Ingredients Of The

Celestial objects are composed, like the Earth and we ourselves, of individual particles of matter called atoms. But the atoms consist largely of seemingly empty space, just as the room you maybe sitting in appears 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 atom's space and govern its chemical behavior.

In the early 20th century, the New Zealand-born British physicist Ernest Rutherford (1871-1937) showed that radioactivity is produced by the disintegration of atoms, and discovered that they emit energetic alpha particles, which consist of helium nuclei; he was awarded the 1908 Nobel Prize in Chemistry for these achievements. By using helium ions to bombard atoms, Rutherford was able to announce in 1911 that most of the mass of an atom is concentrated in a nucleus that is 100,000 times smaller than the atom and has a positive charge balanced by the negative charge of surrounding electrons.

Nearly a decade later, in 1920, Rutherford announced that the massive nuclei of all atoms are composed of hydrogen nuclei, which he named protons. He also postulated the existence of an uncharged nuclear particle, later called the neutron, which was required to help hold the nucleus together and keep it from dispersing as the protons repelled each other. After an eleven-year search, the English physicist James Chadwick (1891-1974) discovered the neutron, in 1932, receiving the 1935 Nobel Prize in Physics for the feat. So, the nucleus of an atom is composed of positively charged protons and neutral particles, called neutrons; both about 1,840 times heavier than the electron.

tte simplest and lightest atom consists of a single electron circling around a nucleus composed of a single proton without any neutrons; this is an atom of hydrogen. tte nucleus of helium, another abundant light atom, contains two neutrons and two protons, and the helium atom therefore has two electrons.

tte atomic ingredients of the Sun can be inferred from dark absorption lines, which are found superimposed on the colors of sunlight (Fig. 1.6). ttey look like a dark line when the Sun's radiation intensity is displayed as a function of wavelength; such a display is called a spectrum, tte term Fraunhofer absorption line is also used, recognizing the German astronomer Joseph von Fraunhofer (1787-1826). By directing the incoming sunlight through a slit, and then dispersing it with a prism, Fraunhofer was able to overcome the blurring of colors from different parts of the Sun's disk, discovering numerous dark absorption lines. By 1814 he had detected and catalogued more than 300 of them, assigning Roman letters to the most prominent.

tte Sun is so bright that its light can be spread out into very small wavelength intervals with enough intensity to be detected, tte instrument used to make and record such a spectrum is called a spectrograph, a composite word consisting of spectro for "spectrum" and graph for "record", tte spectrograph spreads out the wavelengths into different locations, as a rainbow or prism does. Nowadays it is the grooves of a diffraction grating that reflect sunlight into different locations according to color or wavelength. And you can see such a colored display by looking at a compact disk.

FIG. 1.6 Visible solar spectrum A spectrograph has spread out the visible portion of the Sun's radiation into its spectral components, displaying radiation intensity as a function of wavelength. When we pass from long wavelengths to shorter ones (left to right, top to bottom), the spectrum ranges from red through orange, yellow, green, blue and violet. Dark gaps in the spectrum, called Fraunhofer absorption lines, are due to absorption by atoms in the Sun. The wavelengths of these absorption lines can be used to identify the elements in the Sun, and the relative darkness of the lines helps establish the relative abundance of these elements. (Courtesy of National Solar Observatory, Sacramento Peak, NOAO.)

FIG. 1.6 Visible solar spectrum A spectrograph has spread out the visible portion of the Sun's radiation into its spectral components, displaying radiation intensity as a function of wavelength. When we pass from long wavelengths to shorter ones (left to right, top to bottom), the spectrum ranges from red through orange, yellow, green, blue and violet. Dark gaps in the spectrum, called Fraunhofer absorption lines, are due to absorption by atoms in the Sun. The wavelengths of these absorption lines can be used to identify the elements in the Sun, and the relative darkness of the lines helps establish the relative abundance of these elements. (Courtesy of National Solar Observatory, Sacramento Peak, NOAO.)

