1. Good Day Sunshine

Fire of life

The Sun is our powerhouse, sustaining life on Earth. It energizes our planet and fuels the engine of life. The Sun warms our world, keeping the temperature at a level that allows liquid water to exist and keeps the Earth teeming with life. Without the Sunís light and heat, all life would quickly vanish from the surface of our planet.

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The Sun is not only warmth, it is light to. The miracle of plants is their ability to use sunlight to make them grow (Fig. 1.1), and in doing so they create another miracle. Using photosynthesis, plants use the Sunís energy to convert water and carbon dioxide into carbohydrates, which releases the oxygen animals breath. Animals also eat these plants for nourishment. The warmth in every animalís body was once sunlight. We all owe our lives to this savage Sun.

Radiation from the Sun

Describing Sunlight

To understand how the Sun operates we must examine its radiation. It spreads out and carries energy in all directions. Some of the sunlight is intercepted by astronomers at Earth, who use it to deduce physical properties of the Sun. When radiation moves in space from one place to another, it behaves like trains of waves. It is called ďelectromagneticĒ radiation because it propagates by the interplay of oscillating electrical and magnetic fields. Electromagnetic waves all travel through empty space at the same constant speed - the velocity of light. This velocity is usually denoted by the lower case letter c, and it has a value of roughly 300 000 000 meters per second. A more exact value is 299 792 458 meters per second. No energy can be transported more swiftly than the speed of light.

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Different types of electromagnetic radiation differ in their wavelength, although they propagate at the same speed. Like waves on water, the electromagnetic waves have crests and troughs. The wavelength is the distance between successive crests or successive troughs (Fig. 1.2). It is usually denoted by the lower case Greek letter lambda, ?, and measured in meters.

Sometimes radiation is described by its frequency, denoted by the lower case Greek letter nu or ?. Radio stations are, for example, denoted by their call letters and the frequency of their broadcasts. The frequency of a wave is the number of wave crests passing a stationary observer each second, measure in Hertz, abbreviated Hz. One Hertz is equivalent to one cycle per second. The frequency tells us how fast the radiation oscillates, or moves up and down. The product of wavelength and frequency equals the velocity of light, so ? x ? = c. When the wavelength increases, the frequency decreases, and vice versa.

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Although sunlight appears yellowish, it is actually a combination of colors that we designate white light. In the mid-17th century the English scientist Isaac Newton discovered that sunlight could be broken into its spectral colors, using a prism - a specially cut chunk of glass - to display the range of colors. White sunlight is similarly bent into its separate wavelengths when raindrops act like tiny prisms to give us a rainbow or a crystal or a single drop of dew resonate with color (Fig. 1.3). Our eyes and brain translate these wavelengths into colors.

When light is absorbed or emitted by atoms, it behaves not as a wave but as a package of energy, or a particle. These packages are given the name photons. They are created whenever a material object emits electromagnetic radiation, and they are consumed when radiation is absorbed by matter. Moreover, each elemental atom can only absorb and radiate a very specific set of photon energies.

The ability of radiation to interact with matter is determined by the energy of its photons. This photon energy depends on the wavelength or frequency of the radiation. Light waves with shorter wavelengths, or higher frequencies, correspond to photons with higher energy.

Like many hot, gaseous cosmic objects, the Sun behaves like an ideal thermal radiator called a black body. Thermal radiation arises by virtue of an objects heat, and it is characterized by a single temperature. A black body absorbs all the radiation incident on it and reflects none - hence the term black.

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When the emission from a thermal radiator is arranged in order of wavelength, radiation is found at all wavelengths but with a varying intensity (Fig. 1.4). Astronomers call this a continuum spectrum since the different colors or wavelengths run together with no breaks or gaps. The wavelength, ?max, at which the thermal radiation is at a maximum is given by: ?max = 0.003/T meters, where T is the temperature in degrees Kelvin, abbreviated įK. This expression is called the Wien displacement law. It indicates that colder objects radiate most of their energy at longer wavelengths, and hotter objects are most luminous at shorter wavelengths. In other words, as the temperature of a gas increases, most of its thermal radiation is emitted at shorter and shorter wavelengths.

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The total power received at any square meter of the Earthís surface, known as the solar insolation, is much less than the solar constant (Fig. 1.5). This is due to the absorption of sunlight in the terrestrial atmosphere, as well as the time of day. The insolation varies according to the Sunís altitude both because of the varying angle between the normal to the Earthís surface and the Sunís direction and because of the varying amount of the Earthís atmosphere that the sunlight has to shine through. When the Sun is overhead at a location, the atmosphere it shines through is least and there is therefore the lowest amount of attenuation by the atmosphere. That is essentially why the days are hottest near noontime. The total energy received at ground level is then reduced from the solar irradiance value of 1366 watts per square meter to about 1,000 watts per square meter. Of course, the Sun is not out at night, and the insolation is zero.

