1. Good Day Sunshine
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.
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.
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.
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.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University