Astronomy and Cosmology: Nucleosynthesis and Stellar Evolution Bind Humanity to the Cosmos

Introduction

Until the second half of the nineteenth century, astronomy was principally concerned with accurately describing the movements of planets and stars. Developments in the electromagnetic theory of light in the late nineteenth century along with the articulation of quantum and relativity theories in the early twentieth century, however, gave astronomers the tools they needed to probe the inner workings of the sun and other stars. In the first two-thirds of the century, astronomers and physicists unraveled the life cycles of most types of stars and reconciled the predictions of physical theory with astronomical observation. Insights into the birth and death of stars led to the stunning conclusion that Earth and all life upon it, including human beings, are in a direct and physical sense a product of stellar evolution. In astronomy, the term “evolution” is used to name the orderly process by which individual stars change as they age: stellar evolution is unrelated to biological evolution.

Historical Background and Scientific Foundations

The mass of a star—how much matter it contains—is the main factor determining the course of its development and its ultimate fate. Stars that are more massive burn their fuel quicker and lead shorter lives. These facts would have astonished astronomers at the dawn of the twentieth century. At that time, attempts to understand the source of the heat and light released by the sun and other stars were baffled by a lack of understanding of nuclear processes, that is, events that involve the nuclei of atoms. The nature of atomic nuclei was not understood until about 1911, and their transformations through fission and fusion (splitting and joining) took further decades to comprehend.

Many astronomers already understood that stars were formed in clouds of gas and dust (termed nebulae) that were measured to be light years across. These great clouds, so thick they are often opaque in parts, tantalized astronomers. Decades later, after the development of quantum theory, astronomers were able to understand the energetics behind the reddening of light leaving stellar nurseries (hydrogen atoms absorb energy over a range of wavelengths but tend to re-emit it as red light). Astronomers speculated that stars formed when regions of these clouds collapsed (a phenomenon termed gravitational clumping). As a region collapses under the mutual gravitational attraction of its particles—under its own weight, in fact—it accelerates and forms an infant star, a protostar, heated by gravitational friction.

In the pre-atomic age, the stars' source of energy eluded astronomers, who sought to reconcile seeming contradictions regarding how stars were able to maintain their size (i.e., not shrink as they burned fuel) and radiate the tremendous amounts of energy measured. The sun, using conventional energy consumption and production concepts—chemical combustion and heating by gravitational friction during formation—was calculated to be at most a few thousand years old. This was in agreement with dominant religious beliefs based on literal readings of the biblical book of Genesis, but contradicted other science, such as geological evidence showing that Earth must be many millions, perhaps even billions, of years old.

In 1913, Danish Astronomer Ejnar Hertzsprung (1873–1967) and British astronomer Henry Norris Russell (1877–1957) independently developed what is now known as the Hertzsprung-Russell diagram. In the Hertzsprung-Russell diagram, the spectral type (or, equivalently, color index or surface temperature) of each star is placed along the horizontal axis and the absolute magnitude (or luminosity or brightness) along the vertical axis. Accordingly, stars are assigned places top to bottom in order of increasing magnitude (decreasing brightness) and from right to left by increasing temperature and spectral class. (Confusingly for non-astronomers, the dimmer a star is, the higher its magnitude.) The diagram shows that stars fall into a few major groups, most prominently the Main Sequence, which meanders diagonally across the diagram and of which our own sun is a member.

Finding a definite relationship between stellar color and true brightness (not the brightness that a given star happens to have in our own sky, which depends both on its true brightness and its distance from us) was a fundamental advance in modern astronomy. The correlation of color with true brightness eventually became the basis of a widely used method that allowed astronomers to estimate the distance of distant stars from Earth. Estimates from relatively nearby stars could be made using geometrical parallax, that is, the apparent slight shift in position of these stars as Earth moves back and forth across the solar system while circling the sun.