The atmospheric attenuation is much greater at certain wavelengths than at others due to absorption by atmospheric molecules of ozone, oxygen, water, and carbon dioxide (Fig. 1.5). The ozone molecules in our atmosphere absorb the short-wavelength ultraviolet rays from the Sun, thereby filtering out energetic radiation that can be harmful to life on Earth.

Invisible Radiation From the Sun

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There is much more to the Sun than meets the eye! In addition to visible light, there is invisible radiation as well. The solar spectrum extends over an enormous range in wavelength, and different wavelength regions carry different names (Fig. 1.6). The visual or optical region contains the observed colors. The invisible domains include the infrared and radio waves, with wavelengths longer than that of red light, and the ultraviolet (UV), X-rays and gamma (?) rays that are shorter than violet light. They are all electromagnetic waves and part of the same family, and they all move in empty space at the velocity of light, but we canít see them.

The infrared part of the electromagnetic spectrum is located between the radio wave region and visible region (Fig. 1.6). Infrared radiation was discovered in 1840 by the astronomer William Herschel, who was already world-famous for his discover of the planet Uranus. He put a beam of sunlight through a prism to spread it into tits spectral components and he noticed that a thermometer placed beyond the red edge of the visible spectrum of the Sun became warmed by an invisible portion of sunlight. A thermometer placed in this infrared sunlight showed higher temperatures than one placed in normal visible sunlight. The warmth of ha heat lamp is similarly provided by its infrared radiation.

Just beyond violet light is the short-wavelength ultraviolet part of the electromagnetic spectrum (Fig. 1.6), where photons are sufficiently energetic to tear electrons or atoms off many of the molecular constituents of the Earthís atmosphere, particularly in the ozone layer. These energetic photons cannot reach the ground, and if they did they would cause considerable damage to out skin and eyes.

The X-ray region of the electromagnetic spectrum extend from a wavelength of one hundred billionth (10-11) of a meter, which is about the size of a atom, to the short-wavelength side of the ultraviolet (Fig. 1.6). X-ray radiation is so energetic that it is usually described in terms of the energy it carries. . The X- ray region lies between 1 and 100 kilo-electron volts, abbreviate keV, of energy. There are soft X-rays with relatively low energy and modest penetrating power, with energies of 1 to 10 keV. The hard X-rays have higher energy and greater penetrating power, at 10 to 100 keV. As a metaphor, one thinks of the large, pliant softballs and the compact, firm hardballs, used in the two kinds of American baseball games.

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The atmosphere effectively absorbs most of the Sunís ultraviolet radiation and all of its X-rays and gamma rays. To look at the Sun at these invisible wavelengths, we must loft telescopes above the atmosphere. This was done first by using balloons and sounding rockets, followed by satellites that orbit the Earth above the atmosphere. Satellite-borne telescopes now view the Sun at invisible ultraviolet and X-ray wavelengths, above the Earthís absorbing atmosphere in a world where night is often brief or non-existent. One of these solar satellites is the Solar and Heliospheric Observatory, or SOHO for short, launched on 2 December 1995, and another is the Yohkoh, or ďsunbeamĒ, satellite launched on 21 February 1981. They provide entirely new perspectives of the Sun, detecting the hotter parts of the solar atmosphere (Fig. 1.7) and revealing a Sun that is unlike anything we have ever seen before (Fig. 1.8).

Spectroscopy and the ingredients of the Sun

Nowadays scientists use special instruments called spectrographs to spread out the visible portion of the Sunís radiation into its spectral components and separate colors. Such a display of the intensity of radiation as a function of wavelength is called a spectrum. Spectroscopy is the study and interoperation of spectra, especially with a view to determine the chemical composition of and physical conditions in the source of radiation.

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When the spectrum of sunlight is examined carefully, with very fine wavelength resolution, numerous fine dark gaps are seen crossing the rainbow-like display (Fig. 1.9). When coarser resolution is used, the separate colors of sunlight are somewhat blurred together and the dark places are no longer found superimposed on its spectrum.

The dark gaps of missing colors are now called absorption lines. When a cool, tenuous gas is placed in front of a hot, dense one, atoms or ions in the cool gas absorb radiation at specific wavelengths, thereby producing the dark absorption lines. They are called lines because they look like a line in the spectrum.

