000,000 K generation of energy by fusion 200,000 km radius

Envelope radiation zone + convection zone transmission of energy by radiation and

convection 500,000 km total thickness

Photosphere visible “surface” layer from which energy escapes to

space less than 500 km thick temp = 5780 K

Chromosphere lower atmosphere

Corona outer atmosphere temp = 1,000,000 K thin, grades into solar wind at high altitudes

Interior of the Sun• Toward the core,

– increases in • pressure

• density

• temperature – 5800°K (surface), – 15,600,000°K (core),

– All the energy of the sun is created in the core • flows outward through the sun,

• heating the surface which radiates that energy as light and heat.

• Energy flow from hot regions to cold regions occurs by three mechanisms:

– conduction, – radiation, – convection

Energy

• Energy is transmitted from the core to shallower depths in two ways– Radiation – transfer of energy only, in the form of

a wave• Ex: electromagnetic radiation

– Convection – transfer of energy and solar gas; constant upwelling of warm fluid and the simultaneous downward flow of cooler material to take its place

• Convective motion is apparent in the granulation of the photosphere (surface)

Interior of the Sun• Core

– Very high density • 150 g/cc - 150 times denser than water and 10 times denser than lead)

– hot (15 million K)

– Core extends outward 25% of distance from Sun's center.

• Radiation zone – zone of radiative heat transfer composed of ionized gases.

– Extends to ~70% of distance from center.

• Convection zone – zone of active circulation of gases and heat transfer to the sun's surface.

– Extends from 70% of distance from center to the surface.

– The outer shell of the sun that we see from Earth is the top of the turbulent convective zone that appears to be boiling

Sun's Atmosphere• Photosphere

– visible surface of the sun

– yellow that we see from Earth.

– only ~500 km deep

– relatively very thin

– gases are of low enough density to allow light to escape

• (0.1% of air at sealevel on Earth)

– 5500 K gas

Sun’s Atmosphere• Photosphere

– surface mottled by a pattern of bright cells called granulation • each cell about 1000 km across (size of Texas) separated by a dark boundary.

• each granule lasts about 8-10 minutes

• likely the surface expression of underlying convection cells

Sun’s Atmosphere

• Chromosphere – nearly transparent layer of gas

about 1000 times fainter than the photosphere

– very low density gas with red/pink color

– The characteristic red color of the chromosphere is due to hydrogen alpha (6563Å) emission

– rises to heights near 10,000 km, • temps of 1,000,000 K

– only visible during a total solar eclipse

Sun’s Atmosphere• Chromosphere

– spicules are small, flamelike projections from the chromosphere

• 100-1000 km across,

• extending up to 12,000 km from the top of the photosphere

• lasting 5-15 minutes

• cooler regions (10,000 K) extending up into base of much hotter corona (500,000 K)

• spring up from edges of "supergranules“

• charged particles that follow magnetic field lines.

Sun’s Atmosphere• Corona

– faint outer atmosphere, composed of very low density ionized gases

• mostly free electrons and protons

– density at base of corona is 100 billion times thinner than the air we breathe

– milky white glow only visible during total eclipses or with a special telescope outfitted with a disk to cover the photosphere

– "streamers" from the corona extends out to 10-30 "solar radii" (~7.6 - 23 x 106 km)

– temps rise rapidly outward in corona • from 500,00 K low to 3,000,000 K in outer

corona)

EnergyEnergy☼ All of the sun’s energy is created in the core by nuclear fusion

– fusion is a chemical reaction in which light nuclei are combined (fused) into heavier ones– for a main sequence star, this reaction involves the fusion of hydrogen into helium

– 4H → He + energy– the release of energy is most important!

☼ Core-hydrogen burning keeps the sun in hydrostatic equilibrium– state of balance in which pressure’s outward push is exactly counteracted by gravity’s inward pull

Proton Proton Chain

• Two mass-1 isotopes of hydrogen undergo a simultaneous fusion and beta decay to produce a positron, a neutrino, and a mass-2 isotope of hydrogen (deuterium). • The deuterium reacts with another mass-1 isotope of hydrogen to produce Helium-3 and a gamma-ray. • Two helium-3 isotopes produced in separate implementations of steps (1) and (2) fuse to form a Helium-4 nucleus plus two protons. • The net effect is to convert hydrogen to helium, with the energy released going into the particles and gamma-rays produced at each step of the sequence.

• Rates for the PP Chain• The average time required for a nucleus to undergo each step of this sequence in a typical stellar interior is indicated in the figure shown above. Thus, for

example, a hydrogen nucleus waits on the average 1 billion years before it undergoes an interaction with another hydrogen nucleus to initiate the sequence! Since all other steps require much less time than this, it is this initial step that controls the rate of the reaction.

• This incredibly small rate nevertheless accounts for the luminosities of normal stars because there are so many hydrogen atoms in the core of a star that at any one instant many are undergoing the reactions of the PP chain.