In the Hertzsprung-Russell Main Sequence, some stars—namely, those now understood to be burning hydrogen as their primary nuclear fuel—form a prominent band or sequence of stars from extremely bright, hot stars in the upper left-hand corner of the diagram to faint, relatively cooler stars in the lower right-hand corner of the diagram. Most stars fall into the Main Sequence on the Hertzsprung-Russell diagram. Our sun, for example, is a main-sequence star that lies roughly in middle of the diagram among what are referred to as yellow dwarfs.

There are classes of stars that are much larger and brighter than the sun, termed giants and supergiants: the largest supergiant stars are 70 or more times as massive as our sun and hundreds of thousands of times brighter. Russell attempted to explain the existence of giant stars as the result of large gravitational clumps. Stars, according to Russell, would migrate down the Main Sequence on the Hertzsprung-Russell diagram as they burned away their mass as fuel. Stars, Russell theorized, begin life as huge, cool, red bodies, then undergo a continual shrinkage as they heated. Although the Hertzsprung-Russell diagram was an important advance in understanding stellar evolution—and it remains highly useful to modern astronomers—Russell's reasoning behind the movements of stars on the diagram turned out to be exactly the opposite of the modern understanding of stellar evolution, which was later made possible by an understanding of the sun and stars as thermonuclear reactors.

Advances in quantum theory and improved models of atomic structure made it clear to early-twentieth-century astronomers that a deeper understanding of the life cycle of stars and of the structure of the cosmos was tied to advances in our knowledge of how nature works on an atomic and subatomic scale. This linkage between the smallest and largest scales remains vital in cosmology and astronomy in the twenty-first century. A complete understanding of mass conversion in stars was developed from Albert Einstein's (1879–1955) special theory of relativity (1905), which included his relation of mass to energy: famously, E = mc², that is, energy = mass times the square of the speed of light. In brief, a very small amount of matter can be converted into a very large amount of energy. This explained how stars could shine so brightly for millions of years without greatly diminishing in mass or running out of chemical fuel: they shine by converting a fraction of their mass into energy. During the 1920s, based on the principles of relativity and quantum mechanics, British physicist Ralph H. Fowler (1889–1944) determined that, in contrast to the predictions of Russell, a white dwarf would become smaller as its mass increased, converted slowly into radiant energy.

Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910–1995) first articulated the evolution of stars into supernovae, white dwarfs, and neutron stars, and he described the conditions required for the formation of black holes; these bizarre objects were found in the latter half of the twentieth century. In 1939, American physicist J. Robert Oppenheimer (1904–1967), who ultimately supervised the making of the first atomic bombs, made detailed calculations reconciling Chandrasekhar's predictions with general relativity theory.

In the decade that followed, as the mechanisms of atomic fission and fusion worked their way into astronomical theory, astronomers refined concepts regarding stellar birth. Eventually, when a protostar contracts enough, the increase in its core temperature triggers nuclear fusion and the star ignites, becoming visible as it vaporizes the surrounding cocoon of dust and gas. Stars then spend a period of time as main-sequence stars, burning hydrogen as their nuclear fuel, before suffering the ultimate fate dictated by their mass.

Throughout the life of a star, a tug-of-war goes between the compressing force of the star's own gravity and the expanding internal pressures generated by the nuclear reactions at its core. In all cases, a young star converts its hydrogen to helium through fusion. Lowmass stars can fuse only hydrogen, and when the hydrogen is used up, fusion stops. The expended star shrinks to become a white dwarf, radiating away its remaining heat slowly over billions of years.

Medium-mass stars swell to become red giants, blowing off planetary nebulae in massive explosions before shrinking to white dwarfs. A star remnant less than 1.44 times the mass of the sun (a mass threshold termed the Chandrasekhar limit) collapses until the pressure in the increasing compacted electron clouds exerts enough pressure to balance the collapsing gravitational force. Such stars become white dwarfs contracted, to the radius of only a few thousand kilometers—roughly the size of a planet. This will be the fate of the sun, since after its red-giant phase it will have lost enough mass to be below the Chandrasekhar limit.