Each chemical element, and only that element, produces a unique set, or pattern, of wavelengths at which the dark lines fall. It is as if each element has its own characteristic barcode that can be used to identify each element, as a fingerprint might identify a criminal.

Since a greater number of atoms will absorb more light, the relative darkness of the absorption lines establishes the relative abundance of the elements in the Sun. That is, darker absorption lines generally indicate greater absorption and therefore larger amounts of the element. Studies of the absorption lines in the Sunís spectrum showed, in the 1920s, that hydrogen is the most abundant element in the visible solar gases. Since the Sun was most likely chemically homogenous, a high hydrogen abundance was implied for the entire star, and this was confirmed by subsequent calculations of its luminosity.

Helium, the second-most abundant element in the Sun, is so rare on Earth that it was first discovered on the Sun.

Altogether, 92.1 percent of the number of atoms in the Sun are hydrogen atoms, 7.8 percent by number are helium atoms, and all other heavier elements make up only 0.1 percent.

Atoms consist of largely of empty space, just as the room you may be sitting in appears to be mostly empty. A tiny, heavy, positively charged nucleus lies at the heart of an atom, surrounded by a cloud of relatively minute, negatively charged electrons that occupy most of an atomís space and govern its chemical behavior.

Hydrogen is the simplest atom, consisting of a single electron circling around a single proton. The nucleus of helium contains two neutrons and two protons, and so has two electrons in orbit.

Stars are Born, Live and Die

Massive stars have an explosive death. After the core has become hot enough to produce iron, the star has reached the end of the line. It has become bankrupt, completely spending all its nuclear resources, and there is nothing left to do but collapse under the sheer weight of it all. In a matter of milliseconds the central core is crushed into a ball of neutrons about 10,000 meters across, no bigger than a city. The outer layers also plunge in toward the center, but they rebound like a tightly coiled spring. The pent-up energy generated in the collapsing center blasts the outer layers out in a violent explosion called a supernova, littering space with its cinders. These ashes will join the debris from countless other explosions, providing the raw material for the next generation of stars.

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In a galaxy the size of the Milky Way, a supernova explodes on average once every hundred years or so. The energy released in a supernova is immense. For a few weeks it can be brighter than the combined brightness of all the other stars in a galaxy. Then, as the debris expands outwards, it cools and becomes fainter. Astronomers use the name supernova remnant for this expanding shell of gas (Fig. 1.11, Fig. 1.12). This material moves out into interstellar space, seeding it with heavy elements that were forged inside the former star.

Where did the Chemical Elements Come from?

The majority of atoms that we see today were formed at the dawn of time before the stars even existed, in the immediate aftermath of the big-bang explosion that produced the expanding Universe. All of the most abundant element, hydrogen, that is now in the Universe was created back then, about 10 or 20 billion years ago, and so was most of the helium, the second-most abundant element. The hydrogen and helium were synthesized in the earliest stages of the infant Universe, within just a wink of the cosmic eye. As the Universe expanded, it cooled and thinned out, prohibiting primordial nucleosynthesis after the first few hundred seconds of the big bang.

The first stars could not have had rocky planets like the Earth, because there was initially nothing but hydrogen and helium. The only possible planets would have been icy balls of frozen gas. Without carbon, life as we know it could not evolve on these planets.

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Stars that contained only hydrogen and helium are called first-generation stars. Middle-aged stars like the Sun are second-generation stars, meaning that some of their material came from previous stars. Sun-like stars contain heavy elements that were formed inside massive first-generation stars or at the time of their explosions (Fig. 1.14).

During the billions of years before the Sun was born, massive stars reworked the chemical elements, fusing lighter elements into heavier ones within their nuclear furnaces. Carbon, oxygen, nitrogen, silicon, iron, and most of the other heavy elements were made this way. The enriched stellar material was then cast out into interstellar space by the short-lived massive stars, gently blowing in their stellar winds or explosively ejected within supernova remnants.

The Sun and its retinue of planets condensed from this material about 4.6 billion years ago. They are partly composed of heavy elements that were synthesized long, long ago and far, far away, in the nuclear crucibles of stars that lived and died before the Sun was born. The Earth and everything on it have been spawned from this recycled material, the cosmic leftovers and waste products of stars that disappeared long ago.

