February 15, 2024

Supernovae, the glorious deaths of stars

By Charles J.Aouad1

1 Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

1. UNVEILING THE CELESTIAL SHOW

Imagine capturing all the sunlight emitted by the sun over the course of 10 billion years, containing it within a box, and then releasing it all in one burst. This is not just imagination; such objects exist in the Universe, and they are known as supernovae. Supernovae represent some of the most cataclysmic events in the cosmos, marking the grand finale in a star’s life.

Essentially, they are the celestial fireworks celebrating a star’s death.

Stars, much like humans, not only vary in mass, color and size, but they also undergo a life cycle, a journey encompassing birth, life, and eventual death.

Throughout this cycle, they do more than emitting light; they also forge the elements that constitute the very essence of our world.

Figure 1: SN 1994D, a type Ia supernova in NGC 4526, seen through the lens of the Hubble Space Telescope, outshines its host galaxy, which houses billions of stars. Image credit: NASA, ESA, The Hubble Key Project Team, and The High Z Supernova Search Team.

2. A STAR IS BORN: THE SPECTACULAR GENESIS OF STELLAR LIFE

The story begins with the gravitational collapse of an immense cloud of gas, primarily composed of molecular hydrogen. Why hydrogen? Because hydrogen is the predominant element in the Universe. Gravity, the force that pulls objects together due to their mass, starts its relentless work on this cloud. As the cloud contains significant mass, gravity exerts its influence, causing the cloud to contract inward upon itself. This gradual contraction unfolds over millions of years, resulting in a notable rise in temperature—much like how pressure increases temperature in a pressure cooker—soaring to an astonishing 15 million degrees at its core, while pressure builds to 100 billion atmospheres.

Figure 2 : The genesis of a star and its planetary system begins with the condensation of molecular clouds under the gravitational force. As the central stars form, residual gas and dust coalesce into disks, orbiting the newborn stars for millions of years in nearly circular paths. Within these disks, planets gradually take shape, composed of primordial matter from the gas cloud, which itself originated from material processed within older stars that exploded millions of years ago in Supernovae.

(Credit: NASA/FUSE/Lynette Cook)

This sets off a process known as nuclear fusion, where two hydrogen atoms meld together to form one helium atom. Thus begins the star’s main sequence, where the delicate equilibrium between gravity (trying to pull the star inwards) and nuclear fusion causing thermal pressure (trying to push the star’s matter outwards) upholds its stability. Our sun alone converts a staggering 600 million tons of hydrogen into helium every single second!

Nuclear fusion, difficult to achieve on Earth due to the strong repulsion between positively charged atomic nuclei—imagine the challenge akin to trying to force two magnets of the same polarity together— becomes achievable within the extreme conditions at the star’s core. Here, amidst temperatures skyrocketing to millions of degrees and densities reaching astounding levels, the electrostatic force (the force that makes two similar charges repel and two opposite charges attract) is overshadowed by a far stronger force of nature called the strong nuclear force. This force, operating at minuscule distances, is attractive and triumphs over the electrostatic repulsion force, enabling two protons of the same charge (positive) to glue together and form a rigidly bound nucleus. This results in the fusion of two hydrogen atoms (each containing one proton) into one helium atom (containing two protons). This fusion liberates vast amounts of energy, the very essence of a star’s brilliance, including our Sun, which has sustained this process for an astounding 5 billion years and is projected to continue for another 5 billion years.

 

3. OH STAR, ARE YOU REALLY GOING TO DIE?

However, this cannot last indefinitely. The hydrogen fueling the star’s core is finite and will eventually be depleted. With the gradual decline of hydrogen, the pace of nuclear fusion begins to decelerate, allowing gravity to dominate over thermal pressure.

