Free The Life of Super-Earths by Dimitar Sasselov

found all over the Universe (billions of galaxies’ worth) in big and small clumps that just sit there and do nothing—except when some of these clumps get compressed under their own weight and form stars. It just happens that the balance between gravity pull and matter repulsion is achieved at temperatures and densities inside the star that allow the atomic nuclei of hydrogen and helium to fuse. When you fuse atomic nuclei, two important consequences follow: lots of energy is released and new, heavier nuclei are formed. That is how our Sun shines.

    Stars are the queens of fusion—they do it admirably well! They literally light up the place and proceed to transform it from a boring simple gas to the richness of the entire table of the elements. 7 The process is orderly: first, hydrogen fuses into helium until the central regions of the stars are chock-full of helium, which, being heavier than hydrogen, shrinks and heats up. Helium heats up until its threshold for fusion is reached, and then a new stage in the life of the star begins, at least inwardly.
    While fusing hydrogen produces mostly helium (fusion would be a clean, powerful source of energy for humankind, if we ever learn to do it in a controlled fashion), the fusion of helium produces a number of heavy elements, most notably carbon and oxygen. Stars can fuse elements all the way up to iron, at which point they stop, lacking sufficient energy to go any farther—unless the star is big enough to explode. In such a supernova, more fusion can happen that produces many more elements and frees the rest to capture electrons and become the atoms of heavy elements with all the rich chemistry they can cook up.
    Astronomers can observe how the stars enriched the Universe in heavy elements. The large telescopes of the past ten to twenty years have allowed them to peer back to about 12 billion years ago. They see some heavy elements, such as iron; they see patterns in which elements are relatively enriched and that reveal how stars produced them. The picture that emerges is one of generations of stars steadily transforming the hydrogen and helium of the young Universe into all the heavy elements.

    Stars form out of low-density gas, which must cool while being compressed (under its own weight) for a star to be able to condense. Because hydrogen and helium are terribly bad at such cooling, the first stars must have been super-size only—hundreds of times larger than our Sun. The first stars were massive, had short lives, and produced some heavy elements that were dispersed inside the nascent galaxies and made it possible to form smaller, less massive stars. The addition of even a sprinkling of elements to the hydrogen-helium gas helps it cool, so the next generation of stars can be formed from a wider range of gas clumps. With each generation, progressively smaller stars can form—and they do. Today our Galaxy has many stars smaller than our Sun. Partly this is due to the fact that smaller stars live longer by burning their nuclear fuel slowly, but mostly because small stars have formed in increasingly larger numbers as the Universe has evolved.
    Small stars disperse a portfolio of heavy elements when they die, so the enrichment of the Universe with heavy elements continues at a slow, steady pace. In fact, after 13 billion years only about 2 percent of the original mixture has been transformed to heavy elements; the enrichment, as astronomers like to call it, is very slow indeed.
    The brief story of the Universe, then, looks like this: from just hydrogen and helium about 13 billion years ago, generations of stars made enough iron and oxygen, silicon and carbon, and all other elements, to be able to form Earths and super-Earth planets. There are at least two important morals to this story regarding life.

    First, it took a long time before stars anywhere in the Universe could have planets. Stable environments in normal galaxies that were enriched enough to