Plumes of hot gas shoot across the surface on an average day on the Sun as shown in dramatic detail by the Japanese Hinode satellite. Near the horizon, active regions around a sunspot eject hot plasma along the magnetic field lines that connect the sunspot to surrounding regions in the solar atmosphere. Bright regions are hotter and more active. The bubbling granularity and activity of the Sun's photosphere is visible in the foreground. This frame covers a solar region of about 8,000 kilometers.

THE SUN The source of light, heat and (nearly all) energy on the earth.

Diameter: 1,390,000 km (4.6 light seconds) Mass: x kg (333,000 times Earth’s mass) Temperature: 5800 K (“surface”) 15,600,000 K (core) > 99.8% of the Solar System’s mass (Jupiter has most of the rest) Hydrogen 92.1%, Helium 7.8%, other elements: 0.1%

One million Earth’s would fit into a hollow Sun

The Sun has existed for 4.6 billion years. Where does all the Sun’s energy come from? A long history of speculation has preceded our understanding of how the Sun generates energy.

We can constrain the types of energy sources that might fuel the Sun by comparing its age with the length of time various sources of energy are able maintain the Sun’s energy output, i.e., its luminosity.

CHEMICAL ENERGY (Is it on fire)? NO. Coal or more efficient fuels would last the Sun less than 10 6 years. However, geological evidence that the earth (therefore, presumably the Sun) is over 10 8 years old has been convincing since the late 1800's.

GRAVITATIONAL ENERGY (from shrinking)? NO. An object such as a star, planet, or cloud of gas converts gravitational potential energy into other forms of energy as it shrinks. This process provides most of the excess heat from Jupiter and Saturn, but would last fewer than 10 8 years for the Sun.

NUCLEAR ENERGY (from nuclear fusion)? YES! The Sun and all stars generate energy through nuclear fusion. If the mass of an atomic nucleus is LESS THAN the SUM of the masses of two nuclei jammed together to produce that atom, then the difference between initial masses and the final mass is converted to lots of ENERGY, via E = mc 2

Fusion of hydrogen into helium in the core of the Sun provides sufficient energy to power the Sun for 10 billion years. Since the Sun is approximately 4.5 billion years old, it is half- way through its life.

FUSION: Small nuclei stick together to make a bigger one. This is the energy source of stars. There are two types of nuclear reactions FISSION: Big nucleus splits into smaller pieces. This happens with radioactive isotopes and is the basis of power from nuclear power plants. In both fission and fusion, the mass of the product(s) produced by the reaction is less than the mass of the particle(s) before the reaction. This difference in mass, m, is converted to energy: E = mc 2.

Sun releases energy by fusing four hydrogen nuclei (protons) into one helium nucleus. The Proton–proton chain is how hydrogen fuses into helium in Sun

The proton-proton (pp) chain

IN 4 protons OUT 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower.

Fusion Requires Very high temperatures (> 15 million Kelvin in the core of the Sun) to provide high enough velocities to overcome electrical repulsion Very high densities to make collisions frequent At low speeds, electromagnetic repulsion prevents the collision of nuclei At high speeds, nuclei come close enough for the strong force to bind them together

The temperatures and densities required for the proton-proton chain only exist in the core of the Sun.

Why is the core of the Sun so hot and dense? Because the Sun is in “hydrostatic equilibrium” (sometimes referred to as “gravitational equilibrium”)

Hydrostatic equilibrium: The condition where gravitational forces seeking to shrink an object are precisely balanced by pressure seeking to expand an object.

Hydrostatic (Gravitational) equilibrium: Energy provided by fusion maintains the pressure.

Gravitational contraction… provided energy that heated the core as the Sun was forming. Contraction stopped when fusion began replacing the energy radiated into space.

Feedback If nuclear fusion rates varied, so would the Solar temperature –Need a feedback mechanism to keep this in check Rate of nuclear fusion is very sensitive to temperature –Suppose the core temp rose: fusion rate would increase, pressure would push the core apart, make it larger, cooling it down

Solar Thermostat Decline in core temperature causes fusion rate to drop, so core contracts and heats up Rise in core temperature causes fusion rate to rise, so core expands and cools down

What is the Sun’s structure?

