Astronomy Lecture

The Sun: Our Extraordinary Ordinary Star

Filaments Across the Sun

Bulk Properties of the Sun

• Diameter• Mass• Density• Rotation

Periods

• “Surface” Temp.

• Core Temp.

109 Earth Diameters333,000 Earth

Masses1408 kg/m3

Equatorial: 25 DaysPolar: 35 Days5800 K15,500,000 K

Limb Darkening

• The sun is not as bright near the limb as it is in the center. Also it is more yellow, indicating that we are looking through cooler layers near the limb than at the center.

• This is because we see deeper into the photosphere when we look straight down than when we look obliquely.

Mercury

Explanation of Limb Darkening

The Sun’s Atmosphere• The photosphere is the visible layer of the Sun.• The chromosphere is a mostly cooler layer that

lies just above the photosphere. This region creates the Sun’s absorption line spectrum.

• The transition region is a thin region above the chromosphere, where the temperature rises rapidly from about 10,000 K to a million K.

• The corona is the Sun’s outer atmosphere. The temperature of the corona is 1 to 2 million K. The corona extends several times the diameter of the Sun.

Photosphere showing

Solar Granulation

Solar Granulation

High-resolution photographs of the Sun’s surface reveal a blotchy pattern, called granulation. Granules, which measure about 1000 km across, are convection cells in the Sun’s photosphere.

Solar Granulation Video

Spicules and Supergranules in the Chromosphere

Supergranules are regions of rising and falling gas, spanning hundreds of granules

The Solar Corona

The Active Sun• Sunspots and the Sunspot Cycle• Solar Magnetism and the Solar Cycle• Other Atmospheric Phenomena

– Plages– Active Regions– Prominences– Filaments– Coronal Holes– Flares– Coronal Mass Ejections

Sunspots• Sunspots are typically about 10,000 km in diameter –

about the size of the Earth. They have a dark central umbra surrounded by a grayish, structured penumbra.

• They appear dark only by comparison to the brighter surrounding photosphere. They are cooler regions of the photosphere. The temperature of the umbra is about 4500 K and that of the penumbra is about 5000 K.

• A large group of sunspots typically lasts about 50 days.• Galileo determined the Sun’s rotation period by timing

the movement of sunspots. The Sun rotates in 25.4 days at the equator and in 33 days in the polar region.

Motion of Sunspots Video

The Sunspot Cycle

Solar Magnetism• Sunspots are directly linked to intense magnetic fields on

the Sun. When atoms are in magnetic fields, their spectrum lines are split into two or more lines on each side of the central line. This is called the Zeeman effect.

• The strong magnetic field in sunspots lowers their temperature by interfering with the convective flow of hot gas toward the surface.

• Sunspots usually come in pairs with the magnetic field coming out of one member of the pair and going in at the other member. In opposite hemispheres, sunspot pairs are reversed in their polarity.

• Solar Cycle: The 11-year sunspot cycle is ½ the solar cycle. In alternate sunspot cycles, the Sun’s magnetic field reverses direction causing the polarity of the sunspots to reverse.

Zeeman Splitting of Spectrum Lines

Magnetic Field Lines and Sunspot Pairs

Effect of Sun’s Differential Rotation on its Magnetic Field

Other Atmospheric Phenomena• Plages – brighter (hotter) areas in chromosphere• Filaments – dark streaks above the chromosphere.

Huge volumes of gas uplifted into the corona• Prominences – filaments viewed from the side• Coronal Holes – darker (cooler) areas in corona,

visible in X-rays, where gases easily can escape from the Sun

• Flares – violent eruptive events seen in UV & X-rays• Coronal Mass Ejections – huge, balloon-shaped

volumes of high-energy gas being ejected

Solar Prominences

Prominences and the Corona photographed during the solar eclipse of July 11, 1991, near sunspot maximum

Active Sun in Hα

A Solar Prominence from SOHO

Prominences

X-ray picture of a Coronal Hole

UV picture of a Solar Flare

A Coronal Mass Ejection

The Sun’s CompositionThe main elements are hydrogen and helium, as in the gas giant planets and in other stars and nebulae in the universe.

Element by Atoms by MassHydrogen, H 92% 74%Helium, He 8% 25%Others 0.1% 1%

This data is needed for the homework.

Main Regions of the Sun

The Sun’s Interior• The Standard Solar Model. Is a mathematical model of

the Sun, made by combining all available observations with theoretical insight into solar physics. This model shows that the Sun’s interior has three major regions, listed from outside to inside.

• Convection Zone. This zone extends downwards from the photosphere about 200,000 km. The material is in constant convective motion.

• Radiation Zone. Below the convection zone and extending to the core, is the radiation zone, where energy is transported toward the surface by radiation rather than by convection.

• Core. The central core, about 200,000 km in radius, is the site of the nuclear reactions that generate the Sun’s enormous energy output.

Density & Temperature Profiles of the Sun’s Interior

The Sun’s Source of Energy• Solar Constant is 1400 W/m2.• Luminosity of the Sun is 1400 W/m2 4π (1 AU)2 =

41026 W.• The Conversion of Mass to Energy: Mass and energy

are related through Einstein’s equation E = mc2.• Solar Energy from Nuclear Fusion, the combining of light

nuclei into heavier ones. The sum of the masses of the light nuclei is a little greater than the mass of the heavier nucleus that is formed.

• The Sun gets its energy from the Proton-Proton Chain, fusing 4 hydrogen atoms into 1 helium atom.

The Proton-Proton Chain

Hydrogen to Helium Animation

How Energy Gets from the Sun’s Core to Its Surface part 1

• Radiative Transfer.– In the central regions of the Sun, the temperature is

so hot that all the electrons are stripped from the nuclei. Thus there are no bound electrons to move from one state to another, absorbing radiation.

– This region is relatively transparent to radiation, allowing energy to flow out freely.

– Radiation diffuses slowly outward in a haphazard zigzag pattern, taking about 170,000 years on the average to go from the core to the bottom of the convective zone.

How Energy Gets from the Sun’s Core to Its Surface part 2

• Convective Transfer. As the temperature drops outside the inner core, atoms can retain some electrons. This causes the gas to become more and more opaque to radiation. The energy must still get out, but since the radiation is blocked, convection begins and carries the energy away to the surface. This takes about 10 days to reach the photosphere.

• Convection Cells. In the deep solar interior they are thought to be large, perhaps 30,000 km across. At higher levels the convection cells are smaller, about 1000 km across just below the photosphere.

Solar Neutrinos• Neutrinos are “ghostly” particles with no charge

and having an immeasurably small mass.

• They go through matter like it isn’t there. Therefore they are very difficult to detect.

• Solar neutrinos are going from the Sun through the Earth and through your body right now – roughly 100 billion of them per square centimeter each second.

Helioseismology

Sound waves resonating within the solar interior cause the photosphere to move in and out, rhythmically distorting the shape of the Sun. This heaving motion can be described as the superposition of millions of oscillations, like the one shown here.

Sound waves inside the Sun, like seismic waves in the Earth, are refracted back out. The Sun’s surface reflects waves back in. How deep a wave penetrates, and how far around the Sun it goes before it hits the surface depends on its wavelength.

Rising Gasses

Descending Gasses

Summary of Key IdeasKnow and Understand:• The size of the Sun compared with Earth

and its “surface” temperature• The regions of the Sun’s atmosphere• The nature of sunspots and the sunspot

cycle• Where and how the Sun gets its energy• The regions of the Sun’s Interior