Structure & Function

Our Nearest Star

 Core  Comprises about 25% of sun’s interior  site of nuclear fusion  Radiative Zone  Energy produced in the core is carried outward by photons over LONG periods of time  Convective Zone  Energy is carried outward by convection It takes photons produced through fusion from 100,000 to 1,000,000 years to move from the core to the edge of the convective zone

 Lower atmosphere or “surface” of the sun  The only part of the sun we can normally see  Granulation  “Blotchy” appearance  Granules are convection cells in the photosphere  Brighter areas are hotter

 Visible during a solar eclipse or with the use of filters  Appears pink because the gas here only emits certain wavelengths of light, mostly red  “spiky” surface due to jets of gas that surge upward

 Extremely hot temperature and low density  Only seen using special filters (or during an eclipse)

 The immense electromagnetic bubble containing our solar system, solar wind, and the entire solar magnetic field.  It extends well beyond the orbit of Pluto, but within the Oort cloud.

 High-speed charged particles (mostly electrons and protons) constantly blowing off the Sun.  May be viewed as an extension of the outer atmosphere of the Sun (the corona) into interplanetary space. Computer image

 Can be seen on the granular photosphere  Sunspots are cooler than the photosphere at about 3800 K  Indicate magnetic disturbances on the sun.

 Sunspots reach a maximum about every 11 years  By tracking sunspots, astronomers have determined that it takes the Sun 27 days to rotate at the equator, but 31 days at the poles

 large regions of very dense ionized gas ejected from the photosphere and held in place by the sun’s magnetic fields  Return back to the surface of the sun along magnetic field lines

 Prominences may stretch 150,000 Km or more along the sun’s surface (10x the size of the Earth)

 short-term outbursts on the sun, caused by the sudden release of energy stored in twisted magnetic fields in the solar atmosphere.  release up to 1025 joules of energy—the energy equivalent of ten million volcanic eruptions.  They can last just a few minutes or up to several hours.

 Tremendous amounts of energy flung into space, including high-energy particles and electromagnetic radiation  When the radiation and particles reach the Earth's magnetic field, they interact with it to produce auroras.  Solar flares can also disrupt communications, satellites, navigation systems and power grids.

 This is an photograph of the Big Dipper shining through green-colored aurora in the skies above Washington State.  com/news/stunning-time-lapse- reveals-auroras-and-earth-from- space/ com/news/stunning-time-lapse- reveals-auroras-and-earth-from- space/

 Coronal mass ejections can carry up to 10 billion tons of plasma traveling at speeds as high as 2000 km/s.  Near solar maximum we observe an average of 2 to 3 CMEs per day  Thought to arise when the sun’s magnetic fields suddenly rearrange, releasing an enormous bubble of matter

Converting mass to energy

 Complete activity modeling nuclear fusion  Watch this video:   Answer the questions on your wkst

 The luminosity of a star is powered by nuclear fusion taking place in the centre of the star  The temperature and density are sufficient to allow nuclear fusion to occur.  Stars are primarily composed of hydrogen, with small amounts of helium.  They are so hot that the electrons are stripped from the atomic nuclei.  This ionized gas is called a plasma.

 At temperatures above 4 million Kelvin hydrogen nuclei fuse into helium

 The star is kept in a delicate balance between gravity trying to collapse it and radiation pushing it outwards.  As the hydrogen runs out, the energy released from fusion decreases and the gravity causes the star to collapse.  If the star is massive enough the core temperature increases until helium fusion starts.

 At temperatures above 100 million Kelvin helium can be fused to produce carbon. This reaction is called the “Triple Alpha process”

 Helium is fused with carbon to make heavier elements:  oxygen, neon, magnesium, silicon, sulphur, argon, calcium, titanium, chromium and iron  It’s impossible to make elements heavier than iron through nuclear fusion without putting in more energy.

 Eventually the helium is exhausted, and the star collapses again.  If it is massive enough, then the temperature increases enough to allow carbon fusion.  The cycle repeats, fusing heavier elements each time, until the core temperature cannot rise any higher.  At this point, the star “dies”.