Kurzgesagt – In a Nutshell

Sources – Supernova Death

Thanks to our experts –

• Dr. Brian Fields

University of Illinois at Urbana-Champaign

• Dr. Matt Caplan

Illinois State University

– Supernovae are the most powerful explosions in the universe, unleashing enough energy to outshine galaxies.

The electromagnetic power (energy per unit time at all frequencies) radiated by an astronomical object is called “intrinsic luminosity”. The Sun has a luminosity of the order of 10²⁶ Watts (Joules per second), equivalent to 10³³ ergs per second (“erg” is a unit of energy; the unit of energy used in the International System of Units, the joule, equals 10⁷ ergs.):

#NASA (2018): “Sun Fact Sheet” (retrieved 2022)
https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html

Quote: “Luminosity (10²⁴ J/s) 382.8”

The luminosity of a supernova can be billions or trillions of times higher than that. For example, a typical supernova of type Ia will reach a peak luminosity of 10⁴³ erg/s:

#Branch, D. (2003): Supernovae. In: Encyclopedia of Physical Science and Technology, Academic Press
https://www.sciencedirect.com/topics/physics-and-astronomy/type-ia-supernovae
Quote: “At its brightest, a normal Type Ia supernova (SN Ia) reaches an absolute visual magnitude of −19.5 and has a luminosity exceeding 10⁴³ erg/sec, billions of times that of the Sun.”

While the so-called “superluminous supernovae” can reach luminosities of up to 10⁴⁵ erg/s, like the event ASASSN-15lh observed in 2015. This is about a trillion times more luminous than the Sun.

#Sukhbold, T. & Woosley, S. E. (2016): The Most Luminous Supernovae. The Astrophysical Journal Letters, Vol. 820 (2)
https://iopscience.iop.org/article/10.3847/2041-8205/820/2/L38

The luminosity of a supernova is comparable to the luminosity of a whole galaxy. The number of stars in a galaxy like the Milky Way (a big galaxy) is of the order of hundreds of billions:

#NASA (2017): How Many Stars in the Milky Way?
https://asd.gsfc.nasa.gov/blueshift/index.php/2015/07/22/how-many-stars-in-the-milky-way/
Quote: “The most common answer seems to be that there are 100 billion stars in the Milky Way on the low-end and 400 billion on the high end.”

And therefore a rough estimate of the luminosity of a (big) galaxy would be in the range of hundreds of billions of solar luminosities – comparable to (and often lower than) that of a supernova.

Real supernova explosion in a distant galaxy. Most of the stars seen in the picture belong to the host galaxy. A time-lapse video of the event (amounting to one year of observations) can be found here:

#HubbleWebbESA (2020): Time-Lapse of Supernova in NGC 2525
https://www.youtube.com/watch?v=hB2QQyEyF5k

– We have no real metaphor for their power – if the sun were to magically go supernova it would feel like you were being hit by the energy of a nuclear explosion, every second. For weeks.

The Sun will never explode as a supernova since it’s not massive enough for that (only stars at least 8-10 times more massive than the sun can explode as supernovae). But let’s imagine that something like that would magically happen.

Such an explosion would release an energy L of about 10⁴³-10⁴⁵ ergs per second. If the energy is emitted in all directions, by the time it reaches Earth it will be distributed along the surface of a sphere of radius R = 1 AU (the distance of sun-earth, about 150 million km). This implies that, on the earth, you’ll get an energy flux (amount of energy per square meter and per second) of

L / 4πR ² = 10²⁰-10²² ergs/m²/s

Assuming that your body has an area of the order of 1 m², this means that, every second, your body will be bombarded with 10²⁰-10²² ergs of energy.

The explosion of 1000 metric tons of TNT (a “kiloton”) releases about 4·10¹⁹ ergs of energy, so (depending on the luminosity of the supernova) your body would get between 10 kilotons and 1 megaton of energy per second. This is equivalent to the yield of a nuclear bomb: the Hiroshima bomb released about 15 kilotons of energy, and many nuclear warheads today have a typical yield of about 100 kilotons or more:

#ICAN: How destructive are today’s nuclear weapons? (retrieved 2022)
https://www.icanw.org/how_destructive_are_today_s_nuclear_weapons
Quote: “Many of the modern nuclear weapons in Russian and U.S. nuclear weapons are thermonuclear weapons and have explosive yields of the equivalent at least 100 kilotons of dynamite - and some are much higher.”

