The Sun

Our Nearest Stellar Neighbor

• Officially known as Sol, as in Solar System.

• Each star in the sky would be referred to as a sun by (hypothetical) inhabitants of the planets that orbit it.

• Like almost all stars, 74% H and 24% He, with trace amounts of other elements (2%).

Vital Statistics

• 93 million miles away,

• 864,000 miles in diameter (about 100 Earths across, and it would hold 1,000,000),

• 4.567 billion years old, using Uranium/Saramium radiometric dating.

• Good for another 5 billion.

fire Fire FIRE!• Not!• There is no fire on the Sun;• A fire of normal combustion with the Sun’s mass

would only last a few thousand years.• The source of the Sun’s energy is nuclear fusion

—similar to the fusion discussed earlier. • The Lawson Criteria hold.

Structure

• Largely due to the fusion process and the energies it produces.

• The core is where the fusion occurs,

• The radiative zone transmits heat to the,

• Convective zone which moves the heat to the Photosphere, the surface of the Sun we see.

Hydrostatic equilibrium

• The balance between the explosive force of the fusion at the center and the crushing gravity of its 2E30kg

• When it fails the Sun will change drastically

Heat Flow

• The core of the Sun is about 15 million degrees.• Using E = mc2, it converts around 6.5 million

tons of H into energy every second!• This is 400 million million million million Watts

of power, +/- 0.07%.• Visible light output varies to a small degree, but

the UV output varies greatly.• Sunspots a measure of that variability.

Outward

• Above the core, the pressure/temperatures aren’t sufficient for fusion, so heat is radiated through a layer of hot gas.

• Eventually, the pressures are low enough for “normal” convective fluid flow, which brings the heat to the surface, or photosphere.

At the Surface

• Many surface features due to the upwelling of heat.

• Particularly Sunspots and Prominences, as can be seen with a simple telescope and filter.

• Sunspots are magnetic storms that peak every 11 years, causing electromagnetic disturbances on the Earth.

• Streams of hot ionized gas that loop along the sunspot’s field lines are called prominances.

Details

• The hot gases of the Sun are electrically charged, so as it rotates magnetic fields are formed.

• Since the Sun isn’t solid like a planet, its equator rotates faster than its poles, stretching the magnetic field lines deep in the interior of the Sun.

• These get wrapped up, causing Sunspots.

In which century has sunspot activity been the greatest?

Perhaps a Global Connection?

And More Details…

• Sometimes the energy in a prominence is so great that it breaks free of the magnetic field and escapes into space, adding to the Solar Wind.

• The Solar Wind is a constant stream of charged particle blasting outward from the Sun.

• It is both a potential source of propulsion and dangerous radiation.

• The Sun’s atmosphere, called the Corona, is a superheated rarefied gas similar in composition to the Solar Wind.

Formation• The Sun condensed ~5 billion years ago out of an

enormous cloud of hydrogen, dust, and other gases called a Bok Globule.

• As the globule contracted due to gravity, angular momentum was conserved and the cloud spun faster and faster, becoming a protostar.

• Small bits of the protostar were cast out, like water in the spin cycle of a washer, to become the Solar System.

• Bands of this debris condensed, or accreted, into the planets, moons, and other denizens of nearby space.

• For 9 Gyr (billion years) the Sun is pretty much as it is now, though slowly increasing output

• At ~ 10Gyr the H in the core runs out and hydrostatic equilibrium fails

• The Sun expands out to beyond the orbit of Venus as a Red Giant

• Its He core contracts until it flashes into fusion at 100 million K.

• He burns in the core, H burns in a shell

• It will eject a large planetary nebula

• Left behind is its CNO core, known as a White Dwarf

The End of the Sun

• After 11 Gyr our Sun has ceased to do fusion

• About the size of the Earth, radiating due to gravitational heating

• Super dense!

• Will exist as a white dwarf for trillions of years, cooling off and becoming a black dwarf