Supergiant (star) - astronomy.

Supergiant (star) - astronomy. I INTRODUCTION Supergiant (star), extremely large, luminous star that can be seen from vast distances across space. Supergiants are stars that have evolved through several stages, converting the nuclear fuels in their cores to successively heavier elements at each stage. They often explode as supernovas when the nuclear fuels in their cores are exhausted. Supergiants form in the same way that ordinary stars form--a cloud of hydrogen gas and interstellar dust compresses under the gravitational attraction of its matter for itself until the temperature at the center of the cloud is hot enough to fuse hydrogen to form helium (see Star: Evolution of Stars). The central region where hydrogen fusion occurs is called the core. After hydrogen fusion occurs in a new star, electromagnetic radiation is released from the core. The radiation creates an outward pressure that balances the gravitational force, and the cloud eventually stabilizes as a main-sequence star--a star in the first and longest phase of its luminous existence (see Star: Spectral Type ). The primary difference between a star destined to become a supergiant and a more typical star is its mass. Astronomers estimate that a star must be at least six to ten times more massive than the earth's sun in order to have a core massive enough to make the star a supergiant. The additional mass leads to stronger gravitational forces that create the temperature and pressure conditions in the core needed to induce the fusion of heavy elements late in the life of the star. The additional mass also causes higher core temperatures in the star's early phases and results in more rapid nuclear reactions than smaller stars. Astrophysicists estimate that the rate of fusion in a star is roughly proportional to the fourth power of its mass. Thus, a star with a mass of ten times the earth's sun consumes hydrogen about 104 (10,000) times faster than the sun. Such a star will consume the hydrogen in its core within a few million years, whereas a star of the sun's mass will last a thousand times longer. II EVOLUTION OF A SUPERGIANT After the hydrogen in a star's core has been consumed, the outward radiation pressure that supported the star dissipates and the star collapses. The outer layers of the star compress enough to cause fusion of the hydrogen in the layer next to the core. This creates a hydrogen-burning shell that causes the outer layers of the star to expand while the inner core of the star continues to contract. The outer layers cool and turn red as they expand, and the star becomes a red giant. If the mass of a red giant star's core exceeds a critical limit, which is approximately 0.7 to 1.0 times the entire mass of the earth's sun, the core will compress until its temperature reaches 100 million °C--hot enough to induce the fusion of helium atoms to form carbon. The radiation released by helium fusion causes the red giant to swell to 500 times the size of the sun or larger and become a red supergiant. Because the mass of a star's core is only about 10 percent of its total mass, only stars with a mass of six to ten suns or more can become supergiants. Antares is a red supergiant star in the constellation Scorpio. It is so large that if it were placed at the center of Earth's solar system, it would engulf the orbits of Mercury, Venus, Earth, and Mars. Betelgeuse in the constellation Orion is another well-known and easily identifiable red supergiant star. The maximum light output of a red supergiant corresponds to an absolute magnitude of about 9--the equivalent light output of 600,000 suns. Knowing the maximum light output of these bright stars allows astronomers to use them as "standard candles" to judge distances to far parts of the Milky Way or even to other galaxies where the stars can be seen. More massive stars--those with masses in the range of 20 to 60 suns--have cores hot enough to induce more energetic nuclear reactions, and they appear to move more directly from the main-sequence stage to the supergiant stage. These blue supergiants may have surface temperatures in excess of 30,000° C--more than five times hotter than the surface of the sun. Although much hotter than red supergiants, blue supergiants do not grow as large, and they only attain about the same intrinsic luminosity as red supergiants. Several blue supergiants are easily found in the earth's night sky. Alnitak, another star found in Orion, is six times hotter than the sun. Although Alnitak is 350 times farther away than the closest star to our solar system, Alpha Centauri, it is one of the most prominent stars in the earth's sky. III THE END OF A SUPERGIANT When the core of a supergiant exhausts all of its helium, it contracts a third time. If the core is sufficiently massive, it will be able to "burn" the carbon and other elements that it formed in its previous phases and will produce even heavier elements. More massive stars will go through a number of successive phases--each shorter in duration than the last--that each produce heavier and heavier elements. Eventually all stars reach a point when their cores can no longer liberate energy from nuclear reactions. This point is reached either when the star's mass is insufficient to create the temperature and pressure necessary to cause fusion of the elements in its core or when the core has been converted to iron. Although there are many elements heavier than iron, there are no known nuclear fusion reactions using iron or heavier elements as fuel that liberate energy. When the nuclear reactions in the core of a supergiant come to an end, the core collapses a final time. Smaller supergiants have cores that are less than a critical value known as the Chandrasekhar limit, which is about 1.4 times the mass of the sun (see Chandrasekhar, Subrahmanyan). The core material of such stars will collapse into a state known as a degenerate electron state and the core will become a white dwarf star. Stars with cores of mass between the Chandrasekhar limit and about three times the entire mass of the sun will eventually become iron. The degenerate electron state of a white dwarf does not have the mechanical strength to support such a massive dense body, and so the core collapses further until the electrons and atomic nuclei in the core are squeezed together to form neutrons. When this happens, the core is squeezed into a spherical shape only 20 km (12 mi) in diameter to form a neutron star. If the mass of the supergiant's core is more than three times the entire mass of the sun, the neutron star condenses even further--literally disappearing from the visible universe as it becomes a black hole. In both cases, the outer envelope of the star is blown away to form a planetary nebula by a supernova explosion that accompanies the collapse of the core. These planetary nebulas contain hydrogen as well as heavy elements--such as carbon, oxygen, nitrogen, and iron--that were synthesized in the core of the star during its earlier phases. Thus, supergiants are a major source of the heavier elements found in the universe. In fact, astronomers generally agree that the earth and all of the earth's living organisms are made of matter that was blasted into space by a supernova resulting from the collapse of a supergiant star more than 5 billion years ago (see Solar System: Theories of Origin; Planetary Science). Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.