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Stellar Evolution

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Stellar Evolution National Aeronautics and Space Administration a guide to understanding STELLAR EVOLUTION The Milky Way galaxy contains several STELLAR NURSERY hundred billion stars of various ages, sizes Orion Nebula and masses. A star forms when a dense cloud Most stars form as members of star of gas collapses until nuclear reactions begin clusters created by the collapse of cold (10 degrees above absolute zero), dense deep in the interior of the cloud and provide clumps of gas and dust embedded in enough energy to halt the collapse. much larger clouds of cold gas and dust. At a distance of about 1,800 light Many factors influence the rate of evolution, years, the Orion Nebula cluster is the closest large star-forming region to the evolutionary path and the nature of the final Earth. Chandra’s image shows about a remnant. By far the most important of these is thousand X-ray emitting young stars in the initial mass of the star. This guide illustrates the Orion Nebula star cluster. The X-rays are produced in the hot, multimillion- in a general way how stars of different masses degree upper atmospheres of these evolve and whether the final remnant will be a stars. (The dark diagonal lines and the white dwarf, neutron star, or black hole. streaks from the brightest stars are instrumental effects.) Stellar evolution gets even more complicated when the star has a nearby companion. For example, excessive mass transfer from a PROTOSTAR companion star to a white dwarf may cause the white dwarf to explode as a Type Ia supernova. TW Hydrae Protostars and very young stars are usually X-ray data reveal extreme or violent conditions surrounded by disks of dust and gas. where gas has been heated to very high Some of this matter will fall onto the young temperatures or particles have been accelerated star, some may form into planets, and the to extremely high energies. These conditions remainder will be blown away by intense radiation from the star. In TW Hydrae, the can exist near collapsed objects such as white X-ray spectrum provides strong evidence dwarfs, neutron stars, and black holes; in giant that this very young star is pulling in bubbles of hot gas produced by supernovas; in matter from a circumstellar disk. X-rays are produced as the infalling matter collides stellar winds; or in the hot, rarified outer layers, with the surface of the star or coronas, of normal stars. www.nasa.gov www.chandra.si.edu SUN-LIKE STAR BLUE GIANT RED DWARF PLANETARY NEBULA SUPERSHELL Proxima Centauri Cat’s Eye (NGC 6543) Tarantula Nebula (30 Doradus) The nearest star to Earth, Proxima Sun Crescent Nebula (NGC 6888) Eta Carinae After the core-helium-burning giant phase, all The Tarantula Nebula is in one of the most Centauri, is the most common type of The Sun and other stars are balls of gas that shine as When a massive star uses up the hydrogen Eta Carinae, one of the most luminous of a Sun-like star’s available energy resources active star-forming regions in the Local star in the Galaxy – a red dwarf star. a result of nuclear fusion reactions that release energy fuel in its central core, it expands enormously stars known in our galaxy, radiates energy will be used up. The exhausted giant star will Group of galaxies to which the Milky Way Red dwarfs have a mass between deep in their interiors. The Sun is now in a long-lived to become a red giant. In this phase the outer at a rate that is 5 million times that of the puff off its outer layer leaving behind a smaller, belongs. Some massive stars in the Nebula approximately 8% and 50% of the mass phase of its evolution wherein nuclear reactions are layers of the star are ejected, and the star Sun, and is estimated to have a mass of hot star with a surface temperature of about are producing intense radiation and searing of the Sun. Because of their low mass, converting hydrogen to helium in the central core. In becomes a blue giant,or Wolf-Rayet star. about 100 solar masses. The exact nature 50,000 degrees Celsius. When the high winds that carve out gigantic bubbles nuclear fusion reactions that consume all a thick outer shell of the Sun, the gas is in a state of Intense radiation from the blue giant pushes of Eta Carinae is unknown, but it may be speed “stellar wind” from the hot star rams in the surrounding gas. Other massive of the hydrogen in the core of red dwarfs rolling, boiling turmoil called convection. This up and gas away at speeds in excess of 3 million miles an extreme example of a luminous blue into the slowly moving material ejected earlier, stars have exploded as supernovas. The can take 20 billion years or more – longer down motion, coupled with the Sun’s