1 March 2005AST 2010: Chapter 14 1 The Sun: A Garden -Variety Star

1 March 2005AST 2010: Chapter 142 The Sun Biggest object in the solar system diameter 1,392,000 km 109 x Earth’s diameter 10 x Jupiter’s diameter Most Massive 333,000 x Earth’s mass 1,000 x Jupiter’s mass so heavy, everything else orbits around it! so heavy, it makes its own heat and light Temperature of 15,000,000 K in its core nuclear power!

1 March 2005AST 2010: Chapter 143 The Sun’s Profile

1 March 2005AST 2010: Chapter 144 The Sun’s Composition The Sun contains the same elements as the Earth, but not in the same proportions About 73% of the Sun’s mass is comes from hydrogen, and another 25% from helium Other chemical elements make up the rest 2% The fact that the Sun are mostly made up of H and He was first shown by Cecilia Payne-Gaposchkin The 1 st woman to get a PhD in astronomy in the U.S. ElementsFraction Hydrogen92.1% Helium7.8% Oxygen0.061% Carbon0.030% Nitrogen0.0084% Neon0.0076% Iron0.0037% Silicon0.0031% Magnesium0.0024% Sulfur0.0015% All others0.0015%

1 March 2005AST 2010: Chapter 145 The parts of the Sun

1 March 2005AST 2010: Chapter 146 The Sun's Interior From inside out: Core Radiative zone Convection zone Photosphere Chromosphere Transition region Corona

1 March 2005AST 2010: Chapter 147 The Sun's Core The core is the innermost 10% of the Sun's mass generates energy from nuclear fusion has the highest temperature and density temperature 10 million K density = 160 x density of water = 20 x density of iron at this temperature, the core is a gas no molten interior

1 March 2005AST 2010: Chapter 148 How does Heat from the Core Reach Us? Three ways to transfer heat: Conduction: direct contact A spoon in a hot cup of coffee gets warm Convection: moving currents in a fluid Hot air rises Hot current in boiling water Radiation: electromagnetic waves emitted by a heat source and absorbed by a cooler material Electric stove Heat from the Sun reaches us through the EM waves it emits

1 March 2005AST 2010: Chapter 149 Radiative Zone Radiation transfers heat from the interior of the Sun to its "cooler" outer layers The core & radiation zone make up 85% of the Sun The temperature drops from 10 million K at the inner side of the radiative zone to 2 million K at its edge The energy generated in the core is carried by photons that bounce from particle to particle through the radiative zone The photons are too energetic to be absorbed by atoms Each photon bounces so many times that it is estimated to take one million years to reach the outer edge of the region

1 March 2005AST 2010: Chapter 1410 Convection Zone Matter at the base of the convection zone is “cool” enough (2 million K) for the atoms to absorb energy and hold on to it Convection occurs in this region The hotter material near the top of the radiation zone (the bottom of the convection zone) rises while the cooler material sinks — heated below like a pot of boiling water It takes a week for the hot material to carry its energy to the top of the convection zone radiative zone hot cool convection zone

1 March 2005AST 2010: Chapter 1411 Photosphere This is the Sun’s deepest layer that one can see from the outside Photosphere means “light sphere” It is the visible “surface” of the Sun From this layer, photons can finally escape to space The surface is not something one could land or float on The photosphere is about 500 km thick The gas is so dense that you could not see through it The gas emits a continuous spectrum of light It features sunspots

1 March 2005AST 2010: Chapter 1412 Temperature of Photosphere The photosphere temperature is about 5,800 K The sunspots appear darker because they are cooler than their surroundings The center of a typical sunspot has a temperature of 4,000 K The spectrum and energy output of the radiation emitted from the photosphere obey Wien’s Law and Stefan-Boltzmann law

1 March 2005AST 2010: Chapter 1413 Features of Photosphere Sunspots dark spots, 1500 K, cooler than surroundings glow by themselves granules tops of convection cells 700 to 1000 km diameter last 10 minutes centers ~ 100 K hotter than edges

1 March 2005AST 2010: Chapter 1414 Sunspots Discovered by Galileo Galilei Sun's surface sprinkled with small dark regions - sunspots Sunspots are darker because they are cooler by 1000 to 1500 K than the rest of the photosphere Spots can last a few days or as long as a few months Galileo used the longer-lasting sunspots to map the rotation patterns of the Sun Sunspots number varies in a cycle with an average period of 11 years Cycle starts with minimum and most of them are at around 35° from the solar equator At solar maximum (number peaked), about 5.5 years later, most of the sunspots are within just 5° of the solar equator

