2.2 Stars as Element Factories
The basic definition of a star is that it is a huge ball of extremely hot gas consisting mainly of hydrogen and helium. However, this definition does not appropriately encapsulate just how important stars are to the evolution of the universe. Stars are responsible for creating nearly all the elements in the universe, either through nuclear fusion within their cores (a process known as stellar nucleosynthesis) or their death process. Without stars, elements heavier than beryllium would not exist!
Astronomers estimate that the universe could contain up to 1 x 1024—or one septillion—stars. Our Milky Way alone contains more than 100 billion, including our most well-studied star, the Sun. However, not all stars are the same. They come in a variety of different masses, ranging from 1/12 the mass of our Sun (i.e., a solar mass [MSun]) to roughly 100–200 the mass of our Sun. The mass of a star (or in other words the amount of matter in a star) determines how much fuel a star has to drive nuclear fusion in its core. The more massive a star, the more fuel it has and vice versa. In this way, the mass of a star controls how it lives and how it dies.
Remember, in nuclear fusion, two or more light atomic nuclei smash together to produce a heavier nucleus and massive amounts of energy. In the cores of stars, the temperature and pressure is high enough to ignite this process, and the energy that is released is what causes a star to shine! Once a star’s core temperature exceeds 10 to 12 million Kelvin, fusion begins, with hydrogen turning into helium. Our Sun is a relatively average low mass star; it first uses up its hydrogen fuel to make helium and then uses some helium to fuse to make small amounts of beryllium, carbon, nitrogen, oxygen, and fluorine. Stars more massive than our Sun are needed to fuse elements heavier than fluorine inside their core. However, regardless of size, only elements up to iron on the periodic table can be produced through fusion within the core of stars (i.e., stellar nucleosynthesis).
Eventually stars exhaust their nuclear fuel and begin the death process. It’s only through the death of stars that the remaining elements are created. In the future, our lower mass Sun will stop making new atoms in its core, cool down, and bloat until its middle reaches the orbit of Mars. In contrast, higher mass stars end their lives in spectacular fashion, exploding as supernovae and casting off newly formed atoms—including the elements heavier than iron—into space. It took many generations of stars creating heavier elements and casting them into space before heavier elements were abundant enough to form planets like Earth.
The above paragraphs are a very cursory discussion of how stars are element factories, so be sure to watch the below video to learn more about how the lifecycle and death of different sized stars creates all the elements beyond beryllium.
For Further Exploration
For a more detailed explanation of the stellar lifecycle, watch the public talk titled Nucleosynthesis: the story of ‘us,’ as told by star stuff from Cambridge University Astronomy.
Text Attributions
The text of this chapter is partially adapted from:
- Section 2 (Thinking Ahead) of OpenStax’s Astronomy 2e (2022) by Andrew Fraknoi, David Morrison, and Sidney Wolff. Licensed under CC BY 4.0. Access full book for free at this link.
- Section 22.2 by Karla Panchuk in Physical Geology – 2nd Edition (2019) by Steven Earle. Licensed under CC BY 4.0, except where otherwise noted.
