Visible Image of the Sun

The SunThe Sun

•Our sole source of light and heat in the 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 spherical ball of gas, called the Sun.

What would happen if the nuclear reactions (“burning”) stopped?

Main Regions of the Sun

Radius = 696,000 km (100 times Earth)

Mass = 2 x 1030 kg (300,000 times Earth)

Av. Density = 1410 kg/m3

Rotation Period = 24.9 days (equator) 29.8 days (poles)

Surface temp = 5780 K

Solar PropertiesSolar Properties

The Moon’s orbit around the Earth would easily fit within the Sun!

Luminosity of the Sun

= LSUN

Solar constant:

LSUN 4R2

(energy/second/area at the radius of Earth’s orbit)

(Total light energy emitted per second)

~ 4 x 1026 W100 billion one-megaton nuclear bombs every second!

The Solar Interior “Helioseismology”

•In the 1960s, it was discovered that the surface of the Sun vibrates like a bell

•Internal pressure waves reflect off the photosphere

•Analysis of the surface patterns of these waves tell us about the inside of the Sun

How do we know the interior How do we know the interior structure of the Sun?structure of the Sun?

The Standard Solar Model

Energy Transport within the Sun

• Extremely hot core - ionized gas • No electrons left on atoms to capture photons - core/interior is transparent to light (radiation zone)• 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 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 solar spectrum has thousands of absorption lines

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 part of the Sun that forms them (photosphere and chromosphere) but these elements are also thought to be representative of the entire Sun.

Chromosphere

Chromosphere (seen during full Solar eclipse)

Chromosphere emits very little light because it is of low density

Reddish hue due to 32 (656.3 nm) line emission from Hydrogen

Chromospheric Spicules: warm jets of matter shooting out at ~100 km/s last only minutes

Spicules are thought to the result of magnetic disturbances

H light

Transition Zone and Corona

Transition Zone & Corona

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

We see emission lines from highly ionized elements (Fe+5 – Fe+13) which indicates that the temperature here is very HOT

Very low density, T ~ 106 K

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.

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

Most of theSolar luminosity is continuous photosphere emission.But, there is an irregular component

(contributing little to the Sun’s total luminosity).

The Active Sun

UV light

Sunspots

Granulation around sunspot

SunspotsSunspots

• Typically about 10000 km across

• At any time, the sun may have hundreds or none

• 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 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

“tube” of lines bursts through atmosphere creating sunspot pair

Sunspot Cycle

Solar Cycle is 22 years long – direction of magnetic field polarity flips every 11 years (back to original orientation every 22 years)

Solar maximum is reached every ~11 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

Charged particles follow magnetic fields between sunspots:

Solar Prominences

Sunspots are cool, but the gas above them is hot!

Earth

Solar ProminenceTypical size is 100,000 kmMay 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

Coronal activity increases with the number of sunspots.

The Proton-Proton Chain:

4 H4 H

HeHe

What makes the Sun shine?

Nuclear FusionNuclear Fusion

EE = = mm c c 22

((cc = speed of light) = speed of light)

But where does the Energy come from?

c2 is a very large number!

A little mass equals a LOT of energy.

Example:

1 gram of matter 1014 Joules (J) of energy.

Enough to power a 100 Watt light bulb for ~32,000 years!

Mass “lost” is converted to Energy:

Mass of 4 H Atoms =Mass of 4 H Atoms = 6.693 6.693 10-27

kg

Mass of 1 He Atom =Mass of 1 He Atom = 6.645 10-27

kg

Difference =Difference = 0.048 10-27

kg

((% m converted to E% m converted to E) = (0.7%)) = (0.7%)

EE = = mm c c 22

((cc = speed of light) = 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

Nuclear fusion requires temperatures of at least 107 K – why?

Atomic nuclei are positively charged they repel via the electromagnetic

force.

Merging nuclei (protons in Hydrogen) require high speeds.

(Higher temperature – faster motion)

At very close range, the strong nuclear force takes over, binding protons and neutrons together (FUSION).

Neutrinos are one byproduct.

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 gammy rays. These are transformed into visible and IR light by the time they reach the surface (after interactions with particles in the Sun).

Solar Neutrino Problem: There are fewer observed neutrinos than theory predicts (!)

A discrepancy between theory and experiments could mean we have the Sun’s core temperature wrong.

But probably means we have more to learn about neutrinos! (Neutrinos might “oscillate” into something else, a little like radioactive decays…)

Detecting Solar Neutrinos – these light detectors measure photons emitted by rare chlorine-neutrino reactions in the fluid.