Abstract
Supernovae are considered the most spectacular and powerful explosions in the universe, and they occur at the end of a star’s lifetime. They are divided into two main classes, Type I and II, and can be caused by either a core-collapse or thermal runaway. Supernovae play a vital role in the universe by creating and dispersing materials that can eventually become the seeds of new solar systems. Moreover, they are useful for scientists as cosmic mile markers for determining distances to distant objects. Supernovae do not pose a significant danger to Earth since the closest supernovae are not close enough to eradicate the planet itself; however, there are some serious implications that they can cause. Despite their importance, much remains unknown about supernovae. Scientists have studied supernovae for centuries, but there is still no clear understanding of what causes certain supernovae, such as the Type Ia supernovae.
The Tycho Supernova (SN 1572)[15]
Introduction
A supernova is a bright explosion of a star, brought about by a change in a star’s core. Supernovae are the largest explosions that occur in space, flinging matter into space at speeds of up to 25,000 miles per second.[1] In most cases, a supernova explosion occurs at the end of a star’s life, when its core runs out of the fuel it needs to sustain the process of fusion.[2] Usually, a star with the process of fusion in its core still intact would exert an outward pressure to counterbalance the inward gravitational pull. However, when the star\’s core starts to run out of fuel needed for fusion, the outward pressure drops, and the core becomes hotter and denser.[1] At some point, this core grows denser until it becomes unstable and incapable of withstanding its gravitational force. As a result, it collapses and causes a supernova explosion. Regardless of the specific cause of a supernova, the effects can be massive.
Types of Supernovae
Graph of light curves for different types of supernovae[16]
There are two primary categories of supernovae, type I and type II. Type I supernovae are characterised by the absence of hydrogen lines in their spectra, whereas type II supernovae are characterised by the presence of hydrogen lines. Both types are further categorised into more types: type I supernovae are subdivided based on the presence of silicon and helium lines in their spectra, whereas type II supernovae are subdivided based on whether their light curves, which depict brightness as a function of time, demonstrate a linear decline after peak brightness or remain relatively constant for a longer duration.[3][4] Most of these supernovae types occur only in massive stars near the end of their lifespans when they go through a core-collapse, as outlined above.[3] However, type Ia supernovae occur in smaller stars known as white dwarfs and are triggered by runaway thermonuclear reactions.[5] They occur in binary star systems when a white dwarf \’steals\’ matter from its companion star and eventually consumes so much matter that it exceeds the Chandrasekhar limit, the greatest mass a stable white dwarf can theoretically reach, which is 1.4 times the mass of the Sun.[5] At this mass, a white dwarf becomes unstable, and its radius decreases, while its density and temperature increase. This process triggers the uncontrolled fusion of oxygen and carbon, turning the star into a fusion bomb that explodes into a supernova.[5]
The Role of Supernovae in our Universe
One reason why supernovae are essential to our universe is their role in creating materials and scattering them around the universe. While it seems at first that the space between solar systems is a massive vacuum, this area is filled with matter known as the interstellar medium. The interstellar medium has a low density and primarily consists of dust and gas but comprises 15% of the visible matter in our galaxy.[6] Supernovae are crucial to the composition of the universe because they supply the interstellar medium with much of this material. Indeed, supernovae are the primary forces responsible for spreading heavier elements such as copper, mercury, and gold into this medium.[7]
Throughout millions of years, the interstellar medium would receive more elements from the supernovae; thus, the medium’s chemical composition becomes richer. At some point in time, when the interstellar medium becomes sufficiently dense, these elements would clump together and create new solar systems. Materials from supernovae have also had a profound influence on Earth. In fact, without supernovae, life would be impossible as humans would lack the key elements necessary for their survival. For instance, the iron in our haemoglobin in our bodies that is crucial for breathing came from supernovae. The nitrogen in the soil of our planet and the oxygen in our atmosphere also came from supernovae [7]
The Importance of Supernovae to Science
