(V): Where does carbon come from?

世间万物都在运动变化，都有各自的起源和演化规律，碳元素也不例外，也在不断演变和进化，也有其起源和演化的规律。

那么碳是从哪里来的呢？

早在1886年，英国著名科学家克鲁克斯曾发表了一篇论文《元素的产生》，提出所有的元素都是由一种原始物质逐渐凝聚而成的。1932年匈牙利天文学家勒梅特首次提出宇宙大爆炸概念。他根据任何事物都是从简单到复杂的规律，认为整个宇宙最初是聚集在一个“原始原子”中，后来发生了大爆炸，碎片向四面八方散开，形成了现代的宇宙。1956年，美国科学家尤里首先发表了在宇宙间元素分布的数据，发现氢最多，氦次之，再次为碳、氮、氧，并且某一元素随其原子量的增加而迅速减少，但到了铁时又会突然增多，而比铁原子量更重的元素则又逐渐减少。1948年，美籍俄国天体物理学家伽莫夫考虑到，如果按照爱因斯坦的广义相对论的理论再加上一些原子的反应过程，就有可能推算出宇宙的演化历史。伽莫夫和他的学生阿尔文及朋友贝瑟一起将他们的论文《化学元素的由来》投给著名的刊物《物理学评论》，这篇文章最后按照三位作者名字第一个字母的希腊文读音，被称之为αβγ理论。这一文章在勒梅特想法的基础上，完整地提出了他们的大爆炸宇宙模型，提出宇宙起源于大约150亿年前一次巨大的爆炸，由最早时期温度极高且密度极大，体积极小的一个“奇点”迅速膨胀而形成，是一个由热到冷、由密到稀不断膨胀的过程，犹如一次规模极其巨大的超级大爆炸。根据天文观测和爱因斯坦的相对论，现代天文学界和物理学界都一致认同宇宙诞生的大爆炸理论。20世纪60年代，这个预言果然被天文学家在对宇宙进行观测后证实，这也就使大爆炸宇宙模型成为宇宙学界的标准模型。它与粒子物理学中的夸克模型、生物学中的DNA双螺旋结构模型和地质学中的板块模型一起，并称为20世纪科学界最著名的四大科学模型。

在此基础上，1954-56年间福勒（Fowler）与伯比奇夫妇 (E. M. Burbidge和G. R. Burbidge) 以及霍伊尔 (F. Hoyle) 合作，对恒星中的核反应进行了一系列的研究。1957年，他们全面阐述了恒星中化学元素的核合成，在《现代物理评论》上发表了后来简称为B2FH（四作者名的第一个字母）理论的著名论文。该论文指出了恒星的演化方向以及与恒星演化各阶段相应的8种核合成过程，提供了计算恒星内部结构的客观基础，阐明了超新星爆发和大质量恒星演化的关系。

近年来美国宇航局威尔金森微波各向异性探测器(WMAP)向天文学家提供了新的数据，证实大爆炸理论的正确性，它已探测到137亿年前大爆炸后40万年所形成的余辉，发现约40亿年后第一代星球的生成。

