DEATH WITH A BANG - WHEN WILL WE WITNESS THE NEXT EXPLODING STAR?

Of all kinds of violent events that happen within the boundary of our observable universe, from colliding moons and planets to binary stars and black holes, energetic gamma-ray bursts and active galactic nuclei, supernovae are unique in their own respect. When a massive star, tens to hundreds of times the mass of our Sun finally reaches the end stage of its life, its dense stellar core collapses upon itself, wherefore it blows apart a significant fraction of the star's total mass due to the rebounding shockwaves arising from the gravitational collapse. This colossal explosion often means the formation of a neutron star or a black hole. In extreme scenarios, as in some colliding binaries, the progenitor star is itself completely destroyed.

Based on the spectral characteristics, a supernova can be classified into two distinct groups - Type I and Type II, followed by their respective sub-classes. However, without loss of generality, we can skip the scheme of supernova classifications and keep in mind for the time being that broadly speaking supernovae can be of two kinds. The first kind occurs in a binary star system comprising a white dwarf that draws in material from its binary companion, which can be a red giant. As the white dwarf gathers (accretes) additional material, its mass increases up to a limit known as the Chandrasekhar Limit, beyond which the white dwarf becomes unstable and explodes violently. This type of supernovae is classified to be Type Ia. The other kind, in this case, is a Type II-P, meaning a core-collapse supernova resulting from the gravitational collapse of a massive star. 

A schematic (not to scale) representation of the interior layers of a star before going supernova
Image Credits: Public Domain, via Wikimedia Commons

In this article, we are concerned only with the second kind, i.e., the core-collapse type, mainly because of all the stars we see in the night sky, two candidates await their turn to go off in a blaze of glory. 

Any star, big or small, in its adolescent and mature years, powers itself via the fusion of millions of tons of hydrogen into helium every second. This spontaneous fusion releases a copious amount of energy which sustains the star against its own weight. Hydrogen fusion is the most efficient mechanism of powering a star. It so happens that the energy and the pressure of radiation liberated as the by-product of the aforementioned fusion reaction tries to radiate out from the core of the star whereas, on the other hand, because of gravity, all of the total mass which fundamentally constitutes the star forces it to collapse towards the central core. A star is like an equilibrium state existing between the perpetual outward push of radiation and the inward pull of gravity. But as the star age, its hydrogen reserves are depleted, and helium builds up. If the star is massive enough, it will start fusing helium in its core. Massive stars (＞10 solar masses) keep on fusing heavier elements until it reaches the stage where the stellar core becomes nothing but a large piece of iron and some nickel. At this point, the star resembles a cut-out section of an onion with its central iron core surrounded by concentric shells of other elements (below iron in the periodic table). Since the fusion of iron consumes more energy than it liberates, it is impossible for a star to fuse iron into the higher candidates of the periodic table. When the iron core reaches the Chandrasekhar limit, i.e., a mass 1.4 times that our own Sun, it contracts and collapses upon itself while the rebounding shockwaves blows apart the concentric layers, atom by atom, leaving behind either a neutron star or a black hole surrounded by a luminous region of expanding gas clouds.  

A typical Type Ia supernova seen as the bright spot on the lower left. Its brightness is comparable to the galaxy
Image Credits: NASA/ESA

Our Sun is not massive enough to go supernova. As its hydrogen fuel depletes slowly, the Sun would expand in size, becoming a red giant. Within the span of  6-7 billion years from now, the red giant Sun will intermittently expel most of its material and leave behind its remnant core as a white-dwarf star surrounded by a luminous planetary nebula. For a supernova to occur, the progenitor star has to be born massive, so that in due course of its evolution, it would eventually expand into a red supergiant and finally explode. These luminous explosions can exceed the brightness of an entire galaxy, blazing bright for a brief moment before fading away forever. Whenever one occurs within the vicinity of Earth, in the sense that the event achieves naked eye visibility in the night sky, it would outshine the moon and to much awe, be visible also during the daytime as a very bright star. 

