16.1 Physical Properties of the Sun The Sun is the sole source of light and heat forthe maintenance of life on Earth.

It is a star, a glowing ball of gas held together byits own gravity and powered by nuclear fusion at its center.

                                           

                         

                  

SOLAR STRUCTURE

LUMINOSITY

Imagine our detector as having a surface area of one square meter (1 m2) and as being placed at the top of Earth’s atmosphere. The amount of solar energy reaching this surface each second is a quantity known as the solar constant.

Its value is approximately 1400 watts per square meter (W/m2). Most of this energy reaches Earth’s surface. Thus, for example, a sunbather’s body having a total surface area of about 0.5 m2 receives solar energy at a rate of nearly 700 watts, roughly equivalent to the output of a typical electric room heater or about ten 75-W lightbulbs.

LUMINOSITY

The Sun is an enormously powerful source of energy. Every second, it produces an amount of energy equivalent to the detonation of about 100 billion one-megaton nuclear bombs. Six seconds worth of solar energy output, suitably focused, would evaporate all of Earth’s oceans. Three minutes would melt our planet’s crust. The scale on which the Sun operates simply defies Earthly comparison.

With the fury of a 24-megaton atomic bomb, hot volcanic ash exploded 60,000 feet into the sky. The blast killed 57 people. (Grant M. Haller / Seattle Post-Intelligencer)

August 6: At 8:15 a.m., Japanese time, a B-29 bomber flying at high altitude drops the first atomic bomb on Hiroshima. The gun-type uranium bomb, called Little Boy, is detonated 1,900 feet above the city. It has a yield of approximately 15 kilotons TNT.

More than 4 square miles of the city are instantly and completely devastated. From 90,000 to 100,000 persons are killed immediately; about 145,000 persons will perish from the bombing by the end of 1945. (Trinity: Manhattan Report and NF)

16.2 The Solar Interior

Scientists use mathematical models constructed by solar physics To develop a STANDARD SOLAR MODEL

In the 1960s it was discovered that the surface of the Sun vibrates like a complex set of bells. These vibrations are the result of internal pressure (“sound”) waves that reflect off the photosphere and repeatedly cross the solar interior.

Because these waves can penetrate deep inside the Sun, analysis of their surface patterns allows scientists to study conditions far below the Sun’s surface. This process is similar to the way in which seismologists study the interior of Earth by observing the P- and S-waves produced by earthquakes.

Study of solar surface patterns is usually called helioseismology, even though solar pressure waves have nothing whatever to do with solar seismic activity—there is no such thing.

The most extensive study of solar oscillations is the ongoing GONG (Global Oscillations Network Group) project.

By making continuous observations of the Sun from many clear sites around Earth, solar astronomers can obtain uninterrupted high-quality solar data spanning many days and even weeks—almost as though Earth were not rotating and the Sun never set.

The Solar and Heliospheric Observatory (SOHO), launched by the European Space Agency in 1995 and now permanently stationed between Earth and the Sun some 1.5 million km from our planet

Some mysteries remain, however. For example, helioseismology indicates that the Sun’s rotation speed varies with depth—perhaps not too surprising given the surface differential rotation mentioned earlier and the fact that similar behavior has been noted in the outer planets.

What is puzzling, though, is the complexity of the differential motion

A full explanation of the Sun’s rotation currently eludes theorists.

Figure 16.5 shows the solar density and temperature according to the Standard Solar Model, plotted as functions of distance from Sun’s center.

The solar density continues to decrease out beyond the photosphere, reaching values as low as 10-23 kg/m3 in the far corona

The solar temperature also decreases with increasing radius, but notas rapidly as the density.

ENERGY TRANSPORT

In and near the core, the extremely high temperatures guarantee that the gas is completely ionized.

THE CORE

With no electrons left on atoms to capture the photons, however, the deep solar interior is relatively transparent to radiation. Only occasionally does a photon encounter and scatter off a free electron or proton. As a result, the energy produced by nuclear reactions in the core travels outward toward the surface in the form of radiation with relative ease.

