The Sun

Nur Islami

INTRODUCTION

Visible Image of the SunThe SunThe Sun

•Our sole source

of light and heat in

the solar systemthe solar system

•A very common

star: a glowing ball of

gas held together by its

own gravity and powered

by nuclear fusion at its

center.

Pressure (from heat

caused by nuclear

reactions) balances the

gravitational pull

toward the Sun’s center.

Called “Hydrostatic

Equilibrium.

This balance leads to a This balance leads to a

spherical ball of gas,

called the Sun.

What would happen if

the nuclear reactions

(“burning”) stopped?

SOLAR PROPERTIES

Radius = 696,000 km

(100 times Earth)

Mass = 2 x 1030 kg

(300,000 times Earth)

Av. Density = 1410 kg/m3

Rotation Period =

Solar PropertiesSolar Properties

Rotation Period =

24.9 days (equator)

29.8 days (poles)

Surface temp = 5780 K

Element

Hydrogen 70.9%

Helium 27.4%

etc

Luminosity of the Sun

= LSUN

(Total light energy

emitted per second)

~ 4 x 1026 W100 billion one-

megaton nuclear bombs

every second!

Solar constant:

LSUN / 4πR2

(energy/second/area

at the radius of

Earth’s orbit)

The Solar Interior

“Helioseismology”

•In the 1960s, it was

discovered that the

surface of the Sun

vibrates like a bell

•Internal pressure

How do we know the interior How do we know the interior

structure of the Sun?structure of the Sun?

•Internal pressure

waves reflect off the

photosphere

•Analysis of the

surface patterns of

these waves tell us

about the inside of the

Sun

The Standard Solar Model

Energy Transport within the Sun

• Extremely hot core - ionized gas

• Temperature falls further from core - more and more non-ionized atoms

capture the photons - gas becomes opaque to light in the convection zone

• The low density in the photosphere makes it transparent to light - radiation

takes over again

Convection

� Convection takes over when

the gas is too opaque for

radiative energy transport.

� Hot gas is less dense and

rises (or “floats,” like a hot air

balloon or a beach ball in a balloon or a beach ball in a

pool).

� Cool gas is more dense and

sinks

Solar Granulation

Evidence for 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 @ ~1 km/sec (2200 mph; Doppler).

THE SUN ATMOSPHERE

� The solar spectrum has

thousands of absorption

lines

� More than 67 different

elements are present!

� Hydrogen is the most

The Solar Atmosphere

� Hydrogen is the most

abundant element followed

by Helium (1st discovered

in the Sun!)

Spectral lines only tell us about the part of the Sun that forms them (photosphere and chromosphere) but these elements are also thought to be representative of the entire Sun.

Main Regions of the Sun

Visible

surface

The Photosphere

• The visible surface of the sun

• It is not solid, thin layer of gas, less than 500

km deep.

• The pothosphere is less than 1/3000 as dense • The pothosphere is less than 1/3000 as dense

as the air in the earth.

• Below photosphere, the gas is denser and

hotter

• The granulation is appear in the photosphere

GranulesConvection from inside the sun causes the

photosphere to be subdivided into 1000-

2000km cells.

Energy rises to the surface as gas wells up in the cores of

the granules, and cool gas sinks around their edges.

Chromosphere

Chromosphere (seen during full Solar eclipse)

� Chromosphere emits very little light because it is of low density

� Reddish hue due to 3→2 (656.3 nm) line emission from Hydrogen

• The chromosphere’s density ranges from

10000-100 billion times less than the air on

the earth

Chromospheric Spicules:

warm jets of matter

shooting out at ~100 km/s

Spicules are thought to the

result of magnetic

disturbances

Hα light

Transition Zone and Corona

Transition Zone

& Corona

We see emission

lines from highly

ionized elements

Very low density,

T ~ 106 K

� Why does the Temperature rise further from the hot light source?

ionized elements

(Fe+5 – Fe+13) which

indicates that the

temperature here is

very HOT

→ magnetic “activity” -spicules and other more energetic

phenomena (more about this later…)

Corona (seen during full Solar eclipse)

Hot coronal gas

escapes the Sun

→ Solar wind

Solar Wind

Solar Wind

� Coronal gas has enough heat (kinetic) energy to escape the

Sun’s gravity.

� The Sun is evaporating via this “wind”.

�Solar wind travels at ~500 km/s, reaching Earth in ~3 days

� 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.its total mass.

Hot coronal gas (~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

The Active Sun

UV light

Most of theSolar luminosity is continuous photosphere emission.

But, there is an irregular component (contributing little to the Sun’s total luminosity).

SUNSPOT

Sunspots

Granulation around sunspot

SunspotsSunspots

• Typically about 10000 km

across

• At any time, the sun may

have hundreds or none

• Dark color because they • Dark color because they

are cooler than photospheric

gas (4500K in darkest parts)

• Each spot can last from a few days to a few months

• Galileo observed these spots and realized the sun is rotating

differentially (faster at the poles, slower at the equator)

Sunspots &

Magnetic Fields

•The magnetic field in a sunspot •The magnetic field in a sunspot

is 1000x greater 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

The Sun’s differential rotation distorts the magnetic field lines

The twisted and tangled field lines occasionally get kinked, causing the field

strength to increase

Sunspot Cycle

Solar maximum is

reached every ~11 years

Solar Cycle is 22 years long – direction of magnetic field

polarity flips every 11 years (back to original orientation every 22 years)

� Charged particles (mostly

protons and electrons) are

accelerated along magnetic field

“lines” above sunspots.