Since a significant section of our galaxy remains forever obscured behind the dark bands of interstellar clouds of gas and dust, we miss acquiring a first-hand account of the aplenty astrophysical phenomenon mentioned at the start of this article. Sure from our vantage point, we get to see some of the supernovae taking place in other galaxies, although, not without using some state-of-the-art telescopes. To witness a supernova with our mere mortal eyes, it has to happen somewhere within the [naked-eye] visible neighbourhood of our planet. Even then, we have a very dim chance of capturing one in our lifetimes. According to some estimates only one supernova occurs every 50 years in our Milky Way Galaxy or maybe one in a century. However, owing to the large number of stars in the whole universe, and if the estimates are correct then at least one star could be exploding every second somewhere in the observable universe which stretches for 93 billion light-years! In that case, supernovae might not seem to be rare at all rather, seeing one with the naked eye is just a matter of sheer luck. 

A composite image of a supernova in the constellation of Cassiopeia located in our galaxy at a distance of 11,000 light years from Earth.
Image Credits: Chandra: NASA/CXC/RIKEN/T. Sato et al.; NuSTAR: NASA/NuSTAR; Hubble: NASA/STScI

Although the term itself has been coined much later, in arcane times, the appearance of a previously unknown/unseen star was dubbed a nova - Latin for new. Ancient astronomers have left behind written records of so-called guest stars appearing out of nowhere in the night skies, shining brilliantly for a few weeks, maybe months before slowly fading away into the cosmological nothingness from whence it came. However, even though the word supernova readily appeals to us as something cataclysmic, from where we stand, they appear to be sudden bright stars in the night sky. Its total manifestation (like the Crab Nebula shown above) is revealed only through powerful telescopes and complex imaging techniques.  

Throughout the course of written human history, very few supernovae have occurred in our galaxy, and quite unfortunately, some of the records failed to survive through the ages while the rest are debatable. All in all, the list of the best-observed supernovae include that of the SN 1006 in the constellation of Lupus, the SN 1054 in the constellation of Taurus, the SN 1572 in the constellation of Cassiopeia and finally, the SN 1604 in the constellation of Ophiuchus. SN is an abbreviation for SuperNova, whereas the four-digit number indicates the year of its occurrence (in AD). The SN 1572 was previously known as Tycho's supernova, named in honour of Tycho Brahe for publishing an explicit account of his observations in his book De Nova Stella (Concerning The New Star). Similarly, the SN 1604 was named after Johannes Kepler who provided a similarly detailed note of his observations in a reprint of De Stella Nova. However, 400 years have gone by since the invention of the telescope in 1608 and not a single supernova has occurred in our galaxy since then. 

Of the four instances of supernovae mentioned here, only the SN 1054, i.e., the one behind the creation of the famous Crab Nebula has been the only confirmed core-collapse naked-eye supernova observed in all of human history. A pulsar, i.e., a rapidly spinning neutron star left behind at its centre from the progenitor super-giant star, confirms that it was a Type II explosion. 

The Crab Nebula has been so named because for when the English astronomer William Parsons, 3rd Earl of Rosse observed the nebulosity in 1844 and thought that it looked like a Crab. 
Image Credits: NASA, ESA, J. Hester, A. Loll (ASU)

Modern astronomy has identified some of the upcoming core-collapse supernovae expected to go off in a blaze of glory somewhere between 10,000 years to 100,000 years. Even if it seems to be a long wait based on our human timescales but as per the universe is concerned that is just a blip of a moment away. However, if we take into account those that occur in binary systems, then there is always a possibility of the sudden appearance of a bright star in our night sky. In that case, we should keep looking up.  

If humanity survives the next 100,000 years this is what awaits us; 

• Antares: Antares is the brightest star in the constellation of Scorpius and also one of the brightest and the largest star visible to the unaided eye. This red supergiant star with a mass comparable to 14 times that of our Sun and situated at a distance of 550 light-years is pacing gradually towards its ultimate explosive demise. If this star were to replace our Sun, then its photosphere (the visible surface of a typical star) would extend beyond the orbit of Mars. So when it finally explodes within the next 10,000 years, it is expected to put on quite a show. 

