Chapter 9 – The Sun •Our sole source of An image of the Sun and the large sunspot taken on October 22, light and heat in the 2014 solar system •A common star: a glowing ball of plasma held together by its own gravity and powered by nuclear fusion at its center. Nuclear fusion: Combining of light nuclei into heavier ones Example: In the Sun is conversion of H into He Plasma: Ionized material composed of electrons, protons and ions The Stellar balance The outward pressure (from heat caused by nuclear reactions) in the core balances the gravitational pull toward the Sun’s center. This balance is called Hydrostatic equilibrium This balance leads to a spherical ball of plasma, called the Sun. What would happen if the nuclear reactions in the core (“burning”) stopped? Main Regions of the Sun • Core • Radiation Zone • Convection Zone • Photosphere • Chromosphere • Transition Zone • Corona • Solar wind Radius of the Sun = 696,000 km (The thickness of the regions are not to scale) Radius = 696,000 km Solar Properties (100 times Earth’s radius) Mass = 2 x 1030 kg (300,000 times Earth’s mass) Av. Density = 1410 kg/m3 Rotation Period = 25 days (equator) 36 days (poles) Surface temp = 5780 K The Moon’s orbit around the Earth (Radius around 385,000 km) would easily fit within the Sun! Luminosity: Total light Luminosity of the Sun = LSUN energy emitted per second (Power) 26 LSUN ~ 3.96 x 10 W Watt (W) is a unit of power. Power is energy emitted per unit of time. Joule is a unit of energy 1 W = 1 Joule/sec d How do we determine the luminosity of the Sun? - First, we measure the amount of power received from the Sun at the Earth per squared meter per second. This is power in W/m2 It is called the Solar constant = 1400 W/m2 -Second, we multiply this by the surface of a sphere of radius d (4d2 ), where d is the distance between the Earth and Sun (1 AU, ~ 150 million km). In other words, we “integrate” the power over the whole sphere -We assumed here that the Sun emit the same amount of energy in all directions. The standard solar model The Standard Solar Model The temperature of the core must be least 10 million K in order to be able to convert H into He. The Sun’s central core temperature is about 15 million K The temperature of the layer that we see from the Sun (Photosphere) is about 6,000 K 1 g/cm3 = 1000 kg/m3 Energy Transport within the Sun • Extremely hot core , 10-15 million K. All the matter is completely ionized (plasma) • Radiation zone The temperature is so high that no electrons are left on the atoms to be able to capture photons – radiation zone is transparent to light. Energy here is transported by radiation • Convection zone Temperature falls further away from the core – at lower temperatures, more atoms are not completely ionized. The electrons left in the atoms can capture photons – The gas becomes opaque to light. Energy is transported here by convection • Farther out, the low density in the photosphere makes it transparent to light - radiation takes over again Solar Granulation: Evidence of 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 rises/sinks at a rate ~1 km/sec (2200 mph) . Detected by Doppler effect. The Solar Atmosphere . The solar spectrum has thousands of absorption lines (The scale is wavelength in nanometers ) . More than 67 different elements are present! . Hydrogen is the most abundant element followed by Helium (1st discovered in the Sun!) Spectral lines only tell us about the composition of the part of the Sun that forms them. But these elements are also thought to be representative of the entire Sun. The composition of the Sun The chromosphere and the photosphere The chromosphere can only be seen in a total solar eclipse when the size of the disk of the moon is slightly larger than the disk of the Sun so it will block the light from the photosphere The layer of the Sun that we see is the photosphere. The photosphere has higher temperature (5,800 K) and higher density . The chromosphere has lower temperature (4,500 K) and lower density • The photosphere forms the continuous spectrum • The chromosphere produce the absorption lines. (Remember Kirchhoff ‘s laws) Transition Zone and Corona Transition Zone & Corona The Corona has very low density but high temperature T ~ 106 K From the corona we see emission lines from highly ionized elements (Fe+5 – Fe+13) which indicates that the temperature here is very HOT Why does the Temperature rise further from the hot light source? magnetic “activity” - spicules and other more energetic phenomena (more about this later…) Corona (seen only during total Solar eclipse) Because the coronal plasma has high temperature (1,000,000 K), it escapes the gravitational attraction of the Sun Solar wind Solar Wind Solar Wind The radiation (light or electromagnetic waves) emitted by the Sun travel at the speed of light and take about 8 minutes to reach Earth. The plasma (electrons, protons and ions) ejected from the Sun travel slower, ~500 km/s and take a few days (~ 3 days) to reach the Earth Solar coronal plasma has enough temperature (kinetic energy) to escape the Sun’s gravity. This stream of particles ejected from the Sun is called the solar wind Radiation and fast moving particles (electron and protons) continuously leave the Sun . The Sun is evaporating via this “wind” 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 plasma (~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. Open magnetic field line generate the coronal holes CME: Coronal Mass Ejection Ejection of plasma through the coronal holes An example of a coronal hole showing the magnetic field lines structure Coronal hole An example of a CME The animation was recorded by the SOHO (Solar Heliospheric Observatory) spacecraft Sunspots Granulation around sunspot Umbra: dark center of sunspot Penumbra: grayish area around the umbra Sunspots • Size typically about 10,000 km across • At any time, the Sun may have hundreds (around solar sunspot maximum) or none (around a solar sunspot minimum) • Dark color because they are cooler than photospheric plasma (4,500 K in darkest parts, compared to 5, 800 K in the photosphere.) • Each spot can last from a few days to a few weeks or a month • Galileo observed these spots and realized the Sun is rotating differentially (faster at the equator, slower at the poles) Rotation of the Sun: An animation Sunspots & Magnetic Fields •The magnetic field in a sunspot is 1000 times strongest 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. They have opposite polarity in north and south hemisphere Why the sunspots have lower temperature? • The charged particles in the plasma (electrons, protons and ions) from the solar atmosphere interact with the magnetic field and prevent plasma to reach the sunspot zone. A charged particle in a magnetic field will follow helical trajectories. • The plasma in and around the sunspot radiates energy and cool off. • The temperature of a sunspot is around 4,500 K. The temperature of the photosphere is around 5,800 K Why the sunspots look darker? The ratio of the flux F between the photosphere (Fph) and the sunspot (Fss) can be calculated by the Stefan’s Law formula: Fph/Fss = (Tph/Tss)^4 Fph/Fss = (5800/4500)^4 Fsp/Fss =2.76 The photosphere emit 2.76 times more flux than the sunspots The Sun’s differential rotation distorts the magnetic field lines Minimum of Maximum of sunspot cycle sunspot cycle The plasma is rotating and drags the magnetic filed lines. The twisted and tangled field lines occasionally get kinked, causing the field strength to increase A “tube” of lines bursts through atmosphere creating sunspot pair Sunspot Cycle and Solar Cycle . During a solar maximum there is an increase of solar radiation, ejection of solar material, sunspots numbers and flares . Solar maximum is reached every ~11 years ~ 11 years . The Sunspot cycle last for about 11 years . The Solar Cycle is 22 years long. The direction of the magnetic field polarity of the sunspots flips every 11 years (back to original orientation every 22 years) The sunspot number last on average about 11 years but occasionally sunspots may disappear (sunspot number drop to low or zero value over several years) as it happened between 1645 and 1715. This is called the Maunder minimum This period of 70 years of minimum sunspot activity coincided with a period of cold temperatures called the Little Ice Age A recent plot of the sunspot numbers including data until 2018 Some sunspot cycle have two maximum. Heating of the Corona Charged particles (mostly protons and electrons) follow helical path and are accelerated along magnetic field “lines” above sunspots. This type of activity, not light energy, heats the corona. Charged particles follow magnetic fields between sunspots: Solar Prominences Sunspots are cool, but the gas above them is hot! Solar Prominence Typical size is 100,000 km May persist for days or weeks Earth Very large solar prominence (1/2 million km across base, i.e.