When a cool, tenuous gas is placed in front of a hot, dense one, atoms in the cool gas absorb radiation at specific wavelengths, thereby producing dark absorption lines. And when a tenuous gas stands alone and is heated to incandescence, emission lines are produced that shine at precisely the same wavelengths as the dark ones, tte Sun's dark absorption lines and bright emission lines carry messages from inside the atom, and help us determine its internal behavior.

Adjacent lines of the hydrogen atom exhibit a strange regularity - they systematically crowd together and become stronger at shorter wavelengths, tte Swiss mathematics teacher Johann Balmer (1825-1898) published an equation that describes the regular spacing of the wavelengths of the four lines of hydrogen detected in the spectrum of visible sunlight, and they are still known as Balmer lines, tte strongest one, with a red color, is also called the hydrogen alpha line.

In 1913, the Danish physicist Niels Bohr (1885-1962) explained Balmer's equation by an atomic model, now known as the Bohr atom, in which the single electron in a hydrogen atom revolves about the nuclear proton in specific orbits with definite, quantized values of energy. An electron only emits or absorbs radiation when jumping between these allowed orbits, each jump being associated with a specific energy and a single wavelength, like one pure note. If an electron jumps from a low-energy orbit to a high-energy one, it absorbs radiation at this wavelength; radiation is emitted at exactly the same wavelength when the electron jumps the opposite way. ^is unique wavelength is related to the difference between the two orbital energies. Bohr was awarded the 1912 Nobel Prize in Physics for his investigations of the structure of atoms and the radiation emanating from them.

Since only quantized orbits are allowed, spectral lines are only produced at specific wavelengths that characterize or identify the atom. An atom or molecule can absorb or emit a particular type of sunlight only if it resonates to that light's energy. As it turns out, the resonating wavelengths or energies of each atom are unique - they fingerprint an element, encode its internal structure and identify the ingredients of the Sun. In addition, spectral lines yield information about the Sun's temperature, density, motion and magnetism.

Each element, and only that element, produces a unique set of absorption or emission lines, ^e presence of these spectral signatures can therefore be used to specify the chemical ingredients of the Sun (Fig. 1.7). ^e abundance calculations depend upon both measurements of the solar lines and on properties of the elements detected in the terrestrial laboratory, ^e lightest element, hydrogen, is the most abundant element in the Sun and most other stars (Focus 1.1). Altogether, 92.1 percent of the atoms in the Sun are hydrogen atoms, 7.8 percent are helium atoms, and all the other heavier

FIG. 1.7 Abundance and origin of the elements in the Sun The relative abundance of the elements in the solar photosphere, plotted as a function of their atomic number, Z. The abundance is specified on a logarithmic scale and normalized to a value a million, million, orl.O X 1012, for hydrogen. Hydrogen, the lightest and most abundant element in the Sun, was formed about 14 billion years ago in the immediate aftermath of the Big Bang that led to the expanding Universe. Most of the helium now in the Sun was also created then. All the elements heavier than helium were synthesized in the interiors of stars that no longer shine, and subsequently wafted or blasted into interstellar space where the Sun originated. Carbon, nitrogen, oxygen and iron, were created over long time intervals during successive nuclear burning stages in former stars, and also during the explosive death of massive stars. Elements heavier than iron were produced by neutron capture reactions during the supernova explosions of massive stars that lived and died before the Sun was born. The atomic number, Z, is the number of protons in the nucleus, or roughly half the atomic weight. The elements shown, He, C, N, O and Fe, have Z = 2,6,7,8 and 26, with atomic weights of 4, 12, 14, 16, and 56, since each nucleus contains as many neutrons as protons with about the same weight. Hydrogen has one proton and no neutrons in its nucleus. The exponential decline of abundance with increasing atomic number and weight can be explained by the rarity of stars that have evolved to later stages of life. (Data courtesy of Nicolas Grevesse.)

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