Consequently, the star’s core undergoes contraction, resulting in a significant increase in temperature reaching up to 100 million degrees. This initiates a new fusion process wherein three helium atoms (each with two protons) fuse to form one carbon atom (composed of six protons), marking a new phase in the star’s life cycle. This cycle of helium fusion continues, once again releasing energy. However, much like hydrogen, helium will eventually be depleted, leading the star to exhaust its nuclear fuel once again.

Now what happens next, depends much on how massive the star is. For low mass stars, less than about 8 solar masses, the force of gravity is not strong enough to contract the core to conditions suitable for fusion beyond carbon, as carbon fusion requires temperatures in the order of 600 million degrees. In that case the nuclear reactions at the core stop, the outer layers of the star are slowly ejected into to the interstellar medium, forming what is called a planetary nebula. What remains is just a hot dead hot and dense ball of carbon and oxygen, the size of earth called white dwarf. This dead body is meant to radiate its heat slowly and silently until the end of the Universe.

Image 3: Composite X-ray and optical image revealing the outer layers of a low-mass star depleted of helium in its core, marking the conclusion of nuclear fusion. During its final stages, these layers are expelled into space, forming captivating nebulae. Each nebula shelters a white dwarf at its center—a fiercely hot, condensed sphere mainly composed of carbon and oxygen. This provides a glimpse into our sun’s anticipated appearance in 5 billion years, long after life on Earth, or even Earth itself has ceased to exist. Image credit: X-ray: NASA/CXC/RIT/J.Kastner et al.; Optical: NASA/STScI

For stars with greater mass, typically around 8 solar masses or more, the narrative diverges significantly. Due to their substantial mass, gravity exerts a stronger pull, causing the star to undergo further contraction. This intensified compression results in even higher temperatures and pressures, facilitating the fusion process of carbon—an occurrence not achievable in less massive stars. As a result, the same cycle of hydrogen and helium repeats, with increasingly heavier elements like magnesium, neon,  silicon, sulfur, and calcium undergoing fusion within the star throughout its lifespan. (Do you remember the periodic table of elements that you probably studied in school?) Think of it, the Universe started with hydrogen a little helium and traces of beryllium, every other element in this periodic table is forged in the stars. This continuous fusion process progresses at an accelerated pace, culminating in silicon burning, a process that takes less than a day. Silicon burning ultimately produces iron, the celestial stars’ poison, marking the end of a star’s journey.

4. EVOLUTION TO CATASTROPHE: THE DRAMATIC FINALE

Once the star starts accumulating iron in its core, its end becomes near. Unable to sustain fusion beyond iron, the core’s nuclear reactions decelerate, absorbing energy instead of producing. Iron, with its highest binding energy per nucleus among all the elements in nature, marks the end point of the fusion process. Gravity tightens its hold as the core contracts once more, with temperatures skyrocketing to a staggering 2 billion degrees. At this extreme heat, radiation primarily manifests as gamma rays, possessing such intense energy that they disassemble the nuclei crafted by the star over billions of years. In this pivotal moment, the catastrophe becomes inevitable. Within a fraction of a second, about 1.4 solar masses of iron collapse inwards. As densities reach staggering levels, electrons fuse with protons to form neutrons. Meanwhile, the star’s outer layers hurtle towards the dense neutron core at a fraction of the speed of light. This compression initiates a rebound, unleashing a shockwave of immense power that triggers the fusion of elements heavier than iron (This is how heavy elements such as gold and silver are made in the Universe). Consequently, the ejected material is propelled outward at velocities reaching up to 10,000 kilometers per second. Additionally, an astonishing number of neutrinos, approximately 1058 are generated from electron capture, swiftly escaping and carrying away 99% of the star’s total binding energy.

This fusion process also generates radioactive material, primarily nickel-56. Nickel undergoes decay to cobalt and then to iron, emitting gamma rays and positrons in the process. These particles become trapped within the dense expanding ejecta. Over time, they deposit energy and thermalize until the ejecta disperse and become diluted enabling their escape as optical photons. These optical photons travel through space for millions of years until they reach our eyes.

