Visible Image of the Sun The 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?

Made of … 71% Hydrogen 27% Helium 2 % heavier elements Information know from sun spectroscopy

Main Regions of the Sun

Radius = 696,000 km (100 times Earth) Mass = 2 x kg (300,000 times Earth) Av. Density = 1410 kg/m 3 Rotation Period = 24.9 days (equator) 29.8 days (poles) Surface temp = 5780 K Solar Properties The Moon’s orbit around the Earth would easily fit within the Sun!

Calculating the mass of the Sun m= 4  2 a 3 G P 2

m= 4  2 a 3 G P 2 a= orbit radius in AU (convert AU to meters) 1 AU = 1.5x10 11 m

m= 4  2 a 3 G P 2 G=gravitational constant G= 6.67x m 3 kg sec 2

P=revolution period (time) in seconds m= 4  2 a 3 G P 2

Example #1 On Earth m= 4  2 a 3 G P 2 a= orbit radius in AU (convert AU to meters) 1 AU = 1.5x10 11 m G=gravitational constant G= 6.67x m 3 kg sec 2 P=revolution period (time) in seconds

Example #2 Pg. 346 Problem #2 m= 4  2 a 3 G P 2 a= orbit radius in AU (convert AU to meters) 1 AU = 1.5x10 11 m G=gravitational constant G= 6.67x m 3 kg sec 2 P=revolution period (time) in seconds

Example #3 SATURN 9.6 AU years m= 4  2 a 3 G P 2 a= orbit radius in AU (convert AU to meters) 1 AU = 1.5x10 11 m G=gravitational constant G= 6.67x m 3 kg sec 2 P=revolution period (time) in seconds

Luminosity of the Sun = L SUN (Total light energy emitted per second) ~ 4 x W 100 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 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 ~1 km/sec (2200 mph; Doppler).

Chromosphere

Chromosphere (seen during full Solar eclipse)  Chromosphere emits very little light because it is of low density  Reddish hue due to emission from Hydrogen

SOHO SPECTROHELIOGRAMS TODAY

Solar Wind & Corona

Corona (seen during full Solar eclipse) Hot coronal gas escapes the Sun  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. CME Video & Activity

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 Corona is HOTTER than the photosphere

Most of the Solar luminosity is continuous photosphere emission. But, there is an irregular component The Active Sun UV light

Sunspots (video) Granulation around sunspot

Sunspots Typically about 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… what’s really happening? Typical Sunspot seen in the visible light spectrum

Sunspots… what’s really happening? Same image seen using x-rays

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 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.

Solar Flare and Resulting Prominence

Solar Flare, Prominence and Filament

Coronal activity increases with the number of sunspots.

SOLAR-TERRESTRIAL RELATIONSHIPS AURORAE

The Proton-Proton Chain: 4 H He What makes the Sun shine? Nuclear Fusion

E = m c 2 ( c = speed of light) But where does the Energy come from?  c 2 is a very large number!  A little mass equals a LOT of energy. Example:  1 gram of matter  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 =  kg Mass of 1 He Atom = Mass of 1 He Atom =  kg Difference = (Binding Energy, ordinarily expressed in MeV) Difference =  kg (Binding Energy, ordinarily expressed in MeV) (% m converted to E) = (0.7%) (% m converted to E) = (0.7%) 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

 Nuclear fusion requires temperatures of at least 10 7 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! (Read “Solving the Neutrino Puzzle, on page 334) Detecting Solar Neutrinos Detecting Solar Neutrinos – these light detectors measure photons emitted by rare chlorine-neutrino reactions in the fluid.

Go to page 345 and answer questions: #1 – 11, 13-15