Supernova - astronomy.

Supernova - astronomy. I INTRODUCTION Supernova, violent explosion that occurs when gravitation causes a star to collapse onto itself. Supernovas, also called supernovae, can be brighter than all the stars in an entire galaxy combined and can shine for weeks or months. The extreme conditions in supernova explosions forge atomic particles into chemical elements--all the atoms in the universe that are heavier than iron were formed in supernovas. The explosions also spread gas and dust into space where the chemically enriched interstellar material can form new stars and planets--almost every substance on Earth is made in part or in whole from the ashes of supernovas. Supernovas can leave behind some of the strangest objects in the universe--neutron stars and black holes--and may give off bursts of gamma rays and cause ripples in space-time called gravitational waves. A supernova also unleashes intense electromagnetic radiation that could destroy all life on Earth if one exploded too near to our solar system. The extreme brightness of some supernovas allows astronomers to measure the distances and motions of galaxies far back into time, indicating how fast the universe is expanding. Supernovas are very rare events and the vast majority of stars, including our Sun, do not have enough mass to reach a supernova stage. Because the universe is filled with billions of galaxies, astronomers are able to observe a few hundred distant supernovas a year. Throughout the observable universe, in fact, a supernova goes off about every second. However, most of these events are not detected because astronomers cannot scan the entire universe at once. Over the past 1,000 years only about a half-dozen supernovas have exploded in our own Milky Way Galaxy. Supernovas are distinct from novas (from the Latin term nova stella meaning "new star"). Novas occur in double-star systems that have a normal star and a white dwarf star orbiting close together around a common center of gravity. Gas drawn off the normal star explodes on the surface of the white dwarf star in a nuclear reaction that brightens then fades. The process can happen repeatedly. A supernova is a one-time event that affects the core of the star, not only its surface, and is billions of times brighter than a nova. II TYPES OF SUPERNOVAS How a supernova explodes and what kind of object is left by the explosion depends on basic features of the star itself. A star forms from a cloud of gas and dust that contracts in on itself from the force of gravity. If the object at the center of the collapsing cloud has at least 8 percent the mass of the Sun, gravity can compress the gas in the core of the object to a point where the mutual repulsion of atomic nuclei is overcome and nuclear fusion of hydrogen into helium begins. In the process a small amount of matter is turned into large amounts of energy as expressed in Einstein's famous equation E = mc2 (energy equals mass times the speed of light squared). The energy released at the star's core heats and pushes back the outer layers of gas, and the star begins to shine. A star's existence is a constant battle between gravity pulling gases in toward the center and energy pushing back. Over a star's life, lighter elements are fused together into heavier elements. The star's final fate is tied to how long it can sustain nuclear fusion. As a basic rule, the more massive the star, the hotter and faster it will burn up its nuclear fuel, and the more cycles of burning heavier elements it can reach. An extremely massive star may last only for a few million years, compared to the 10-billion-year life span of our Sun. See also Life Cycles and Ages of Stars: Star (astronomy). In very large stars known as supergiants, the fusion processes can continue to create and burn nuclei of heavier elements including carbon, oxygen, nitrogen, neon, and silicon. Fusion of different elements occurs at different layers in the stars, giving them an onion-like structure. If these stars are less than 10 times the mass of the Sun, they generally will shed enough of their outer layers to end up as white dwarf stars. Supergiant stars that have at least 8 to 10 times the mass of the Sun can explode as supernovas. A Classification of Supernovas by Light Spectrum The spectrum of the light from supernovas is an important source of information that can be used to help classify supernova explosions. Spectral lines indicate what chemical elements are present and what temperatures occur in the explosion. These spectral lines vary as the brightness of a supernova fades. Astronomers use the term Type I for supernovas that do not show spectral lines of hydrogen and Type II to describe supernovas that do. It is thought that Type I supernovas explode after the stars have lost their outer layers of hydrogen gas. Type II supernovas explode at a stage when the stars still have hydrogen gas in their atmospheres. Scientists recognize a number of subtypes for Type I supernovas, known as Type Ia, Type Ib, and Type Ic. Like Type Ia supernovas, Types Ib and Ic lack hydrogen lines in their spectra. However, they result from a different