The life and death of stars form the chemical elements that make up Earth, making stars critical to

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Look up at the stars. They may seem like permanent fixtures in the night sky, but did you know that stars eventually die? The life and death of stars form the ingredients that make up Earth, making stars critical to life as we know it. The early universe contained nothing but the chemical elements hydrogen, helium, and tiny amounts of lithium and beryllium. During their life cycles, stars create elements with low atomic masses. These are the first 26 elements in the periodic table up to and including iron. When most stars die, these light elements spread across the universe, including to planets like Earth. How are stars born? Early in the history of the universe, before stars and planets existed, giant clouds of hydrogen and helium began to form. Slowly, these clouds collected enough mass for their own gravity to form. This created extremely dense balls of gas. In other words, they formed stars. When a new star is formed, its core is exposed to very strong gravitational forces. This force is so great that the star is in danger of collapsing in on itself. Luckily, nuclear fusion provides the energy the star needs to push back against the collapsing core. Nuclear fusion is a process where the nuclei of two or more elements combine to produce nuclei of heavier elements. Nuclear fusion also releases energy. In the core of a newly formed star, hydrogen nuclei begin to fuse into helium. The inward pull of gravity and the outward push of nuclear fusion eventually balance out. For a time, hydrogen fusion prevents the collapse of the star. When the young star runs out of hydrogen, its core will once again begin to collapse. The extreme forces on the core causes it to heat up. Soon, the core is hot enough that it can begin to fuse helium into carbon and oxygen. Once again, nuclear fusion pushes back against gravity to prevent the star from collapsing. One by one, the star fuses each new element. This successively produces elements with low atomic masses like carbon, oxygen, and neon. Not only does nuclear fusion keep stars from collapsing, it enabled the first stars in the universe to create new elements that had never existed before! Depending on their size, stars can create elements through fusion, up to iron, which has an atomic number of 26. But there are 118 elements in the periodic table. So, where do all the elements with an atomic number higher than iron come from? From the death of stars. How do stars die? Even though stars are not living things, they have “life cycles” and at some point they are said to “die.” How a star lives and dies depends on how large it is. The smallest stars, brown dwarf stars, are too large to be considered planets, but too small to be considered stars. They are unable to sustain the fusion of hydrogen because of their low mass, and are often called "failed stars." The small, slow-burning red dwarf stars have very long lives. Their lives last between one and ten trillion years! Scientists believe that when red dwarf stars eventually collapse, they will become white dwarf stars. These are very dense stars that no longer burn fuel. Scientists also believe that eventually, the white dwarf stars in the universe will cool off and become black dwarf stars. When mid-sized stars, like the Sun, run out of hydrogen, their cores will contract and heat up. The outer layers of gas will expand and the stars will become red giant stars. Eventually when the core of a red giant star cools, the remaining gas will float into space, forming a planetary nebula. Each planetary nebula has a white dwarf star at its core. The very largest stars first become blue supergiant stars before dying in a dramatic fashion. In fact, they create the biggest explosions in the universe when they collapse. We call these explosions supernovas. A supernova is so bright that it can outshine an entire galaxy of a hundred billion stars! The initial explosion of a supernova has so much energy that it can split atoms apart at the core, sending protons and neutrons flying into the universe. In the moments following the explosion, these particles crash into each other with enough energy to fuse back together. Light elements continue colliding with protons and neutrons in this way, constantly growing larger and larger. This process, which is similar to nuclear fusion, is called nucleosynthesis. The nucleosynthesis that occurs during the explosion of a supernova produces elements with a higher atomic number than iron, which cannot be created by nuclear fusion. No matter how a star dies, its life cycle can transform the universe. Without stars, the universe would contain nothing but clouds of hydrogen and helium. It is the life and death of stars that are responsible for the elements that make up everything you see on Earth!When the first stars died out this way, brand new elements, including gold, were formed. Eventually, those elements ended up here on Earth. After a supernova explodes, the core that remains becomes a neutron star. This is an extremely small and dense type of star. For the largest stars of all, the remaining core is so massive and has such a strong gravitational pull that not even light can escape. This is called a stellar black hole.

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