An explosive beauty of a cosmic catastrophe - Supernova

What is a nova?

Supernova is an explosive death of any star. The event releases an immense amount of energy and the scattering of it’s entire stellar matter into space. Supernovae are among the brightest and most luminous events in the universe. It marks the end of a star's life cycle with an extraordinary explosion. When a star exhausts its nuclear fuel, it undergoes dramatic changes that ultimately drive it to a colossal explosion. These ‘cosmic fireworks display’ releases an immense amount of energy! The luminosity of a supernova explosion is extreme enough to briefly outshine an entire galaxy. Such unbelievable luminosity makes them visible across vast cosmic distances of light years.

These cataclysmic events not only captivate astronomers but also people who able to witness it since they are extremely bright and most times, visible during daytime. They play a very pivotal role in the cosmos. Supernovae enrich the interstellar space with heavy elements on the periodic table and greatly trigger the formation of new stars. Supernovae are however rare events. In a galaxy like the Milky Way, they happen roughly once every 50 years.

A supernova captured recently by the James Webb Space Telescope

Please remember, the term supernova is a singular word meaning 1 event. It’s plural term is supernovae meaning all supernova events collectively. In this blog post, we will delve into the few details of supernovae, explore their types, causes, effects and the profound impact they have on the universe.

 

The life-cycle of a star

To understand supernovae, its vital to understand the life cycle of a star. Stars are born from vast clouds of gas and dust called nebulae. In these gigantic nebulae, somewhere where there is a lot of material or density, gravity begins to coalesce the dust and gas together. Gravity then begins pulling nebulae matter together and the core gets denser as more matter fall in. Under the influence of gravity, these clouds collapse and become protostars. With the core becoming very dense beyond a point, it triggers the ignition of nuclear fusion. A new star is born. From here on, it begins to release energy and producing light throughout it’s existence.

A star maintains a delicate balance between the outward pressure of nuclear fusion and the inward pull of gravity. This fusion of hydrogen into helium releases energy, counteracting gravitational collapse and stabilizing the star. This activity will go on for billions of years. But as a star ages, its core eventually runs out of it’s main fuel, which happens to be hydrogen. The end of hydrogen inside a star triggers a chain reaction that causes the star to expand and become a red giant. This is the life of a greatest number of stars in the universe.

However, some stars are not your average size in mass and volume. They are several times larger or massive than our sun. Our is a medium-sized star for reference. These colossal stars die out in fashion. The extinction of hydrogen in them triggers a gravitational collapse that cannot be stopped by any process. The implosion sends out explosive energy outwards in all direction destroying the star entirely. The energy is so great that unbelievable number of photons are released making it luminous than an entire galaxy. This explosive event is the supernova. After the supernova, the star is no more. It either becomes a variety of other cosmological bodies that we’ll read about later on.

 

Generic types of Stars

1) Low-to-intermediate massive stars - These stars have up to nearly 8 times masses to that of our Sun. While dying, they undergo helium fusion inside them. Fusing of helium atoms create heavier elements like carbon and oxygen. Eventually, they shed out their outer layers which continues forming into planetary nebulae. All they leave behind are their cores which become white dwarves.

2) Massive stars - Stars with masses greater than 8 times that of our Sun fall into this category of stars. They undergo successive stages of fusion unlike their counterparts. Such kind of fusion ends up creating elements up to iron in their cores. Iron fusion is endothermic which means it absorbs energy rather than releasing it. Iron cannot be fused further in these cosmic cauldrons. This leads to the core's collapse under its own gravity which ends up triggering one of the many types of supernovae.

 

Types of supernovae

Supernovae are broadly classified into 2 main types namely Type 1 supernovae and Type 2 supernovae. They are based on their spectral lines and the presence or absence of hydrogen in the explosions. Let's see how they are and how they differ from each other.

 

Type I Supernovae

The Type 1 supernovae are characterized by the absence of hydrogen lines in their spectra. They are further subdivided into three further categories. 

a) Type 1A - These supernovae happen as a result of thermonuclear explosion by a white dwarf in a binary star system. When the white dwarf accretes enough stellar matter from its companion star, it crosses a critical mass called ‘The Chandrasekhar Limit’. At this point, carbon fusion ignites explosively throughout the star which leads to a thermonuclear runaway reaction. This causes it to suffer a powerful supernova explosion.

The fusion front propagates as a deflagration (subsonic burning) or a detonation (supersonic burning). Either of these is going to consume the white dwarf within a matter of seconds. The white dwarf is then completely disrupted and the energy release ejects the star's material into space. Such supernovae from a white dwarf in a binary star system is Type 1A Supernova.

b) Type 1B - These supernovae originate from massive stars that have lost their outer hydrogen layers. They may have lost it due to strong stellar winds or from the interactions with a companion star. The collapsing cores of these “stripped” stars result in a powerful supernova. They are always characterized by the presence of helium lines in their spectra. So, Type 1B supernova happen with very massive stars which have lost their hydrogen envelopes because of various factors.

c) Type 1C - These are similar to Type 1B supernovae. But, Type 1C supernovae results from the collapsing cores of massive stars that have lost both their hydrogen and helium layers. Their spectra lack both hydrogen and helium lines. Thus, these supernovae become distinct from other types of supernovae.

