Supernova

• 1. Introduction

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  A supernova can briefly shine as bright as an entire galaxy. The peak optical luminosity of one of these explosions rivals all the stars of a galaxy combined, before fading over several weeks or months. In our own galaxy, such an event is expected on average about once every 61 years. Yet the last one observed here was Kepler's Supernova, seen in 1604. Several thousand more turn up each year, but they flare in distant galaxies far beyond naked-eye reach. So what is actually exploding, and why does the light linger for months when the blast itself is sudden? How did stargazers in China, Japan, Iraq, Egypt and Europe come to record the same flash a thousand years ago? And why do astronomers say that the oxygen in your blood and the iron in your veins were forged inside these dying stars? The answers run from a carving on a Kashmiri rock to a burst of neutrinos detected in 1987.

• 2. Two Ways To Explode

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  Theoretical studies point to two basic triggers behind nearly every supernova. One is the sudden re-ignition of nuclear fusion in a white dwarf. The other is the gravitational collapse of a massive star's core. In the white dwarf case, the object's temperature climbs high enough to set off runaway fusion that completely disrupts the star. This can follow an accumulation of material drawn from a binary companion through accretion, or a stellar merger. A massive star follows a different path to ruin. Its core collapses once fusion can no longer produce enough energy to counteract the star's own gravity. That moment must arrive once the star begins fusing iron, though it may come during an earlier stage of metal fusion. The progenitor, as astronomers call the original object, faces three possible fates. It can collapse to a neutron star, collapse to a black hole, or be destroyed entirely into a diffuse nebula. Whatever the outcome, the blast expels several solar masses of material at speeds reaching several percent of the speed of light. That expelled shell, sweeping up surrounding gas and dust, becomes the supernova remnant that later generations of astronomers will study.

• 3. Naming The New Star

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  The word supernova comes from the Latin nova, the name for what looks like a temporary new bright star. Adding the prefix super distinguishes these explosions from ordinary novae, which are far less luminous. Walter Baade and Fritz Zwicky coined the term, using it in astrophysics lectures at Caltech in 1931. The word first reached print as "super-Novae" in a 1933 journal paper by Knut Lundmark, who may have arrived at it independently, and again in a 1934 paper by Baade and Zwicky. By 1938 the hyphen had dropped away and the modern spelling was in use. A formal shorthand grew up alongside it: SN for a single supernova, SNe for several. The naming itself follows a strict convention. Discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which issues a circular with an assigned name. The name pairs the prefix SN with the year of discovery, then a letter suffix. The first 26 of a year take capital letters A through Z. After that come lower-case pairs like aa and ab, so SN 2005nc marks the 367th supernova reported in 2005. Historical events keep the simplest form: SN 1572 is also known as Tycho's Nova, and SN 1604 as Kepler's Star. The Asiago Supernova Catalogue ran until the 31st of December 2017, when its final entry bore the designation SN 2017jzp.

• 4. A Thousand Years Of Watching

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  A rock carving in the Burzahama region of Kashmir, dated to 4500 and possibly showing nova HB9, is the earliest of many claimed but unverifiable prehistoric records. The first widely recorded example was SN 1006, seen in AD 1006 in the constellation of Lupus and described by observers across China, Japan, Iraq, Egypt and Europe. SN 1054, recorded by Chinese astronomers in AD 1054, produced the Crab Nebula. Analysis of the historical record suggests that, apart from telescope findings, fewer than 10 supernovae have been seen over the last 2,000 years. Two of those naked-eye events reshaped European astronomy. SN 1572 and SN 1604 were used to argue against the Aristotelian idea that the heavens beyond the Moon and planets were static and unchanging. Johannes Kepler began watching SN 1604 at its peak on the 17th of October 1604, tracking its brightness until it faded from view a year later. It was the second such event in a generation, following Tycho Brahe's observation of SN 1572 in Cassiopeia. Some events left no written trace at all. The youngest known supernova in our galaxy, G1.9+0.3, appears to have occurred in the late 19th century, more recently than Cassiopeia A from around 1680, yet neither was noted at the time. For G1.9+0.3, heavy dust along the plane of the galactic disk could have dimmed the flash enough for it to slip by unseen.