Core: The innermost 20% by radius. Essentially all of the sun's energy is produced by fusion reactions in the solar core, in the region where the temperature ranges from ~ 6 to 15 million K

Radiative zone: No energy is produced here, but the huge power generated in the core is carried outwards by photons, whose average energy slowly decreases from X-ray into UV as the temperature and density slowly decline.

Convective zone: While photons are still wending their ways outwards through this relatively low density region, the only way all of the luminosity can be carried out is if blobs of hot plasma flow outward and colder blobs sink inward.

Convective Zone: Convection (rising hot gas) takes energy to the surface.

Granules: the top of the convection zone. At the centers of granules hot solar gas rise and rapidly radiate heat into space; the gas then is diverted horizontally, and sinks back into the Sun in the darker inter-granular lanes.

Time lapse series of solar granulation (35 minutes). Sizes of the granules: km, with an average diameter of 1300 km. Lifetimes of granules: typically 8 to 15 minutes. Horizontal and vertical velocities of the gas motion: 1 to 2 km/s.

Photosphere: Visible surface of Sun ~ 6,000 K

Chromosphere: Middle layer of solar atmosphere ~ 10 4 –10 5 K

Corona: Outermost layer of solar atmosphere ~1 million K

Solar wind: A flow of charged particles from the surface of the Sun

How do we know what is happening inside the Sun? making mathematical models. observing solar vibrations. observing solar neutrinos.

Helio-seismology: Patterns of vibration on the surface of the Sun tell us what the interior of the Sun is like. This is very similar to how seismology tells us what the interior of the Earth is like.

Data on solar vibrations agree with mathematical models of solar interior.

Neutrinos interact very weakly with matter. Neutrinos created in the core during fusion immediately escape from the Sun. Observations of these solar neutrinos can tell us what’s happening in the core.

Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number.

Solar neutrino problem: More recent observations find the right number of neutrinos, but some have changed form. There are three types of neutrinos; the early searches could only detect one of these forms.

What causes solar activity?

Solar activity is like “weather” All are related to magnetic fields. Solar prominences Solar flares Sunspots

Sunspots … Regions with strong magnetic fields Cooler than other parts of the Sun’s surface (4,000 K) and thus not as bright as their surroundings. Recall: Cooler objects emit less radiation per surface area.

Sun Spots 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.

How do we know they have strong magnetic fields?  A magnetic field can cause spectral lines to be split into several components (the process is called the “Zeeman effect”).  The details of the splitting depends upon the strength and geometry of the magnetic field.  Examination of spectral lines from the surface of the Sun shows that the magnetic field varies with location on the Sun.  This same effect is important in MRI (magnetic resonance imaging) done in medical tests.

We can measure magnetic fields in sunspots by observing the splitting of spectral lines Zeeman Effect The strong magnetic field inside a sunspot splits that line into multiple lines Outside a sunspot, a single spectral line

Loops of bright gas often connect sunspot pairs, leading one spot to have one magnetic polarity and the other the opposite polarity.

Magnetically Linked Spots

solar flare: a violent explosion in the sun’s atmosphere Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space.

An active region on the Sun that produced a powerful flare shown over a period of about four hours. The glowing gas flowing around the relatively stable magnetic field loops above the Sun's photosphere has a temperature of over ten million degrees Celsius. These flows occurred after violently unstable magnetic reconnection events above the Sun produced the flare.

Magnetic activity also causes solar prominences that erupt high above the Sun’s surface.

The corona appears bright in X-ray photos in places where magnetic fields trap hot gas.

An active region on the Sun that produced a powerful flare shown over a period of about four hours. The glowing gas flowing around the relatively stable magnetic field loops above the Sun's photosphere has a temperature of over ten million degrees Celsius. These flows occurred after violently unstable magnetic reconnection events above the Sun produced the flare. How does solar activity affect humans?

Charged particles streaming from the Sun can disrupt electrical power grids, disable communications satellites, and cause aurora (“northern and southern lights”).

Coronal mass ejections send bursts of energetic charged particles out through the solar system.

How does solar activity vary with time?

The number of sunspots rises and falls in 11-year cycles.

The sun rotates faster at its equator than away from the equator. The sunspot cycle arises from the winding and twisting of the Sun’s magnetic field produced by this differential rotation.

Active Sun: Coronal Loops Coronal loops are tracing magnetic field lines in the corona Coronal loops are rooted in the photosphere, one end with positive polarity, and the other end negative polarity