Lastly, a supernova explosion doesn’t last for one second, but typically for more than a month:

#Branch, D. (2003): Supernovae. Encyclopedia of Physical Science and Technology, Academic Press
https://www.sciencedirect.com/topics/physics-and-astronomy/type-ia-supernovae
Quote: “The light curve of a SN Ia consists of an initial rise and fall (the early peak) that lasts about 40 days, followed by a slowly fading tail. The tail is nearly linear when magnitude is plotted against time, but since magnitude is a logarithmic measure of brightness, the tail actually corresponds to an exponential decay of brightness, The rate of decline of the SN Ia tail corresponds to a half-life of about 50 days.”

– While supernovae are the engines of creation, forging the elements that enable life, they also burn sterile whole regions of galaxies.

With the exception of hydrogen and some amounts of helium and lithium, which were created right after the Big Bang, all chemical elements are created by stars. Throughout the life of a star, progressively heavier chemical elements are formed by the same nuclear fusion processes that keep the star alive and burning: fusion of hydrogen nuclei produces helium, fusion of helium nuclei produces carbon, and so on. But this fusion chain of progressively heavier elements generally stops at iron.

The creation of elements heavier than iron requires more energetic astrophysical phenomena, like supernova explosions. And apart from heavy elements, supernova explosions create many other atoms, many of which are essential for planet formation and for life itself.

#Johnson, J. A. (2019): Populating the periodic table: Nucleosynthesis of the elements. Science, Vol. 363
https://www.drkohn.org/uploads/1/4/0/9/14095127/science_-_populating_the_periodic_table_-_nucleosynthesis_of_the_elements_science.aau9540.pdf

– There are roughly speaking two ways to make a supernova. Either the core of a massive star implodes, or, less common, a white dwarf gains mass to the point where it ignites explosive nuclear fusion. The outcome is the same: a supernova explosion.

These two types of supernovae are known as “core-collapse” or “type II” supernovae and “type Ia”, respectively:

#NASA (2021): Supernovae
https://imagine.gsfc.nasa.gov/science/objects/supernovae2.html
Quote: “Supernovae are divided into two basic physical types:

Type Ia: These result from some binary star systems in which a carbon-oxygen white dwarf is accreting matter from a companion. (What kind of companion star is best suited to produce Type Ia supernovae is hotly debated.) In a popular scenario, so much mass piles up on the white dwarf that its core reaches a critical density of 2 x 10⁹ g/cm³. This is enough to result in an uncontrolled fusion of carbon and oxygen, thus detonating the star.

Type II: These supernovae occur at the end of a massive star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star's iron core is massive enough, it will collapse and become a supernova.”

A depiction of the process leading to a Type Ia supernova. The white dwarf is at the left, sucking mass from its companion. When a certain threshold is reached, the white dwarf will undergo uncontrolled nuclear fusion and will explode. An animated video of the image above can be found here:

#ESO (2009): Artist's impression of vampire star
https://www.eso.org/public/videos/eso0943b/

A depiction of the process leading to a Type II (core-collapse) supernova.

#Brau, J. (2016): Two Types of Supernovae. Astronomy 122 Mid-term Exam 4 Review, University of Oregon
https://pages.uoregon.edu/jimbrau/BrauImNew/Chap21/7th/AT_7e_Figure_21_09b.jpg

The core-collapse supernovae are more common than the type Ia class:

#Harvard-Smithsonian Center for Astrophysics (2020): Introduction and Background: Type II and Type Ia Supernovas. Chandra X-Ray Obervartory https://chandra.harvard.edu/edu/formal/snr/bg4.html
Quote: “Type II supernova events – core collapses of massive stars – are more common than Type Ia events – the thermonuclear explosion of white dwarfs.”

– Hot and dangerous gas rushes outward at speeds of 10,000 km/s through the near vacuum of space, sweeping up the sparse gas of the galaxy.

