Are we made of stars? This video segment adapted from NOVA features scientists studying light emissions from exploding stars, called supernovas. Their observations and analyses can tell us the origin of the elements found on Earth and throughout the universe.
All liquids, gases, and solids found on our planet are made from one or more of 92 naturally occurring elements. From what they have observed, scientists have determined that these same 92 elements are found throughout our universe. This suggests that a common process leads to their creation. But how are these elements created? How did they get they so widely disseminated?
Currently, the most popular theory states that the nuclei of hydrogen and helium, the lightest and most abundant elements in the visible universe, were created in the moments following the Big Bang. All other naturally occurring elements were — and continue to be — generated in the high temperature and pressure conditions present in stars. Elements are composed of tiny particles called atoms that are indivisible under normal conditions. However, when exposed to high heat and pressure, atoms can either break apart or fuse together. Under these conditions, the nucleus of one element can fuse with the nucleus of a different element, creating the nucleus of a heavier element. When elements lighter than iron form, the mass of the new nucleus is less than the combined mass of the two original nuclei. The difference in mass between the two is released as energy. In stars, this kind of reaction is referred to as stellar nucleosynthesis, but it is more commonly known as nuclear fusion. Nuclear fusion is used today on Earth in the nuclear explosives called hydrogen bombs. Many people hope that one day nuclear fusion will be used for peaceful energy production.
Stars are fueled by nuclear fusion reactions, which take place in their deep interiors, or cores. Hydrogen nuclei fuse, forming helium nuclei. The energy produced by these fusion reactions prevents the star from collapsing under its own gravity. Mature stars contain enough hydrogen nuclei to last billions of years. When a star's hydrogen fuel supply is spent, however, its core begins to contract. The contraction is so intense that it creates conditions under which helium nuclei fuse. In this way, helium becomes the star's next fuel source. The fusion of helium nuclei produces carbon and oxygen nuclei, and in the process sufficient energy is released to temporarily sustain the star.
Once helium runs out, the nuclei of carbon, oxygen, and other elements begin to fuse. These new fuel sources are depleted at faster and faster rates. Since the heaviest element created in a star by nuclear fusion reactions is iron, a large iron core eventually forms at the center of everything. At this point, gravity becomes overwhelming, the core collapses, and an explosion occurs, during which outer layers of gas and heavy elements are ejected to space. Such explosions, called supernovas, occur about once a century in our galaxy. The energy created by supernovas produces nuclei heavier than iron. This process is known as supernova nucleosynthesis.
Long ago, scientists learned that they could heat atoms of an element to a glow and direct the light though a prism to produce a set of colored lines. These spectral lines are unique for each element: No two elements produce the same colors and line positions along a spectrum. By using instrumentation that reads light signatures from far away — a technique known as spectroscopy — scientists today know with great certainty which elements a planet or a star, or even a star's dispersed remnants, contains.
To learn about how certain elements are created in a star, check out The Elements: Forged in Stars.
To learn about carbon, one of the universe's most common elements and the most important one for life on Earth, check out Ingredients for Life: Carbon.
Consider in this NOVA classroom activity how scientists use indirect observations to infer conclusions about objects they cannot directly measure.
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