In addition to providing much of the matter in our world and the universe, supernovae are of special interest to scientists because of the clues they offer about the properties of the universe. Type Ia supernovae are particularly important because astronomers use them frequently to measure distances in the universe owing to the fact that they are formed by exploding white dwarfs.[8] It is assumed that these white dwarfs explode at a uniform mass, which means that their peak luminosity would be consistent.[8] This consistency has led scientists to dub these supernovae \’standard candles\’ or \’cosmic mile markers.\’[9] In short, because luminosity decreases as distance increases, scientists can determine the distance to a supernova by calculating the difference between the observed and predicted brightness.[9] This unique property of Type Ia supernovae has led to important scientific breakthroughs. Scientists have discovered that these distant supernovae seem dimmer than expected, which would not be possible if the universe had been expanding at a constant velocity for the past several billion years. This observation indicates that distances to supernovae have increased, suggesting that the expansion of the universe is accelerating. Though the cause of this acceleration is still unclear, scientists theorise that this is driven by a mysterious repulsive force, which we now call dark energy.[10]
Possible Effects of Supernovae on Earth
Given that supernovae are the largest explosions in space, many have speculated how one can affect life on Earth. Fortunately for our planet, the Sun would not explode in a supernova. It would need to be at least 20 times more massive to become the kind of supernova that results in a black hole. While smaller stars can explode in supernovae resulting in neutron stars, for this explosion to happen, the Sun would still need to be roughly ten times its current mass. Thus, the Sun would eventually turn into a white dwarf and slowly lose its fuel until it becomes dark.[11] Even though the Sun would not explode into a supernova, there are nearby stars that can explode into supernovae and potentially affect Earth.
In fact, a recent study speculates that the Devonian mass extinction, which occurred around 359 million years ago and resulted in the extinction of roughly 80% of species on Earth, may have been because of a nearby supernova.[12] It was observed that between the Devonian period and the Carboniferous period, plant spores were burnt by ultraviolet rays, which suggests that the ozone layer of Earth at that time got depleted. Researchers speculate that a nearby supernova was likely responsible for this damage.[13]
Could a supernova-induced mass extinction happen again? It is possible, but Earth has little to worry about because the Milky Way is a massive galaxy, so a nearby supernova is unlikely. Betelgeuse is a possibility, but it is said that it would not explode until 100,000 years later, and even if it does explode sooner, it is far enough from Earth that it will not pose much of a risk.[13] Supernovae must be around 25 light-years or closer to pose a significant threat to human life on Earth.[13] However, if Betelgeuse were to explode into a supernova, there would be several implications, such as the fact that the explosion would be as bright as the half-moon for several months. It can impact life on Earth in other significant ways. Animals, from seals to dung beetles, that use the moon for navigation would be confused since there would be another object as bright as the moon.[14] Additionally, astronomers would have a difficult time observing the sky because there would be no clear dark sky for them to observe. Studying Betelgeuse would be an issue as well because its brightness would overwhelm astronomers’ instruments.[14]
Conclusion
Even if we have a lot of information about supernovae, several questions remain unanswered. In particular, there is a controversy about the cause of Type 1a supernovae. Although most white dwarfs explode when they reach the Chandrasekhar limit, this is not always the case.[9] There is a possibility that a white dwarf will remain in a dense ball state at the Chandrasekhar limit for billions of years.[9] This can be due to electron degeneracy pressure, which is essentially the pressure that prevents the electrons from being pushed further into each other and counteracts the gravity that would cause the white dwarf to collapse. It is unclear what causes this balance to be disrupted and triggers a supernova.[9] Moreover, astronomers have long thought that for a Type Ia supernova to be triggered, the white dwarf in the binary star system would have to have a companion star similar to a red giant, something large with an ample supply of gas that can be taken. While evidence suggests that this particular situation can indeed result in a supernova, recent research provides evidence that these supernovae can also be the result of a pair of white dwarfs, also known as a double-degenerate system.[9] This suggests that Type Ia supernovae are more diverse than initially believed, which can present limitations to their usefulness as standard candles. At any rate, it seems that more research would be needed to understand the inner workings of supernovae and their causes better.
