Supernovae are not rare. On the basis of some reasonable assumptions, our Milky Way galaxy contains about 100 billion stars of all types, from tiny red dwarfs weighing a meager 0.01 solar masses to colossally massive ones weighing upwards of 250 solar masses. A typical Hubble Ultra Deep Field image taken in any part of the empty sky gives astronomers an estimated 100 - 200 billion galaxies in the known universe. Again, relying on some standard estimates (theoretical and backed by observational data), 2 - 3 supernovae are expected to occur in any galaxy in every century, one in every 50 years. Thus, putting all the numbers, at the least, with each passing second, 30+ stars explode somewhere in the visible galaxy. If that number looks unrealistically high, then at least one star explodes every second.
And just like that, nearly two months ago, on the night of 19th May, a star flared up in a blaze of glory, 21 million light years from Earth in the Pinwheel Galaxy (Messier Catalogue M101) in the direction of the constellation of Ursa Major, 21 million light-years away. 2023ixf could have passed undetected like a bazillion other events that occur sporadically across the humanely unfathomable extent of the visible universe. Except it turned out to be otherwise. Its discoverer Koichi Itagaki, an amateur astronomer, just happened to be at the right place and at the right time. By pure chance, he had pointed his telescope to M101.
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In this image, taken one day after its discovery, SN2023ixf (position marked) continues to get brighter.
Image Source: Wikimedia Commons.
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2006 Hubble image of the Pinwheel galaxy.
Image Credit: European Space Agency and NASA. |
A star may explode in one of two ways. What astronomers describe as Type Ia occurs in binary star systems, where a white dwarf suddenly begins to hijack stellar material from its ill-fated companion.
White dwarfs are the remnant cores left behind by low and intermediate-mass stars, including stars like our Sun. As they run low on hydrogen, they gradually inflate into a red giant, with hydrogen burning in a shell enveloping the helium/carbon-rich core, and shed their outer layers in a brilliant planetary nebula. White dwarfs come with a critical mass limit of 1.44 solar masses, also known as the Chandrasekhar limit, named after its discoverer Subramanyam Chandrasekhar, beyond which it (the white dwarf) can no longer support itself from collapsing gravitationally under its own weight by the counter opposing electron degeneracy pressure. Left behind as a remnant core, a white dwarf shines dimly by the residual heat of spent nuclear fusion reactions that once powered its former self. Ordinarily, a white dwarf sits idle, radiating its heat over trillions upon trillions of years before becoming a cold, dark, and dead chunk of matter.
In a binary star system of two main sequence stars, when the more massive of the two evolves into a white dwarf, the denser white dwarf may begin to attract stellar material from its companion. When it has gathered enough mass – overshooting the Chandrasekhar limit of 1.44 solar masses, nuclear fusion sets in once again. The white dwarf enters runaway, uncontrolled nuclear fusion; it becomes a "stellar" bomb. Following a cataclysmic explosion, the white dwarf obliterates itself out of existence in an expanding fireball of luminous plasma.
The other kind, described as a Type II supernova, is the fate common to massive stars – stars weighing from 8 solar masses to upwards of 250 times the mass of the Sun. While stars weighing less than 8 solar masses don't get to go beyond helium/carbon fusion in their cores, massive stars can climb the ladder across the periodic table all the way up to iron. For all stars out there, whether they be luminous giants or dim dwarfs, hydrogen fusion is the most efficient mode of energy production through which they get to sustain themselves against their own gravity – maintaining hydrostatic equilibrium between two forces - the inward pull of gravity and the outward push of radiation pressure. Hydrogen fusion returns significantly more energy compared to what it takes to bind four hydrogen nuclei (protons) into a helium nucleus. At the point of iron fusion, a star has to supply an exorbitant amount of energy but receives far less than what it needs to maintain hydrostatic equilibrium. Unable to meet this energy demand, the star gives up. It collapses. It implodes. As the entire mass of the star falls inward, the outer layers of the core reach speeds exceeding 70,000 km/s.
The collapse of an 8-plus solar mass star compresses its core to unimaginable densities (trillions of times denser than liquid water) – solidifying it into a ball of neutrons only. In other instances, the star may collapse directly into a black hole. The shockwaves rebounding from the initial collapse shatter the star atom by atom in a brilliant planetary nebula – much like the red-green-blue fireworks we enjoy when the clock hits 00 hours on New Year's Eve. The supernova remnant initially grows at a rate close to 30,000 m/s and continues unabated over thousands to millions of years till it dissipates into the surrounding interstellar medium.
