Our Star, The Sun
Solar Formation
The Sun formed from the Solar Nebula
Theorized by Kant and Laplace
A vast cloud of gas and dust
Mass slightly larger then our sun
Nebula initially in equilibrium
Needs a contracting mechanism
Contraction causes matter to concentrate in the center
Called the Protosun
Protosun eventually forms into the sun
Planets form in the sparser outer regions of the nebula
Total mass of planets is only 1% of the sun’s mass
Kelvin-Helmholtz Contraction Theory
Gravitational energy is converted to thermal energy
Gravity causes objects to accelerate
KE converted to thermal energy
Causing temp to increase inside the nebula
Not the major sauce of the Sun’s energy
Could only be 25 Million years old
Geological and fossil records are much older
Chemical energy heating (burning fuel)
Sun contains about 1057 atoms
Chemical reactions release about 10-19 j per atom
Sun emits 3.9 x 1026 j/s
Thus requires 3.9 x 1045 atoms per second
Only have enough atoms for 3 x 1011 seconds = 10,000 years
Earth much older
Nuclear Fusion
After about 107 years of contracting the center reaches a few 106 degrees K
The Sun’s low density requires the sun to be made of light particles
Made of mostly Hydrogen and Helium
Temp of sun easily ionizes the atoms
Find H nuclei and electrons in center of the protosun
Core temp high enough to fuse hydrogen nuclei
called hydrogen burning or thermonuclear fusion
Einstein’s Mass – Energy Equation
E = mc2
E = energy released
M = mass converted
c = speed of light (3 x 108 m/s)
A small amount of matter can release an enormous amount of energy.
Most of the energy released in thermonuclear fusion appears as gamma rays.
Electron – positron disintegrations also create gamma rays in the sun.
Example:
4 hydrogen atoms = 6.693 x 10-27 kg
-1 helium atom = -6.645 x 10-27 kg
mass lost = 0.048 x 10-27 kg
About 0.7% of the mass is converted to energy
E = mc2
E = (0.048 x 10-27 kg) x (3 x 108 m/s)
E = 4.3 x 10-12 j of energy per formation of one He atom
This is about 107 times larger then the energy released in a chemical reaction.
Stellar Structure
Hydrostatic Equilibrium is reached
Inward gravitational force = Outward pressure force
Else star would collapse
Three Main Outer Layers
Photosphere – Lowest layer (Visible Surface)
Chromosphere – Middle layer
Corona – Upper layer
Everything below the photosphere is called the Solar Interior
99% of the sun is composed of H & He
Photosphere
Bright visible surface (From which light escapes)
Shell of hot, opaque gas
Can’t see through this layer
300 – 400 km thick
Completely transparent above
About 4500 K; 99.5 % of photons escape
Completely opaque below
About 6000 K; 4% of the photons escape
Outside rim of photosphere defines the diameter of the sun
Pressure in photosphere about 10% of the sea level pressure on Earth.
Place where the continuous spectrum produced
Granulation
Pattern of light and dark areas
Light areas hotter then dark areas
Pattern changes with time
Looks like boiling gas
Mat’l bubbling out of bright areas
5800 K
Chromosphere
Color Sphere
Lies immediately above the photosphere
Reddish glow seen during solar eclipse
Coronagraph – an occulting disk
Creates most absorption lines found in solar spectrum
About 2500 km thick
Density of chromosphere decreases with altitude
Temp increases from 4500 K to 10,000 K with altitude
Transition Region
Temp increases from 10,000 K to about one million K
About 20 to 40 Km thick
Spicules
Needlelike filamentary structures
Transport heat
Rise and fall in about ten minutes
Speed ~ 30 km/s
Reach heights of 5000 and 20,000 km above photosphere
Sun’s lowest temp at base of chromosphere ~ 3000 to 3500 K
Temp rises to 30,000 K at upper levels
Continues to rise to nearly 800,000 K
The Chromosphere can be seen during a solar eclipse.