RADIATION ZONEAs we move outward from the core, the temperature falls, atoms collide less frequently and less violently, and more and more electrons manage to remain bound to their parent nuclei.

electrons absorb the outgoing radiation

the gas in the interior changes ,becoming almost totally opaque

photons produced in the Sun’s core have been absorbed by theouter edge of the radiation zone.

Not one of them reaches the surface

What happens to the energy they carry?

The photons’ energy must travel beyond the Sun’s interior

The energy reaches the surface by convection

Hot solar gas physically moves outward while cooler gas above it sinks, creating a characteristic pattern of convection cells.

energy is transported to the surface by physical motion of the solar gas.

ZONE OF CONVECTION

the zone of convection is much more complex than we have just described

Heat is then successively carried upward through a series of progressively smaller-sized cells, stacked one on another until, at a depth of about 1000 km, the individual cells are about 1000 km across.

GRANULATION

The visible surface is highly mottled, or granulated, with regions of bright and dark gas known as granules

Each bright granule measures about 1000 km across

. Supergranulation is a flow pattern quite similar to granulation except that supergranulation cells measure some 30,000 km across

Together, several million granules constitute the top layer of the convection zone, immediately below the photosphere

Each granule forms the topmost part of a solar convection cell.

Spectroscopic observation within and around the bright regions shows direct evidence for the upward motion of gas as it “boils” up from within. This evidence proves that convection really does occur just below the photosphere.

Spectral lines detected from the bright granules appear slightly bluer than normal, indicating Doppler-shifted matter approaching us at about 1 km/s

Conversely, spectroscopes focused on the darker portions of the granulated photosphere show the same spectral lines to be redshifted, indicating matter moving away from us.

16.3 The Solar Atmosphere

Astronomers can glean an enormous amount of information about the Sun from an analysis of the absorption lines that arise in the photosphere and lower atmosphere

Solar Spectrum A detailed spectrum of our Sun shows thousands of Fraunhofer spectral lines which indicate the presence of some 67 different elements in various stages of excitation and ionization in the lower solar atmosphere. The numbers give wavelengths, in nanometers.(Palomar Observatory/Caltech)

Spectral lines arise when electrons in atoms or ions make transitions between states of well-defined energies, emitting or absorbing photons of specific energies (that is, wavelengths or colors) in the process

When we look at the Sun, we are actually peering down into the solar atmosphere to a depth that depends on the wavelength of the light under consideration.

The existence of the Fraunhofer lines is direct evidence that the temperature in the Sun’s atmosphere decreases with height above the photosphere.

Strictly speaking, spectral analysis allows us to draw conclusions only about the part of the Sun where the lines form—the photosphere and chromosphere

The 10 most common elements in the Sun.

THE CHROMOSPHERE

Above the photosphere lies the cooler chromosphere, the inner part of the solar atmosphere

This region emits very little light of its own and cannot be observed visually under normal conditions.

The relative dimness of the chromosphere results from its low density—large numbers of photons simply cannot be emitted by a tenuous gas containing very few atoms per unit volume

The chromosphere’s characteristic reddish hue is plainly visible. This coloration is due to the red Ha (hydrogen alpha) emission line of hydrogen, which dominates the chromospheric spectrum

The chromosphere is far from tranquil. Every few minutes, small solar storms erupt, expelling jets of hot matter known as spicules into the Sun’s upper atmosphere

Spicules are spikes of glowing gas and shaped by the magnetic field of the Sun.

These long, thin spikes of matter leave the Sun’s surface at typical speeds of about 100 km/s, reaching several thousand kilometers above the photosphere.

They cover only about one percent of the total area, tending to accumulate around the edges of supergranules

Scientists speculate that the downwelling material there tends to strengthen the solar magnetic field and spicules are the result of magnetic disturbances in the Sun’s churning outer layers.

THE TRANSITION ZONE AND CORONA

With the photospheric light removed, the pattern of spectral lines changes dramatically

The intensities of the usual lines alter

Changes occur in; a. composition b. temperature c. the spectrum changes from absorption to emission creating a new set of lines d. discovered in the 1920’s e. called coronium, a new nonterrestrial element

Absorption spectraIf you look more closely at the Sun's

spectrum, you will notice the presence of dark lines. These lines

are caused by the Sun's atmosphere absorbing light at certain

wavelengths, causing the intensity of the light at this wavelength to drop and appear dark. The atoms and molecules in a gas will absorb only certain wavelengths of light.

