Sea Of Stars Download
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Astronomers believe that molecular clouds, dense clouds of gas located primarily in thespiral arms of galaxies are the birthplace of stars. Denseregions in the clouds collapse and form "protostars". Initially, thegravitational energy of the collapsing star is the source of its energy. Once the starcontracts enough that its central core can burn hydrogen to helium, it becomes a"main sequence" star.
Main sequence stars are stars, like our Sun, that fuse hydrogen atoms together to makehelium atoms in their cores. For a given chemical composition and stellar age, a stars'luminosity, the total energy radiated by the star per unit time, depends only on its mass.Stars that are ten times more massive than the Sun are over a thousand times more luminousthan the Sun. However, we should not be too embarrassed by the Sun's low luminosity: it isten times brighter than a star half its mass. The more massive a main sequence star, thebrighter and bluer it is. For example, Sirius, the dog star, located to the lower left ofthe constellation Orion, is more massive than the Sun, and is noticeably bluer. On theother hand, Proxima Centauri, our nearest neighbor, is less massive than the Sun, and isthus redder and less luminous.
Since stars have a limited supply of hydrogen in their cores, they have a limitedlifetime as main sequence stars. This lifetime is proportional to f M /L, where f is the fraction of the total mass of the star, M, available for nuclear burning in the core and Lis the average luminosity of the star during its main sequence lifetime. Because of thestrong dependence of luminosity on mass, stellar lifetimes depend sensitively on mass.Thus, it is fortunate that our Sun is not more massive than it is since high mass starsrapidly exhaust their core hydrogen supply. Once a star exhausts its core hydrogen supply,the star becomes redder, larger, and more luminous: it becomes a red giant star. Thisrelationship between mass and lifetime enables astronomers to put a lower limit on the age of the universe.
Meanwhile, the core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core and the star becomes a red supergiant. At this stage, the Sun will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which lasts only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking "planetary nebula", much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star.
Massive stars burn brighter and perish more dramatically than most. When a star tentimes more massive than Sun exhaust the helium in the core, the nuclear burning cyclecontinues. The carbon core contracts further and reaches high enough temperature to burncarbon to oxygen, neon, silicon, sulfur and finally to iron. Iron is the most stable formof nuclear matter and there is no energy to be gained by burning it to any heavierelement. Without any source of heat to balance the gravity, the iron core collapses untilit reaches nuclear densities. This high density core resists further collapse causing theinfalling matter to "bounce" off the core. This sudden core bounce (whichincludes the release of energetic neutrinos from the core) produces a supernova explosion.For one brilliant month, a single star burns brighter than a whole galaxy of a billionstars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up toiron into interstellar space. They are also the site where most of the elements heavierthan iron are produced. This heavy element enriched gas will be incorporated into futuregenerations of stars and planets. Without supernova, the fiery death of massive stars,there would be no carbon, oxygen or other elements that make life possible.
The fate of the hot neutron core depends upon the mass of the progenitor star. If theprogenitor mass is around ten times the mass of the Sun, the neutron star core will coolto form a neutron star. Neutron stars are potentially detectable as "pulsars",powerful beacons of radio emission. If the progenitor mass is larger, then the resultantcore is so heavy that not even nuclear forces can resist the pull of gravity and the corecollapses to form a black hole.
It is shown that the most important source of energy in ordinary stars is the reactions of carbon and nitrogen with protons. These reactions form a cycle in which the original nucleus is reproduced, viz. C12+H=N13, N13=C13+ε+, C13+H=N14, N14+H=O15, O15=N15+ε+, N15+H=C12 +He4. Thus carbon and nitrogen merely serve as catalysts for the combination of four protons (and two electrons) into an α-particle (7).
The agreement of the carbon-nitrogen reactions with observational data (7, 9) is excellent. In order to give the correct energy evolution in the sun, the central temperature of the sun would have to be 18.5 million degrees while integration of the Eddington equations gives 19. For the brilliant star Y Cygni the corresponding figures are 30 and 32. This good agreement holds for all bright stars of the main sequence, but, of course, not for giants.
It is shown further (5-6) that no elements heavier than He4 can be built up in ordinary stars. This is due to the fact, mentioned above, that all elements up to boron are disintegrated by proton bombardment (α-emission!) rather than built up (by radiative capture). The instability of Be8 reduces the formation of heavier elements still further. The production of neutrons in stars is likewise negligible. The heavier elements found in stars must therefore have existed already when the star was formed.
A star is a sphere of gas held together by its own gravity. The closest star to Earth is our very own Sun, so we have an example nearby that astronomers can study in detail. The lessons we learn about the Sun can be applied to other stars.
A star is born, lives, and dies, much like everything else in nature. Using observations of stars in all phases of their lives, astronomers have constructed a lifecycle that all stars appear to go through. The fate and life of a star depends primarily on it's mass.