• 5. The Telescope Era

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  SN 1885A, in the Andromeda Galaxy, was the first supernova found with an astronomical telescope. A decade later came SN 1895A and SN 1895B, in NGC 4424 and NGC 5253. Through the 1920s, what looked like a new category of novae drew names such as "upper-class Novae", "Hauptnovae" and "giant novae". Rudolph Minkowski and Fritz Zwicky began building the modern classification scheme in 1941. During the 1960s, astronomers realised the maximum intensities of supernovae could serve as standard candles, turning them into measuring sticks for cosmic distance. Some of the most distant ones observed in 2003 looked dimmer than expected, supporting the view that the universe's expansion is accelerating. Records have grown into vast catalogues. The Pantheon data set, assembled in 2018, detailed 1048 supernovae. By 2021 it had grown to 1701 light curves for 1550 supernovae drawn from 18 different surveys, a 50 percent increase in under three years. A few captures stand out for their timing. SN 2013fs was recorded just three hours after the explosion on the 6th of October 2013 by the Intermediate Palomar Transient Factory, with spectra beginning six hours after detonation. SN 2016gkg was caught by amateur astronomer Victor Buso of Rosario, Argentina, on the 20th of September 2016, the first time the initial "shock breakout" from an optical supernova was seen. Astronomer Alex Filippenko remarked, "Observations of stars in the first moments they begin exploding provide information that cannot be directly obtained in any other way." The most luminous supernova on record, ASASSN-15lh, lies 3.82 gigalight-years away. First detected in June 2015, it peaked at twice the bolometric luminosity of any other known supernova, though its true nature remains debated.

• 6. Reading The Spectra

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  Astronomers sort supernovae by their light curves and by the absorption lines of chemical elements in their spectra. The deciding question is hydrogen. A spectrum showing the Balmer series of hydrogen makes a supernova Type II; without it, the event is Type I. Type I splits further on its spectral lines. Type Ia shows a strong singly ionised silicon line at 615.0 nanometers near peak light, the signature of thermal runaway. Type Ib displays a non-ionised helium line at 587.6 nanometers, while Type Ic shows weak or no helium. Both Ib and Ic come from core collapse, despite resembling the thermal Type Ia. Type II carries its own subdivisions. Type II-P reaches a plateau in its light curve, a stretch where visual brightness holds roughly constant for several months. Type II-L instead shows a linear decline. Some events, such as SN 2005gl, display narrow spectral lines and are labelled Type IIn, the "n" standing for narrow. A few change character entirely: SN 1987K and SN 1993J showed hydrogen early, then became dominated by helium lines over weeks to months, earning the Type IIb label. Zwicky once defined rarer classes from single examples. SN 1961i in NGC 4303 was the sole Type III, SN 1961f in NGC 3003 the sole Type IV, and SN 1961V in NGC 1058 the Type V, a faint event whose maximum lasted many months and recalled the Eta Carinae Great Outburst. These would now be treated as peculiar Type II supernovae, though whether SN 1961V was a true supernova or an impostor is still debated.

• 7. What Powers The Glow

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  The gases thrown off by a supernova would dim quickly without some source to keep them hot, and that source was at first a puzzle. The answer turned out to be radioactivity, calculated on nucleosynthesis grounds in the late 1960s. Confirmation came with SN 1987A, the first event whose gamma-ray lines were directly observed. The decay chain releases gamma-ray photons, primarily at energies of 847 and 1238, which the ejected gas absorbs and re-radiates as visible light from several weeks to several months out. SN 1987A pinned down the details. Energy for the peak of its light curve came from a decay with a half-life of 6 days, while the later curve matched closely the 77.3-day half-life of a second decaying nucleus. Space gamma-ray telescopes later caught the small fraction of gamma rays that escaped the remnant without absorption, confirming which radioactive nuclei drove the brightness. For cosmology's standard candles, the Type Ia supernovae, the diagnostic 847 and 1238 gamma rays were not detected until 2014. The energy budget hides a surprise. Although supernovae are known as luminous events, their light is almost a minor side effect. In core collapse, the vast majority of the energy pours into neutrino emission, and more than 99 percent of those neutrinos escape the star within the first few minutes of collapse. SN 1987A delivered the only measurements of astronomical neutrinos other than the Sun's, and was attributed to the explosion of a blue supergiant star.

• 8. Seeding The Universe

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  Supernovae scatter heavier elements across the interstellar medium, from oxygen through to rubidium. Type Ia events produce mainly silicon and iron-peak metals such as nickel and iron. Core collapse events eject far less of the iron-peak elements but larger masses of light alpha elements like oxygen and neon, along with elements heavier than zinc. The heaviest nuclei form through rapid neutron capture, the r-process, during the collapse itself, accounting for about half of all the isotopes beyond iron, though neutron star mergers may dominate for many of these. The expanding remnant does more than enrich its surroundings. Its kinetic energy can compress nearby dense molecular clouds and trigger the birth of new stars, although too much turbulent pressure can also prevent star formation if the cloud cannot shed the excess energy. Evidence from short-lived radioactive isotopes shows that a nearby supernova helped shape the composition of the Solar System 4.5 billion years ago, and may even have triggered its formation. The reach extends to our own planet. A near-Earth supernova close enough to affect the biosphere could lie as far as 3,000 light-years away. In 1996 researchers theorised that past supernovae might leave metal isotope signatures in rock strata, and iron-60 enrichment was later found in deep-sea rock of the Pacific Ocean. In 2009, elevated nitrate ions appeared in Antarctic ice, coinciding with the supernovae of 1006 and 1054. The Supernova Early Warning System now watches for the next nearby blast, using a network of neutrino detectors to catch the particles that race out of a collapsing core ahead of its light.