Antares is probably going to be our next core-collapse supernova. Even for that humanity has to survive the next 10,000 years. This is a reconstructed image of the surface of Antares by using the data gathered from ESO's VLTI
 Image Credits: ESO/K. Ohnaka, CC BY 4.0, via Wikimedia Commons

• Betelgeuse: The red star comprising Orion's shoulder, i.e., Betelgeuse, is one of the most easily recognisable star in our night skies. Like Antares, Betelgeuse is a massive star which amounts to nearly 20 solar masses ( i.e., twenty times the mass of our own Sun) and if it were to replace our Sun then its photosphere would extend up to the orbit of Jupiter and beyond. Situated at a distance of 640 light years away from Earth, it is estimated that when this red-supergiant star finally erupts in a supernova at the end of its life sometime within the next 100,000 years it would be brighter than an ordinary full moon. Since both Antares and Betelgeuse are not that far away (in terms of astronomical scales) when they finally explodes, their brightness would be comparable to that of the entire galaxy (of course for some alien civilisation looking from a vantage point outside our galaxy) and will even be visible even during the day time as a brilliantly shining star. 

Betelgeuse is nearing the point of its final demise. It could happen any moment from now within the next 100,000 years.
Image Credits: ALMA(ESO/NAOJ/NRAO)/E. O'Gorman/P. Kervella

• Eta Carinae: The last best candidate for our future naked-eye supernovae will be Eta Carinae which is in fact, a binary star system located at a distance of 7,500 light years away from the Sun in the constellation of Carina. The primary is an extremely massive, 100 solar mass star and is expected to erupt in a cataclysmic core-collapse supernova in the astronomically near future. This stellar-system is quite unique in the sense that the binary pair is surrounded by nebulous matter known as the Homunculus Nebula which is nothing but matter ejected intermittently from the primary. Eta Carinae is sometimes referred to as a supernova impostor due to the fact that it appeared to nearly explode as a supernova during the 19th century which has been dubbed as the Great Eruption of Eta Carinae. 

The Homunculus Nebula surrounding the Eta Carinae star system. The primary of the two is expected to go supernova in the astronomically near future.
Image Credits: Public Domain, via Wikimedia Commons

A good concluding remark would be to mention that nearly thirty-five years ago a star exploded in the Large Magellanic Cloud which is Milky Way's satellite galaxy. The 1987 supernova, designated as SN 1987 A was visible in regions across the Southern hemisphere as a very bright red star according to the New York Times article A Star Went Supernova In 1987. Where Is It Now?. This supernova sparked much sensation among the astronomical community, for the astronomers finally have had a chance to observe in detail the final moments of a star's death in 400 years since Kepler's nova of 1604 and also for the first time since the invention of the telescope. 

We are long due witnessing something as spectacular and as violent and as beautiful as a supernova, and in our dark times, all we wish is a little light. 

Sources: 

1. https://www.nytimes.com/2020/01/09/science/astronomy-supernova-betelgeuse.html
2. https://www.nasa.gov/mission_pages/chandra/supernova-ejected-from-the-pages-of-history.html
3. https://www.forbes.com/sites/startswithabang/2020/01/23/this-is-what-well-see-when-betelgeuse-really-does-go-supernova/?sh=7ca1bf4643a2
4. https://www.theatlantic.com/science/archive/2016/01/rare-supernova-crab-nebula/424125/
5. https://nightsky.jpl.nasa.gov/docs/SNStarMaps.pdf
6. https://www.e-education.psu.edu/astro801/content/l6_p5.html
7. https://astronomy.stackexchange.com/questions/46737/how-can-we-know-if-a-star-which-is-visible-in-our-night-sky-goes-supernova
8. https://www.wtamu.edu/~cbaird/sq/2012/12/11/how-does-a-supernova-completely-destroy-a-star/
9. Carroll, Bradley W. and Dale A. Ostlie. An Introduction To Modern Astrophysics. Pearson Education Limited, 2014.