1 March 2005AST 2010: Chapter 1415

1 March 2005AST 2010: Chapter 1416

1 March 2005AST 2010: Chapter 1417 Sunspots and Magnetic Field Sunspots = regions of strong magnetic fields Found by observation of Zeeman effect

1 March 2005AST 2010: Chapter 1418 Sun Rotates Galileo discovered sunspots sunspots moved  sun rotates Rotation – speed depends on latitude equator once/25 days 30º N once/26.5 days 60º N once/30 days Jupiter also does this

1 March 2005AST 2010: Chapter 1419

1 March 2005AST 2010: Chapter 1420 Chromosphere Visible during solar eclipses as a thin pink layer at the edge of the dark Moon Colorful layer – “color sphere” Color due to hydrogen bright emission line Also shows yellow emission due to helium – discovered in 1868 – new element previously not seen on Earth Helium was found on Earth in 1895 The chromosphere is only 2,000 to 3,000 km thick Temperature rises outward away from the photosphere – from 4,500 K to 10,000 K

1 March 2005AST 2010: Chapter 1421 Transition Region It’s a thin region (about 10 km thick) in the Sun’s atmosphere where temperature changes from 10,000 K to nearly 1,000,000 K

1 March 2005AST 2010: Chapter 1422 Solar Weather The Sun has complex and violent weather patterns Chromosphere contains jet-like spikes of gas – called spicules Spicules rise vertically through the chromosphere Last 10 minutes Consist of gas jets, at 30 km/s Rise to heights of 5000 to km T ~ chromosphere

1 March 2005AST 2010: Chapter 1423 Corona (1) The outermost part of the Sun’s atmosphere is called the corona It is visible during total solar eclipses as a pearly-white glow around the dark Moon The corona has a very high temperature of ~1-2 million K It is known to be very hot because it contains multiply ionized atoms At very high temperatures, atoms like iron can have 9 to 13 electrons ejected (the atoms become ionized) 9-times ionized iron is only produced at a temperature of 1.3 million K 13-times ionized iron means the temperature gets up to 2.3 million K! Total solar eclipse in 1973

1 March 2005AST 2010: Chapter 1424 Corona (2) Most of the corona is trapped close to Sun by loops of magnetic field lines In X-rays, those regions appear bright Some magnetic field lines do not loop back to the Sun and will appear dark in X-rays These are called coronal holes More details visible at short wavelengths A solar eclipse photographed in the extreme ultraviolet taken by the SOHO spacecraft

1 March 2005AST 2010: Chapter 1425 X-rays from the Corona

1 March 2005AST 2010: Chapter 1426 Prominences Bright clouds of gas forming above the sunspots Quiet prominences 40,000 km above surface Last days to several weeks Eruptive prominences 700 km/s Rare Surge prominences Last up to a few hours Shoot gas up to 300,000 km Gas speed ~1300 km/s

1 March 2005AST 2010: Chapter 1427 Prominences follow magnetic-field loops

1 March 2005AST 2010: Chapter 1428

1 March 2005AST 2010: Chapter 1429

1 March 2005AST 2010: Chapter 1430 Solar Flares Solar flares are eruptions more powerful than surge prominences Flares last from a few minutes to a few hours A lot of ionized material is ejected in a flare Unlike the material in prominences, the solar-flare material moves with enough energy to escape the Sun's gravity When such a burst of ions reaches the Earth, it interferes with radio communication Sometimes a solar flare will cause voltage pulses or surges in power and telephone lines Brownouts or blackouts may result Humans traveling outside the protection of the Earth's magnetic field will need to have shielding from the powerful ions in a flare

1 March 2005AST 2010: Chapter 1431

1 March 2005AST 2010: Chapter 1432 Solar Wind Fast-moving charged particles (mostly protons and electrons) can escape the Sun's gravitational attraction The stream of particles is called the solar wind They move outward at a speed of about 400 km/s They can reach the farthest reaches of the solar system Solar-wind particles passing close to a planet with a magnetic field are deflected around the planet Some are deflected to the planet's magnetic poles As the particles hit the planet's atmosphere, they cause the molecules in the atmosphere to produce beautiful curtains of light called the auroras Aurora borealis in the northern hemisphere Aurora australis in the southern hemisphere

1 March 2005AST 2010: Chapter 1433 Is the Sun a Variable Star? What is more certain than that the Sun will rise tomorrow? We’ve already seen that sunspots follow an 11-year cycle On longer time scales, the Sun undergoes changes in overall activity Changes are only about 0.1%! Yet this is enough to affect our climate In the mid 1600’s the Sun’s output was particularly low  the “Little Ice Age” Other stars are seen to vary by 0.3%, up to 1%