 

Type 2 Supernovae 

Type 2 supernovae are characterized by the presence of hydrogen lines in their spectra. They occur when massive stars (often more than 8 times the mass of our Sun) exhaust their nuclear fuel (which is hydrogen) and undergo uncontrolled gravitational collapse. They are unable to lose enough stellar matter to become white dwarves. Type 2 supernovae are again divided based on their light curves. 

a) Type 2P - These supernovae exhibit a plateau in their light curve. This means that their luminosity keeps rising up before suddenly remaining almost constant for a long period of time. Then, their luminosity begins to dip gradually before completely fading away. The plateau phase is caused by a recombination of hydrogen in the expanding supernova shell.

b) Type 2L - These supernovae show a linear decline in their light curve without a plateau phase. Meaning, they rise up in luminosity and then fade away without keeping a tendency of constant shine for some time. They are less common than Type 2P supernovae and are theorized to happen from stars with different pre-supernova structures.

 

Mechanics of a supernova

The core collapse of a massive star (for Type 2, 1B and 1Cc supernovae) or the thermonuclear runaway in a white dwarf (for Type 1A supernovae) are the primary mechanisms behind supernova explosions. In core-collapse supernovae, the iron core of a massive star collapses within seconds. The core suddenly reaches extremely high densities and temperatures. This process leads to several key events and some of them are mentioned below.

1) Neutron star or a black hole formation - The core's collapse halts when it reaches nuclear densities which ends up forming a neutron star. In some cases, the resulting supernova might form a black hole if the core mass is sufficiently large.

2) Shockwave generation - As the infalling stellar matter rebounds off the dense core, there is an equally powerful shockwave that propagates outward. This energy or shockwave will ultimately lead to the outer layers of the star to be carried away or expelled at unbelievably high velocities in seconds.

Actual image of a supernova shockwave of Supernova 1987A | Source : Hubble Telescope, NASA (4954621859)

3) Nucleosynthesis - The supernova produces a wide range of heavy elements through nuclear reactions and pressure. By overcoming the pull of gravity because of the supernova explosion what happens is, the enriching of surrounding interstellar medium with the scattering of these new elements.

 

The benefits of a supernova

Supernovae play a very important role in the universe. They are responsible for creating many of the elements heavier than iron such as gold, silver, platinum, uranium etc. The more massive the stars responsible for supernovae are, the heavier-than-iron come into existence due to nuclear fusion process. These elements are scattered throughout space by the supernova explosion. Only powerful explosions like supernovae can distribute and scatter various elements into light years of space. These heavier elements go onto eventually become part of new stars, star systems, planets and even life itself. All elements on Earth were once part of some stars. In the words of Prof. Lawrence Krauss, the atoms of both our hands could’ve possibly come from 2 different stars.

Star's shells showing what elements form when nuclear fusion intensifies upto the core

Supernovae also have a profound impact on their surrounding environment. The shockwaves from the explosion can trigger the formation of new stars. The intense searing radiation from this cosmic destruction can ionize gas clouds and dust leading to the creation of new molecules. Supernovae are the sources of elemental enrichment in our universe.

The remnants of a supernova explosion are complex structures and evolving that continue to impact their immediate surroundings. Supernovae play a crucial role in the cosmic cycle of matter by synthesizing and dispersing heavy elements. This enrichment process is essential for the formation of planets, stars and the development of life.

 

Aftermath of supernovae

All the expelled matter from a supernova forms an expanding shell known as a Supernova Remnant (SNR). These remnants provide detailed insights into understanding the explosion mechanics and the interaction of the ejected material with the interstellar medium. Famous examples of SNR that we can see are the Crab Nebula (SN 1054) and Cassiopeia A (SN 1680). When a star explodes as a supernova, it leaves behind a complex and fascinating remnant of stellar matter.

A supernova remnant | Source : NASA

The remnants in these expanding shells of gas and debris continue to evolve for thousands or millions of years are they travel. Thanks to modern telescope technology of our times, we can take images of these supernovae and give them colours based on accurate spectroscopy. Their vibrant pictures give us awe and detailed visuals of such mega cosmic events.

 

Types of Supernova Remnants (SNRs)

1) Pulsar Wind Nebulae - This kind of cosmic wind is formed by rapidly spinning neutron stars (pulsars) which begin emitting high-energy particles. These high-energy particles or pulsar winds are characterized by bright, non-thermal emissions which are often in X-ray and gamma-ray wavelengths. Examples include the Crab Nebula and Vela Pulsar.

The 1st Pulsar Wind Nebular captured by astronomers around a rare ultra-magnetic neutron star

2) Shell-Type Supernova Remnants - They appear as spherical or nearly-spherical structures. You can tell by looking at them that they are composed of ejected matter of a star during it’s supernova explosion. They often display complex structures due to interactions with their surrounding interstellar medium. Examples for these include Cassiopeia A and Tycho's Supernova Remnant.

