What Is A Hypernova?

Have you ever felt the urge to escape the bustle of city life, away from all the hubbub and flashing neon billboards? Ever felt the desire to dim the lights and stare up into the night sky for hours? Well, if you were lucky enough to get rid of the lights and got the chance to lounge luxuriously with only the sky above you, you would be mesmerized by the enormous number of stars sparkling in our cosmos. What’s even more fascinating is the fact that half of them might be dead by now!

Yes, you heard that right. What you see in the night sky is just the light the stars radiated out, from millions of light-years away, before dying. However, not all the stars are dead.

cartoon looking stars

How do stars die anyway?

Just like living beings, stars have various ways to pass away, but they only do so after several billion years of gleaming glory.

How do stars die?

Stars die when they’ve depleted their nuclear fuel. The events that follow, the fate of a dying star, depend solely on the star’s massiveness. The tiniest stars, known as ‘red dwarfs’, burn their nuclear fuel so slowly that they might live to be 100 billion years old, which is much older than the current age of our Universe. The closest star to our sun, Proxima Centauri, is a red dwarf that isn’t visible to the naked eye.

RedDwarfNASA

Red dwarf (Photo Credit: NASA/Walt Feimer/Wikimedia Commons)

Average-sized stars (up to about 1.4 times the mass of the Sun) die less dramatically than others. When the core runs out of hydrogen fuel, it contracts under the weight of gravity. However, some hydrogen fusion (forming helium) will occur in the upper layers, where the effect of gravity is less As the core contracts, it will start heating up under increased pressure. This causes the upper layers to expand, thus forming a red giant.

Eventually, the core heats back up, causing the helium to fuse into carbon. When the helium fuel runs out, the core expands and cools down. Finally, the core will cool into a white dwarf, and eventually into a black dwarf. The whole process takes a few billion years. If you’ve been wondering, yes, this is also the fate of our sun! Luckily, we won’t live long enough to see it take its final breaths.

red dwarf white dwarf and black dwarf

Red giant, white dwarf and black dwarf

Now comes the exciting part, which is why you were reading this article in the first place. But first, do you understand what a Supernova is?

Supernova

Supernovae come into existence when truly massive stars (at least 5 times as large as our sun) quickly use up their hydrogen fuel. This produces tons of energy, thus heating up the core. Heat generates pressure, and the pressure created by a star’s nuclear processes also keeps that star from collapsing.

crab-nebula

The Crab Nebula, a pulsar wind nebula associated with the 1054 supernova (Image Source: Wikimedia Commons)

A star is in perpetual balance between two opposite forces. The star’s gravity tries to squeeze the star into the smallest and tightest possible space, while the nuclear fuel burning in the star’s core creates a strong outward pressure.  For massive stars like these, when they run out of fuel, they begin cooling off. This reduces the outward pressure from the burning of nuclear fuel and gravity wins out. There is a sudden collapse of all the mass of the star towards the center, creating enormous shock waves, thus causing the outer layers of the star to explode. This dramatic event is a supernova.

Hypernova

A superluminous supernova, better known as a hypernova, is an extremely energetic supernova thought to result from a severe core collapse. This happens so quickly that the outer layers of the star are unaware of what has taken place, so the star subsequently explodes with vigorous winds of strong shockwaves, quite similar to a supernova.

-HyperNova1_LG

Hypernova (Photo Credit: NASA/GSFC/Dana Berry/Wikimedia Commons)

The difference comes when the star is more than 30 times as massive as our sun. This humongous size leads to exaggerated conditions for a supernova. Hypernovae are generally mistaken for supernovae, but in reality, these two are completely different entities.  A hypernova is a process that starts after the occurrence of a supernova. As soon as the limit of a supernova is exceeded, it begins releasing high-energy electromagnetic radiations known as a gamma-ray bursts. This marks the onset of the hypernova. Hypernovae can be regarded as the second stage of a massive supernova.

What is a gamma-ray burst?

Gamma-Ray Bursts are the brightest and most violent explosions in our universe, with some releasing more energy in 10 seconds than what our Sun will emit in its entire 10 billion-year lifetime.  

