From Stardust to Supernova
The Prelude of Stellar Radiance:
Imagine a billion-year cosmic dance in which stars are born out of the blackness of interstellar clouds, burn brightly for billions of years, and then meet their dramatic end. The story of stars' evolution is fascinating, revealing the mysteries of creation and destruction on a cosmic scale, from the birthplace of stellar nurseries to the blazing spectacle of supernovae.
Stars are the hot balls of gases that produce their energy. They are the reason behind the light our eyes perceive in the sky. Stars form when interstellar clouds of dust, also known as stellar nurseries, collapse under gravity, increasing the density and temperature of stellar clouds. After the collapse, a nuclear fusion reaction takes place inside the core, where existing hydrogen atoms fuse to form bigger atoms like helium. This process serves as the origin of many of the elements that fill out the periodic table, and an enormous amount of energy is released that lights up the star (Stellar Structure and Evolution | Center for Astrophysics | Harvard & Smithsonian, n.d.). This is the same reason why our sun radiates light and heat. It’s just nuclear energy reaching us from a safe distance. Now, the star is formed and it will not collapse anymore. Gravity exerts a force inwards, and an equal amount of force is exerted by the nuclear reactions that push the star outwards. A star achieves stability when hydrostatic equilibrium is established, balancing the inward force of gravity with the outward force of nuclear reactions." (Imagine the Universe!, n.d.).
Contemplating a clear night sky can raise questions such as: 'Why do these stars emit different lights compared to others?”. The answer to this might seem complex to some readers. Let us recall a scenario first; many have seen metal heating up inside a furnace somewhere. At first, it will heat up and turn red, but as we continue to heat it more and more, it will turn yellow and eventually white. Therefore, the whiter or bluer a star appears, the hotter it is relative to those with red or orange hues.
Taking the aforementioned observation into account, the astronomer Anne Jump Canon classified stars into seven groups called O, B, A, F, G, K, and M stars. In this classification, the most massive and hottest stars belong to Class O, while the coolest and lighter ones belong to Class M. The M-type stars are abundant, while the O-type stars are less common. Our sun is a yellow G-type star with a temperature ranging from 5,200 - 6,000 Kelvin. The Al-nitak in the Orion constellation is an O-type star and has a temperature of around 30,000 - 50,000 Kelvin. Similarly, The Betelgeuse in the same Orion constellation is a red M-type star with a temperature of less than 3,700 Kelvin (Stellar Classification | The Schools’ Observatory, n.d.)
Credits: physicsfeed.com https://physicsfeed.com/post/harvard-spectral-classification/
The Prelude to Supernova:
In the grand cosmic ballet of creation and destruction, stars are born from the whispers of interstellar dust, only to meet their fiery demise in a dazzling spectacle known as a supernova. All the stars live a thermonuclear life and die, leaving fossils of oxygen, carbon, iron, magnesium, and many other elements. A supernova is just an explosion of a star undergoing a collapse that throws out its outer layers and releases a fierce amount of energy. The Crab Nebula supernova of 1054 AD was the first observed from Earth, shining as brightly as Venus in the night sky (Burrows, 2000). Recorded in Chinese historical records, it appeared as a "guest star" in the constellation Taurus, visible even during daylight for weeks. Later, in 1930, this event was linked to a supernova (Gaud, n.d.).
The onset of a star’s death is marked when it begins to run out of its nuclear fuel and starts fusing the remaining helium to form heavier elements. This process leads to the dominance of gravity, causing the star to collapse—a phenomenon known as Core-Collapse Theory. The stars with masses of 0.5 ~ 1 Solar mass tend to expel their outer core and expand to form a red giant, which sends shock waves all across the star and makes planetary nebulae and ultimately results in a white dwarf star (How White Dwarfs Are Born, n.d.). The fate of our sun is the same, it is in a stable state as of now, but after some 5 billion years, it will run out of its fuel and will form a red giant star. It will expand and may engulf various planets of the solar system, including Earth (Will our sun ever burn out?, 2022).
