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Kaler, James B. Last reviewed: March The large-scale, systematic, and irreversible changes over time of the structure and composition of a star over time.

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Kaler, James B. Last reviewed: March The large-scale, systematic, and irreversible changes over time of the structure and composition of a star over time. All stars have "lives" in the sense that they are born, age, and die. A stellar "death," or end stage of evolution, ranges from an eons-long smolder as a white dwarf for typical stars, to explosive self-destruction as a supernova for massive stars. The initial mass of a star is the overwhelmingly determinative property of the evolutionary path that the star will follow Fig.

See also: Star. Dozens of different types of stars populate the Milky Way Galaxy and the universe at large Fig. The most common are main-sequence dwarfs like the Sun that fuse hydrogen into helium within their cores. Dwarfs run the full gamut of stellar masses, from perhaps as much as solar masses M down to the minimum of 0. Within the main sequence, dwarf stars break into two broad groups: those under 1.

Below the end of the main sequence masses less than 0. Brown dwarfs shine both from gravitational energy and from fusion of their natural deuterium. See also: Brown dwarf ; Carbon-nitrogen-oxygen cycles astrophysics ; Deuterium ; Nuclear fusion ; Proton-proton chain ; Spectral type. Scattered among the dwarfs are different kinds of stars that are distinguishable through comparison with the main sequence dwarfs by their sizes and luminosities: luminous giants with maximum radii equaling that of the inner solar system , brilliant supergiants maximum radii comparable to the orbits of Jupiter and Saturn , subgiants which fall between giants and dwarfs , and dim white dwarfs the size of Earth.

All these classifications may be broken into subclasses. The list also includes the central stars of planetary nebulae, double stars, novae, supernovae, neutron stars, and black holes. The study of stellar evolution seeks to determine where they all came from. Do they stand alone, or are they somehow connected? If the latter, what kinds of stars precede and succeed what other kinds? Stars age so slowly that it is impossible—except under rare circumstances—to see the transformations take place.

Instead, stellar theorists, using the laws of physics, build numerical models of stars and then age them. The procedure is simple in principle, but difficult in practice. At the start, a star is in hydrostatic equilibrium, in which each layer supports the layers above it, with the pressure difference across a layer equaling the weight of the layer, and is internally supported by thermonuclear fusion.

Other equations involve the mass within a given radius, the rate at which energy is generated at a given radius, and the temperature gradient, which controls convection. As fusion proceeds and the rate of energy generation and the internal chemical composition change, the structure of the star changes as well.

When appropriate mass loss is taken into account, most of the different kinds of stars fall into place. Stars are born from compact knots of gas and dust within molecular clouds Fig.

If dense enough, the knots start to contract under their own gravity. Conservation of angular momentum demands that as the knots collapse, they must spin faster. Magnetic fields transport some of this angular momentum away, and over time, a flattened disk of material—known as a protoplanetary disk—forms around the collapsing, budding star, which at this point in its evolution is known as a protostar.

The protoplanetary disk contains the materials from which planets later accumulate. See also: Angular momentum ; Molecular cloud ; Planet ; Protostar. That and heat convection, which brings in fresh deuterium from outside the nuclear-burning zone, bring some stability, and a star can now be said to have been born. Stars like the Sun shrink at constant temperature until deuterium fusion dies down. Then the young stars heat at roughly constant luminosity until the full proton-proton chain begins, which provides the stars' luminosity and support and stops the gravitational contraction.

The stars then settle onto the zero-age main sequence from which they will later evolve. At the same time, the surrounding dusty cloud is clearing, allowing new, accreting, and highly active so-called T Tauri stars to be seen flocking around their birth clouds Fig. The whole process takes only 10 or so million years, with the mature stars eventually leaving their birthplaces as they orbit within their host galaxy.

High-mass stars proceed similarly, but at such a great pace that the death process begins even as the birth process is ending. See also: Heat convection ; T Tauri star. The main sequence is that zone on the HR diagram in which stars are stabilized against gravitational contraction by fusion. The higher the stellar mass, the greater the internal compression and temperature, and the more luminous the star. Hydrogen fusion is highly sensitive to temperature, with a small increase in stellar mass meaning a much higher fusion rate.

Although greater mass means a greater nuclear-burning core mass and therefore a larger fuel supply, the increased fusion rate more than offsets this and thereby shortens stellar life. A star's fuel supply is proportional to its mass M , while the rate of fusion is expressible by the luminosity L. While the new Sun was destined to survive on the main sequence for 10 billion years, a 0.