High-mass stars can either undergo carbon detonation or additional fusion cycles that create and then burn increasingly heavier elements as nuclear fuel. These fusion cycles can only use heavier elements up to iron (the main product of silicon fusion). Eventually, as iron accumulates in the core, the core can exceed the Chandrasekhar mass limit and collapse. Electrons are driven into protons in the super-hot environments at the hearts of stars, neutrons are formed and energy is released as gamma rays and neutrinos. After blowing off its outer layers in a supernova explosion (type II), the remnants of such a star form a neutron star, which may also be a pulsar (discovered in the late 1960s).

Although Chandrasekhar did not completely understand the nuclear mechanisms involved in stellar evolution, his work allowed the prediction that neutron stars would be only a few kilometers in radius and that within such a star the nuclear forces and the repulsion of the compressed atomic nuclei would balance the crushing force of gravity. In even more massive stars, however, there was no known force that could withstand gravitational collapse. Such stars would continue their collapse to form a singularity—a point of infinite (or near-infinite) density not accurately described by any

laws of physics yet known. According to general relativity, the gravitational field of such a singularity warps space-time so intensely that not even light can escape from its vicinity: the result is termed a black hole. A black hole is not a hole but a region of intense gravity surrounding a singularity formed by collapsed mass. (The modern terminology presented here was not the language of early twentieth century astronomers.)

German astronomer Karl Schwarzschild (1873–1916) made important early contributions to the properties of geometric space around a singularity when warped according to Einstein's general relativity theory.

Modern Cultural Connections

Several concepts stemming from the evolution of stars have impacted science, philosophy, and popular culture. Most importantly, the articulation of the stellar evolutionary cycle had profound effects on the cosmological theories developed during the first half of the twentieth century, which culminated in the big bang theory. This was proposed by Russian physicist Alexander Friedmann (1888–1925) and Belgian astronomer Georges-Henri Lemaître (1894–1966) in the 1920s and subsequently modified by Russian-born American physicist George Gamow (1904–1968) in the 1940s.

According to the big bang theory, the universe began in a state of extremely small size and high energy density. As its size grew, its energy density lessened, until a point was reached when atoms could form, and our present-day universe of stars and galaxies began to take shape. The theoretical understanding of nucleosynthesis—the formation of new elements under conditions of high pressure, radiation, and temperature—that was developed by astronomers during their efforts to understand the evolution of stars indicates that only hydrogen, helium, and traces of lithium and beryllium were produced in the big bang. As this became understood, scientists realized that all atoms of heavier elements, such as carbon and oxygen, up to and including iron and nickel, must have had their genesis in the cores of massive stars in the final stages of their evolution. The energy released in supernova explosions created shock waves that gave birth via fusion to elements with still heavier nuclei, allowing the creation of radioactive isotopes.

The philosophical implications of this new cosmological world-story were as startling, in their own way, as the quantum and relativity theories that underpinned the model. Essentially, all elements heavier than hydrogen have been literally cooked into existence in the hearts of stars. The universe's first generation of stars consisted almost entirely of hydrogen. Blown back into interstellar space by the violent process of stellar death, the new-forged atoms of heavier elements drifted for many billions of years before regrouping to form second-generation solar systems such as our own, including

Earth. The oxygen, carbon, silicon, metals, and other heavier elements that make up most of our environment, and indeed our own bodies, are recycled star-matter.

By the mid-twentieth century, people could look into the night sky with the knowledge that they are, as American astronomer Carl Sagan (1934–1996) once observed, literally stardust.

See Also Astronomy and Cosmology: Big Bang Theory and Modern Cosmology; Astronomy and Space Science: Stellar Astronomy; Physics: Nuclear Physics;

bibliography

Books

Hawking, Stephen. A Brief History of Time. New York: Bantam Books, 1998.

Hoyle, Fred. Astronomy and Cosmology. San Francisco: W.H. Freeman, 1975.

Sagan, Carl. Cosmos. New York: Random House, 1980.

Trefil, James S. Space, Time, Infinity: The Smithsonian Views the Universe. New York: Pantheon, 1985.

K. Lee Lerner