Nova - astronomy. I INTRODUCTION Nova, explosion on a star that makes the star increase rapidly in brightness. The star then slowly declines to its original level (see Star). Novas are members of a much larger class of binary, or double, star systems called cataclysmic variables. Early astronomers used the term nova (Latin nova stella, "new star") to describe all astronomical objects that suddenly became visible to the naked eye. Astronomers now differentiate between novas and supernovas. Supernovas are very bright outbursts that radically change the original star. A nova explosion is caused by the interaction of two stars that are close to each other. The gravitational pull of one star pulls off and drags the atmosphere of the other star onto its own surface. When the collected layer of atmosphere becomes thick enough, it explodes. After a nova outburst, the stars return to their original state relatively unchanged, and the system returns to its original brightness. A nova outburst is one of the most dramatic events in astronomy. A nova appeared in the constellation Cygnus (the Swan) in 1975, rising to almost the brightness of Deneb (the brightest star in Cygnus). It stayed bright for about three days. While the brightness of most novas increases up to 1 million times that of the normal star, the brightness of this nova, called Nova Cygni 1975, increased at least 100 million times. II THE FORMATION OF A NOVA SYSTEM Nova systems consist of two stars orbiting each other. One star is a small--but very dense--hot star called a white dwarf. The other is a normal star, usually about the size of the Sun. Researchers designate the white dwarf the "primary star" and the other star the "secondary star." A white dwarf star has about as much mass as the Sun, but its radius is about equal to that of Earth. This means that the white dwarf has a very high core density--more than a million times that of water. A thimbleful of material from the center of a white dwarf would weigh more than a ton. White dwarfs are the endpoints of stellar evolution for stars like the Sun. The existence of a white dwarf in a double star system indicates that the system is old and has had enough time for one component to complete its evolution. See also Binary Star. The evolution of a star like the Sun begins with the conversion of hydrogen into helium in its central region. This occurs through the process of thermonuclear fusion (see Nuclear Energy: Nuclear Fusion). In thermonuclear fusion, the heat and pressure of the star's core force hydrogen atoms together, forming helium atoms and releasing huge amounts of energy. As the core converts into helium, the outer layers of the star begin to expand. After about 10 billion years, the star becomes a red giant. (When the Sun reaches this stage of evolution, its surface will extend past the orbit of Mars.) After further evolution, the helium fuses to carbon, oxygen, and sometimes neon. During these later stages, the core becomes more and more dense, and the outer layers extend more and more until they disperse into space. A single star at this stage has a short-lived shell of gas around it and is called a planetary nebula (see Nebula). The star has used up its hydrogen and helium fuel, and the heavier elements require too much energy to fuse. Therefore, no further thermonuclear processes occur in the star, and it has become a white dwarf surrounded by a gaseous nebula. The primary star in a nova system undergoes the evolution described above. However, because the primary star is so close to its neighbor, the secondary star, its evolution and the gravitational pull of the secondary star work together to draw the two stars closer and closer together. The gravitational pull of the secondary star strips the ejected outer layers off of the primary star until the primary star is reduced to its central white dwarf core. This is called common envelope evolution. The secondary is still a normal, Sun-like star, but the primary, with all its hydrogen and helium fuel spent, is a gradually cooling white dwarf star. The white dwarf consists of a carbon and oxygen core surrounded by a very thin layer of helium. As the secondary star evolves, it begins to expand. Eventually the outer layers of the secondary star extend far enough that the gravitational forces of the white dwarf pull the gases of the secondary star's atmosphere into a torus, or doughnut shaped, ring of gas around the white dwarf. Astronomers call this torus an accretion disk. The material in the accretion disk is mostly hydrogen. Material from the inner part of the accretion disk falls onto the surface of the white dwarf star. The gas falling onto the surface of the white dwarf forms a thicker and thicker layer as the gas from the secondary star flows onto the accretion disk and then onto the white dwarf. The pressure of the accumulating material compresses and heats the bottom of the layer to temperatures at which fusion can occur. Fusion is a basic nuclear process that converts four hydrogen nuclei to one helium nucleus and releases a large amount of energy. This is the same process that provides our sun's energy. III THE CAUSE OF THE OUTBURST After decades or thousands of years of accretion (depending on the system), the bottom of the accreted layer becomes so dense that the electrons in the gas lose their attachment to any particular atom. Instead, they move freely among the atoms, just like the electrons in a typical metal on Earth. This makes the gas in the deeper layers of the white dwarf behave like a metal and prevents the gas from expanding and cooling the growing explosion. Heat pours into the accreted layer from the nuclear fusion in the core, increasing the temperature to more than 100,000,000° C (200,000,000° F). The inner layers become so hot that they overcome their resistance to expansion. The hot accreted layers begin to expand. The expanding layers increase the brightness of the star, creating