Most distant Gamma ray burst

Gamma-ray Burst (2 beams can be seen radiating outwards, along the axis) (Photo Credit: ESO/A. Roquette / Wikipedia Commons)

The light from these gamma-ray bursts have been traveling for over half the age of the universe, and they are among the most distant objects ever observed. However, to be so far away and still be the brightest things in the sky means that an incredible amount of energy must be producing these flashes. Due to the sudden collapse of a dying star’s core, the intense energy being produced can no longer be contained in the star. This energy is instead released as high-frequency radiation in an initial bright flash of gamma rays. Soon after, a much longer-lived “afterglow” is emitted at lower frequencies (X-ray, ultraviolet, visible, infrared, microwave and radio).

Image result for if gamma rays were visible to naked eye

Stars tend to rotate on a particular axis, and when they die, this spinning grows more rapid, due to the concentration of all the matter in a smaller region. This in-falling material is whipped up into a swirling frenzy, forming a disk deep inside the star. In the ensuing vortex, superheated plasma is ensnared by highly twisted magnetic fields. Like an electromagnetic cannon, jets of hot plasma and gases blast through the poles of the star and eject into space. The tunnel through the star forces the plasma streams into narrow beams, tightly focusing the energy of the collapse. However, what if one of these beams is directed towards our planet?

How deadly is a gamma-ray burst?

Despite the obvious doom and gloom associated with mass extinctions, they still tend to ensnare our imagination. After all, the sudden demise of the dinosaurs, presumably due to an asteroid strike, is quite an enthralling story in itself. Moreover, researchers have no reason to shun the idea that a gamma-ray burst, 440 million years ago, might have contributed to the Ordovician mass extinction, which wiped out two-thirds of all species on the planet.

Diorama_of_a_Cincinnatian_seafloor_(Late_Ordovician)_-_nautiloid_eating_an_Isotelus_trilobite

Ordovician organisms (Photo Credit: James St. John/Wikimedia Commons)

The intense radiation of a gamma-ray burst might have depleted up to 40% of the ozone layer, which would take roughly ten years to recover from such a blast. The loss of such a large fraction of the protective ozone layer would have allowed harmful ultraviolet radiation to reach Earth for years. Marine organisms that dwelt close to the surface would have been exposed the most to the UV radiation, and thus would have been killed at higher rates than those organisms living deeper. Indeed, geological evidence confirms that species living near the water surface were hit hardest in the Ordovician extinction.

If your eyes could detect gamma rays, you would observe brilliant bursts of light in the sky about once every day. These flashes would be so dazzling that they would momentarily outshine everything else in your vision, including our sun. Fortunately, a nearby gamma-ray burst, beamed directly at Earth, is highly unlikely. However, in a hypothetical situation, if one did occur, the amount of damage would depend on where the burst began. Assuming that one occurs in our Milky Way galaxy, but very far away from our own solar system, things might not be too bad.

GRB_illustration_Earth,gamma ray burst affecting earth

(Photo Credit : NASA /Wikimedia Commons)

With gamma-rays beamed directly at Earth, the gamma radiation would destroy a significant portion of our atmosphere, specifically the ozone layer. The highly energetic photons streaming along with the burst would cause massive chemical reactions, leading to photochemical smog, which would further deplete our protection from cosmic rays. The lethal doses of radiation that surface life would experience would result in a mass extinctions of most species of life on our planet. Basically, while gamma-ray bursts and hypernovae are fascinating to study, it’s best to keep them at a very healthy distance!

References:

  1. Swinburne University (Link 1)
  2. Environmental Defense Fund
  3. Swinburne University (Link 2)
  4. NASA.Gov
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About the Author:

Dev is an undergraduate (Bachelor of Science) from St. Xavier’s College (India). He watches a lot of anime and documentaries on the universe and wildlife. He spends most of his time engaging in sports and exercise. He has represented his state at the National Volleyball tournament, in India. Along with that, he has a flair for sketching, photography and would be glad to take up gardening as a career.

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