词条 氢聚变
描述: 核聚变是一个总称，指的是较轻的原子核相互碰撞并合并形成一或多个较重的原子核的所有反应。在天文学中，氢核聚变是将氢核（由一个质子组成）转化为氦-4 核（由两个质子和两个中子结合在一起组成）的核聚变反应。氦-4核的质量小于它由质子和中子组成的质量之和。根据爱因斯坦著名的公式 E=mc2，质量差相当于能量差。当质子和中子聚变形成氦-4 时，就会释放出与该差值相对应的能量。通过这种方式，氢聚变成为了像太阳这样的主序星的能量来源。至少在一段时间内，这类恒星处于平衡状态：它们核心中由氢聚变释放的能量量与这些明亮恒星以光和其他类型的电磁辐射以及粒子形式释放的能量量相对应。
氢聚变要经过几个中间步骤。对于质量与太阳相当或更小的恒星来说，氢聚变是通过所谓的质子-质子链（pp 链）进行的。在该反应链的最简单版本中，两个氢核（质子）融合产生氘核（各一个质子和一个中子），然后氘核与另外一个氢核融合产生氦-3（两个质子，一个中子）。两个这样的氦-3 核融合后产生氦-4 和剩余的两个氢核。在质量超过太阳 1.3 倍的恒星中，一种叫做碳-氮-氧（CNO）循环的替代过程成为将氢熔化成氦的主要方式。地球上的科学家已经制造出了制造核聚变反应的装置，并希望将来它能成为一种可行的能源生产方式。氢聚变不仅发生在恒星中，在宇宙大爆炸的早期阶段也曾发生过。
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其他语言版本
- 阿拉伯语: اندماج الهيدروجين
- 德语: Wasserstoffbrennen
- 英语: Hydrogen Fusion
- 法语: Fusion de l'hydrogène
- 意大利语: Fusione di idrogeno
- 巴西葡萄牙语: Fusão de hidrogênio
- 繁体中文: 氫聚變
相关图表
Stellar Structure
图注： Stars are balls of plasma. For most of a star’s life it burns hydrogen into helium in its core. This phase of a star’s life is known as the main sequence. Burning hydrogen into helium produces heat, that heat travels out of the star’s core eventually reaching the star’s photosphere (often referred to as the “surface” of the star). From here the heat can radiate into space as various forms of electromagnetic radiation. However, how heat travels from the core to the photosphere depends on the star’s mass.
Imagine a parcel of gas rising inside a star. As it rises, it moves into an area of lower pressure, so it cools down and expands. If the parcel is still hotter, and therefore less dense than its surroundings, it keeps moving upward due to buoyancy. Eventually, it will rise far enough to cool and sink back down. This rising and sinking cycle is called convection. Whether convection occurs depends on how quickly temperature changes as you move away from the star’s core. If the temperature in a star drops rapidly, rising parcels of gas are more likely to stay hotter than their surroundings, so convection dominates as the mode of energy transfer in this part of the star. Conversely if the temperature drops more slowly (i.e. if the temperature gradient is small) then heat will mostly be transferred by radiation (photons).
In the most massive main sequence stars (more massive than about 1.5 times the mass of the Sun, seen here on the left), hydrogen is burned into helium using the CNO cycle. This is highly temperature dependent and thus energy production is concentrated near the center of the star. This leads to a larger temperature gradient and thus a convective core. Further out the temperature gradient becomes smaller and heat transport is dominated by radiation. This is called the radiative zone.
For lower mass stars like the Sun (between 0.3 and 1.5 solar masses, seen here in the middle) hydrogen is burned to helium using a different process (the pp chain). This depends less on the internal temperature than the CNO cycle and so energy production is more distributed in the star’s core. This leads to a smaller temperature gradient and thus a radiative core where convection occurs surrounded by a radiative zone. Going further out the gas becomes cool enough for some elements to hang to on some of their electrons, i.e. not being completely ionised. This partially ionised gas is more opaque to photons, trapping heat. This leads to a large temperature gradient and thus convection.