explosion mechanism from Type Ia--core collapse of a giant star rather than from the thermonuclear explosion of a white dwarf star. Different types of supernovas also have distinct light curves or patterns of brightness over time. Type Ia supernovas are typically brighter than other supernovas, while Type Ib and Type Ic are generally dimmer. The expanding debris from Type I supernovas gets its energy for shining from the radioactive decay of isotopes created in the explosion. Type II supernovas are thought to shine as the envelope of material that surrounded the giant star before it exploded is heated by the shock wave created in the core collapse event. B Classification of Supernovas by Explosion Mechanism Another way to classify supernovas is by how they explode. Two basic mechanisms involving gravitation are thought to cause a star to collapse onto itself, generating a supernova explosion: (1) core collapse and (2) excess matter added to a white dwarf, causing a thermonuclear explosion. B1 Core-Collapse Supernovas Giant stars that contain more than about 10 times the Sun's mass can form elements all the way up to iron in their cores. When the iron fuses to form still heavier elements, the process uses up energy instead of releasing it. When no more energy is released in the star's core, the outward pressure that countered the inward pull of gravity disappears. The core of the giant star collapses in seconds. Gravity causes the rest of the star to crash in toward the core, resulting in a catastrophic explosion. Core-collapse supernovas can leave behind superdense neutron stars or black holes. All Type II supernovas result from core collapse. Type Ib and Type Ic supernovas are also thought to explode from core collapse, but only after much of their outer layers were expelled. Type Ib supernovas have shed their hydrogen and show spectral lines of helium. Type Ic supernovas have also shed the helium layer, revealing the carbon and oxygen layer below by their spectral lines. The term hypernova has been proposed for an extremely massive core-collapse supernova--possibly more than 100 times the mass of the Sun. A hypernova is thought to form a black hole. Just before it explodes, a hypernova may release a huge burst of gamma rays in a jet from the rotating black hole at its center. These jets may explain the so-called long gamma-ray bursts detected by astronomers. According to some researchers, massive stars with over 40 solar masses may sometimes collapse directly into a black hole without generating an explosion. Such an event might generate gravitational waves. Because giant stars have short lives that last only a few million years, the huge stars that explode as core-collapse supernovas are only found in regions of galaxies that have populations of new stars. New stars can form in irregular galaxies and in the spiral arms of spiral galaxies, both places where core-collapse supernovas have been observed. However, core-collapse supernovas are not seen in elliptical galaxies, which are populated by older stars and lack active star-forming regions. The supernovas that occur in elliptical galaxies result from a different explosion mechanism, which can also cause supernovas in irregular and spiral galaxies: thermonuclear explosions. B2 Thermonuclear Explosion Supernovas A very different kind of supernova is thought to come from a white dwarf star that explodes when too much extra matter is added to its surface. The extra matter is drawn off a nearby companion star in a double-star system. The white dwarf cannot support the added mass and begins to collapse on itself. The mass limit at which this collapse occurs is 1.4 times the mass of our Sun and is called the Chandrasekhar limit after the scientist who proposed it. The inward pull of gravity violently fuses the atomic nuclei in the white dwarf together, resulting in a thermonuclear explosion that incinerates the entire white dwarf. Only a nebula of gas and dust remains, called a supernova remnant. The heat from the radioactive decay of the unstable isotope nickel-56 created in the fusion processes makes the debris shine for weeks or months. Such explosions appear as Type Ia supernovas and can occur in older star regions of all types of galaxies, including elliptical galaxies. Because Type Ia supernovas occur within very strict mass limits, all Type Ia supernovas explode with about the same level in brightness, making them very useful tools for astronomers. Because they all have the same luminosity, the apparent brightness of a Type Ia supernova indicates its distance. III FORMATION OF CHEMICAL ELEMENTS Supernovas play a fundamental role in the formation of the chemical elements, a process known as nucleosynthesis. The first nucleosynthesis occurred for a few minutes in the extreme conditions following the big bang, creating hydrogen, helium, and a small amount of lithium. This material was later processed into heavier elements by stars. Stars burn by fusing lighter elements into heavier elements--hydrogen into helium, helium into carbon, carbon into oxygen. The orderly burning processes in stars can only build up chemical