1 The Sun - Our Star Sun’s diameter 100 times the Earth’s Sun’s mass is 700 times the mass of all the planets put together The energy source of the Sun is the conversion of hydrogen atoms into helium atoms through nuclear fusion in the core of the Sun. Without this energy source the Sun would collapse. Composition of the Sun is 71% Hydrogen, 27% Helium, 2% heavier elements

2 How do we know what we know? Our understanding of the Sun is a combination of measurements of observed quantities (diameter, surface temperature, spectra) and computer models based on physical laws (internal temperature and density). Models are correct so far as they can predict the observed properties of the Sun.

3 Solar Seismology Solar seismology is the study of the Sun’s interior by analyzing wave motions on the Sun’s surface and atmosphere P-mode (compression) waves only The wave motion can be detected by the Doppler shift of the moving material –absorption line shifts in the photosphere are driven by processes in the convective region below the surface The detected wave motion gives temperature and density profiles deep in the Sun’s interior These profiles agree very well with current models

4 Solar Seismology

5 Solar Structure The Sun is completely gaseous. The atoms in this gas strongly absorb light. Because of this absorption energy travels very slowly through the Sun. It takes about 100,000 years for energy to travel from the core to the solar surface. Above a certain height however the gas is too thin to absorb light effectively. The point at which the gas becomes transparent is called the photosphere. The photosphere is considered the “surface” of the Sun. Surface temperature about 6000 Kelvin.

6 Solar Structure At the core of the Sun, where nuclear fusion occurs, temperatures are about 15 million degrees Kelvin. As this energy propagates outwards it initially is carried by photons of light. This region of the Sun is called the radiative zone. Near the photosphere this process of energy transfer becomes less efficient and energy begins moving outward through convective motions of the gas. As gas heats it rises to the surface. Once it reaches the surface it releases heat and sinks. This region is called the convective zone.

7 Solar Granules

8 Caused by convection Red regions show rising gas Black regions show falling gas White specks show regions of concentrated magnetic fields White bar is 5000km long Astronomy Picture of the day from 16 April 2010

9 Hydrostatic Equilibrium Requires that the pressure generated by the fusion reactions in the core of the Sun must be in exact balance with the weight of material falling inwards due to the Sun’s gravity. Without this balance the Sun would either collapse (gravity wins) or explode (pressure wins).

10 The Sun is not burning. If it were burning fuel like coal it would have exhausted its fuel long ago. The slow collapse of the Sun was once thought to be the energy source but that wouldn’t have lasted more than a few million years. It wasn’t until the 20th century that physicists understood the process behind what powers the Sun. The Sun’s Energy Source

11 Einstein’s Mass-Energy Relation Albert Einstein ( ) In 1905 Albert Einstein recognized that mass and energy were related through the formula: E=mc 2 (m =mass, E=energy, c=speed of light) What this means is that a small amount of mass could be converted into an enormous amount of energy. The means by which the Sun generates this energy is through nuclear fusion.

12 Nuclear fusion involves combining two atomic nuclei together to form one, larger nucleus. This can not occur under normal conditions because the electrical repulsion between protons prevents the nuclei from fusing. At extremely high temperatures however this electrical repulsion is overcome and the nuclei fuse. Nuclear Fusion

13 Nuclear Fusion The specific steps of nuclear fusion follow a process called the proton-proton chain Through this process 2 neutrinos, 2 positrons, 2 1 H and a 4 He is created by the fusion of 6 1 H. The mass of all the particles created is less than the sum of the masses of colliding particles. This difference in mass was converted into energy through E=mc 2.

14 Studying the Solar Interior The neutrinos generated in the fusion process escape rapidly through the Sun. These neutrinos are very difficult to detect but they can be detected. This type of neutrino “telescope” can check whether our understanding of the solar interior is correct. It seems that the number of neutrinos measured match computer models but only if – neutrinos have mass –neutrinos can oscillate among three different states or types A neutrino detector in Japan. This is a giant tank of water buried deep underground. Tiny bursts of light due to neutrinos from the Sun are detected by light sensitive detectors on the walls of the tank.