Corona
Outer most layer of the Sun
Extends millions of kilometers above the photosphere
Tenuous layer of gas ~109 atoms/cc (Earth sea level ~ 1019 atoms/cc)
Causes light to be scattered in our direction
About one millionth as bright as the sun
Temp in access of one million K
Shape changes from month to month
Spherical at sunspot maximum
Hypothetical element Coronium (Unknown spectral lines seen)
Solved in 1930’s – highly ionized atoms produced obs spectrum
The composition of the sun as derived from spectroscopic techniques refers to the outer most layers
The sun radiates energy from the photosphere
The energy originates from the interior
The interior must be hotter or the energy would flow inward
Most of the sun’s energy originates from within the core
Core ~ 10% of the volume of the star
Energy output
The Solar Wind
Escaping coronal gases
Consists of mostly protons and electrons
Travel at about 400 km/s (Measured at 450 km/s at the Earth)
Reaches Earth in about five days
Detected at the outer planets (about 300 km/s)
First detected by its effects on comets
Sun loses about 107 tons of material/yr
Solar wind comes mainly from coronal holes
Coronal Holes
Large, relatively quiet regions of the corona
Usually found at the polar regions
Extremely low density
Cause of observed empty space in picture below
Source of solar wind
Earth’s Magnetic Field protects us
Strong solar activity trigger gusts of solar winds
Causing dramatic auroral displays
Granulation
Granules typically 700 km to 1000 km in diameter
Motion verified by the Doppler shift
Bright granules are rising columns of gas (v~2 – 3 km/s)
Intergranular regions about 50 to 100 K cooler
Cooler gas falls back into the interior and is the reheated
Heating by convection
Overlapping sunspots; Umbra & Penumbra; Granulation
Sunspots
Can be seen with unaided eye (with sufficient haze)
Do not look at the sun, can cause eye damage!
Cooler then the surrounding gasses by about 1500 K
Sunspots would glow if extracted and placed in space
Lifetimes of a few hours to a few months
Move with the photosphere surface
Rotation rate of sun about 25 to 30 days; depending on latitude
The number of sunspots changes year to year
11 year sunspot cycle
Individual sunspots are short lived but the total number of sunspots varies in an 11 year cycle
The Zeeman Effect
The strength of the sun’s magnetic field is measured by observing the spectral line separation. Magnetic fields cause atomic energy levels to split proportionally to the strength of the magnetic field.
Plages
Bright clouds in the chromosphere
Located near sunspots
Often appearing prior to and above a sunspot
Regions of higher temp and density (relative to the chromosphere)
Seen when observing using Ca or H filters
Contain all the elements in the sun but H and Ca most easy to see
Prominences
Distinctive shape results from looping magnetic field lines
Once were mistaken as filaments
Extend from the photosphere into the corona
Similar but smaller objects are spicules
Last from hours to days and can extend to heights of about 106 km
Solar Flares
The most violent event on the surface of the sun
Largest called Coronal Mass Ejections
Can last for several hours
Emit enough energy to power the US for 105 years
Typically lasts from 5 to 10 minutes
Equivalent to 106 hydrogen bombs
Occurs most often during sunspot maximum
Active Regions
Associated with magnetic fields
Include sunspots and flares,
Located near sunspots
Variations in the Number of Sunspots
Clear 11 year variations
1645 – 1715; period of low sunspot activity
Called the Maunder Minimum
High sunspot activity yields
High aurora activity on Earth
Greater chance the sun will eject material
Strong solar magnetic fields shielding the Earth from cosmic rays
Cosmic Rays are high energy charged particles; protons, a, b part.
The data for the above sunspot table was collected using radioactive isotope C14 studies.
The shape of the corona changes from spherical to irregular
Solar Variability and the Earth’s Climate
Strong historical evidence linking solar activity to Earth’s climate
The Maunder Minimum was a time of low temp in Europe
Little Ice Age
Cool global climate from 1400 – 1510
Period of low solar activity
Patterns of human migration seem to follow solar activity
Evidence suggests that on average the Earth is cooler during low solar activity
Why? Does the Sun emit less energy?
The sun is 0.1% brighter during solar maximum
Problem – Sun should be fainter when covered by many sunspots
Answer? Get more UV causing Ozone and then temp to increase
An ejected solar prominence
The solar interior
Mapping the solar magnetic field
The field guides the out flowing particles which emit radio waves indicating the position of the field.
Spicules are shown in the chromosphere of the sun.
Spicules are jets of gas that surge upwards into the sun’s corona.