WMAP探测到的宇宙膨胀过程

Therefore, according to the recent standard cosmological model and B2FH theory, there is a relatively consistent view that the universe is in a continuous cycle of expansion and contraction, with neither a beginning nor an end. The cycle we are in was produced about 13.7 billion years ago by the explosion of small particles that contracted to very dense particles. In a very short time (10-30 seconds) after the big bang, the universe accelerated inflation, and is currently in the early stages of slow expansion and is still expanding. A few million seconds after the explosion, the temperature reaches 0.1 billion degrees (1011K), forming an ultra-hot, extremely dense, composed of particles called quarks (quarks) and gluons (gluons), which wander around and go on a rampage. Then a small number of electrons, photons and other lighter elementary particles joined in. At this time, the temperature of the mixture can reach trillions of degrees Celsius, more than 100000 times hotter than the core of the sun. However, the temperature will plummet as the universe expands, just as a blob of ordinary gas today cools when it expands rapidly. The speed of quarks and gluons also slowed so much that some of them began to temporarily stick together. As the universe expanded, as it cooled to 0.1 billion degrees, the strong forces transmitted by quarks through gluons formed subatomic particles, protons and neutrons, which in turn formed atomic nuclei. When the electromagnetic force works, the nuclei capture electrons to form some of the lightest hydrogen and helium atoms, a process that takes only three minutes. 1 billion years after the Big Bang, when the universe cooled to 4000K, the initially formed elements such as hydrogen, neon and helium formed primitive interstellar matter and contracted by gravity to form primitive stellar clusters. At this time, the temperature gradually increased. When the temperature reached 7 million K, the electrons in the atoms were removed, leaving only the nuclei. Under high temperature and high pressure, they collided due to thermal motion, and the two light nuclei fused together to form a heavy nucleus. In a star with a temperature of 110~11.1 million K, the proton-proton thermonuclear reaction is mainly produced, and the hydrogen nuclear fusion is converted into helium. When the temperature and pressure at the center of the star increase to the point where the helium is ignited, the helium core contracts and the outer layers expand and cool, becoming a red giant. According to the B2FH theory, three helium atoms can be fused into one carbon atom through the 3α process. This process is completed in two steps. First, two alpha particles (ie, 4He nuclei) form 8Be, but the latter has a lifetime of only 10-16 seconds, and then another alpha particle is captured to form 12C. Carbon is therefore the first lightest element to form completely inside the planet. If the core of the planet shrinks further to a high temperature above 0.6 billion degrees, carbon will continue to fuse to produce nuclei such as oxygen and magnesium. Without the 3α process to form carbon, further formation of other heavy elements is not possible. After the formation of iron with a proton number of 26, nuclear fusion is no longer an exothermic reaction. At this time, the shock wave generated by the core collides with the outer material, releasing a large amount of energy to produce more thermonuclear reactions, thereby forming more than iron. Heavy elements. The outer layers of the star are blown up, forming a so-called supernova explosion, and various elements generated by a series of thermonuclear reactions are also scattered throughout the universe. It is estimated that there are 215 carbon atoms per 106 hydrogen nucleus in the sun and 85 carbon atoms in 106 hydrogen nucleus in the universe. however, the carbon-rich stars with carbon/oxygen> 1 in the envelope of stars and their periphery can produce solid c at a rate of about 0.002M ⊙(M ⊙ is the mass of the sun) per year.

在温度为 ~5×107K的最早恒星中，会以更高速率产生必要量的碳来催化氢的燃烧。太阳中心温度为1500万度，而质量>1.5M⊙的恒星，其核心温度可达3000万K以上，此时主要是：“碳氮氧循环”（也称“碳循环”）反应。在此反应中，碳作为中间物起催化作用，所以碳在恒星发光发热过程中也起着重要作用。1938年贝特（Bethe）提出这一碳氮氧循环理论,并因此获得1967年的诺贝尔奖。

从现代观点看，整个宇宙都是大爆炸后热核反应的产物，无论是我们每天看到的太阳，还是我们目前生活的地球都莫不如此。作为物质的一种的碳元素当然也不例外。可以说，我们人体中的每一个碳原子也都是宇宙大爆炸后，在恒星热核反应中燃烧 “灰烬”的产物。

碳是宇宙前期中形成的关键元素,也是地球上最活跃的元素,其存在形式复杂而且在不断地演化:从单一键态，演化到复合键态的多形式碳；从早期的原生碳，演化到无机碳,从无机碳演化到有机碳；碳的单质形态，也从链型向层型、管型、球型和配位型不断转化。那么宇宙中的碳有哪些形态？请看研炭翁说碳(六)宇宙中的碳。

本文来自王茂章科学网博客。

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