15 Sunspots This sunspot is twice the size of the Earth! Sunspots are dark splotches on the photosphere of the Sun. Sunspots are dark because they are cooler than their surroundings. The reason they are cooler is because intense magnetic fields prevent hot, ionized gases from entering so the region cools forming a spot

16 Solar Magnetic Activity Like several of the planets the Sun generates a magnetic field. Unlike the planets the strength of the field strongly varies with time and with location on the Sun The most easily detectable evidence for solar magnetic activity are the Sunspots

17 Prominences & Flares Prominences are associated with pairs of sunspots are are arcs of magnetic field with glowing gas trapped inside. Flares are explosive events where strong bursts of X-rays are emitted. These are due to rapid changes in the magnetic field.

18 The Sun’s Atmosphere The extremely low-density gases that lie above the photosphere make up the Sun’s atmosphere

19 The Chromosphere The lower part of the atmosphere is referred to as the chromosphere –The chromosphere appears as a thin red zone around the dark disk of a totally eclipsed Sun –The red is caused by the strong red emission line of hydrogen H  –The chromosphere contains millions of thin columns called spicules, each a jet of hot gas

20 The Corona and Solar Wind The outer atmosphere of the Sun is called the corona. The temperature of the corona can be several million degrees. Magnetic fields are thought to be responsible for these very high temperatures. The corona is so hot it expands outward along with the Sun’s magnetic field and streams through the Solar System. This is known as the Solar Wind. Depending on the Sun’s magnetic activity level, the Solar Wind may change in velocity and density. The corona is only visible from Earth during a Solar Eclipse.

21 Heating of the Chromosphere and Corona While the Sun’s magnetic field cools sunspots and prominences, it heats the chromosphere and corona Heating is caused by magnetic waves generated in the relatively dense photosphere –These waves move up into the thinning atmospheric gases, grow in magnitude, and “whip” the charged particles found there to higher speeds and hence higher temperatures –Origin of waves may be from rising bubbles in convection zone

22 Heating of the Chromosphere and Corona

23 The Solar Cycle Sunspots, prominences, flares and the strength of the solar wind all vary over an approximately 11- year cycle The magnetic field of the Sun will strengthen and weaken over 11 years. The polarity of the Sun’s field will also flip. The North pole becomes the South pole and vice versa. It takes 22 years to return to its original polarity. A plot of the number of sunspots with time from A solar cycle is the time between two minima (low sunspot counts).

24 Maunder's Sunspot Diagram

25 Changes in the Solar Cycle The cycle may vary from 6 to 16 years Considering the polarity direction of the sunspots, the cycle is 22 years, because the Sun’s field reverses at the end of each 11-year cycle Leading spots in one hemisphere have the same polarity, while in the other hemisphere, the opposite polarity leads

26 Solar Seismology Reveals Difference in rotational speeds of convective zone and radiative zone –Contributes to magnetic dynamo Jet streams in convective zone deep below solar surface Map far side of solar surface –Sunspots on far side affect doppler shifts of absorption lines on near side Cyclic time variation in differential rotation (torsional oscillation) is similar to solar cycle

Solar Cycle and Climate Midwestern United States and Canada experience a 22- year drought cycle Few sunspots existed from , the Maunder Minimum, the same time of the “little ice age in Europe and North America

Solar Cycle and Climate Number of sunspots correlates with change in ocean temperatures

29 Recent Solar Observatories SOHO - Solar & Heliospheric Observatory –Hurricane-sized whirlpools spotted on the Sun (YouTube) Stereo – two satellites –direct simultaneous far and near side observations starting in 2011 –Mercury sodium (?) tail movie SDO – Solar Dynamics Observatory –YouTube Channel: spicules, prominences, flares, internal solar dynamic model animations

30 Solar System by Voyager 1

31 Extent of Solar Wind

32 Bow Shock Around Star R Hydrae