Rotation Period = 25 days at the equator & 29 days near the pole Composition = 99% hydrogen and helium State = gaseous (plasma)

Energy Source: Thermonuclear Fusion For 4.5 billion years, the Sun has been emitting 4 x Watts. H + H + H + H - ---> He The Helium nucleus weighs slightly less than four times the Hydrogen nucleus, so there is a slight loss of mass, this is converted into energy Nuclear Fusion: Energy comes from the conversion of mass into energy following Einstein's Equation: E = m c 2

Energy Production and Transport Zones 3 Interior Layers The core produces the fusion energy The radiative zone The convective zone Temperature of the core is 15 million degrees K –temp 7 million degrees K –temp 2 million degrees K

Convection inside the Sun causes the photosphere to be subdivided into km cells.

Granulation Typically 1000 km across Bright center, dark edge Center moving out, edges moving inward

The Sun’s Structure 3 Atmosphere Layers – Photosphere (surface, part we see) – Chromosphere (Next outward) – Corona ( The outer atmosphere) Since the Sun gives off most of its radiation in the yellow-green region, humans have evolved so that our eyes are most sensitive to that light.

The Photosphere Our sunlight originates from this layer. This is what we see, it is only 300 km thick, and we refer to this as the sun’s surface. Temperature 5,800 k Absorption lines reveal the 67 elements have been identified The innermost of the sun’s atmosphere layers. H, and He gas finally cool enough that it becomes transparent to light.

The Chromosphere 2 nd atmosphere layer. Several thousand kilometers thick. Glows in red H-  light, which is an n=3 to n=2 transition and glows red in color. This is a region of red flames. Temp 10,000 K

Chromosphere also contains spicules -- jet- like spikes: 30 km/s jets rising 5,000-20,000 km above photosphere. Last about 10 minutes each. Give chromosphere a ragged “edge”

Actual color of photo- sphere … Should be slightly greenish. Chromosphere glows in red. This tends to filter out the slightly greenish color of the photosphere, so we see yellow light from sun.

The Corona Millions of kilometers thick, but extremely low density. The sun’s magnetic field agitates the corona, and raises temperature back up to about 1,000,000 K. We can only study the corona during a total solar eclipse, or from space with specially designed telescopes. The Corona merges with the solar wind

Features on the Sun’s Surface All features are produced by The sun’s wacky magnetic field. Sunspots Prominences & Flares Coronal Holes Coronal Mass Ejections

Gases can only travel along the field lines. The magnetic field holds the gases down and they cool with the results being a darker, cooler area…a sunspot. A kink in magnetic field lines pushes up through surface with one polarity and where it enters the surface has the opposite charge.

Sunspots Umbra Penumbra Granulation Darker and cooler than surrounding photosphere

Close up of Sunspot Sunspots are still about 4000 K – hot enough to melt anything on the earth, but 2000 K cooler than the surrounding surface.

Limb darkening: - the Sun looks darker near its apparent edge, or limb, than near the center of its disk. (Universe, 2005) Granulation: - Mottled appearance of the solar surface - Caused by rising (hot) and falling (cool) material in convective cells just below the photosphere. (Astronomy Today, 2002)

Prominences When a loop of the sun’s magnetic field projects out from the surface, charged particles from the photosphere flow along the field lines in arcs or loops, called prominences. (Red gas) These are the more gentle eruptions on the Sun’ surface. Temp~ 10,000 K.

Plagues also originate from sunspots, and have a little more energy than prominences. They are white, or lighter in color. Filaments are the same thing as prominences. They are seen against the surface, rather than on the edge. They look darker against the bright surface.

Solar Flares Sometimes, the magnetic field lines disconnect from the sun, when the magnetic field rearranges itself. Hot gases trapped inside the loop of magnetic field travels outward from the sun as a solar flare. Violent release of energy. T~5,000,000 K

Prominences: Loop or sheet of glowing gas ejected from an active region on the solar surface - under the influence of the Sun’s magnetic field (Astronomy Today, 2002) ) Flares: - Explosive event occurring in or near an active region on the Sun. The area tends to be whiter, and hotter. (Astronomy Today, 2002)

Coronal Holes Another surface feature is the coronal hole. These are actual holes or windows in the sun’s corona, where solar wind from the photosphere can easily blow through. The Solar Wind, is electrons and positive ions (charged particles ), streaming from the sun. (V ~ 500 km/sec) When one of these points towards the earth, the velocity and density of the solar wind increases.

Coronal Mass Ejection Total Time for these events is several hours Can impact the Earth causing electrical and communications disruptions as well as auroras. Emission of lots of of charged particles, dangerous.

Sunspot Cycle The number of sunspots varies with a period of about 11 years from maximum sunspots, to minimum, and back to maximum again. This is called the sunspot cycle. Sunspots occur in pairs of (+) and (-) magnetic polarity. The Magnetic Field Cycle is 22 years; the time for the magnetic fields of a sunspot pair to change from + - to -+ and back to + - again.

Number of sunspots peaks every 11 years Sunspot Cycle Some peaks are higher and we don’t know why. It is not a constant 11 years each time, it may be 10, or even 12, but 11 is the average. The Maunder Minimum was a period of low solar activity from ,

During a maximum they are closer At the start of a new cycle, sunspots always appear within 30º of the solar equator Sunspot Cycle During a minimum they are further from equator

Radiation Zone: Photons “random walk” through many layers of the Sun

Differential Rotation If the sun were solid and magnetic field rotated in an orderly way, there would be no storms or surface features on the sun, but… …differential rotation winds up and tangles the sun’s magnetic field, resulting in surface storms. Process is not very well understood.

These changes are caused by convection and the Sun’s differential rotation

THE SOLAR WIND Earth and all the planets in the Solar System are immersed in the Sun's expanding outer atmosphere. This out flowing of solar material is called the solar wind. Spacecraft measurements show that the solar wind is fast (about a million miles per hour), thin (a few particles per cubic centimeter), and hot (several hundred thousand degrees).

The problem was that not as many neutrinos were being detected from the sun as had been calculated. Finally it was discovered that neutrons could change from one type to another known as “flavors” and there are 3 flavors. The chlorine had only detected one type. To be able to change types means that neutrinos must have mass. The detector is a large tank of chlorine that reacts to neutrinos