Photo of a shell-type SNR

3) Supernova Remnant with Pulsar Wind Nebula - This would be a combination of the two types mentioned above. They are characterized by a central pulsar wind nebula surrounded by a larger shell-type remnant. They show different stages in the evolution of a supernova remnant.

Vela Pulsar Wind Nebula and a supernova remnant image sourced from NASA

4) Neutron Stars - Core-collapse supernovae often leave behind compact remnants like neutron stars. Neutron stars are incredibly dense entities with masses atleast 1.4 times that of our Sun. But all this mass would be packed into a sphere with a radius of about 10 to 25 kilometres only.

NASA's rendition of a neutron star

5) Black Holes - Black holes are formed when the star’s core mass overcomes the limit for neutron stars. They are one of the most mysterious and least known cosmic bodies in the universe because they have been theorized to have infinite density. Their gravitational fields are so strong that not even light can escape and are only detectable from the gravitational lensing they cause. They are the devourer of stars and eaters of worlds. Their power reaches out into multiple light years. They even devour each other and grow larger. Some of these blackholes are several times larger than our entire solar system.

Actual image of the black hole Messier 87 taken in the year 2018

Evolution of Supernova Remnants (SNRs)

Supernova remnants undergo several stages in their evolution.

1) Free Expansion Phase - This is an initial phase. During this, the expanding debris encounters little resistance from the surrounding medium and almost freely travel away into the void.

2) Adiabatic Phase - The supernova remnants slow down because it has to interact with the interstellar medium on the way, which eventually ends in heating them up.

3) Radiative Phase - The supernova remnants start cooling down and also emit radiation. They gradually begin to blend with their surrounding interstellar medium, whatever that happens to be.

 

Spotting these supernovae

While supernovae are incredibly bright, they are relatively rare events and very far away. However, with the aid of very powerful telescopes, astronomers can detect and study these cosmic explosions happening in distant galaxies. By analyzing the light coming from supernovae, scientists can learn about the properties of the exploding star, the conditions in the interstellar medium and their expansion rate of the universe. Supernovae continue to fascinate stargazers and inspire scientists alike. Their explosive power and all the various elements that they create are essential to our understanding of the universe and our place in the cosmos.

Supernovae can be observed across different wavelengths between radio waves and gamma rays. Modern telescopes and observatories, both ground-based and space-based, have significantly advanced our understanding of these events. Unless a supernova happens close by, we may need our machines to spot them. In the 10^th century, there was one such event and there was a bright light in the sky for many days. The Chinese have written records about it. No other close by supernovae has occurred since whereby we could see them directly in the sky without any aid. The tools for observing supernovae at our disposal are as below.

1) Optical Observations - Optical telescopes capture the visible light coming from supernovae. These instruments allow astronomers to study their light curves, spectra and evolution over time.

Picture of an optical observation telescope or an observatory

2) Radio Observations - Radio telescopes detect the synchrotron radiation produced by supernova remnants. Synchrotron data provide insights into the magnetic fields and particle acceleration within these supernova remnants.

The radio telescopes of CSIRO (Australia)

3) X-ray and Gamma-ray Observations - High-energy telescopes observe the X-rays and gamma rays emitted by the hot gas and energetic particles in supernova remnants. They unravel details about the explosion mechanics and the interaction with the particles or objects in their vicinity.

Chandra X-Ray Observatory which is now orbiting the Earth in space

HESS II Gamma Ray Telescopes

Supernovae in our own backyard

Historical records and modern observations have documented several supernovae within our Milky Way galaxy. Some notable examples of these are below.

1) SN 1006 - Was observed in the year 1006. This supernova was one of the brightest in recorded history. It was visible even during the brightness of daytime for many days.

Remnant of the SN 1006 supernova today

2) SN 1054 - This supernova explosion gave birth to the Crab Nebula. This nebula has become a well-studied supernova remnant that continues to provide valuable insights into supernova dynamics.

The Crab Nebula sourced from James Webb Space Telescope

3) SN 1604 (Kepler's Supernova) - This supernova was discovered by Johannes Kepler in the year 1604. This supernova is the last unaided-eye supernova observed in our Milky Way galaxy to date.

Conclusion

Supernovae are not just spectacular astronomical events. They are fundamental to the evolution of our universe. By forging and scattering heavy elements, triggering star formation and leaving behind intriguing remnants like neutron stars and black holes, supernovae shape our cosmos in many profound ways. As our observational techniques and theoretical models continue to improve and progress, we will undoubtedly uncover even more about these cosmic explosions and their role in the grand tapestry of our observable universe. Also, supernova remnants play a crucial role in shaping the interstellar space. They inject energy and heavy elements into their galaxy and enrich the materials from which new stars and planets can form. The shockwaves from supernova remnants can also trigger the formation of new stars and celestial bodies.

Hope this post was able to shed brief light on supernova. For more technical details on a supernova, read scientific papers or talk with an astrophysicist. Hope that we get to see one supernova somewhere in our lifetime. Have a great day.

Disclaimer : All the pictures in this post is licensed under Creative Commons Attribution 2.0 Generic license.