The lowest mass stars (below 0.3 solar masses, seen here on the right) have no radiative zone and are fully convective.
The arrows in the radiative zone are shown as wavy lines heading out of the star. However, a photon’s journey out of a star is much more complex with each individual photon travelling only a short distance before being deflected by some of the charged particles that make up the plasma of the star’s interior. This leads to a long and winding road that takes millennia instead of the few seconds it would take if the photon did not interact with particles in the plasma.
来源： Based on a vector diagram by Wikimedia user Д.Ильин which itself is based on a diagram from sun.org
Stellar Evolution
图注： This diagram shows the life cycle of stars of different masses. The mass of the different types of star increases from bottom to top with time going from left to right.
The life cycle of a star depends on its mass, with lower mass stars have longer lifetimes. All stars form from clouds of gas that collapse under their own gravity. As the star collapses, its core becomes hotter and denser. If the star has a mass greater than 0.08 solar masses (0.08 times the mass of the Sun), the pressure of the star’s mass pushing down on its core creates a high enough core temperature for hydrogen fusion to ignite. This burns hydrogen into helium in the star’s core, providing a heat source to power the star and to stop its core from collapsing further. If the collapsing object has a mass below 0.08 solar masses then it does not ignite hydrogen fusion in its core. It continues to cool and slowly contract. Such substellar objects are known as brown dwarfs, shown here in the lowest row.
After stars have formed, they burn hydrogen in their cores and begin their so-called main sequence phase. The most massive stars (>25 solar masses, shown here at the top) have very high core temperatures and thus burn through their hydrogen fuel more quickly. This means they may only spend a few million years on the main sequence burning hydrogen in their cores. Once the hydrogen in the core is exhausted the star’s core contracts, becomes hotter and helium burning starts in the core. While the core contracts, the outer layers of the star expand and it becomes a supergiant. For the most massive stars strong stellar winds strip off the cooler outer layers, leading to the star being very large and very hot, a blue supergiant. Once helium is exhausted in the core, carbon is burned, and then heavier elements. Eventually the star ends with an iron core. Fusing iron into heavier elements does not generate energy so at this point fusion stops in the core. Once this core of iron is massive enough, it and the surrounding matter suddenly collapses to form a black hole and the outer layers are flung off in a supernova explosion.
Slightly lower mass stars (between 8 and 25 solar masses, seen here second top) evolve in a similar way although they do not have strong enough winds to push their outer layers away and become blue supergiants, instead it evolves into a red supergiant. While such stars also collapse and create supernova explosions. The remnant of the star’s core is not massive enough to collapse into a black hole. Instead, its electrons and protons combine to form neutrons and it is supported by a quantum mechanical effect called neutron degeneracy pressure. This results in the remnant of the star being a tiny neutron star, several solar masses in mass but only a few kilometres across.
For stars similar in mass to the Sun (between 0.4 and 8 solar masses, seen here in the middle row), the star burns hydrogen in its core until the hydrogen in its core is exhausted. At this point a hydrogen burning shell forms around the core. Eventually the core will become hot enough to burn helium into carbon and oxygen. After this the star is left with a carbon and oxygen core surrounded by shells burning helium and hydrogen. These shells are unstable producing thermal pulsations that convulse the star. Eventually these pulsations become so extreme that the star’s outer layers are thrown off. This leaves the carbon and oxygen core as a white dwarf supported by electron degeneracy pressure. The outer layers of the star form what is known as a planetary nebula (which doesn’t actually have anything to do with planets despite the name).
The lowest mass stars (seen here in the second bottom row) are so low in mass that their evolutionary timescales are much longer than the age of the universe. This means that none have evolved beyond the main-sequence. Low mass stars are fully convective meaning material in the core is constantly being mixed with material above. This means that all the hydrogen in the star would eventually be burned in the core, but this will take trillions of years.
来源： Danielle Futselaar/IAU OAE