The pattern of these lines is unique to each element and tells us what elements make up the atmosphere

of the Sun. We usually see absorption spectra from regions in

space where a cooler gas lies between us and a hotter source. We usually see absorption spectra from

stars, planets with atmospheres, and galaxies.

Emission spectraAn emission spectra occurs

when the atoms and molecules in a hot gas emit

extra light at certain wavelengths, causing bright lines to appear in a spectra. As with absorption spectra, the pattern of these lines

are unique for each element. We can see

emission spectra from comets, nebul and certain

types of stars.

A peak in the plot shows the position of an emission line and dip shows where an absorption line is. The spacing and location of these lines are unique to each atom and molecule.

Coronium does not exist.

a. The new lines of the spectra arise because atoms in the corona have lost several more electrons than atoms in the photosphere

b. The coronal atoms are much more highly ionized

c. The cause of this extensive electron stripping is the high coronal temperature

d. The temperature of the solar corona, where even more ionization is seen, is higher than the upper chromosphere

What inconsistency exists here?

What might be a plausible answer to this phenomena?

THE SOLAR WIND

Electromagnetic radiation and fast-moving particles—mostly protons and electrons—escape from the Sun all the time.

The radiation moves at the speed of light taking only8 minutes to reach earth.

The particles move more slowly at a speed of 500km/sAnd reach the earth in a few days

This constant stream of escaping solar particles is the solar wind.

a. The solar wind results from the high temperature of the corona

b. About 10 million km above the photosphere, the coronal gas is hot enough to escape the Sun’s gravity

c. At the same time, the solar atmosphere is continuously replenished from below.

d. If not, the corona would disappear in a day

e. The sun is constantly shedding mass through solar wind

f. Although it carries away over a million tons of solar matter each second, less than 0.1 percent of the Sun’s mass has been lost since the solar system formed 4.6 billion years ago.

THE SUN IN X-RAYS

The hotter coronal gas radiates at much higher frequencies—primarily in X-rays

X-ray telescopes have become important tools in the study of the solar corona

In the mid-1970s, instruments aboard NASA’s Skylab space station revealed that the solar wind escapes mostly through solar “windows” called coronal holes

Such structures are simply deficient in matter

16.5 The Heart of the Sun

SOLAR ENERGY PRODUCTION

In round numbers, the Sun’s luminosity is 4 1026 W and its mass is 2 1030 kg. We can quantify how efficiently the Sun generates energy by dividing the solar luminosity by the solar mass:

This simply means that, on average, every kilogram of solar material yields about 0.2 milliwatts of energy—0.0002 joules (J) of energy every second.

This is not much energy—a piece of burning wood generates about a million times more energy per unit mass per unit time than does our Sun. But there is an important difference: The wood will not burn for billions of years.

To appreciate the magnitude of the energy generated by our Sun, we must consider not the ratio of the solar luminosity to the solar mass but instead the total amount of energy generated by each gram of solar matter over the entire lifetime of the Sun as a star.

3 x 1013 J/kg

This is the average amount of energy radiated by every kilogram of solar material since the Sun formed. It represents a minimum value for the total energy radiated by the Sun

NUCLEAR FUSION

We can represent a typical fusion reaction symbolically asnucleus 1 + nucleus 2      nucleus 3 + energy.

Albert Einstein showed at the beginning of the twentieth century that matter and energy are

interchangeable. One can be converted into the other, in accordance with Einstein’s famous

equation:E = mc2,

The law of conservation of mass and energy states that the sum of mass and energy (properly converted to the same units) must always remain constant in any physical process. There are no known exceptions. According to this law, an object can literally disappear, provided that some energy appears in its place

In the case of fusion reactions in the solar core, the energy is produced primarily in the form of electromagnetic radiation. The light we see coming from the Sun means that the Sun’s mass must be slowly but steadily decreasing with time.

Review of properties of the sun