elements from helium (2 protons) to as high as iron (26 protons) on the Periodic Table. Every atom of elements in nature heavier than iron--from cobalt (27 protons) to uranium (92 protons)--comes from a supernova explosion. The range of elements created in a supernova explosion depends on the type of supernova. Core-collapse supernovas can create elements up to uranium. They also release large amounts of oxygen. Thermonuclear supernovas generally create elements as heavy as iron, cobalt, and nickel. In fact, most of the iron on Earth is thought to come from Type Ia explosions. The supernova explosion blasts the elements created inside the star out into space, at the same time transforming some of that material into even heavier elements. The extreme conditions in the supernova explosion allow different processes to create new elements. Because the number of protons in an atomic nucleus determines the identity of an element, processes that add or remove protons can change one element into another. Enormous quantities of neutrons are released in a core-collapse supernova explosion. Free neutrons are not stable--they decay into protons and electrons with a lifetime of about 10 minutes. The neutrons also bombard some of the atomic nuclei that were formed by the regular fusion processes inside the star, creating isotopes with extra neutrons. If the number of neutrons added to a nucleus is much greater than the number of protons, the extra neutrons can become unstable and decay into protons, raising the atomic number of the nucleus to create heavier elements. In addition to neutrons, alpha particles (the same as helium nuclei, 2 protons + 2 neutrons) can fuse to other atomic nuclei, accounting for an excess of new elements whose numbers of protons are multiples of four, such as oxygen (8 protons), magnesium (12 protons), sulfur (16 protons), calcium (20 protons), chromium (24 protons), and nickel (28 protons). In a Type Ia supernova, carbon and oxygen begin a series of fusion reactions that lead up to iron and nickel, also creating silicon, calcium, magnesium, and sulfur. Type Ia supernovas do not create heavy elements such as lead or uranium. IV OBJECTS LEFT BY SUPERNOVAS A Supernova Remnant Nebulas Supernovas are often surrounded by nebulas of material shed by the star before the explosion. The debris ejected by the final supernova explosion may collide with the older material, which can also be heated by the blast. The discovery of a pulsar, interpreted as a rotating neutron star, at the center of the Crab Nebula, a well-known supernova remnant, clinched both the identification of pulsars with neutron stars and the identification of certain types of nebulas as supernova remnants. Supernova remnants from Type Ia supernovas, however, have no such objects left at their centers. B Neutron Stars, Pulsars, and Magnetars As a massive star collapses, the overlying layers add force to the collapsing core. Though the outer layers explode as the visible supernova, the core continues to collapse. The collapse stops if less than four or five solar masses of material remain, leaving a neutron star supported by neutron degeneracy. Neutron stars are only about 20 km (about 12 mi) in diameter and can rotate at hundreds of times a second. The extreme rotation rate results from the angular momentum of the rotating star drawn into a smaller and smaller area, much like ice skaters speeding up a spin by pulling in their arms. In some cases neutron stars can become pulsars, which emit pulsing radio waves into space as they rotate. Magnetars are neutron stars that have magnetic fields a thousand trillion times stronger than Earth's magnetic field. C Black Holes A core-collapse can also result in a black hole in which matter is crushed into a point of infinite density called a singularity. For a black hole to form instead of a neutron star, more than four or five solar masses must be left after a core-collapse supernova explosion. Gravitation overcomes neutron degeneracy and the collapse event does not stop when neutrons form. The size of the black hole that results depends on the mass of the object. The gravitational radius or Schwarzschild radius at which an object becomes a black hole is 3 km (1.8 mi) for each solar mass. A black hole with from 5 to 10 times the mass of our Sun would have a diameter of from 30 to 60 km (20 to 40 mi). The rotating black hole left by an extremely massive supernova (a so-called hypernova) may emit an extremely powerful gamma-ray burst from a jet of matter expelled by the black hole. V A EFFECTS OF SUPERNOVAS Enrichment of Interstellar Matter Supernovas eject material that contains both the elements created over the history of the star and new heavier elements resulting from the explosion. Because so much of the material that once made up the original star is dispersed into space in the explosion, supernovas can add much more to the interstellar medium than regular stars that shed their outer layers as they age. This supernova material provides the most of the elements that go into the later generation of stars and their surrounding planets. B Shock Waves Supernovas may