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THE LIFE CYCLE OF STARS Stars are born in nebulae. Huge clouds of dust and gas collapse under gravitational forces, forming protostars. These young stars undergo further collapse, forming main sequence stars. Stars expand as they grow old. As the core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright. This is a red giant or a red super giant (depending on the initial mass of the star). It will eventually collapse and explode. Its fate is determined by the original mass of the star; it will become a black dwarf, neutron star, or black hole. THE BIRTH OF STARS Accretion Disk: Stars are formed in nebulae, interstellar clouds of dust and gas (mostly hydrogen). These stellar nurseries are abundant in the arms of spiral galaxies. In these stellar nurseries, dense parts of these clouds undergo gravitational collapse and compress to form a rotating gas globule. The globule is cooled by emitting radio waves and infrared radiation. It is compressed by gravitational forces and The Eagle also by shock waves of nebula, a stellar pressure from supernova nursery or the hot gas released illuminated by from nearby bright stars. ultraviolet light These forces cause the which is roughly-spherical globule emitted from to collapse and rotate. The the newborn process of collapse takes stars. from between 10,000 to 1,000,000 years. A Central Core and a Protoplanetary Disk: As the collapse proceeds, the temperature and pressure within the globule increases, as the atoms are in closer proximity. Also, the globule rotates faster and faster. This spinning action causes an increase in centrifugal forces (a radial force on spinning objects) that causes the globule to have a central core and a surrounding flattened disk of dust (called a protoplanetary disk or accretion disk). The central core becomes the star; the protoplanetary disk may eventually coalesce into orbiting planets, asteroids, etc. Protostar: The contracting cloud heats up due to friction and forms a glowing protostar; this stage lasts for roughly 50 million years. If there is enough material in the protostar, the gravitational collapse and the heating continue. If there is not enough material in the protostar, one possible outcome is a brown dwarf (a large, not-veryluminous celestial body having a mass between 1028 kg and 84 x 1028 kg). A Newborn Star: When a temperature of about 27,000,000°F is reached, nuclear fusion begins. This is the nuclear reaction in which hydrogen atoms are converted to helium atoms plus energy. This energy (radiation) production prevents further contraction of the star. Young stars emit jets of intense radiation that heat the surrounding matter to the point at which it glows brightly. These narrowly-focused jets can be trillions of miles long and can travel at 500,000 miles per hour. These jets may be focused by the star's magnetic field. The protostar is now a stable main sequence star which will remain in this state for about 10 billion years. After that, the hydrogen fuel is depleted and the star begins to die. Life span: The most massive stars have the shortest lives. Stars that are 25 to 50 times that of the Sun live for only a few million years. Stars like our Sun live for about 10 billion years. Stars less massive than the Sun have even longer life spans. THE DEATH OF SUN-LIKE STARS (with a mass up to 1 1/2 times that of the Sun) A stars expands as it grows old. As the core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright; this is a red giant. After expanding and reaching the enormous red giant phase, the outer layers of the star continue to expand. As this happens, the core contracts; the helium atoms in the core fuse together, forming carbon atoms and releasing energy. The core is now stable since the carbon atoms are not further compressible. Now the outer layers of the star start to drift off into space, forming a planetary nebula (a planetary nebula has nothing to do with planets). The star loses most of its mass to the nebula. The star cools and shrinks; it will eventually be only a few thousand miles in diameter! The star is now a white dwarf, a stable star with no nuclear fuel. It radiates its left-over heat for billions of years. When its heat is all dispersed, it will be a cold, dark black dwarf - essentially a dead star (perhaps replete with diamonds, highly compressed carbon). NOVA A nova is a white dwarf star that suddenly increases in brightness by several magnitudes. It fades very slowly. A White dwarf star: (circled) The Egg nebula: a in the globular planetary nebula formed a cluster M4. few hundred years ago. THE DEATH OF HUGE STARS (from 1.5 to 3 times the mass of the Sun) Betelgeuse, a red supergiant in Orion. Supernova SN1987A: the beginning of a supernova. When huge stars grow old, they become even more enormous red supergiants (as their core fuses all the hydrogen into helium). Their core shrinks, becoming hotter and denser. With these changes, different nuclear processes occur; fusion now produces heavier elements (this temporarily stop the core's shrinking). Eventually this core collapses (in an instant). As the iron atoms are crushed together in this gravitational collapse, the core temperature rises to about 100 billion degrees. The repulsive electrical forces between the atoms' nuclei overcomes the gravitational forces, causing a massive, bright, short-lived explosion called a supernova. During the explosion, shock waves, blow away the star's outer layers. Supernova N132D: 3,000 years after a supernova, ejecting stellar material If the star's remaining mass is between 1 1/2 to 3 times the mass of the Sun, it (including oxygen-rich will collapse into a small, dense neutron star (about ten miles in diameter, gas) in luminescent about 1.4 times the mass of the Sun, with an extraordinarily strong magnetic shock fronts. It is located field, and rapid spin). in the Large Magellanic Cloud (169,000 lightIf the star's remaining mass is greater than three times the mass of the Sun, the years from Earth). star contracts tremendously and becomes a black hole (incredibly dense with a gravitational field so strong that even light cannot escape). The next stage depends on the star's remaining mass: EVOLVED STAR An evolved star is an old star that is near the end of its existence. Its nuclear fuel is mostly gone. The star loses mass from its surface, producing a stellar wind (gas that is ejected from the surface of a star). The Crab nebula: the remnant of a supernova in A.D. 1054. It has a rapidly spinning neutron star. THE DEATH OF GIANT STARS When huge stars grow old, they become even more enormous red supergiants (as their core fuses all the hydrogen into helium). Their core shrinks, becoming hotter and denser. With these changes, different nuclear processes occur; fusion now produces heavier elements (this temporarily stop the core's shrinking). Eventually this core collapses (in an instant). As the iron atoms are crushed together in this gravitational collapse, the core temperature rises to about 100 billion degrees. The repulsive electrical forces between the atoms' nuclei overcomes the gravitational forces, causing a massive, bright, short-lived explosion called a supernova. During the explosion, shock waves, blow away the star's outer layers. The next stage depends on the star's remaining mass: If the star's remaining mass is between 1 1/2 to 3 times the mass of the Sun, it will collapse into a small, dense neutron star (about ten miles in diameter, about 1.4 times the mass of the Sun, with an extraordinarily strong magnetic field, and rapid spin). If the star's remaining mass is greater than three times the mass of the Sun, the star contracts tremendously and becomes a black hole (incredibly dense with a gravitational field so strong that even light cannot escape). EVOLVED STAR An evolved star is an old star that is near the end of its existence. Its nuclear fuel is mostly gone. The star loses mass from its surface, producing a stellar wind (gas that is ejected from the surface of a star).