what astronomers call a nova. The basic process described above is sufficient to produce slow novas, but an additional factor is needed to create a fast nova. In fast novas, the proportion of carbon and oxygen nuclei in the gas must be far larger than it is in the Sun. The carbon and oxygen act as a catalyst, causing the nuclear reactions to proceed much faster than they would without the presence of carbon and oxygen. The presence of carbon and oxygen also produces a much stronger explosion. The extra carbon, oxygen, and sometimes neon come from the mixing of the accreted layers with the material originally on the white dwarf. Astronomers do not yet know how this mixing takes place. IV THE OUTBURST A nova reaches its maximum brightness as layers of gas expand off of the white dwarf after fusion reactions begin to occur in the layer of accreted material. The explosion that carries away the accreted material is so powerful that most of the gas escapes the gravitational pull of the nova system and drifts off into space. The time that a nova takes to reach its maximum brightness and return to normal levels varies widely. So-called fast novas reach maximum brightness within a few days and stay at maximum brightness from a few hours to a week. They decline in brightness very rapidly, returning to normal levels within a few months. Slow novas rise more slowly to maximum, spend more time at their maximum, and decline more slowly. Slow novas also tend to be more erratic than fast novas in their brightening. For example, Nova Herculis, which exploded in 1934, stayed at or near maximum for almost three months. It then dropped rapidly in brightness for about a month, recovered slightly, and continued in slow decline for a number of years. Another very slow nova, Nova Delphini 1967, stayed near maximum for almost a year. Astronomers use a technique called spectroscopy to study nova systems. Using spectroscopy, astronomers can separate the light of a star into the wavelengths that make it up. The separated light is called a spectrum. Spectroscopy allows astronomers to learn about the composition and temperature of a star by examining its spectrum. The wavelengths, or color, of light that a star emits are a measure of its temperature. Different chemical elements absorb and emit light at different wavelengths, so astronomers can tell which elements are present by looking for especially bright or especially dark lines in the spectrum. When first discovered, the spectrum of a nova shows that the expanding layers of gas that cause the brightening have temperatures of 40,000° to 50,000° C (70,000° to 90,000° F)--about eight times as hot as the surface of the Sun. By the time a nova reaches maximum brightness, the temperature of the material has fallen to about 10,000° C (about 20,000° F), or lower. Just after maximum brightness, the escaping cloud of gas cools and expands enough to become transparent. This transparency allows astronomers to view all of the gas that the star has ejected. They have found that a typical nova outburst blows away about 0.01 percent of the mass of the star. The material that the star loses is very different from the material found in the atmosphere of a normal star. The expanding cloud of gas contains much higher levels of helium and lower levels of hydrogen. Carbon, nitrogen, oxygen, and sometimes neon exist in much higher levels in the nova cloud than in the atmosphere of a normal star. Astronomers have also discovered a relationship between the speed of the nova and the amount of heavier elements, such as carbon and nitrogen, in the cloud. Fast novas generally (but not always) eject clouds that contain larger proportions of carbon, nitrogen, and oxygen than the clouds of slow novas. V SPECIAL NOVAS The classical novas described above are just one part of a group of stellar systems called cataclysmic variable stars. All cataclysmic variables are systems of two stars, one of which is a small dense star and the other a normal star. Two of the other cataclysmic variables most closely related to classical novas are dwarf novas and recurrent novas. Dwarf novas brighten and return to normal on an irregular cycle of weeks to months. Their maximum brightness is much less than that of classical novas. Recurrent novas brighten on a cycle on the scale of decades. Their maximum brightness is again less than that of classical novas, but their return to normal brightness is more sudden. A Dwarf Novas A dwarf nova's brightest point is only about 100 times brighter than the system at minimum. These outbursts reach maximum in a matter of hours and stay bright for only a few days. A dwarf nova's outbursts are not caused by the same runaway thermonuclear reactions that fuel classical novas. Instead, the brightness of the system changes according to how fast the gas in the accretion disk flows onto the primary star. When the flow of gases from the secondary to the primary star is fast, the system glows brighter. When the flow of gases slows, the system dims. B Recurrent Novas All novas have repeated outbursts, but it will take hundreds or thousands of years for the hydrogen fuel to flow onto the white dwarf and build up the explosive layer to fusion temperatures. Astronomers call novas that go through the explosion on the scale of decades recurrent novas. Recurrent novas brighten more than dwarf novas but less than classical novas. In a recurrent nova system, the secondary star is a red giant star. Red giants lose material faster than the normal stars of the classical novas, so the layers in the accretion disk build up faster and, thus, they explode sooner than in classical novas. Recurrent novas build up another explosive layer every few decades. They eject only a small fraction of the accreted layers during the explosion and the white dwarf grows in mass until it explodes as a supernova. Contributed By: Sumner Starrfield Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
