The Sun

Our Sun is an ordinary star. Many stars are much more massive. Many stars are much less massive. What is the name of our Sun? What is the difference between a star and a sun?

Where the Sun came from? How old is the Sun? If the Sun has a surface? What are the layers of the Sun’s interior and exterior? The nature of sunspots? The connection between sunspots and the Sun’s magnetic field? What powers the Sun? Do You Know…….

The Solar Nebula

The Cosmic Abundance

Angular Momentum Cloud Cloud Cloud Collapses Begins Flattens Slowly to Flatten in to disk

Solar Data Table

What powers the Sun?

Nuclear Fusion E = mc 2

Know as the “P – P Chain” Matter is turned into anti-matter and energy

Hydrostatic Equilibrium What Keeps the Star From Collapsing?

The outward pressure force balances the inward gravitational force everywhere inside the Sun.

How Do We Know? Is thermonuclear fusion really taking place? Detect the subatomic particles – Neutrinos Nuclear burning produces ~10 38 neutrinos/s ~ neutrinos pass thru each m 2 of Earth/s ~10 12 neutrinos pass thru your body each second neutrinos have no electric charge Neutrinos interact very weakly with matter Rarely, a neutrino hits a neutron converting it into a proton 37 Cl + neutrino => 37 Ar Isotope reaction Rate of formation of 37Ar is proportional to neutrino flux Found 1 neutrino per day Expected 3 per day

1,000 tons of heavy water 9,600 light detectors

The Solar Interior Three main interior layers Core Radiation Zone Convective Layer

Core Central region of the Sun Nuclear fusion occurs (also called hydrogen burning) Star is born when fusion begins (about 1 million degrees) Temp of core now about 15 million degrees

Radiative Zone Section directly above the core Energy moves by “radiation transport” Photons travel ~1 cm before being absorbed Photons take 10 6 to 10 8 yrs to reach surface Random walk

Convective Region Volume between radiative zone and surface Energy is transported by convection Cooler near the surface Brings heat from interior to the surface Similar to a pot of boiling water

The Surface of the Sun Called the Photosphere Bright visible surface from which light escapes 300 to 400 km thick Shell of hot, opaque gas Can’t see through this layer Completely transparent above Completely opaque below Photosphere pressure ~ 10% of Earth’s sea level Place where continuous spectrum is produced Not a solid surface, density gradually increases w/depth

Limb Darkening

The Sun’s three interior layers. The Sun also has a surface and two atmospheric layers. Can you name them?

Computer Model Showing Solar Oscillations Blue zones moving outward Red zones moving inward Oscillations extend to core Study of Helioseismology Sun vibrates at many frequencies

The Atmosphere of the Sun Chromosphere Corona

Chromosphere Color Sphere, red Caused by bright red emission line Lies immediately above the photosphere Difficult to observe Reddish glow seen during a solar eclipse Coronograph – an occulting disk Source of most of the solar emission lines Composed of hot transparent gases – mostly H About 2,500 km thick Density decreases with altitude Temp increases from about 4,400 K to 25,000 K with altitude 100,000,000 times less dense then out atmosphere

Corona Outer most layer of the Sun Extends millions of km above the photosphere Tenuous gas layer ~10 9 atoms/cc Earth ~10 19 atoms/cc About 1 millionth as bright as the photosphere Temp in access of one million degrees Shape changes from month to month Spherical at sunspot maximum

Granulation Pattern of light and dark areas on the photosphere Are the light or dark areas hotter? Lifetime about 5 to 10 minutes Granules typically 700 km to 1000 kn in size Granules are the tops of the convection currents of rising gas through the photosphere Rise at about 2 to 3 km/s

Granulation

Chromosphere Jets of gas surging upward into the corona Last about 15 minutes Speed ~ 20 km/s

Solar Wind The Sun’s gravity keeps most gases from escaping But high coronal temp cause speeds of 10 6 km/hr The highest speed gases can escape Sun ejects 10 6 tons of mass into the solar wind /s Solar wind originates from coronal holes

Coronal Hole Movie Clip Coronal hole is dark – devoid of material

Coronal Holes are usually found at the polar regions

12 X-Ray Images From 1991 – day Intervals

Sunspots

Sunspot Formation

Sunspot Groups

Overlapping Sunspots

1999 By observing the same group of sunspots, Galileo determined that the Sun rotates about once per week.

The Sunspot Cycle

Where Do Sunspots Form?

Sunspots and Earth Temperature

The Corona Changes Shape Spherical at sunspot maximum Irregular at sunspot minimum

How Do We Know the Sun has a Magnetic Field?

Magnetic Fields deflect charged particles

Dark Blue = Strong North Pole Yellow = Strong South Pole Highly magnetic areas in (B) correlate with (A)

Differential Rotation Sunspots are used to measure solar rotation rates 25 days at the equator 28 days at 40 degrees latitude 36 days at 80 degrees latitude The Sun does not rotate like a rigid body

Differential rotation cause B in the photosphere to become wrapped around the Sun. As a result, B becomes concentrated at certain latitudes on both sides of the equator.

Rotation of the Solar Interior

Magnetic Reconnection – Adds Energy to the Corona One arch contains as much as E as a power plant generates in 10 6 yrs.

Plages are hot spots in the chromosphere forming just before sunspots.

Plages are created when B push upward from the Sun’s interior, compressing and heating a portion of the chromosphere.

Eielson Air Force BaseEielson Air Force Base, Alaska — The Aurora Borealis, or Northern Lights, shines above Bear LakeAlaskaAurora BorealisBear Lake

Transient of Mercury 11/15/1999 UV image Visible

Recap – Parts of the Sun

Key Ideas Hydrogen Fusion in the Sun’s Core: The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus. The energy released in a nuclear reaction corresponds to a slight reduction of mass according to Einstein’s equation E = mc 2. Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 10 7 K. In the Sun, fusion occurs only in the dense, hot core.

Key Ideas Models of the Sun’s Interior: A theoretical description of a star’s interior can be calculated using the laws of physics. The standard model of the Sun suggests that hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius. The core is surrounded by a radiative zone extending to about 0.71 solar radius. In this zone, energy travels outward through radiative diffusion. The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure. In this zone, energy travels outward primarily through convection.

Key Ideas Solar Neutrinos and Helioseismology: Conditions in the solar interior can be inferred from measurements of solar neutrinos and of solar vibrations. Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected. Recent neutrino experiments explain why this is so. Helioseismology is the study of how the Sun vibrates. These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun.

Key Ideas 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. Its spectrum is similar to that of a blackbody at a temperature of 5800 K. Convection in the photosphere produces granules.

Key Ideas Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere. Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules. The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density. Activity in the corona includes coronal mass ejections and coronal holes. The solar corona blends into the solar wind at great distances from the Sun.

Key Ideas The Active Sun: The Sun’s surface features vary in an 11- year cycle. This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity. Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field. The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next. Two such cycles make up the 22-year solar cycle.

Key Ideas The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field. These changes are caused by convection and the Sun’s differential rotation. 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.