Fritz Zwicky and Walter Baade proposed that a neutron star forms during the explosion of a supernova.

The Greek philosopher Aristotle taught that the stars, the Sun, and the planets were located on crystal spheres that moved around the stationary Earth. On rare occasions, a new star would appear in the sphere, shining so brightly that it was visible during the day, and finally growing dim over several months. These were called “novas” (or “novae”) from the Latin word for “new,” because they seemed to be new stars. (Centuries later, research would reveal how wrong that name is.) [kw]Zwicky and Baade Propose a Theory of Neutron Stars (1934)
[kw]Baade Propose a Theory of Neutron Stars, Zwicky and (1934)
[kw]Neutron Stars, Zwicky and Baade Propose a Theory of (1934)
[kw]Stars, Zwicky and Baade Propose a Theory of Neutron (1934)
Astronomy;stars
Stars;neutron
Supernovas
Neutron stars
[g]United States;1934: Zwicky and Baade Propose a Theory of Neutron Stars[08540]
[c]Science and technology;1934: Zwicky and Baade Propose a Theory of Neutron Stars[08540]
[c]Astronomy;1934: Zwicky and Baade Propose a Theory of Neutron Stars[08540]
Baade, Walter
Zwicky, Fritz
Landau, Lev Davidovich

In the past two thousand years, only seven bright novas Novas remained visible in the northern sky for at least six months each. In 185 c.e., for example, the Chinese recorded the appearance of a “guest star” that lasted for twenty months. Another such star in 393 c.e. lasted for eight months. The nova of 1006 was visible for several years and was recorded by the Chinese, Japanese, Europeans, and Arabs. The nova in 1054 lasted twenty-two months and was noted by the Chinese and Japanese; there is evidence from several petroglyphs in the American Southwest that Native Americans also observed the event.

A nova that appeared in 1572 was observed by the last great astronomer before the age of telescopes, Tycho Brahe. Brahe, Tycho It was Brahe who gave novas their name. Because of his study of that nova, known as Brahe’s star, and his subsequent book titled De Nova Stella, Brahe’s reputation was made. The German astronomer Johannes Kepler, Kepler, Johannes who was Brahe’s assistant in later years, studied the 1604 nova, Kepler’s star. After Brahe’s death in 1601, Kepler used the astronomer’s data to explain the motion of the planets. The appearance of the nova drove Kepler to greater efforts that resulted in three laws of planetary motion.

A neutron star’s strong magnetic field generates radiation that can be detected on Earth as radio waves.

Over the centuries, advances in telescopes and other astronomical instruments led to a better understanding of novas. Studies suggested that novas were not a simple class of stars. Some were bright and rare, whereas others were much fainter and more common. Fritz Zwicky and Walter Baade recognized in 1934 that a division was necessary and renamed the brighter novas “supernovas.”

Stars are “born” from collapsing clouds of gas and dust. As they become older, the interior pressure and temperature increases, producing chemical reactions in which hydrogen fuses into helium. Energy is released from this reaction in the form of light and other electromagnetic radiation. The length of a star’s life cycle is determined by its mass. Low-mass stars such as the Sun fuse the hydrogen slowly and have lifetimes of tens of billions of years. The most massive stars have lifetimes of tens of millions of years. As the star “dies,” it can do so in one of several ways. The low-mass star depletes its hydrogen supply, grows in size to become a “red giant,” Giant stars and then collapses to a “white dwarf” phase. Dwarf stars
White dwarf stars It shines by its stored heat until eventually it cools and reaches the “black dwarf” stage. Black dwarf stars A star the size of the Sun will shrink to the size of Earth.

A binary star system Binary stars is a pair of stars that orbit a common center of gravity. A binary star system whose stars are near the end of their life cycles is the common source of nova explosions. Material from one of the stars is pulled onto the surface of the other star. When enough of it accumulates, it will fuse to helium and produce the brightening that can be seen as a nova. That is why the word “nova” is not really correct—a nova is not a new star, but rather the death of a star in a binary system.

A supernova, however, is the very rapid explosion of a very massive star near the end of its life cycle. This is evident because in the constellation Taurus, the location of the 1054 nova, lies a gaseous mass known as the Crab nebula. Crab nebula This gas cloud is expanding outward. Calculations of the velocity of the cloud’s gas show that, after the explosion, it started its outward journey in 1054.

When a massive star depletes its supply of hydrogen, it collapses and its internal heat and pressure increase until helium is converted to carbon. Elements with increasingly higher atomic numbers are formed as the collapse continues. Once the core becomes the element iron, the process cannot continue until more energy is added. At this point, the collapse continues because of gravity; in the last stages, the star’s outer layers hit the core and bounce. The star explodes, sending a large part of its mass into space. The remainder of the supernova collapses to become a neutron star or a black hole, Black holes depending on its mass.