rotation, twists per hour. The collision between the high speed variable. Such stars are violently unstable the collision creates a complex and graceful combined activity of many stellar winds and than the estimated 14 billion-year age of the magnetic field and increases its strength. Twisted, “stellar wind” and the previously ejected red and likely explode as supernovas, or even filamentary shell called a planetary nebula. supernovas create expanding supershells the Universe. A red dwarf has a turbulent magnetized loops of hot gas rise high above the giant material creates a spectacular nebula, possibly “hypernovas”– a type of super- A composite image of the Cat’s Eye from that can trigger the collapse of clouds of interior that tangles the magnetic field surface of the Sun, where they make up the corona such as the Crescent Nebula. The massive star supernova explosion that may produce Chandra (purple) and Hubble (red & green) dust and gas to form new generations and heats the star’s corona, sometimes – the outermost layers of the Sun’s atmosphere. The that has produced the nebula appears as the gamma-ray bursts. The X-rays in this image shows where the hot, X-ray emitting gas of stars. explosively. For this reason, red dwarfs Sun’s X-rays (too intense for Chandra to observe) are bright yellow dot near the center of this image, may be caused by the collision of stellar appears in relation to the cooler material seen are observed to be strongly variable X-ray produced in these loops, which can also be the site of just outside the composite X-ray (blue)/optical winds rushing away from Eta Carinae and a in optical wavelengths. sources. solar flares. (red & green) image. suspected companion star. RED GIANT SUPERNOVAS NEUTRON STAR WHITE DWARF BROWN DWARF Beta Ceti A solar-type star becomes a red giant after nuclear fusion reactions that convert hydrogen to helium have consumed all the hydrogen in the core of the star. The core collapses until hydrogen fusion begins in a hot, gaseous shell around the core. Energy generated by hydrogen fusion in the shell causes the star’s diameter to expand about a hundredfold. As the gas expands, it cools, and the star becomes a red giant. During this period, the star emits X-rays weakly. Eventually the core contracts and heats until fusion reactions begin to convert helium to carbon, and the star becomes a core-helium- burning giant. Beta Ceti is an example of such a giant star, which can be X-ray active. Sirius B Type II Supernova /G292.0+1.8 Type IA supernova / Tycho’s SNR Crab Nebula Pulsar TWA 5B The central star of a planetary nebula will A Type II supernova occurs when a Subrahmanyan Chandrasekhar, the During a supernova, the core of a An object that has a mass of less than about 8% eventually collapse to form a white dwarf BLUE SUPERGIANT massive star has used up its nuclear Chandra X-ray Observatory’s namesake, massive star can be compressed to form of the mass of the Sun cannot sustain significant star. In the white dwarf state, all the material fuel and its core collapses to form used relativity theory and quantum a rapidly rotating ball composed mostly nuclear fusion reactions in its core. This marks contained in the star, minus the amount blown Zeta Orionis either a neutron star or a black hole. mechanics to show that if the mass of a of neutrons that is only twelve miles in the dividing line between red dwarf stars and off in the red giant phase, will be packed into When compared to the Sun, the blue Gravitational energy released by this white dwarf becomes greater than about diameter. A teaspoonful of such neutron- brown dwarfs. The brown dwarf TWA 5B has a volume one millionth the size of the original supergiant Zeta Orionis has 20 times process blows the rest of the star 1.4 times the mass of the Sun—called the star matter would weigh more than a mass estimated at about 3% that of the Sun. star. An object the size of an olive made of the diameter, 30 times the mass, apart. The expanding stellar material Chandrasekhar limit—it will collapse. If a one billion tons! Young, rapidly rotating The turbulent interiors of young brown dwarfs this material would have the same mass as an and 100,000 the total power output. produces shock waves that heat a white dwarf is a member of a binary star neutron stars can produce beams of can combine with rapid rotation to produce automobile! For a billion or so years after a star The enormous power output of this multimillion-degree shell of gas that system, a nearby companion star could radiation from radio through gamma- a tangled magnetic field that can heat their collapses to form a white dwarf, it is white-hot star is driving the outer layers of its glows in X-rays for thousands of years.
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