play an important role in causing new stars and solar systems to form beyond supplying the raw materials. Shock waves from the explosion can compress and disturb the interstellar medium, causing clouds of dust and gas to collapse. The collapse can result in the birth of new stars and solar systems. C Gamma-Ray Bursts Powerful gamma-ray bursts (GRBs) coming from space were first detected by military spy satellites in the 1960s. The sources of these GRBs remained elusive until the late 1990s, when astronomers first matched the locations of several gamma-ray bursts with supernovas in other galaxies. Bursts that last over two seconds are thought to come from the collapse of massive stars in Type II supernova explosions that form black holes. See also Gamma-Ray Astronomy. D Cosmic Rays Cosmic rays are protons, alpha particles, and atomic nuclei in space that have been accelerated to extremely high velocities, sometimes reaching a significant percentage of the speed of light. Their source remains a mystery but some cosmic rays may be generated by the most powerful supernova explosions. Interactions with magnetic fields left in the supernova debris and other processes may then accelerate the particles to higher energy levels. VI STUDYING SUPERNOVAS American astronomer Fritz Zwicky of the California Institute of Technology first proposed the term "supernova" in 1934 to distinguish such extremely bright, short-lived stars from regular novas. He also theorized that gravitation caused a supernova to explode. As late as the 1970s astronomers detected supernovas using photographs of galaxies taken with telescopes on film or on glass plates. The photographic plates had to be manually scanned to find a bright spot in a distant galaxy. Researchers now have many tools available to study supernovas. Computers can quickly compare electronic images of galaxies to find new supernova explosions. Optical telescopes with special equipment can study the spectra of supernovas to find their distance and chemical makeup. Satellites and space telescopes can study electromagnetic radiation that is blocked by Earth's atmosphere such as ultraviolet radiation, X rays, and gamma rays that are emitted by supernovas or by the remnants of exploded supernovas. Radio waves can also reveal details about supernovas and their remnants, including pulsars. More unusual instruments for studying supernovas include neutrino and gravitational wave detectors. Supercomputers can help model the complex events that cause supernovas to explode. Astronomers name supernovas with the abbreviation SN followed by the year and a letter or pair of letters to indicate the order in which the supernova was discovered. For example, SN1987A was the first supernova recorded in 1987. The system for naming supernovas first uses single capital letters such as A, B, and C, then pairs of lower case letters such as aa, ab to az, followed by ba, bb on to bz, and so on. So many supernovas are now detected in a year that double letters are commonly used as with SN2006gy. A Early Observations The earliest historical observations of supernovas come from Chinese and Korean chronicles. The supernova of AD 185 is the earliest documented for our galaxy. The supernova of 1006 may have been the brightest ever, shining visibly in the daytime for months. The supernova of 1054, which left behind the Crab Nebula, was observed in China but is not recorded in records in Europe. Tycho Brahe's supernova of 1572 and Johannes Kepler's of 1604 round out the reported optical sightings. B Discovery of Supernova Nucleosynthesis It was only in the 1950s that scientists fully recognized the fundamental role stars and supernovas play in the formation of chemical elements. Some previous theories had proposed that the heavier elements formed in the big bang or in interstellar space. C Supernova SN1987A Astronomers equipped with telescopes, satellites, and other modern technologies had not been able to record and study a nearby supernova until 1987. In February of that year supernova SN1987A was detected 170,000 light-years away in the Large Magellanic Cloud, a companion galaxy to the Milky Way that is visible in the Southern Hemisphere but not in the United States. The supernova surprised astronomers in many ways. SN1987A had hydrogen lines in its spectrum, making it a Type II supernova--but when the star was found in old photographs, it was a blue star, not an old red supergiant. Type II supernovas are also thought to form neutron stars or sometimes black holes. However, careful searches have found no evidence yet of such a remnant object from SN1987A. Researchers continue to study exactly how SN1987A exploded. The remnant of supernova 1987A shows a ring of matter very close to the exploded star's position. The ring was first seen with the high-resolution images of the Hubble Space Telescope. The ring is composed of gas emitted long before the supernova explosion and is brightening as a supersonic shock wave from the supernova reaches it. As a result supernova 1987A may return to naked-eye brightness. D Supernova Remnants Since nearby supernovas are such rare events, finding