An Introduction to the Physics of Our Star

An Introduction to the Physics of Our Star: The Sun • Distance from Earth – 1 AU • Travel time for Light to Earth – About 8 minutes • Travel time for solar wind to 1 AU – A few days • Mean surface temperature – 5800 K • Temperature in the Center – 1. 55 x 107 K Solar Science Workshop 6/8/05

The Sun • Mass – 333, 000 Earth Masses – More mass than all of the other objects in the solar system combined • Diameter – 218 Earth Diameters • Average Density – 1410 kg/m 3 • Composition (by mass) – 74% Hydrogen, 25% Helium, 1% other elements Sun seen in Xrays Solar Science Workshop 6/8/05

The Sun Seen From Earth by Amateurs • COMMON SENSE WARNING … NEVER LOOK DIRECTLY AT THE SUN WITHOUT A PROPER FILTER !! – Looking directly at the Sun without the proper filter WILL cause permanent eye damage • Safest to use the method of projection – Project the image onto a piece of paper – Can see sunspots, and even “graph” them easily Solar Science Workshop 6/8/05

The Sun Seen From Earth by Amateurs • Using a Baader solar filter – Inexpensive – Many “solar observing glasses” are made out of this material – Used to see sunspots – Similar to Mylar Solar Science Workshop 6/8/05

The Sun Seen From Earth by Amateurs • Hα filters – Much more pricey • 100 s-1000 s of dollars – Can see features in the solar chromosphere – Used to see Prominences, filaments, flares – Very impressive Sun seen with an Hα filter Solar Science Workshop 6/8/05

How Old is the Sun ? • Probably about the same age as Earth • Has it been shining brightly the entire time? – The luminosity has probably increased with time, but it has always been very bright • What is the “fuel” that keeps it shining this long? Solar Science Workshop 6/8/05

The Sun’s Energy Source is Thermonuclear Fusion in its Core • Proton-proton chain – Four hydrogen nuclei “fuse” to form a single helium nucleus – There is a slight loss of mass in this process which is converted to energy according to Einstein’s famous equation E = mc 2 • Thermonuclear fusion occurs only at the very high temperatures at the Sun’s core • Fusion should not be confused with fission ! • Will continue to heat the Sun for another 5 billion years Solar Science Workshop 6/8/05

The Proton-Proton Chain Solar Science Workshop 6/8/05

The Structure of the Sun • The Interior – Core – Radiative zone – Convection zone • The Surface and Atmosphere – Photosphere – Chromosphere – corona Solar Science Workshop 6/8/05

• Hydrogen fusion takes place in a core extending from the Sun’s center to about 0. 25 solar radius The Sun’s Interior • The radiative zone extends to about 0. 71 solar radius – Here energy travels outward through radiative diffusion • The convective zone is a rather opaque gas – Here energy travels outward primarily through convection Solar Science Workshop 6/8/05

Where do the Neutrino’s and γ-ray photons go ? • Neutrino’s exit the Sun, unimpeded – Can be used to probe the solar interior – Early attempts at detecting them found that their were about 3 times less seen at Earth than there should be – THE SOLAR NEUTRINO PROBLEM ! • The gamma rays collide with matter and take millions of years to exit the Sun Solar Science Workshop 6/8/05

Detecting Solar Neutrinos • Underground detectors are used to avoid interference from cosmic rays Solar Science Workshop 6/8/05

The Solution to the Solar Neutrino Problem • Particle Physics gave us the answer – Solar neutrinos oscillate and the original detectors could only see certain parts of the oscillations and not all of them – New detectors were built to observe all neutrinos – Two physicists won a Nobel Prize for their work • Modern detectors are placed at different depths within the Earth to observe the actual oscillations Solar Science Workshop 6/8/05 Raymond Davis Jr. Masatoshi Koshiba

Solar Structure • The standard solar model – Theoretical model used to determine the physical properties of the Sun’s interior – Assumes hydrostatic and thermal equilibrium Solar Science Workshop 6/8/05

Solar Oscillations • Waves can propagate through the Sun causing a variety of vibrations – Like sound waves • These are used to infer pressures, densities, chemical compositions, and rotation rates within the Sun Solar Science Workshop 6/8/05