The gas and materials ejected by the supernova explosion are expelled at speeds of the order of 10,000 km/s (3% of the speed of light). This phase of “free expansion” lasts for about 100 or 200 years. After this time, the remnants will have created a cloud of gas (called a “supernova remnant”) of about 10 light years in radius.

#Swinburne University of Technology (retrieved 2022): Supernova remnant. COSMOS - The SAO Encyclopedia of Astronomy
https://astronomy.swin.edu.au/cosmos/s/supernova+remnant
Quote: “When the supernova first explodes, a shock wave is sent out through the star. Once it has passed through the stellar material, it continues to expand into the surrounding ISM [interstellar medium] creating a shock wave in the interstellar gas in the forward direction, and also a shock in the reverse direction, back into the supernova ejecta. This shocked material is heated to millions of degrees Kelvin resulting in the emission of thermal X-rays.

The shock wave also accelerates the ISM into an expanding shell which outputs copious amounts of synchrotron radiation due to the acceleration of electrons in the presence of a magnetic field. This expanding shell surrounds an area of relatively low density, into which the supernova ejecta expands freely, typically with velocities of around 10,000 km/s. This free expansion phase lasts for around 100 – 200 years until the mass of the material swept up by the shock wave exceeds the mass of the ejected material.”

#NASA (2011): Introduction to Supernova Remnants: NASA’s HEASARC: Education & Public Information
https://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html
Quote: “As the ejecta expand out from the star, it passes through the surrounding interstellar medium, heating it from 10⁷ to 10⁸ K, sufficient to separate electrons from their atoms and to generate thermal X-rays. The interstellar material is accelerated by the shock wave and will be propelled away from the supernova site at somewhat less than the shock wave's initial velocity. This makes for a thin expanding shell around the supernova site encasing a relatively low density interior.

While the material swept up by the shock is much less than the mass of the stellar ejecta, the expansion of the stellar ejecta proceeds at essentially a constant velocity equal to the initial shock wave speed, typically of the order of 10,000 km/s. This is known as the "free expansion" phase and may last for approximately 200 years, at which point the shock wave has swept up as much interstellar material as the initial stellar ejecta. The supernova remnant at this time will be about 10 light years in radius.”

The Crab Nebula, probably the most iconic supernova remnant. The explosion was recorded by Chinese astronomers in the year 1054. The span of the cloud is about 10 light-years. In comparison, the whole solar system would be no more than a single pixel in the picture.

#NASA (2021): Messier 1 (The Crab Nebula). Hubble’s Messier Catalog
https://www.nasa.gov/feature/goddard/2017/messier-1-the-crab-nebula

To get an order of magnitude estimate of how the explosion develops in this first phase, assuming an average expansion speed of 10,000 km/s gives the following radii of the explosion/”tsunami” (all numbers are rounded off)

At day 0 (star before explosion) R₀ ~ a few million km

After 1 day R_d ~ 1 billion km

After 1 week R_w ~ 7 billion km

After 1 month R_m ~ 25 billion km (~ 0.003 light-years)

After 50 years R₅₀ ~ 2 light-years

After 100 years R₁₀₀ ~ 4 light-years

After 200 years R₂₀₀ ~10 light-years

Such an simple average gives a plausible example with the right orders of magnitude:

#Micelotta, E. R. et al. (2018): Dust in Supernovae and Supernova Remnants II: Processing and Survival. Space Science Reviews, Vol. 214 (53)
https://link.springer.com/article/10.1007/s11214-018-0484-7

– This wall of gas expands for tens of thousands of years and will eventually span up to dozens of light-years until it finally cools off, and disperses its substance back into the galaxy.

After the phase of “free expansion” (100-200 years) the cloud of gas enters into another phase of slower expansion lasting between 10,000 and 20,000 years.

#Swinburne University of Technology (retrieved 2022): Supernova remnant. COSMOS - The SAO Encyclopedia of Astronomy
https://astronomy.swin.edu.au/cosmos/s/supernova+remnant
Quote: “Adiabatic (Sedov-Taylor) Phase: As the mass of the ISM swept up by the shock wave increases, it eventually reaches densities which start to impede the free expansion. Rayleigh-Taylor instabilities arise once the mass of the swept up ISM [interstellar medium] approaches that of the ejected material. These instabilities mix the shocked ISM with the supernova ejecta and enhance the magnetic field inside the SNR [supernova remnant] shell. This phase lasts between 10,000 and 20,000 years.”