References
[1] \”Supernovae Information and Facts.\” Science. National Geographic, May 04, 2021. Accessed July 29, 2021. https://www.nationalgeographic.com/science/article/supernovae.
[2] May, Sandra. “What Is a Supernova?” NASA. NASA, June 01, 2015. Accessed July 31, 2021. https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-supernova.html.
[3] Swinburne University of Technology. “Supernova Classification.” Cosmos. Accessed July 30, 2021. https://astronomy.swin.edu.au/cosmos/s/supernova+classification.
[4] Swinburne University of Technology. “Type II Supernova.” Cosmos. Accessed September 26, 2021. https://astronomy.swin.edu.au/cosmos/t/Type+II+Supernova.
[5] Ryden, Barbara. “Supernovae.” Lecture 20: Supernovae, Ohio State University, February 04, 2003. Accessed July 30, 2021. http://www.astronomy.ohio-state.edu/~ryden/ast162_5/notes20.html.
[6] University of New Hampshire. \”What Is the Interstellar Medium?\” The Interstellar Medium: An Online Tutorial. Accessed July 28, 2021. http://www-ssg.sr.unh.edu/ism/what1.html.
[7] “Background: Dispersion of Elements.” NASA. NASA, March 08, 2017. Accessed July 31, 2021. https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-elements.html.
[8] Kohler, Susanna. “Finding Distances to Type Ia Supernovae.” AAS Nova, February 19, 2016. Accessed July 30, 2021. https://aasnova.org/2016/02/19/finding-distances-to-type-1a-supernovae/.
[9] Drake, Nadia. “Type 1a Supernovae: Why Our Standard Candle Isn\’t Really Standard.” Science. National Geographic, May 03, 2021. Accessed July 30, 2021. https://www.nationalgeographic.com/science/article/type-1a-supernovas-cosmic-candle-mystery.
[10] “Distant Supernovae Indicate Ever-Expanding Universe – Eso Astronomers Contribute towards Resolution of Cosmic Puzzle.” www.eso.org, December 15, 1998. Accessed July 29, 2021. https://www.eso.org/public/news/eso9861/.
[11] Hatfield, Miles. “Why the Sun Won\’t Become a Black Hole.” NASA. NASA, September 25, 2019. Accessed July 28, 2021. https://www.nasa.gov/image-feature/goddard/2019/why-the-sun-wont-become-a-black-hole.
[12] Baraniuk, Chris. “Earth – The Devonian EXTINCTION Saw the Oceans Choke to Death.” BBC. BBC, June 23, 2015. Accessed July 31, 2021. http://www.bbc.com/earth/story/20150624-the-day-the-oceans-died.
[13] Buongiorno, Caitlyn. “A Nearby Supernova Could Have Caused the Devonian Mass Extinction.” Astronomy.com, September 01, 2020. Accessed July 29, 2021. https://astronomy.com/news/2020/09/a-nearby-supernova-could-have-caused-the-devonian-mass-extinction.
[14] Betz, Eric. “What would a Betelgeuse Supernova Look like from Earth?” Discover Magazine. Discover Magazine, April 17, 2020. Accessed July 30, 2021. https://www.discovermagazine.com/the-sciences/what-would-a-betelgeuse-supernova-look-like-from-earth.
[15] “Photo Album :: Tycho\’s Supernova Remnant :: 22 Sep 05.” Chandra. Accessed October 2, 2021. https://chandra.harvard.edu/photo/2005/tycho/.
[16] “Category:Supernovae.” Wikimedia Commons, June 4, 2012. Accessed October 2, 2021. https://commons.wikimedia.org/wiki/Category:Supernovae.