National Aeronautics and Space Administration

a guide to understanding

The contains several STELLAR NURSERY hundred billion of various ages, sizes and masses. A 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 Many factors influence the rate of evolution, , the cluster is the closest large star-forming region to the evolutionary path and the 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 . The X-rays are produced in the hot, multimillion- in a general way how stars of different masses degree upper of these evolve and whether the final remnant will be a stars. (The dark diagonal lines and the dwarf, star, or . streaks from the brightest stars are instrumental effects.) Stellar evolution gets even more complicated when the star has a nearby companion. For example, excessive from a companion star to a may cause the white dwarf to explode as a Type Ia . TW Hydrae 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 , and the to extremely high energies. These conditions remainder will be blown away by intense from the star. In TW Hydrae, the can exist near collapsed objects such as white X-ray 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 ; 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.

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Proxima Centauri ’s Eye (NGC 6543) Tarantula Nebula (30 Doradus) The nearest star to Earth, Proxima Sun Crescent Nebula (NGC 6888) Eta Carinae After the core--burning giant phase, all The Tarantula Nebula is in one of the most Centauri, is the most common type of and other stars are balls of gas that shine as When a massive star uses up the 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 star. a result of 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 will Group of 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 . 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 ,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 “” from the hot star rams in the surrounding gas. Other massive of the hydrogen in the core of red dwarfs rolling, boiling turmoil called . 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- age of the 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 . A red dwarf has a turbulent magnetized loops of hot gas rise high above the giant material creates a spectacular nebula, possibly “”– 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 . 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 WHITE 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 , 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. B Type II Supernova /G292.0+1.8 / Tycho’s SNR 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 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 —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 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. dump enough material onto the white dwarf ray energies. Like a rotating lighthouse upper atmospheres, or coronas, to a few million with surface temperatures of about 20,000 atmosphere away at speeds in excess The G292.0+1.8 to push it over the Chandrasekhar limit. beam, the radiation can be observed as a degrees Celsius. The X-rays from TWA 5B are degrees Celsius. After that it slowly cools of 4 million miles per hour. This wind shown here has an estimated age The resulting collapse and explosion of the powerful, pulsing source of radiation, or likely due to this process. to become an undetectable “”. speed is not steady, so rapidly moving of 1,600 years. The neutron star is white dwarf are believed to be responsible pulsar, as in the case of the Crab Nebula The bright source in this image is the white groups of particles slam into slower ones, producing shock waves. the white dot below and to the left of for a Type Ia supernova – the type that pulsar shown here. The jets and rings are dwarf Sirius B. (The spike-like pattern is an These shock waves are the likely source of most of the X-rays from center. produced Tycho’s supernova remnant. caused by high-energy particles flowing instrumental artifact.) Zeta Orionis, though hot gas trapped in magnetic fields near the away from the pulsar. surface of the star may also produce X-rays. BLACK HOLE

Cygnus X-1 XTE J1118+480 If the core of a collapsing star has a XTE J1118+40 has a nearby companion mass that is greater than three , star. About 20 confirmed cases of these no known force can prevent it from binary systems have been identified in our forming a black hole. If a black hole Galaxy. This Chandra image was made by an has a nearby companion star, gas instrument which sorts the X-rays according pulled away from the companion will be to their energy, producing a spectrum which heated to tens of millions of degrees, appears as the bright line extending from producing X-rays as it falls toward the the upper left to the lower right. The central black hole. Radiation from the hot gas bright dot of X-radiation marks the location can be detected until the gas passes of the black hole. The other lines and beyond the event horizon of the black hole. The spectrum of X-1 spokes in the image are instrumental artifacts. An analysis of the X-ray spectrum shows the effect of on radiation from about 70 miles from the revealed that the disk of matter around this black hole stops about 600 miles event horizon. from the black hole.

DEFINITIONS End phases A star’s ultimate fate depends on its mass. It can fade into obscurity (brown dwarf or red dwarf), become a white dwarf (sun-like stars), STELLAR NURSERY explode as a supernova and leave behind a neutron star or a black hole Large cold clouds of dust and gas where stars form. (massive to very massive stars), or be disrupted entirely (white dwarfs in close binary systems, or extremely massive stars). PROTOSTARS The stage in the formation of a star just before nuclear reactions ignite. PLANETARY NEBULA due to the of too much BROWN DWARF A nebula produced after an material or merger with another An object with a mass less than about 8% of the exhausted giant star puffs off its white dwarf. mass of the Sun, but about 10 times greater than outer layer and leaves behind a TYPE II SUPERNOVA that of . smaller, hot star. A supernova that occurs when RED DWARF WHITE DWARF a massive star has used up its A star with a mass between about 8% and 50% phase of a Sun-like star nuclear fuel and its core collapses the mass of the Sun. in which all the material contained to form either a neutron star or a in the star, minus the amount black hole, triggering an explosion. SUN-LIKE STAR blown off in the red giant phase, is PAIR-INSTABILITY A star with mass between about 50% and 10 times packed into a volume one millionth SUPERNOVA that of the Sun the size of the original star. A rare type of explosion predicted BLUE SUPERGIANTS NEUTRON STAR to occur as a consequence of the Stars much more massive than the Sun. An extremely extremely high temperatures in the produced by the collapse of the interiors of stars having masses of RED GIANT core of a massive star in the about 200 suns. A phase in the evolution of a star after nuclear fusion supernova process. SUPERSHELLS reactions that convert hydrogen to helium have consumed all the hydrogen in the core of the star, BLACK HOLES The combined activity of many and energy generated by hydrogen fusion in the If the core of a collapsing star has stellar winds and supernovas shell causes the star’s diameter to greatly expand a mass that is greater than three create expanding supershells that and cool. Suns, no known force can prevent can trigger the collapse of clouds it from forming a black hole. of dust and gas to form new BLUE GIANT generations of stars. After a massive red giant star ejects its outer layers, TYPE IA SUPERNOVA its hot inner core is exposed, and it becomes a blue An explosion produced when a giant star. white dwarf becomes unstable

For more information: http://chandra.si.edu/xray_sources/stellar_evolution.html