by Michelle Sekili
Have you ever wondered how there came to be billions and billions of stars in the sky? Or why some stars violently explode, while some just quietly fade out of existence? The answer to these questions lies in the life cycle of a star, a long but magnificent journey. It all begins inside a nebula.
Nebulae are the nurseries of stars. They are large, colorful clouds of cosmic dust and gases. One of such is the Orion Nebula, whose beautiful colors are formed by neutral and ionized gas particles of elements such as hydrogen and oxygen drifting in the cloud. Inside of a nebula, significant turbulence causes dust and gases to pile up unevenly into a central clump. When the clump is massive enough, it begins to collapse under the pull of its own gravity. [1] For the next few million years, with enough heat and pressure, it continues to collapse, until the hot core in the center of a star—the protostar—forms.
The star’s core is made up of mainly hydrogen. Therefore, when it gets hot enough, nuclear fusion begins. During fusion, the tremendous amount of heat and pressure in the core forces hydrogen atoms together and overcomes the natural repulsion of protons, creating helium. This fusion in the core releases an enormous amount of energy and outward gas pressure that counter the inward force of gravity. As long as the outward and inward forces are equal, a star remains the same in size. [2] Hydrogen fusion marks the beginning of the longest phase of a star’s life: the Main Sequence stage. In fact, stars spend about 90% of their lives in this stage and most of the stars in the universe are in this stage, including our Sun. [3] It has been calculated that the Sun will remain in its Main Sequence stage for about 5 billion more years. [4] Specifically, this calculation predicts how long it will take hydrogen in the Sun’s core to run out and stop the fusion process, in which the star would transition into the next phase of its life cycle: the Red Giant Phase.
In this Red Giant phase, a star is like an onion. It is composed of several layers: the center is a hot and dense helium core, while each additional layer is a shell containing hydrogen. When fusion stops in the core, the inward force of gravity overpowers the declining outward pressure, forcing the core to shrink. First, we’ll take a look at what happens in the shells, or layers surrounding the core of the star. The innermost layer of hydrogen begins heating up and eventually begins fusion. As fusion in the shells produces additional energy, the increased outward pressure overcomes the inward force of gravity, causing the less dense outer layers of the star to expand and cool down. Meanwhile, the helium produced by the shells’ hydrogen fusion is dumped on the shrunken core, thus increasing its mass. [5] Once the core reaches a high enough temperature, helium atoms now have enough energy to overcome their natural repulsion. Helium fusion begins, producing carbon and releasing remarkable amounts of energy and pressure. The process cascades through the layers: each layer takes turns carrying out hydrogen and helium fusions, and the pressure produced pushes the cooler, less dense outer layers further outward, resulting in the massive size of the Red Giant stars observed today. For example, when the Sun becomes a Red Giant, it will be about a hundred times its current size! [5] Red giants will continue to enlarge as long as there is helium in the core. Once this helium runs out, the star—depending on its mass—transitions to either one of two final stages.
Stars that are less than three times the mass of our Sun are considered low mass stars. In a star like this, fusion stops when helium runs out and all that is left in the core is carbon. The extremely dense leftover carbon core of a low mass star is called a white dwarf. In the outer layers, clouds of gas are only weakly held to the star by gravity, so they eventually drift away and form new planetary nebulae. Though fusion no longer produces an outward force in the core, the star does not collapse further because another new force balances out gravity. This force, called electron degeneracy pressure, is created when electrons, crammed in a small space, are forced into higher energy levels and thus move faster. [1] White dwarfs are relatively small—they are about the size of Earth—and lose their luminosity over time. Isolated white dwarfs will eventually fade quietly out of existence, dying a quiet and peaceful death. However, if a white dwarf happens to be orbiting another white dwarf, the stronger gravity of one white dwarf will suck up the mass of the other one and add to its own mass. Gravity from too much mass pushing down on the core will overwhelm the electron degeneracy pressure and crush the core in seconds. The core, then, explodes as a brilliant white dwarf supernova, marking the end of the star’s life.
On the other hand, we have high mass stars—stars with mass greater than or equal to three times the mass of the Sun. These stars have enough gravity to continue fusion of elements heavier than carbon. Each time a fusion reaction occurs in the core or layers, the released outward pressure pushes the outer layers further, creating enormous stars known as supergiants. In supergiants, fusion reactions which produce oxygen, then silicon and other heavy elements in the core happen so quickly that the star becomes unstable. Nevertheless, when it is iron that makes up the core, fusing the heavy iron atoms together uses up more energy than it produces, and gravity pulling inward overcomes the opposing outward pressure. [1] The core collapses in a fourth of a second, becoming so dense that when the outer layers fall down on the core, they “bounce” off rapidly into space in a giant supernova explosion. In the remaining hyper-dense core, protons and electrons actually merge together to form neutrons. The core is now a neutron star. Similar to electron degeneracy pressure, neutron degeneracy pressure in neutron stars combats gravity. However, if gravity manages to overwhelm neutron degeneracy pressure, gravity wins the final battle of the forces and the star forever collapses on itself in what is known as a black hole. Meanwhile, the blown off outer layers of the supergiant become nebulae, where a cosmic recycling process occurs: gravity regathers remnants and debris to form new stars and planets.
Thus, after billions of years, the life cycle of a star comes to an end. Nonetheless, remnants of many stars live on in us. Supernovae explosions have created the elements and life as we know it. Even our planet and the oxygen we breathe were created in a high mass star supernova. There are bits of the stars inside us. In the words of famous astrophysicist Neil Degrasse Tyson, “we are stardust.” [6] When we are gone, the cycle continues, going from dust to dust.
Works Cited:
[1] Bolles, Dana. “Stars.” NASA, NASA, https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve.
[2]“How Is a Star Born?” Scientific American, Scientific American, 6 Dec. 1999, https://www.scientificamerican.com/article/how-is-a-star-born/.
[3] Zuckerman, Catherine. “Everything You Wanted To Know About Stars.” Science, National Geographic, 3 May 2021, https://www.nationalgeographic.com/science/article/stars.
[4] Redd, Nola Taylor. “Red Giant Stars: Facts, Definition & the Future of the Sun.” Space.com, Space, 28 Mar. 2018, https://www.space.com/22471-red-giant-stars.html.
[5]“Post-Main Sequence Stars.” Post-Main Sequence Stars, Australia Telescope National Facility, 21 July 2021, https://www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_postmain.html.
[6] Tyson, Neil deGrasse, and Gregory Mone. Astrophysics for Young People in a Hurry. W.W. Norton & Company, Incorporated, 2019.
[7] “Infographic: Life Cycles of Stars.” 3.0—How Were Stars Formed? Khan Academy 2016,
https://cdn.kastatic.org/KA-share/BigHistory/KU3.0.9_Life_Cycle_of_Stars.pdf