Zwicky and Baade, and independently Lev Davidovich Landau, postulated that, after this explosion, the pressure of the star’s collapse overcomes the atoms’ electrical forces and fuses protons and electrons into neutrons. This explanation was not verified experimentally until Jocelyn Bell Bell, Jocelyn , a graduate student at Cambridge, discovered the first pulsar Pulsars in 1967. A pulsar is a neutron star that spins very rapidly, emitting radio waves from its rotating magnetic field. First thought to be signs of extraterrestrial intelligence, these pulsars were the first observational evidence of Baade and Zwicky’s theory.

For thousands of years, people have tried to figure out how the planets came into being. The discovery and understanding of neutron stars and supernovas have helped scientists to solve the puzzle. The “big bang theory” Big bang theory suggests that the universe began about fifteen billion years ago. At that time, all the energy and matter in the universe were contained within a small sphere that exploded. As the universe—the pieces of matter sent flying by the explosion—cooled and expanded, hydrogen and helium began to form. Eventually, clouds of hydrogen and helium collapsed to form stars, and these formed into galaxies.

A supernova is born at the end of a massive star’s life.

As these stars aged, the more massive ones exploded as supernovas. The blast spewed into the surrounding space all the chemical elements contained in the star, enriching the interstellar medium with essential ingredients. After enough stars had become supernovas, planets could begin to form. So, too, could the next generation of stars, which would benefit from the enriched gas and dust clouds of the planets. Before this time, planets could not exist. Life as it is now known could not exist because the essential elements on which life depends—carbon, nitrogen, and others—did not exist. Astronomy;stars
Stars;neutron
Supernovas
Neutron stars

• Bethe, Hans, and Gerry Brown. “How a Supernova Explodes.” Scientific American 252 (May, 1985): 60-68. Bethe was the first person to show how stars convert mass into energy. Discusses the mechanics of the massive star’s implosion and resulting explosion. Also explores the star’s preexplosion history. Rich with figures and diagrams. Intended for informed readers.
• Charles, Philip, and J. Leonard Culhane. “X-Rays from Supernova Remnants.” Scientific American 233 (December, 1975): 36-46. Points out that when a supernova occurs, huge amounts of energy in the form of electromagnetic radiation are sent into space. X rays produced by the explosion are analyzed for information about the explosion.
• Herbst, William, and George Assousa. “Supernovas and Star Formation.” Scientific American 241 (August, 1979): 138-145. Shows how theory and observations support the idea that supernovas trigger the formation of other stars. Also discusses the formation of the spiral structure of some galaxies.
• Kirshner, Robert. “Supernovas in Other Galaxies.” Scientific American 235 (December, 1976): 89-101. Points out that, because supernovas are rare phenomena in Earth’s galaxy, scientists must look to other galaxies such as the Andromeda galaxy to find examples to study. This reveals a difference in supernovas that leads to the classifications of Type I and Type II supernova explosions.
• Mitton, Simon. The Crab Nebula. New York: Charles Scribner’s Sons, 1978. An in-depth study of the 1054 supernova in the constellation Taurus. Covers the historical records of the Asian and American Indian observers, the telescopic observations of the Crab nebula, the formation of a neutron star, and how pulsars pulse.
• Motz, Lloyd, and Jefferson Hane Weaver. The Story of Astronomy. New York: Plenum, 1995. Presents the history of astronomy from ancient times to the end of the twentieth century. Chapters 16 and 17 include discussion of supernovas and galactic research in general. Features bibliography and index.
• Schramm, David, and Robert Clayton. “Did a Supernova Trigger the Formation of the Solar System?” Scientific American 239 (October, 1978): 124-139. Explores the implications of small amounts of decay product from short-lived radioactive isotopes found in primitive meteorites. Concludes that the original isotopes were formed in a nearby supernova explosion that triggered the formation of the solar system.
• Seward, Frederick, Paul Gorenstein, and Wallace Tucker. “Young Supernova Remnants.” Scientific American 253 (August, 1985): 88-96. Reports on the X-ray data obtained from the Einstein X-Ray Observatory and its relationship to supernova remnants. Shows that the X-ray spectrums for Type I and Type II supernovas are different.
• Silk, Joseph. The Big Bang. 3d ed. New York: W. H. Freeman, 2000. Presents a sweeping account of the formation and evolution of the universe. Recounts the history of astronomical speculation about the universe and examines evidence for the big bang theory. Includes glossary and index.
• _______. On the Shores of the Unknown: A Short History of the Universe. New York: Cambridge University Press, 2005. A history of the universe and the development of humankind’s knowledge about it. Accessible to lay readers. Includes illustrations and index.
• Stephenson, F. Richard, and David Clark. “Historical Supernovas.” Scientific American 234 (June, 1976): 100-107. Reports on seven supernovas of the last two millennia and the search for their remnants. Also discusses the recorded observations of various cultures.
• Wheeler, J. Craig, and Robert Harkness. “Helium-Rich Supernova.” Scientific American 259 (November, 1988): 50-58. Reveals that there is a Type Ib subcategory supernova, similar to a nova explosion because it occurs in a binary star system. A massive companion star expands and drives matter into the other star, ripping that star’s outer layer away and exposing the core, which subsequently collapses and explodes.

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