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| Almost all elements, except those past Plutonium, are created in a supernova. Exploding massive stars, white dwarf mergers, neutron star mergers, and even the low mass stars that pop out into planetary nebulae saturate the universe with all kinds of heavier elements. As the renowned astronomer Carl Sagan once said, ''We are star stuff''. |
No matter the type, when a star explodes, as already mentioned, it puts out as much energy as our Sun gets to produce in its entire lifetime. Typical energy output for a Type 1a supernova falls in the range of 1-2 × 10⁴⁴ Joules or 10⁵¹ Ergs. Astronomers use a specific unit, known as a foe to quantize the amount of energy liberated by a supernova. Foe is an acronym for [ten to the power of] fifty-one-ergs. This is, however, the energy output in the few seconds following the implosion.
Anytime a star blows up, it shines brighter than the combined luminosity of billions of stars in its home galaxy; its light travelling unabated through the dark depths of interstellar space to intergalactic space spanning over tens to hundreds of millions of light years, if not billions. In 2015, astronomers came across a supernova event designated ASASSN - 15 h (SN 2015 L), located some 3.8 billion light-years from Earth. You can try to pronounce it as "assassin". Being a super-luminous supernova, at its peak brightness, it was 570 billion times brighter than the Sun and 20 times brighter than the combined light of all the stars emitted by our Milky Way galaxy.
The handful of stars we see in our night sky (of course, with the naked eye) all lie within a distance of 1,000 light-years. In a sense, they are in our ''cosmic backyard''. As we turn towards the hazy arching band of the Milky Way, we are looking at the combined luminosity of an almost numberless multitude of billions of stars. If any of these were to explode, it would appear, out of the blue, as a bright star in our night skies. For the past 1,000 years, our forebears have witnessed probably more than a dozen of these never-seen-before stars. Since these stars appeared out of the blue, they were known as ''nova stars''. The most recent Milky Way supernova appeared in our night skies on 9th October 1604. Johannes Kepler performed extensive studies on this new star in the constellation of Ophiuchus, determined that its origins lay far beyond the Moon, and published all his observations De Stella Nova In Pede Serpentarii (On the New Star in the Foot of the Serpent Handler). The word ''supernova'' was coined much later by Walter Baade and Fritz Zwicky in the 1930s.
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Remnant of Kepler's supernova (SN 1604);
false-color composite image rendered using data
from Hubble, Chandra, and Spitzer space
telescopes. 20,000 light years distant.
Image Credits: Public Domain,
via Wikimedia Commons.
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Remnant of Tycho's Supernova (SN 1572) as
seen in X-ray by the Chandra X-ray observatory.
Located 8 - 10,000 light years away from Earth.
Image Credits: Public Domain,
via Wikimedia Commons. |
Almost three decades earlier, in 1572, another new star appeared in the constellation of Cassiopeia. Kepler's contemporary (Kepler was only one year old), Tycho Brahe, published his observations under the name De nova et nullius aevi memoria prius visa stella (Concerning the star, new and never before seen in the life or memory of anyone).
Four centuries have passed since we saw a star flare up in our own galaxy. A handful of our backyard stars, Antares, Rigel, Spica, Eta Carinae, and Betelgeuse, are all set to explode in the distant future. Betelgeuse, the bright red star in Orion, is almost on the verge of going supernova. It may happen tomorrow or anytime in the coming 100,000 years. Antares is due for 10,000 years. The rest are not going to explode in millions of years. So unless Nature decides to surprise us, we are down for a long wait.
Thanks to the vast number of galaxies out there, astronomers routinely come across thousands of extragalactic supernovae.
SN2023ixf is the closest extragalactic supernova to Earth in this decade since 2014. A Type II core-collapse event, SN2023ixf was discovered at an apparent magnitude of +14.9. In a mere span of 11 hours, its brightness went up to +13.5. By 21st May, its brightness peaked around +11.0. When a star flares up, it remains bright for months. As the expanding fireball continues to rush towards interstellar space, its brightness steadily decreases before it fades away from sight. In the brilliant planetary nebula following the collapse of a massive star, astronomers expect to find either a neutron star or a black hole.
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