“stellar evolution”的概念、定义、翻译、参考文献-科学参考

stellar evolution

Physics

The changes that occur to a star during its lifetime, from birth to final extinction. A star is believed to form from a condensation of interstellar matter, which collects either by chance or for unexplained reasons, and grows by attracting other matter towards itself as a result of its gravitational field. This initial cloud of cold contracting matter, called a protostar, builds up an internal pressure as a result of its gravitational contraction. The pressure raises the temperature until it reaches 5 − 10×10⁶ K, at which temperature the thermonuclear conversion of hydrogen to helium begins. In our sun, a typical star, hydrogen is converted at a rate of some 10¹¹ kg s⁻¹ with the evolution of some 6×10²⁵ J s⁻¹ of energy. It is estimated that the sun contains sufficient hydrogen to burn at this rate for 10¹⁰ years and that it still has half its life to live as a main-sequence star (see Hertzsprung–Russell diagram). Eventually, however, this period of stability comes to an end, because the thermonuclear energy generated in the interior is no longer sufficient to counterbalance the gravitational contraction. The core, which is now mostly helium, collapses until a sufficiently high temperature is reached in a shell of unburnt hydrogen round the core to start a new phase of thermonuclear reaction. This burning of the shell causes the star’s outer envelope to expand and cool, the temperature drop changes the colour from white to red and the star becomes a red giant or a supergiant if the original star was very large. The core now contracts, reaching a temperature of 10⁸ K, and the helium in the core acts as the thermonuclear energy source. This reaction produces carbon, but a star of low mass relatively soon runs out of helium and the core collapses into a white dwarf, while the outer regions drift away into space, possibly forming a planetary nebula. Larger stars (several times larger than the sun) have sufficient helium for the process to continue so that heavier elements, up to iron, are formed. But iron is the heaviest element that can be formed with the production of energy and when the helium has all been consumed there is a catastrophic collapse of the core, resulting in a supernova explosion, blowing the outer layers away. The current theory suggests that thereafter the collapsed core becomes a neutron star or a black hole, depending on its mass.

Astronomy

The series of changes that stars undergo during their lifetimes, the time-scale of which depends strongly on the star’s mass and also, to some extent, on its initial composition. The progress of a star during its evolution can be followed on a graph called the Hertzsprung–Russell (HR) diagram.
A star is born when a dense region of a cloud of gas collapses under its own gravity. A star first shines because the gravitational potential energy lost in this collapse is released as heat and light. Eventually the temperature at the centre of the protostar reaches 1 million K, igniting nuclear reactions involving deuterium (an isotope of hydrogen), and for some time the energy from this is sufficient to prevent further collapse. Once the deuterium has been exhausted, the collapse continues, and the star is classified as a pre-main-sequence object, following a characteristic path on the HR diagram (see hayashi track; henyey track). For a star the mass of the Sun, this phase lasts several million years.
Eventually the core of the star reaches temperatures of around 10 million K, hot enough to initiate the nuclear reactions that convert hydrogen to helium, and the star joins the main sequence on the HR diagram. This hydrogen-burning phase will last from a few million years, in the most massive stars, to (potentially) more than the present age of the Universe for low-mass stars. Once the hydrogen in the core has been exhausted, the core contracts under its own gravity until, in stars of more than 0.4 solar masses, the core temperature reaches 100 million K, initiating further reactions which transform helium into carbon (the triple-alpha process).
Subsequent evolution depends on the star’s mass. In stars of similar mass to the Sun and greater, while helium burning proceeds at the centre, hydrogen burning may continue in a shell outside the core. In this post-main-sequence phase the star is cooler, larger, and brighter than it was on the main sequence, and is classified as a giant or, for the most massive stars, a supergiant. Once the helium in the core is exhausted, the process of core contraction, followed by the initiation of a new set of nuclear reactions, may be repeated several times. Thus the more massive giants and supergiants can develop a layered structure, with the heaviest fuel burning in the centre and overlying layers containing lighter fuels from previous burning cycles. Throughout these processes the stars become larger and brighter. Eventually, however, either the contraction of the core fails to bring about a high enough temperature for further nuclear reactions or, in supergiants, the point is reached at which the core consists of iron, which cannot be used as a nuclear fuel. At this point, with no more energy being produced at the star’s centre, the core collapses. The collapsing core becomes a neutron star, or possibly a black hole, while the outer layers are ejected explosively in a Type II supernova explosion.
In less massive stars, evolution proceeds rather differently, in part because their cores are dense enough for degeneracy effects to be important. When helium ignites in a degenerate core it does so explosively in a helium flash, causing the core to expand. Thereafter, with the star on the horizontal branch of the HR diagram, helium continues to burn non-explosively in the core while hydrogen burns in a surrounding shell. Once helium is exhausted in the core, it continues to burn in a shell during the asymptotic giant branch phase. Details of later evolutionary phases are uncertain. However, it is thought that the outer layers of the red giant are puffed off to form a planetary nebula, leaving the core of the star exposed as a white dwarf. Hence the end-point of stellar evolution, in both high- and low-mass stars, is that much of the star is dispersed into interstellar space, leaving a collapsed remnant of spent nuclear fuel.