the remains of older supernovas can still provide important information. Few supernova remnants are known in the optical spectrum. But supernova remnants show up much more strongly in the radio spectrum. NASA's Chandra X-ray Observatory, with its high-resolution X-ray imaging and energy discrimination, allows the mapping of many supernova remnants and, in a number of cases, their identification with neutron stars. In 2007 Chandra identified Kepler's supernova of 1604 as Type Ia based on its remnant. E Measuring the Universe with Supernovas Astronomers have found Type Ia supernovas especially useful as tools for measuring distances to other galaxies. All Type Ia explosions are thought to have the same peak intrinsic brightness. As a result, the dimmer the explosion appears to an observer on Earth, the further away and further back in time the explosion must have been. Using a 19th-century mining analogy, astronomers call such objects of known, standard brightness "standard candles." In reality, there are small differences between the brightness of particular Type Ia supernovas, but these differences can be adjusted for. Since supernovas are very bright, they can be seen quite far across the universe (or, equivalently, quite far back in time). For example, the Hubble Space Telescope has detected Type Ia supernovas as far as 8 billion light-years away, when the universe was about only one-third its present age. The study of Type Ia supernovas at great distances, therefore, is a key to understanding the rate of the expansion of the universe. Studies in late 1990s of Type Ia supernovas in very ancient and distant galaxies showed that the expansion of the universe is accelerating. Previously, astronomers had assumed that the expansion of the universe was slowing down from the mutual gravitational pull of objects in the universe. The accelerating expansion may be caused by an unknown substance, a property of space that has become known as "dark energy." F Future Supernovas in the Milky Way Galaxy A supernova too near the Earth could threaten human existence, if not all life on the planet. Bursts of ultraviolet radiation, X rays, or gamma rays from the explosion would disrupt or partially destroy Earth's atmosphere, and the high-energy radiation could be lethal to living creatures. Nearby supernova explosions may have caused mass extinctions of life on Earth in the distant past. The safe distance from most supernova explosions is thought to be beyond about 160 light-years. However, a massive gamma-ray burst from a "hypernova" could be deadly from a much farther distance. The star Eta Carinae in the southern constellation Carina ("the keel") is over a million times brighter than our Sun. It has been undergoing major eruptions thought to precede a supernova explosion. Since Eta Carinae is 7,500 light-years from us, only astronauts and spacecraft would be severely affected by radiation from the explosion, although Earth's ozone layer might be damaged. Other candidates for future supernovas include the Pistol Star at 25,000 light-years away and LBV 1806-20 at 45,000 light-years away. Both are massive enough to form black holes when they explode and possibly give off an enormous burst of gamma rays. Nearer and smaller stars are also candidates for supernovas. The red supergiants Betelgeuse and Antares are between 400 and 600 light-years away. Both are also large enough, with estimated masses of about 15 to 18 times that of the Sun, to one day become core-collapse supernovas. They may form neutron stars, but are likely not massive enough to unleash gamma-ray bursts that could endanger Earth. G Remaining Mysteries Supernovas are one of the most important areas of research in modern astronomy. All the processes that make supernovas explode and that create chemical elements are still not understood in detail. The role of neutrinos in particular is under study. Other areas of ongoing research include what the very first supernovas were like after the big bang--these gigantic stars were made of pure hydrogen and helium that made them burn differently from modern giant stars that contain additional elements (called "metals" by astronomers). Also to be answered is whether all Type Ia supernovas explode at the same stage or if rotational speeds affect such explosions. New space telescopes and satellites with greater power and sensitivity are being built or planned to help astronomers find answers to many of these questions. In 2006, astronomers witnessed the brightest supernova ever recorded, in a galaxy in the constellation Perseus. Designated SN2006gy, the supernova gave off 100 times more energy than other supernovas. Some astronomers theorize that the event may represent a new mechanism of supernova, with energy provided by the production of electrons and positrons out of high-energy gamma rays. Other astronomers are trying to modify core-collapse models to account for SN2006gy's extremely energetic output. The unusual features of SN2006gy show that many more mysteries about supernovas remain to be solved. Contributed By: Jay M. Pasachoff Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.