Perhaps the most fascinating aspect of stellar alchemy is its implications for life on Earth. Most of the chemical elements in our bodies, from the calcium in our teeth to the iron that makes our blood red, were created billions of years ago in the hot interiors of long-vanished stars. We are true children of the stars, for we are all made of star stuff. If the Universe were not very, very old, there would not have been time enough to forge the necessary elements of life in the ancient stellar cauldrons. The same nuclear powerhouse that synthesized heavy elements from light ones, and made these stars shine, is now at work inside the Sun.

How the solar system came into being

Age of the Solar System

Precisely when did the solar system originate? Its precise age is determined by examining primitive meteorites, ancient rocks returned from the Moon, and deep ocean sediments. These relics have remained unaffected by the erosion that removed the primordial record from most terrestrial rocks.

Their ages can be determined by measuring the relative amounts of radioactive materials and their non-radioactive products. When this ratio is combined with the known rates of radioactive decay, the time since the rock solidified and locked in the radioactive atoms is found.

Rounding off the numbers and allowing for possible systematic errors, we can say that the Earth, Moon and meteorites solidified at the same time some 4.6 billion years ago, with an uncertainty of no more than 0.1 billion years. If the solar system originated as one entity, then this should also be the approximate age of the Sun and the rest of the solar system.

Regular Planetary Orbits and the Nebular Hypothesis

Although Newtonís and Keplerís laws describe the present behavior of the solar system, they cannot explain the remarkable arrangement. Some additional constraints are required, that describe the state of affairs before the planets were formed and set in motion. These initial conditions are provided by the nebular hypothesis, in which the Sun and planets formed out of a single collapsing, rotating cloud of interstellar gas and dust, called the solar nebula (Fig. 1.13).

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Although Newtonís and Keplerís laws describe the present behavior of the solar system, they cannot explain the remarkable arrangement. Some additional constraints are required, that describe the state of affairs before the planets were formed and set in motion. These initial conditions are provided by the nebular hypothesis, in which the Sun and planets formed out of a single collapsing, rotating cloud of interstellar gas and dust, called the solar nebula (Fig. 1.13).

Modern versions of the nebular hypothesis provide additional caveats, but the basic tenants of the original idea are still valid. Billions of years ago the spinning solar nebula, attracted by its own gravity, fell in on itself, getting denser and denser, until its middle became so packed, so tight and hot, that the Sun began to shine. The planets formed at the same time, within a flattened rotating disk centered on the contracting proto-Sun.

Observing the Sun

Astronomers could not obtain scientific understanding of the Sun until they developed techniques to observe the Sun indirectly with telescopes. Important new insights were obtained each time that a new part of the electromagnetic spectrum was observed in this way. Indeed, solar astronomy is primarily an observational science, that has both led to, and been driven by, revolutionary scientific discoveries. Early indirect observations of the Sun, using optical telescopes at visible wavelengths, allowed scientific study of the Sun to begin, showing that our star is a dynamic changing body. The development of optical spectroscopy permitted investigation of the magnetic fields, atmospheric motions, and composition of the Sun, as well as a new understanding of the internal structure of the atom and the chemistry of the cosmos. Radio telescopes provided a unique, high-resolution perspective of the changing, million-degree solar atmosphere, powerful explosions on the Sun, and violent activity that characterizes much of the Universe. The development of artificial satellites and other spacecraft then allowed scientists to study the Sun above the Earthís atmosphere, permitting a full and continuous view of the Sunís ultraviolet and X-ray radiation, and direct sampling of energetic particles and magnetic fields flowing from it.

Ground-based optical observing

The Sun has been observed with optical telescopes for centuries, gathering all the visible colors of sunlight. These telescopes are used to resolve spatial details that we cannot detect with the unaided eye. Since the Sun is a quarter of a million times closer to us than the next nearest star, it permits a level of detailed examination that is not possible on any other star.

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There are two types of optical telescopes, the refractor and the reflector (Fig. 1.15), and both of them are used to observe the Sun. Never look directly at the Sun through either kind of telescope; it can cause permanent damage to your eyesight. The safest way to observe the Sun with a telescope is to project the Sunís image on a surface and view it there.

In a refractor, sunlight is bent by refraction at the curved surface of a lens, called an objective, toward a focal point where the different rays of light meet (Fig. 1.15). If we place a detector at the focal point, in the plane parallel to the lens, we can record an image of the Sun. The distance from the lens to the focal point is called the focal length, which determines the overall size of the solar image. The critical thing is the diameter, or aperture, of the light-gathering lens. The larger the aperture, the more light is gathered and the finer the detail that can be seen.

