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Twinkle Twinkle
Natural History, Feb, 2000 by Allan Sandage
Also explode explode, collapse collapse, nucleosynthesize nucleosynthesize. It turns out that our nightly companions do more than just sparkle--and therein lies the tale of our own origins.
Historians of science a hundred years hence will remember twentieth-century astronomy for two main accomplishments. One is the development of a cosmology of the early universe, from creation through consequent expansion. The other is the understanding of stellar evolution.
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Although not as well known among nonscientists as the big bang is, the notion of the evolution of stars provided the foundation upon which astronomers built the grand synthesis of cosmological origins. The idea that stars change as they age and that these changes in turn alter their local environment and the chemical makeup of their parent galaxy--an idea that has developed only within the past fifty years--stands in the same relation to astronomy as the Darwinian revolution does to biology. It is a conceptual breakthrough that makes possible the modern understanding of the origin, evolution, and fate of the universe and that influences even questions of life and eschatology.
The theory of stellar evolution had its beginnings when the American physicist Jonathan Homer Lane, in 1869, and the German physicist A. Ritter, from 1878 to 1883, derived equations that described gaseous spheres, or stars, as chemical configurations held together by their own gravity and obeying the known gas laws of thermodynamics. The German mathematician Robert Emden published a remarkable book on the subject--Gaskugeln (Gas Spheres)--in 1907, summarizing the work of Lane and Ritter and adding much to the early theory. Well into the 1950s, the so-called Lane-Emden equation was the starting point for much of the theoretical work on the structure of stars: their central temperatures and pressures, their masses, and their equilibria.
But did the stars actually do what the equations said? Yes--and the fact that we can determine what the conditions are in the deep interior of the Sun and other stars with far greater precision than we can manage for most other regions of the visible universe still amazes most of us.
In the early years of the twentieth century, the Danish astronomer Ejnar Hertzsprung, working in the Netherlands, and the U.S. astronomer Henry Norris Russell, working at Princeton, invented a graph that would turn out to be the Rosetta stone of stellar evolution. When you plot the temperatures of stars (which can be inferred from their colors) against their absolute luminosities (which can be calculated from their distances and apparent brightnesses), a striking pattern emerges. Figure out why that pattern should exist at all, and you have the life history of the stars.
As stellar astronomers filled in the so-called Hertzsprung-Russell (HR) diagram with observational data during the first half of the twentieth century, two dense collections of data points emerged. The main sequence shows stellar luminosities ranging from 10,000 times greater than to 10,000 times less than the intrinsic brightness of the Sun. In this thin, wavy, highly populated region of the graph, the corresponding surface temperatures of the stars range from 100,000 [degrees] Kelvin to cooler than 3,000 [degrees]. The other principal branch on the HR. diagram is occupied by stars that are all 100 times more luminous than the Sun yet cooler than 4,000 [degrees] Kelvin. It is easy to use fundamental equations of physics to show that stars such as those in the second group must have enormous radii, exceeding that of the Sun by more than a hundredfold, and, in addition, are of exceedingly low density.
The stars in this second group are now called giants, while stars on the main sequence (which include the Sun), with radii that range from only one-fifth to ten times that of the Sun, are appropriately named dwarf.
But while this diagram showed that stars clearly belonged to certain distinct types, astronomers still didn't understand how the types corresponded to one another or how (and even whether) stars changed. Then, in 1938, Hans Bethe in the United States and, independently, Carl-Friedrich von Weizsacker in Germany wrote down the nuclear reactions by which hydrogen converts to helium in the high-temperature, high-density realm of deep stellar interiors. With that discovery, nuclear astrophysics was born and, with it, the understanding that hydrogen's burning into helium is the first stage of an evolutionary process.
Over the next two decades, physicists described the way in which other elements are newly created from atomic reactions within stars. In this process, called nucleosynthesis, first hydrogen burns into helium, then helium turns into carbon, carbon into nitrogen, nitrogen into oxygen, and so on through all the other heavy elements, up to and including iron. At each stage, mass is lost, the stellar structure changes, and the star recycles chemical elements into space. Sometimes this process ends in the catastrophic explosion of a supernova and, with it, the formation and expulsion into space of iron and all the heavier elements.
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