� This type of activity, not light

energy, heats the corona.

Heating of the Corona

energy, heats the corona.

SOLAR PROMINENCES

Charged particles follow magnetic fields between sunspots:

Solar Prominences

Sunspots are cool,

but the gas above

them is hot!

Earth

Solar ProminenceCloudlike structure, loop shape

Large amount of gases

Looks like a huge flame

coming out of the sun

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.

Solar Flares – much more violent magnetic instabilities

5 hours

Particles in the flare are so energetic, the magnetic field cannot bring them

back to the Sun – they escape Sun’s gravity

SOLAR FLARE

Solar Flares

• A solar flare is a sudden brightening observed over the Sun's surface, which is interpreted as a large energy release of up to 6 × 1025 joules of energy (about of energy (about 160,000,000,000 megatons of TNT)

• They are mainly followed by a colossal coronal mass ejection.

• The flare ejects clouds of electrons, ions, and atoms through the corona of the sun into space.

• These clouds typically reach Earth a day or two after the event.

• Solar flares affect all layers of the solar atmosphere (photosphere, chromosphere, and corona)

• When the plasma medium is heated to tens of millions of kelvins the electrons, protons, and heavier ions are accelerated to near the speed of light. accelerated to near the speed of light.

• Flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior.

• Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona.

• Solar flares strongly influence the local space weather in the vicinity of the Earth.

• They can produce streams of highly energetic particles in the solar wind, known as a solar proton event, or "coronal mass ejection" (CME).

• These particles can impact the Earth's magnetosphere, • These particles can impact the Earth's magnetosphere, and present radiation hazards to spacecraft and astronauts.

• Massive solar flares are sometimes associated with CMEs which can trigger geomagnetic storms that have been known to knock out electric power for extended periods of time.

SOLAR WIND

• The solar wind is a stream of charged particles released from the upper atmosphere of the Sun.

• It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV.

The stream of particles varies in density, • The stream of particles varies in density, temperature, and speed over time and over solar longitude.

• These particles can escape the Sun's gravity because of their high kinetic energy and the high temperature of the corona.

• The solar wind flows outward supersonically

to great distances.

• Other related phenomena include

geomagnetic storms that can knock out power geomagnetic storms that can knock out power

grids on Earth, the aurora (northern and

southern lights), and the plasma tails of

comets that always point away from the Sun.

• As the solar wind approaches a planet that has a well-developed magnetic field (such as Earth, Jupiter and Saturn), the particles are deflected by the Lorentz force.

• This region, known as • This region, known as the magnetosphere, causes the particles to travel around the planet rather than bombarding the atmosphere or surface.

• The magnetosphere is roughly shaped like a hemisphere on the side facing the Sun, then is drawn out in a long wake on the opposite side.

• The boundary of this region is called the magnetopause, and some of the particles are able to penetrate the magnetosphere through this region by partial reconnection of the magnetic field lines.

Solar Wind (conclusion)

• Blows charged particles and magnetic fields away from the Sun

• Charged particles captured by Earth’s magnetic field

• Create Auroras or Northern and Southern Lights

Image at http://solarscience.msfc.nasa.gov/the_key.shtml

SUN’S INFLUENCE ON THE EARTH

Influences on Earth

• Gravity

• Light (Radiation)• Light (Radiation)

• Solar Wind (already discussed)

Gravity

• Orbits

– The Sun’s powerful gravity keeps the planets in orbit

Radiation• Our Sun (and all active stars) emits

radiation– Radio, infrared, visible, ultraviolet, x-ray and even some gamma

rays

– Most of the sunlight is yellow-green visible light or close to it

The Sun at X-ray wavelengths

Sun’s Radiation at Earth

• The Earth’s atmosphere filters out some

frequencies– Ozone layer protects us from some ultra-violet, and most x-rays

and gamma rays

– Water and oxygen absorb some radio waves

– Water vapor, carbon dioxide, and ozone absorbs some infrared

Electromagnetic spectrum

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/what_is_ir.html

.

Sun as a Source of Energy• Light from the Sun is absorbed by the Earth, unevenly

to:– drive wind bands – which drive surface currents

– drive deep ocean currents

– drive water cycle

– drive weather

– Long ago????

NASA image at http://visibleearth.nasa.gov/view_rec.php?id=107 Credit: NASA GSFC Water and Energy Cycle

http://www.nasa.gov/centers/jpl/news/grace-20061212.html

Sun as a Source of Energy

• Plants need light for photosynthesis

• Without its heat, the only inhabitable areas on

Earth would be near volcanic vents

Task 2 (group)

• Make one paper (3-5 pages) that discuss

about: Nuclear fusion in the sun and its

implication.

• Submit the paper at next lecture• Submit the paper at next lecture

• One group (chosen randomly) will present it

within 20 minutes (15 minutes for

presentation + 5 minutes Q&A)

Task 3 (group)

• Make one paper (3-5 pages) that discuss

about: The origin of the solar system.

• No need to print (just waste the paper and

ink)ink)

• Submit it via this email (nris74@yahoo.com)

• The presenter is group: ……