The other type of telescope, the reflector, uses a concave mirror with a parabolic shape to gather and focus the sunlight. The diameter of this primary mirror determines the telescopeís resolution and light gather ability. The prime focus is back in the path of the incoming sunlight, so secondary mirrors are sometimes used to reflect the light to another place of observation. There are three types of secondary mirrors called the Cassegrain, Coudť, and Newtonian mirrors, that can focus light to different locations (Fig. 1.15).

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The McMath solar telescope is a fine example of a modern high-resolution instrument (Fig. 1.16). It has a mirror at the top, called a heliostat, that follows the Sun, and sends a beam of sunlight into the side of a mountain to another mirror that takes the beam and focuses it, forming an image (Fig. 1.17). With a focal length of 82 meters, the Sunís image is about 0.77 meters across. The tube that encases the telescope is kept at a cold temperature by pumping water through external tubes around it. This reduces air turbulence inside the tube, resulting in a sharper image.

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At first sight you would think there is plenty of light coming from the Sun. Because of its closeness, it is a hundred billion times brighter than any other star. But you obtain much less light if just a narrow section of the solar spectrum is isolated and observed for short times, as with a spectroheliograph (Fig. 1.18). Large lenses or mirrors might then be needed to obtain a strong signal, permitting detailed studies of spectral components such as absorption or emission lines.

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A special type of optical telescope is the coronagraph (Fig. 1.19), first developed by the French astronomer Bernard Lyot around 1930. It produces an artificial eclipse of the Sun by means of an occulting disk inside the telescope, blocking out the intense glare of the photosphere. This permits the detection of the faint light of the corona around the perimeter of the occulting disk. Special precautions must be taken to reduce instrumental scattered and diffracted light.

Ground-based radio observations of the Sun

Because of its proximity, the Sun is the brightest radio object in the sky, and radio telescopes have therefore been used to study our home star for decades. Moreover, the Earthís atmosphere does not distort radio waves that are less than about one meter in wavelength, so we can observe the radio Sun on a cloudy day, just as radio signals are used to communicate with satellites even when it rains or snows outside.

If you want to examine the radio Sun in fine detail, a bigger telescope is needed. Since radio waves are millions of times longer than those of light, a radio telescope needs to be at least a million times bigger than an optical telescope to obtain the same resolving power. For this reason, the first radio telescopes provided a very myopic, out-of-focus view. These views have been compared to looking at the Sun through the bottom of a glass bottle.

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Nowadays, relatively small radio telescopes, separated by large distances, are combined and coordinated electronically, achieving radio images of the Sun that are as sharp as optical ones. Because it is spread out, an array of small telescopes has the property that is crucial for high resolving power, namely great size relative to wavelength.

The technique is known as interferometry because it analyzes how the waves detected at the telescopes interfere once they are added together. The simplest example is a pair of telescopes with a computer to reconstruct the waves from the combined data (Fig. 1.20).

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The Very Large Array, abbreviated VLA, is an example of a modern interferometric radio array that is used to observe the Sun (Fig. 1.21). It consists of 27 radio telescopes placed along the arms of an Y-shaped array at separations of up to 34 thousand meters (Fig. 1.22). The telescopes are connected electronically and linked to a central computer, providing a total of 351 pairs of telescopes. When the telescopes are all pointed at the Sun, the received signals are combined to create images of temperature and magnetic structures on the Sun with an angular resolution of up to one tenth of a second of arc, equal to or better than any ground-based optical telescope.

Observing the Sun from space

Radio telescopes do not provide our only window on the invisible Sun. There are the invisible gamma-rays, X-rays, and ultraviolet radiation. They are all absorbed in our atmosphere and must be collected by telescopes in satellites that orbit the Earth above its atmosphere These space telescopes measure the intensity of the incoming signal and convert these measurements into radio transmissions that are sent to radio telescopes on the ground. Spacecraft can also directly sample the particles and magnetic fields flowing from the Sun.

Solar astronomy from space has several advantages over ground-based observations. The weather in space is always perfectly clear, and the images are not blurred by the atmosphere. Moreover, if the space telescope is placed in the right location, it can view the Sun 24 hours a day, every day, for years.

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Seven modern spacecraft, Yohkoh, Ulysses, Wind, the Solar and Heliospheric Observatory, abbreviated SOHO, ACE, TRACE, RHESSI, Hinode, and STEREO, have recently made revolutionary discoveries about the Sun, (Fig 1.23, 1.24, 1.20). Major new instruments aboard these spacecraft have traced the flow of energy and matter from down inside our star to the Earth and beyond, providing insights that are vastly more focused and detailed than those of previous solar missions. Indeed, we may have obtained more essential new information about the Sun from these spacecraft than the entire previous century of investigations.