Main-sequence life, while stable, is not altogether quiet. Even there, stars change and evolve. The pressure of a gas depends on the number of particles per unit volume, not on their kind. The result is a slow shrinkage of the core, which increases the temperature, raises the fusion rate, and causes the core to eat into the surrounding hydrogen envelope, incorporating fresh fuel. As a result, main-sequence dwarfs slowly brighten and eventually expand and cool some at their surfaces.

The Sun will more than double in brightness, dooming life on Earth, long before its core hydrogen is gone. As a result, the main sequence spreads itself into a band toward the cool side to the right of the zero-age main sequence in an HR diagram, with the band widening toward the top. Clusters of stars are born together in both space and time, most of them presumably with fully intact main sequences. As a cluster ages, stars peel off the main sequence to become giants and supergiants from the top down.

A star cluster can be dated by where its main sequence ends. Open clusters, which occupy galactic disks, range from just-forming to about 10 billion years old, which gives the disk's age. Globular clusters, however, and for instance those in the Milky Way Galaxy's halo, have burned their main sequences down to around 0.

This process takes roughly 12 billion years, which is close to the age of the Galaxy. The main sequence is divided into three parts. Below 0. Low-mass K and M dwarf evolution is therefore of only academic interest.

Their fates depend again on mass. Between 0. Above this range classes O and B0 , they explode. Binary, trinary, and even higher numbers of closely associated stars in a shared star system contribute further to the richness of stellar phenomena. Main-sequence life lasts until the core hydrogen is almost gone, at which time hydrogen fusion shuts down rather suddenly.

With no support, the now-quiet helium core can contract more rapidly under gravity's force. It heats, causing hydrogen fusion to spread into a thick, enclosing shell that runs on the carbon cycle. With a new though temporary energy source, the star first dims somewhat while it expands and cools at the surface, changing its spectrum to class K. The transition takes only a few hundred million years or less, leaving few stars in the middle of the HR diagram, with those stars with lower masses appearing as F, G, and K subgiants.

The consequences again depend on mass. As core contraction proceeds beyond the rightward transition in the HR diagram, stars from about 1 to 5 solar masses still fusing hydrogen in a shell suddenly and dramatically increase their luminosities. The future Sun will eventually grow times brighter than it is today, and a 5-solar-mass star which begins at about solar luminosities will reach nearly solar.

At the same time, the stars swell to become red giants. While the core roughly half a solar mass of the future Sun shrinks to the size of Earth, the radius will expand to that of Mercury's orbit, or even beyond.

When the core temperature climbs to million K, helium nuclei alpha particles begin fusing to unstable beryllium 8 Be , which quickly decays back to alpha particles, setting up an equilibrium.

The tiny amount of 8 Be present reacts with additional alpha particles to create carbon via the triple alpha process:.

The initiation of the triple-alpha process is explosive in stars like the Sun, because the core electrons become degenerate, like those that support white dwarfs. The sudden burst of energy is absorbed and does not reach the stellar surface. Fusion with additional helium nuclei creates oxygen and even neon.

Above 2 solar masses, helium burning starts more quietly Fig. See also: Nucleosynthesis. The star, now stabilized by a helium-burning core that is surrounded by a hydrogen-fusing shell, retreats about halfway down the red giant branch. Energy-generating fusion reactions will try to proceed toward iron the most stable of all nuclei. From 5 solar masses up, evolution proceeds similarly, but instead of settling into a distinct location on the HR diagram, the stars loop to the blue higher temperatures , where they fuse helium as class G, F, and A giants.

This change in evolutionary style is not abrupt, but gradual with increasing mass. See also: Giant star. When the helium has fused to carbon and oxygen, the core again contracts. Helium fusion spreads outward into a shell, and for the second time the star climbs the HR diagram's giant branch. Because the second climb is roughly asymptotic to the first, the second climb creates the asymptotic giant branch. The shrinking carbon-oxygen core is now surrounded by a shell of fusing helium, while the old hydrogen-fusing shell expands, cools, and shuts down.

Eventually, however, the helium shell runs out of fuel, and the hydrogen shell reignites. Hydrogen burning feeds fresh helium into the space between it and the carbon core, and when there is enough of it, helium burning reignites explosively in a helium flash or thermal pulse that can affect the star's surface.

The flash squelches hydrogen fusion, and the whole process starts again, with helium flashes coming at progressively shorter intervals.

AGB stars become larger and brighter than before, passing into the cool end of class M, where they eventually become unstable enough to pulsate as long-period variables Miras. The Sun will become times brighter than now and will reach out to the Earth's orbit, perhaps destroying the planet. More massive Miras can exceed the size of the Martian orbit.

