Brain Teaser What makes our Sun shine bright? What life cyle do you believe our sun will undergo?

Visible Image of the Sun The Sun: Why is it important? 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.

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!

Energy Transport within the Sun The positively charged hydrogen nuclei in the core would then collide with one another with such tremendous forces that would allow them to fuse. This process, called nuclear fusion, results into the formation of helium. The energy released by nuclear fusion prevents the star from collapsing further. At this point, when nuclear fusion occurs, what was once a cloud of gas then becomes a star. In our case, the Sun.

Radiation Zone The radiation zone or radiative zone is a layer of a star's interior where energy is primarily transported toward the exterior by means of radiative diffusionradiative diffusion

Convection zone The convective zone, which is the final 30 percent of the sun's radius, is dominated by convection currents that carry the energy outward to the surface.

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).

 The solar spectrum has thousands of absorption lines  More than 67 different elements are present!  Hydrogen is the most abundant element followed by Helium (1 st discovered in the Sun!) The Solar Atmosphere Spectral lines only tell us about the part of the Sun that forms them (photosphere and chromosphere) but these elements are also thought to be representative of the entire Sun.

Electromagnetic Spectrum The electromagnetic spectrum from lowest energy/longest wavelength (at the top) to highest energy/shortest wavelength (at the bottom). (Credit: NASA's Imagine the Universe) wavelength

Chromosphere During a total solar eclipse the moon blocks light from photosphere. A reddish glow is visible.

Chromosphere (seen during full Solar eclipse)  Chromosphere emits very little light because it is of low density  Reddish hue due to 3  2 (656.3 nm) line emission from Hydrogen

Chromospheric Spicules: warm jets of matter shooting out at ~100 km/s last only minutes Spicules are thought to the result of magnetic disturbances H  light

Transition Zone and Corona During a total solar eclipse an even fainter layer of the sun becomes visible. This outer layer looks like a “halo”.. Gradually thins into streams of electrical charged particles called solar wind!

Transition Zone & Corona  Why does the Temperature rise further from the hot light source? We see emission lines from highly ionized elements (Fe +5 – Fe +13 ) which indicates that the temperature here is very HOT Very low density, T ~ 10 6 K  magnetic “activity” -spicules and other more energetic phenomena (more about this later…)

Corona (seen during full Solar eclipse) Hot coronal gas escapes the Sun  Solar wind

Solar Wind Particles that are normally blocked by earths atmosphere and magnetic field can enter through North and South Poles creating auroras- gas molecules glowing!!

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.

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

Most of theSolar luminosity is continuous photosphere emission. But, there is an irregular component (contributing little to the Sun’s total luminosity). The Active Sun UV light

Sunspots Granulation around sunspot

Sunspots Typically about km across.. They usually occur in groups. Huge reddish loops of gas called “Prominences” often link different parts of sunspot regions. 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 & 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 Flares – much more violent magnetic instabilities 5 hours Particles in the flare are so energetic, the magnetic field cannot bring them back to the Sun – they escape Sun’s gravity

Coronal activity increases with the number of sunspots.

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 = Difference =  kg (% 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! (Neutrinos might “oscillate” into something else, a little like radioactive decays…) Detecting Solar Neutrinos – these light detectors measure photons emitted by rare chlorine-neutrino reactions in the fluid.