单词	stellar evolution
释义	stellar evolution Physics The changes that occur to a star during its lifetime, from birth to final extinction. A star is believed to form from a condensation of interstellar matter, which collects either by chance or for unexplained reasons, and grows by attracting other matter towards itself as a result of its gravitational field. This initial cloud of cold contracting matter, called a protostar, builds up an internal pressure as a result of its gravitational contraction. The pressure raises the temperature until it reaches 5 − 10×10⁶ K, at which temperature the thermonuclear conversion of hydrogen to helium begins. In our sun, a typical star, hydrogen is converted at a rate of some 10¹¹ kg s⁻¹ with the evolution of some 6×10²⁵ J s⁻¹ of energy. It is estimated that the sun contains sufficient hydrogen to burn at this rate for 10¹⁰ years and that it still has half its life to live as a main-sequence star (see Hertzsprung–Russell diagram). Eventually, however, this period of stability comes to an end, because the thermonuclear energy generated in the interior is no longer sufficient to counterbalance the gravitational contraction. The core, which is now mostly helium, collapses until a sufficiently high temperature is reached in a shell of unburnt hydrogen round the core to start a new phase of thermonuclear reaction. This burning of the shell causes the star’s outer envelope to expand and cool, the temperature drop changes the colour from white to red and the star becomes a red giant or a supergiant if the original star was very large. The core now contracts, reaching a temperature of 10⁸ K, and the helium in the core acts as the thermonuclear energy source. This reaction produces carbon, but a star of low mass relatively soon runs out of helium and the core collapses into a white dwarf, while the outer regions drift away into space, possibly forming a planetary nebula. Larger stars (several times larger than the sun) have sufficient helium for the process to continue so that heavier elements, up to iron, are formed. But iron is the heaviest element that can be formed with the production of energy and when the helium has all been consumed there is a catastrophic collapse of the core, resulting in a supernova explosion, blowing the outer layers away. The current theory suggests that thereafter the collapsed core becomes a neutron star or a black hole, depending on its mass. Astronomy The series of changes that stars undergo during their lifetimes, the time-scale of which depends strongly on the star’s mass and also, to some extent, on its initial composition. The progress of a star during its evolution can be followed on a graph called the Hertzsprung–Russell (HR) diagram. A star is born when a dense region of a cloud of gas collapses under its own gravity. A star first shines because the gravitational potential energy lost in this collapse is released as heat and light. Eventually the temperature at the centre of the protostar reaches 1 million K, igniting nuclear reactions involving deuterium (an isotope of hydrogen), and for some time the energy from this is sufficient to prevent further collapse. Once the deuterium has been exhausted, the collapse continues, and the star is classified as a pre-main-sequence object, following a characteristic path on the HR diagram (see hayashi track; henyey track). For a star the mass of the Sun, this phase lasts several million years. Eventually the core of the star reaches temperatures of around 10 million K, hot enough to initiate the nuclear reactions that convert hydrogen to helium, and the star joins the main sequence on the HR diagram. This hydrogen-burning phase will last from a few million years, in the most massive stars, to (potentially) more than the present age of the Universe for low-mass stars. Once the hydrogen in the core has been exhausted, the core contracts under its own gravity until, in stars of more than 0.4 solar masses, the core temperature reaches 100 million K, initiating further reactions which transform helium into carbon (the triple-alpha process). Subsequent evolution depends on the star’s mass. In stars of similar mass to the Sun and greater, while helium burning proceeds at the centre, hydrogen burning may continue in a shell outside the core. In this post-main-sequence phase the star is cooler, larger, and brighter than it was on the main sequence, and is classified as a giant or, for the most massive stars, a supergiant. Once the helium in the core is exhausted, the process of core contraction, followed by the initiation of a new set of nuclear reactions, may be repeated several times. Thus the more massive giants and supergiants can develop a layered structure, with the heaviest fuel burning in the centre and overlying layers containing lighter fuels from previous burning cycles. Throughout these processes the stars become larger and brighter. Eventually, however, either the contraction of the core fails to bring about a high enough temperature for further nuclear reactions or, in supergiants, the point is reached at which the core consists of iron, which cannot be used as a nuclear fuel. At this point, with no more energy being produced at the star’s centre, the core collapses. The collapsing core becomes a neutron star, or possibly a black hole, while the outer layers are ejected explosively in a Type II supernova explosion. In less massive stars, evolution proceeds rather differently, in part because their cores are dense enough for degeneracy effects to be important. When helium ignites in a degenerate core it does so explosively in a helium flash, causing the core to expand. Thereafter, with the star on the horizontal branch of the HR diagram, helium continues to burn non-explosively in the core while hydrogen burns in a surrounding shell. Once helium is exhausted in the core, it continues to burn in a shell during the asymptotic giant branch phase. Details of later evolutionary phases are uncertain. However, it is thought that the outer layers of the red giant are puffed off to form a planetary nebula, leaving the core of the star exposed as a white dwarf. Hence the end-point of stellar evolution, in both high- and low-mass stars, is that much of the star is dispersed into interstellar space, leaving a collapsed remnant of spent nuclear fuel.
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