Stars with masses greater than 1.5 solar masses undergo explosions, emitting excessive gamma rays that can illuminate the parent galaxy for months. Its temperature and mass determine the properties of a star. According to the Chandrasekhar limit [Threshold for collapse], stars with masses between 1.5~3 solar masses will become neutron stars after undergoing a supernova. Neutron stars are one of the most extreme objects in the whole universe, as they are incredibly dense and hot but approximately 10-15 km across. A single teaspoon of a neutron star would have a mass of about a trillion kilograms (ESA/Hubble, n.d.). They are called neutron stars because the core collapse leads to extreme pressure, causing protons and electrons to combine into neutrons, giving rise to a neutron-rich composition. Intense magnetic fields accompany them. They form pulsars [short for pulsating radio source] when there is a rapid increase in its rotation along with the release of radio waves and various other electromagnetic radiations. Astronomer Jocelyn Bell Brunel discovered them in 1967 when she received radio signals with the help of radio telescopes from a distant source in the sky. Initially, it puzzled the scientists and perceived it as if an extra-terrestrial civilization was contacting us through radio signals. Those signals, pulsing every 1.3 seconds, sparked consideration of extraterrestrial intelligence, nicknamed "LGM-1" (Little Green Men 1). Further analysis in 1968 revealed the signals were natural, not artificial, leading to the announcement of pulsars – rotating neutron stars emitting electromagnetic radiation (What Are Neutron Stars?| Astronomy.Com, n.d.).
Credit - An artist's impression of the 76s pulsar (in magenta) compared to other more rapidly spinning sources. (c) Danielle Futselaar (artsource.nl)
If the burning-out stars are more massive than three solar masses, they collapse and end their days; their gravity is so great that even light cannot escape, and a black hole results (Chandrasekhar, 2018). The idea of the black hole has been theorized through Einstein’s General Theory of Relativity. Inside the center of every galaxy, there is a supermassive black hole that came into being due to the merging of other intermediate-sized black holes. The black hole situated at the center of our galaxy, Milky Way, is known as Sagittarius A* black hole. It was first discovered in 1974, but the Event Horizon Telescope captured its image on May 12, 2022. This mysterious phenomenon is an important area of study for astronomers and astrophysicists, due to its enormous gravitational force and mind-boggling characteristics.
Sagittarius A* is a black hole located at the center of the Milky Way galaxy.
Credit: EHT Collaboration https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy
In the vast cosmos, the journey from stardust to supernova is a cosmic symphony of creation and destruction. Stars, born from interstellar whispers, illuminate the mysteries of existence. From their serene birth to a fiery end, they sculpt the universe anew. Each chapter, from neutron stars to black holes, reveals the universe's boundless complexity. Ultimately, we're reminded of our fleeting place in the cosmic destiny—a moment in the grandeur of eternity.
Works Cited
Burrows, A. (2000). Supernova explosions in the universe. Nature, 403(6771), 727-733.
Chandrasekhar: Black holes from absurd to fact. (2018, November 21). https://cosmosmagazine.com/space/chandrasekhar-black-holes-from-absurd-idea-to-fact-of-nature/
How white dwarfs are born. (n.d.). Retrieved March 24, 2024, from https://www.skyatnightmagazine.com/space-science/white-dwarf
Imagine the Universe! (n.d.). Retrieved March 23, 2024, from https://imagine.gsfc.nasa.gov/science/objects/stars1.html
Stellar Classification | The Schools’ Observatory. (n.d.). Retrieved March 23, 2024, from https://www.schoolsobservatory.org/learn/astro/stars/class
Stellar Structure and Evolution | Center for Astrophysics | Harvard & Smithsonian. (n.d.). Retrieved March 23, 2024, from https://www.cfa.harvard.edu/research/topic/stellar-structure-and-evolution
J. W. last. (2022, January 7). Will the Sun Ever Burn Out? Space.Com. https://www.space.com/14732-sun-burns-star-death.html
What are neutron stars? The cosmic gold mines, explained | Astronomy.com. (n.d.). Retrieved March 24, 2024, from https://www.astronomy.com/science/what-are-neutron-stars-the-cosmic-gold-mines-explained/
Gaud, W. S. (n.d.). Crab Nebula. Northern Arizona University. Retrieved from https://www2.nau.edu/~gaud/bio301/content/crab.htm#:~:text=The%20supernova%20that%20created%20the,first%20supernova%20observation%20ever%20recorded.