1H + 1H 2H + e+ + e + 0.42 MeV The positron immediately annihilates with one of the hydrogen's electrons,

and their mass energy is carried off by two gamma ray photons.

e+ + e- 2 + 1.02 MeV After this the deuterium produced in the first stage can fuse with another hydrogen

to produce a light isotope of helium, 3He:

2H + 1H 3He + + 5.49 MeV Finally, after millions of years, two of the helium nuclei 3He produced can fuse

together to make the common helium isotope 4He, releasing two hydrogen nuclei to start the reaction again through three different paths called PP1, PP2 and PP3:

PP1: (69% of the time)

3He +3He 4He + 1H + 1H + 12.86 MeV

The complete PP1 chain reaction releases a net energy of 26.7 MeV. PP1 chain is dominant in temperatures of 10-14 million Kelvin. Below 10 million Kelvin, the PP chain does not produce much 4He.

PP2: (31% of the time)

       3He + 4He 7Be +        7Be + e- 7Li + e

       7Li + 1H 4He + 4He

PP2 chain is dominant in temperatures of 14-23 million Kelvin.

PP3: (0.3% of the time)

       3He + 4He 7Be +        7Be + 1H 8B +        8B 8Be + e+ + e        8Be 4He + 4He

PP3 chain is dominant if the temperatures exceeds 23 million Kelvin.

Sun’s Magnetic Field

• Sun's magnetosphere (its shell of magnetic field lines produced by a dynamo effect in the sun's core) extends well beyond Pluto

• The Sun’s differential; rotation wraps and distorts the magnetic field lines

Sun’s Magnetic Field

• Magnetic lines of force leave the sun and wrap through space

Sunspots• cool dark spots on the solar surface (photosphere) that contain intense magnetic

fields – dark because they are cooler than surrounding photosphere

– center of sunspot (umbra) is about 4000 K

– outer border (penumbra) is about 6000 K

– average size is about 2 Earth diameters

– tend to form in groups of up to 100 individual spots,

– last as long as 2 months or more, most about a week

Sunspots

• why are there sunspots? – magnetic field within sunspots 1000 times greater than background solar

average

– perhaps rising currents of hot gas just under the photosphere are inhibited by the intense magnetic fields in sunspots,

• cause a decrease in temp and a dark spot

• higher than average temps have been measured in the immediate margin of sunspots,

• as if the light and heat can't escape through the intense magnetic field, so it is deflected around the edges

Sunspots

• Sunspots tend to occur in pairs. – one spot will be a magnetic north

pole• where magnetic field lines exit the

sun

– the other spot will be a magnetic south pole

• where magnetic field lines enter the sun

• may relate to interaction between magnetic field lines and differential rotation of the sun

– field lines become tangled and pop through the surface with a "north pole" and a "south pole"

Sunspots• Sunspot cycle

– sunspots vary over an 11 year average cycle

– from maximums of up to 100 visible spots to minimums of only a few spots

– tend to first form in latitudes of 20 degrees then progressively more towards the equator.

– for a cycle of 11 years, • leading sunspots in the northern hemisphere will have the same polarity,

• those in the southern hemisphere will have the opposite polarity,

• then reverses for the next 11 tear cycle.

• SO, the cycle is actually 22 years.

– last maximum in 2001

The Active Sun• Prominences

– red protrusions of ionized gases from the chromosphere

– often arch-shaped, controlled by magnetic fields

– seem to represent twisted magnetic fields emanating from the active regions around sunspots

The Active Sun• Solar flares

– chromosphere eruption of • ionized gases

• x-rays,

• uv light

• visible light

– rises rapidly in a few minutes then decays over an hour or less

– may release the energy equivalent of 2 billion megatons of TNT

– clearly linked to magnetic field, because occur of sunspot groups

– significant effects on Earth - most flares become "gusts" in the solar wind and reach Earth after a few hours or days

– creates a magnetic storm on Earth that affects communications and increased auroral activity near the poles

The Active Sun

• Changing Solar Cornea– Varies with sunspot

activity

SOHO

• composite image combines Extreme Ultraviolet Imaging Telescope (EIT)images from three wavelengths(171, 195 and 284 angstrom) into one that reveals solar features unique to each wavelength.

• SOHO web site

Solar Wind• the rapidly moving outward extension of the corona.

• composed of ionized hydrogen (protons with free electrons) called plasma

– Carries over 1 million tons of solar matter per second

• Extends into interplanetary space

• travels at measured rates of 300-770 km/sec past the Earth

• Recent data from the spacecraft Ulysses show that the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, that it does at lower latitudes.

– The composition of the solar wind also appears to differ in the polar regions

• density measured at 50 particles per cc (not g/cc)

• "gusts" of the solar wind cause magnetic disturbances on Earth, which is mostly protected by its magnetosphere

Solar Wind

• Affects entire solar system

Interaction of Solar Wind and Earth’s Magnetic Field

Solar Wind• Interaction of the Solar

wind with the Interstellar wind

• Heliopause – gradual boundary

between the heliosphere and the interstellar gas outside our solar system.

• Heliosphere – The area in space that

contains our solar system, solar wind, and the entire solar magnetic field.

– It extends well beyond the orbit of Pluto, out to the heliopause.

ItIt’’s all s all over over

-- but the but the

fireworksfireworks

Which is your Which is your homework. homework.