Stellar evolution

Radio astronomy has helped astronomers explore the life stories of stars, and here is what we have learned so far. The color of a star is an indicator of its temperature. The cooler stars are brown to dark red, barely warmed enough to glow, like the cooling embers in a fire. The nearest star we can study is our Sun. It is a very average star, which means that galaxies in the Universe contain smaller and larger, brighter and dimmer, and hotter and cooler stars than our Sun. The Sun and the bulk of the stars in the Universe are called dwarf stars.

M assive stars distinguish themselves from their lower mass cousins by their eventual fate. Their subsequent evolution may lead to an explosion in the form of a core-collapse supernova, a long duration gamma-ray burst GRB or direct collapse to a black hole. But our physical understanding of transients remains rather patchy, and is reliant upon the far less glamorous field of stellar astrophysics. In this review, I shall set out some of the current issues relating to massive stars, involving their formation, evolution and ultimate demise. Extensive review articles on these topics include Zinnecker and Yorke , Maeder and Meynet , Smartt and Woosley and Bloom


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Birth, life and death of a star

In Grades 6 and 8 learners covered material regarding the solar system including the Sun. In Grade 7, they focused on the system which includes the Sun, Earth and Moon. Learners should be familiar with the fact that the Sun is a star and produces heat and light energy via nuclear reactions.

Birth, life and death of a star

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 Какой ключ. Стратмор снова вздохнул. - Тот, который тебе передал Танкадо. - Понятия не имею, о чем. - Лжец! - выкрикнула Сьюзан.  - Я видела твою электронную почту.

Я тебя разбудил. Она улыбнулась и поудобнее устроилась в постели. - Ты мне только что приснился. Приходи поиграть. - На улице еще темно, - засмеялся. - А-ах, - сладко потянулась.  - Тем более приходи.

Ошеломленный потерей жены и появлением на свет неполноценного, по словам медсестер, ребенка, которому скорее всего не удастся пережить ночь, он исчез из больницы и больше не вернулся. Энсея Танкадо отдали в приемную семью. Каждую ночь юный Танкадо смотрел на свои скрюченные пальцы, вцепившиеся в куклу Дарума note 1и клялся, что отомстит - отомстит стране, которая лишила его матери, а отца заставила бросить его на произвол судьбы. Не знал он только одного - что в его планы вмешается судьба. В феврале того года, когда Энсею исполнилось двенадцать, его приемным родителям позвонили из токийской фирмы, производящей компьютеры, и предложили их сыну-калеке принять участие в испытаниях новой клавиатуры, которую фирма сконструировала для детей с физическими недостатками. Родители согласились. Хотя Энсей Танкадо никогда прежде не видел компьютера, он как будто инстинктивно знал, как с ним обращаться.

Star Life and Death

Я могу вам помочь. - Спасибо, не. Мне нужен консьерж.

 Черт возьми, - тихо выругался Стратмор, - мой новый пейджер, - и с отвращением посмотрел на коробочку, лежащую у него на ладони. Он забыл нажать кнопку, которая отключила звук. Этот прибор он купил в магазине электроники, оплатив покупку наличными, чтобы сохранить анонимность.

Глядя на оживающий монитор, он подумал, известно ли Стратмору, что в лаборатории систем безопасности нет ни души. Подходя к шифровалке, он успел заметить, что шторы кабинета шефа задернуты. Это означало, что тот находится на рабочем месте.

Два часа. Три. Она перевела взгляд на пустую шифровалку.

Мрачный голос Стратмора вывел ее из задумчивости. - Единственный луч надежды во всей этой печальной истории - то, что Танкадо путешествовал. Есть шанс, что его партнер пока ничего не знает. Испанские власти обещали придержать информацию - столько, сколько смогут. Мы узнали об этом лишь благодаря оперативности КОМИНТа.

ГЛАВА 18 Стоя у громадного окна во всю стену своего кабинета в токийском небоскребе, Нуматака с наслаждением дымил сигарой и улыбался. Он не мог поверить в свою необыкновенную удачу. Он снова говорил с этим американцем, и если все прошло, как было задумано, то Танкадо сейчас уже нет в живых, а ключ, который он носил с собой, изъят.

Нуматака закрыл трубку ладонью и громко засмеялся. Однако он не смог удержаться от вопроса: - Сколько же вы хотите за оба экземпляра. - Двадцать миллионов американских долларов. Почти столько же поставил Нуматака. - Двадцать миллионов? - повторил он с притворным ужасом.

4 Comments

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Stellar evolution is the process by which a star changes over the course of time.

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A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.

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