Helioseismology • The branch of science that studies solar oscillations is known as Helioseismology • The movie shows evidence of seismic activity on the Sun as seen by the SOHO MDI experiment Solar Science Workshop 6/8/05

The Convection Zone • The convection zone is just outside the radiative zone. – Thickness is about 200, 000 km • Turbulent convective motions occur, similar to a pot of boiling water. • Overturning (bubbling) motions inside the Sun are responsible for the granulation pattern seen on the Sun’s surface. Solar Science Workshop 6/8/05

Convection Cells Solar Science Workshop 6/8/05

Solar Granulation • Convection cells that are about 1000 km wide • These are part of the Sun’s atmosphere known as the Photosphere Solar Science Workshop 6/8/05

Recent Highresolution Images of granulation Solar Science Workshop 6/8/05

“Flowers” and Ribbons • Produced by magnetic fields Solar Science Workshop 6/8/05

The photosphere is the lowest of three main layers in the Sun’s atmosphere • The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona • Everything below the solar atmosphere is called the solar interior • The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere • The photosphere undergoes “differential” rotation Solar Science Workshop 6/8/05

Limb Darkening • The edges of the Sun appear darker than that seen “straight on” • This is called limb darkening • It is due to the fact that the temperature in the photosphere decreases with altitude Solar Science Workshop 6/8/05

The Origin of Limb Darkening • The light we see at the limb originated higher up in the atmosphere where it is cooler – Thus it will be less bright there Solar Science Workshop 6/8/05

Sunspots • Low temperature regions – How do we know this ? • Darkest part is called the “umbra” • Just outside the umbra is the penumbra • Associated with Intense magnetic fields Solar Science Workshop 6/8/05

Solar Science Workshop 6/8/05

Sunspots Often Come in Groups Solar Science Workshop 6/8/05

The Chromosphere • Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere “Color Sphere” • characterized by spikes of rising gas • Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules Solar Science Workshop 6/8/05

• Filaments are dark, threadlike features seen in the red light of hydrogen (Halpha). – Dense cooler material suspended at high altitudes by magnetic fields Chromospheric Features: Plages and Filaments • Plage (the French word for beach) are bright patches surrounding sunspots that are best seen in H-alpha. – associated with concentrations of magnetic fields Solar Science Workshop 6/8/05

Chromospheric Features: Prominences • Same as filaments, except that they are seen from the side rather than “straight on” Solar Science Workshop 6/8/05

The Corona • The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density • The solar corona blends into the solar wind at great distances from the Sun Solar Science Workshop 6/8/05

The 11 -year Sunspot Cycle Number of Sunspots versus time – they come and go every 11 years Number of Sunspots versus latitude – forms a “butterfly pattern” Solar Science Workshop 6/8/05

The Maunder Minimum • Complete absence of sunspots for 50 years corresponds to a mini ice age • There is a loose correlation between global man temperature and sunspots Solar Science Workshop 6/8/05

These changes are caused by convection and the Sun’s differential rotation: The Solar Dynamo Solar Science Workshop 6/8/05

The Buildup of magnetic field energy must be released – how? • Coronal Mass Ejections and Flares – Releases an enormous amount of energy – A solar flare is a brief eruption of hot, ionized gases from a sunspot group – A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona • These storms can interact with the Earth and create huge geomagnetic storms • They also accelerate particles to very high energies Solar Science Workshop 6/8/05

Coronal loops expand from the surface of the Sun following a solar explosion (solar flare) on April 21, 2002 TRACE movie Solar Science Workshop 6/8/05

Solar Science Workshop 6/8/05

The Halloween 2003 Flares were the Largest in Modern History Perhaps the most extreme flare ever seen erupted on Nov. 4. . . IMAGES: SOHO/NASA/ESA Solar Science Workshop 6/8/05 . . . and then this coronal mass ejection was hurled into space.

Solar Science Workshop 6/8/05

Aurora The pattern of auroral light around the north and south magnetic poles is called the auroral oval. It expands and contracts over a period of hours and days, depending on geomagnetic activity. Solar Science Workshop 6/8/05

Aurora in Tucson Solar Science Workshop 6/8/05

Shock Waves in Space • Analogy with sonic booms • Can accelerate charged particles to very high energies • Radiation Environment ! • Space Weather Solar Science Workshop 6/8/05