Known as a core collapse supernova, this event shines as brilliantly as a billion suns reaching its peak luminosity within just 20 days, before gradually dimming into obscurity. The ejected material expands over millions of years in space, forming what is commonly referred to as a nebula. Eventually, these nebulae recycle within the interstellar medium, awaiting the collapse of new gas clouds, thus giving birth to new stars of a new generation in a perpetual cycle of cosmic re-birth.

Figure 4: Diagram illustrating the lifecycle of stars. A star’s existence is a perpetual struggle against gravity. Heat in its core generates pressure, countering gravity’s pull, establishing hydrostatic equilibrium. This balance supports the star during its main sequence, where nuclear reactions power its core. The star’s mass dictates its lifecycle. Stars akin to the Sun burn fuel slowly, leading longer lives, while high-mass stars consume their fuel rapidly, culminating in a spectacular explosion known as a supernova. Image credit: NASA and Night Sky Network.

5. ECHOES OF STARDUST : REFLECTION ON LIFE’S COSMIC INGREDIENTS

Consider for a moment that our existence owes its presence to a star that, millions of years ago, underwent the intricate dance of life and death, ejecting its remains into the cosmos. From the remnants of this cosmic spectacle, our own sun emerged, accompanied by a myriad of planets, among them our beloved Earth. Within this cosmic theater, all the necessary ingredients coalesced, from the water we drink, to the oxygen we breathe, even our own bodies, and all what makes us alive, the calcium in our bones, the iron in our blood, the silicone in our microchips and even the apple pies that we cook. We are literally stardust, but what is even more amazing is that we are aware of it all, pondering about our existence and trying to understand the universe from which we emerged. This process of stars birth, life and death, has been at play since the universe formed, 13.7 billion years ago and will continue to happen until the end of the Universe.

What remains after this explosion, is a neutron star, a body so dense that a spoonful of its material weighs as the Himalaya Mountains, or for very massive stars a black hole, one of the most mysterious objects in the whole universe.

6. SUPERNOVAE TYPE Ia AND COSMIC MYSTERIES

Meanwhile, do you remember the white dwarf? Initially expected to quietly cool until the Universe’s end? Sometimes, this white dwarf has a companion in a two-star system. Matter from the companion feeds into the white dwarf, increasing its mass and core temperature. In some cases, two white dwarfs merge.

In either scenario, temperatures rise enough for carbon to burn explosively, leading to a supernova type Ia. These events, ten times brighter than core collapse supernovae, explode with more or less the same mass, thus emitting the same luminosity. 

This makes them useful as distance indicators or what the astronomers call “standard candles”. The use of Supernovae type Ia has led to the discovery of dark energy, a mysterious form of energy pushing on the Universe and causing its accelerating expansion.

What is dark energy? What secrets does it keep? How will it affect the fate of the Universe? These answers are hidden in the vast unknown, inviting curious minds to explore. But for now, their story is wrapped in mystery, waiting for future scientists or explorers to uncover it.

Figure 5: The Crab Nebula: A spectacular remnant of a type II supernova (core collapse) explosion observed by ancient astronomers in 1054 AD. This cosmic wonder is located about 6,500 light-years away. Different colors capture the diverse elements and energetic processes at play. Reds indicate hydrogen emissions, while blues signal oxygen presence. Image credit: NASA, ESA, J. Hester, and A. Loll (Arizona State University)

Charles Aouad, is a PhD candidate and astrophysics researcher at Liverpool John Moores University’s Astrophysics Research Institute (ARI). He holds a DES in architecture from ALBA-BALAMAND (1998), an MBA from ESA-ESCP/EAP (2003), and an MSc in astrophysics with distinction from LJMU (2017). Specializing in theoretical numerical simulations of type Ia supernovae spectra, his research aims at gaining a better understanding of the nature and the diversity of these stellar explosions.

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