THE SUN The star we see by day

Goals Summarize the overall properties of the Sun. What are the different parts of the Sun? Where does the light we see come from? The scientific method: solar neutrinos.

The Sun, Our Star The Sun is an average star. From the Sun, we base our understanding of all stars in the Universe. No solid surface.

Vital Statistics Radius = 100 x Earth (696,000 km) Mass = 300,000 x Earth (1.99 x 1030 kg) Surface temp = 5,800 K Core temp = 15,000,000 K Luminosity = 4 x 1026 Watts Solar “Day” = 24.9 Earth days (equator) 29.8 Earth days (poles) Which are observations? Which inferences?

Interior Properties Core = 20 x density of iron Surface = 10,000 x less dense than air Average density = Jupiter Core = 15,000,000 K Surface = 5800 K

Scientific Method How do we know this stuff? Three examples: Fusion in the core (core temperature). Different zones in interior. Solar activity and Earth

1. The Core Scientific Method: Observations Make hypothesis (a model) Models make predictions Test predictions Compare results of predictions with observations Revise model if necessary.

Testing the Core Observe Sun’s: Use physics to make a model Sun. Mass (how?) Composition (how?) Radius Luminosity (total energy output) Use physics to make a model Sun. Predict: Surface temp/density (how do you test?) Surface Luminosity (how do you test?) Core temp/density  Fusion Rate  neutrino rate (test?)

In The Core Density = 20 x density of Iron Temperature = 15,000,000 K Hydrogen atoms fuse together. Create Helium atoms.

Nuclear Fusion 4H  He The mass of 4 H nuclei (4 protons): 4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg The mass of He nuclei: = 6.643 x 10-27 kg Where does the extra 4.7 x 10-29 kg go? ENERGY!  E = mc2 E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2 E = hc/l  l = 4.6 x 10-14 m (gamma rays) So: 4H  He + light!

Concept Question How do the nuclear reactions in the Sun’s core produce energy? The mass of the product of the reaction is greater than the mass of the hydrogen atoms that enter the reaction. The mass of the product of the reaction is less than the mass of the hydrogen atoms that enter the reaction. The chemical potential energy of the product of the reaction is greater than the chemical potential energy of the hydrogen atoms that enter the reaction. The chemical potential energy of the product of the reaction is less than the chemical potential energy of the hydrogen atoms that enter the reaction. B. The mass of the product of the reaction is less than the mass of the hydrogen atoms that enter the reaction.

2. Helioseismology Continuous monitoring of Sun. Ground based observatories One spacecraft (SOHO) Surface of the Sun is ‘ringing’ Sound waves cross the the solar interior and reflect off of the surface (photosphere).

Solar Interior Core Radiation Zone Convections Zone Only place with fusion Radiation Zone Transparent Convections Zone Boiling hot

Convection A pot of boiling water: Hot material rises. Cooler material sinks. The energy from the pot’s hot bottom is physically carried by the convection cells in the water to the surface. Same for the Sun.

Solar Cross-Section Progressively smaller convection cells carry the energy towards surface. See tops of these cells as granules.

The Photosphere This is the origin of the 5,800 K thermal radiation we see. l = k/T = k/(5800 K)  l = 480 nm (visible light) This is the light we see. That’s why we see this as the “surface.”

Concept Question How can we observe nuclear fusion in the Sun’s core? We can’t. We can observe neutrinos produced in the core. We can observe positrons produced in the core. We can observe high energy gamma rays produced in the core. B. We can observe neutrinos produced in the core.

3. Solar Activity and Earth Is there a connection between Solar Activity and Earth’s Climate? Observation: Little Ice Age Maunder Minimum

What is Solar Activity? Sunspots Magnetic Fields Coronal Mass Ejections Solar Wind Magnetic Storms Aurora Other effects?

Sunspots 11-year sunspot cycle. Center – Umbra: 4500 K Edge – Penumbra: 5500 K Photosphere: 5800 K Sunspots

Magnetic fields and Sunspots At kinks, disruption in convection cells. Sunspots form.

Magnetic fields and Sunspots Where magnetic fields “pop out” of Sun, form sunspots. Sunspots come in pairs.

Concept Question Why are sunspots cooler than the rest of the photosphere? They are where cooler gas sinks as part of the convection cells bringing heat to the photosphere. They are areas of slightly different composition that absorbs radiative energy from below less efficiently than the rest of the photosphere. They are areas of magnetic fields that inhibit convective transport of heat from below. They are regions of denser gas. They are at higher altitudes, where temperatures are slightly lower, than the surrounding photosphere. C. They are areas of magnetic fields that inhibit convective transport of heat from below.

Prominences Hot low density gas = emission lines

Stereo

Corona and Solar Wind Hot, low density, gas emits the radiation we see as the Corona: 1,000,000 K Solar Wind: Like steam above our boiling pot of water, the gas ‘evaporates’. Carries away a million tons of Sun’s mass each second! Only 0.1% of total Sun’s mass in last 4.6 billion years.

Solar Cycle Increase in solar wind activity - Coronal Mass Ejections Increase in Auroral displays on Earth Increase in disruptions on and around Earth. Courtesy of SOHO/LASCO/EIT consortium.

Aurora The solar wind passes out through the Solar System. Consists of electrons, protons and other charged particles stripped from the Sun’s surface. Magnetic fields herd charged particles into atmosphere at poles. Charged particles excite electrons in atoms.  Light!

2003 CME Oklahoma 10/29/2003 Credit: E. Woldt

Homework #9 Due Friday 24: Read Bennett Ch9 Do Ch9 Quiz online