All stars begin their lives from the collapse of material in a giant molecular cloud. These clouds are clouds that form between the stars and consist primarily of molecular gas and dust. Turbulence within the cloud causes knots to form which can then collapse under it's own gravitational attraction. As the knot collapses, the material at the center begins to heat up. That hot core is called a protostar and will eventually become a star.
The cloud doesn't collapse into just one large star, but different knots of material will each become it's own protostar. This is why these clouds of material are often called stellar nuseries they are places where many stars form.
These high-mass stars go through some of the same steps as the medium-mass stars. First, the outer layers swell out into a giant star, but even bigger, forming a red supergiant. Next, the core starts to shrink, becoming very hot and dense. Then, fusion of helium into carbon begins in the core. When the supply of helium runs out, the core will contract again, but since the core has more mass, it will become hot and dense enough to fuse carbon into neon. In fact, when the supply of carbon is used up, other fusion reactions occur, until the core is filled with iron atoms.
About 75% of the mass of the star is ejected into space in the supernova. The fate of the left-over core depends on its mass. If the left-over core is about 1.4 to 5 times the mass of our Sun, it will collapse into a neutron star. If the core is larger, it will collapse into a black hole. To turn into a neutron star, a star must start with about 7 to 20 times the mass of the Sun before the supernova. Only stars with more than 20 times the mass of the Sun will become black holes.
Wow! Stars evolve, or change, over time. It may take millions of years or it may take billions of years for a star to complete its life cycle. A star is a brilliantly glowing sphere of hot gas whose energy is produced byan internal nuclear fusion process. Stars are contained in galaxies. A galaxy contains not only stars, but clouds of gas and dust. These clouds are called nebulae, and it is in a nebula where stars are born. In the nebula is hydrogen gas which is pulled together by gravity and starts to spin faster. Over millions of years, more hydrogen gas is pulled into the spinning cloud. The collisions which occur between the hydrogen atoms starts to heat the gas in the cloud. Once the temperature reaches 15,000,000 degrees Celsius, nuclear fusion takes place in the center, or core, of the cloud. The tremendous heat given off by the nuclear fusion process causes the gas to glow creating a protostar. This is the first step in the evolution of a star. The glowing protostar continues to accumulate mass. The amount of mass it can accumulate is determined bythe amount of matter available in the nebula. Once its mass is stabilized,the star is known as a main sequence star. The new star will continue to glow for millions or even billions of years. As it glows, hydrogen is converted into helium in the core by nuclear fusion. The core starts to become unstable and it starts to contract. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and starts to glow red. The star has now reached the red giant phase. It is red because it is cooler than the protostar phase and it is a giant because the outer shell has expanded outward. All stars evolvethe same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths the star will take.
The Pleiades
The Cat's Eye Planetary Nebula
MEDIUM STARS As a red giant, the hydrogen gas in the outer shell continues to burn as the temperature in the core continues to rise. At 200,000,000 degrees Celsius, the helium atoms fuse to form carbon atoms in the core. The last of the hydrogengas in the outer shell is blown away to form a ring around the core. This ring is called a planetary nebula. When the last of the helium atoms in the core are fused into carbon atoms, the medium size star begins to die. Gravity causes the last of the star's matter to collapse inward and compact. This is the white dwarf stage which is extremely dense. White dwarfs shine with a white hot light but once all of their energy is gone, they die. The star has now reached the black dwarf phase.MASSIVE STARS Once massive stars reach the red giant phase, the core temperature continues to increase as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull together the carbon atoms in the core until the temperature reaches 600,000,000 degrees Celsius. At this temperature, carbon atoms form heavy elements such as oxygen and nitrogen. The fusion and production of heavy elements continues until iron starts to form. At this point, fusion stops and the iron atoms start to absorb energy. This energy is eventually released in a powerful explosion called a supernova. A supernova can light the sky up for weeks. The temperature in a supernova can reach 1,000,000,000 degrees Celsius. This high temperature can lead to the production of new elements which may appear in the new nebula that results after the supernova explosion. The core of a massive star that is 1.5 to 4 times as massive as our Sun ends up as a neutron star after the supernova. Neutron stars spin rapidly giving off radio waves. If the radio waves appear to be emitted in pulses (due to the star's spin), these neutron stars are called pulsars. The core of a massive star that has 10 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core, so it is swallowed by its own gravity. It has now become a black hole which readily swallows any matter and energy that comes too near it. Some black holes have companion stars whose gases they pull off. As the gases are pulled down into the black hole, they heat up and give off energy in the form of X-rays. Black holes are detected by the X-rays which are given off as matter falls down into the hole.A Question What determines just how large a star becomes?
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The Answer