By the end of the expansion, the total size of the supernova remnant will typically reach a few dozens of light years. This work studied the properties of supernova remnants in the galaxy M33 and found that their most typical diameters were 20-50 pc, equivalent to 60-160 light years:

#Long, K.S. et al. (2010): The Chandra ACIS survey of M33: X-ray, optical, and radio properties of the supernova remnants. The Astrophysical Journal Supplement Series, Vol. 187 (2)
https://iopscience.iop.org/article/10.1088/0067-0049/187/2/495/meta

And this other work found very similar results for the Large and Small Magellanic Clouds:

#Badenes, C. et al. (2010): On the size distribution of supernova remnants in the Magellanic Clouds. Monthly Notices of the Royal Astronomical Society, Vol. 407 (2)
https://academic.oup.com/mnras/article/407/2/1301/1127347

To get an order of magnitude estimate of how the supernova remnant evolves in this second phase, we can assume a typical radius of 100 light-years after 20,000 years. This would give:

After 5,000 years R_5k ~ 25 light-years

After 10,000 years R_10k ~ 50 light-years

After 20,000 years R_20k ~ 100 light-years

– Humans witnessed dozens of supernovae but all of them were thousands of lightyears away. They appeared as new stars, some outshining the moon, twinkling for a few days, and disappearing. Aside from looking very pretty at this distance they don’t do much to us.

Since the advent of modern astronomers, humans have witnessed many supernovae in distant galaxies. But to be seen with the naked eye, a SN explosion has to take place inside the Milky Way. Current models predict that, in the Milky Way, we should have about 1-3 core-collapse supernovae per century and about 1-2 type Ia supernovae per century:

# Rozwadowska, K. et al. (2021): On the rate of core collapse supernovae in the milky way. New Astronomy, Vol. 83
https://arxiv.org/abs/2009.03438
Quote: “A conservative treatment of the errors yields a combined rate R = 1.63 ± 0.46 (100 yr)⁻¹; the corresponding time between core collapse supernova events turns out to be T = 61 +24 −14 yr.”

#Adams, S. M. et al. (2013): Observing the next galactic supernova. The Astrophysical Journal, Vol. 778 (2)
https://iopscience.iop.org/article/10.1088/0004-637X/778/2/164/meta
Quote: “Based on our modeled observability, we find a Galactic ccSN [core-collapse] rate of 3.2 +7.3 –2.6 per century and a Galactic SN Ia [type Ia] rate of 1.4 +1.4 –0.8 per century for a total Galactic SN rate of 4.6 +7.4 –2.7 per century is needed to account for the SNe observed over the last millennium.”

However most of these supernovae won’t be visible from Earth since many of them will be obscured by the large amounts of dust present in the galactic plane. In fact, before the era of modern astronomy, only five supernovae have been unambiguously identified as such in the historical record:

#Murphey, C. T. et al. (2020): Witnessing history: sky distribution, detectability, and rates of naked-eye Milky Way supernovae. Monthly Notices of the Royal Astronomical Society
https://arxiv.org/abs/2012.06552
Quote: “The Milky Way hosts on average a few supernova explosions per century, yet in the past millennium only five supernovae have been identified confidently in the historical record. [...] We calculate the percentage of all supernovae bright enough for historical discovery: ≃13% of core-collapse and ≃33% of Type Ia events.”

Therefore, if we have 20 core-collapse supernovae in the Milky Way every 1000 years, only about 2 or 3 will be visible from earth. A comparable figure holds for type Ia supernovae. Most of them may have not been documented, but these numbers indicate that, in the last 10,000 or 15,000 years, humans may have witnessed dozens of supernova explosions with the naked eye.

As an example, in the year 1006 AD the supernova SN 1006 appeared in the sky. The explosion happened at about 7000 light years from earth, and sources of the time described the phenomenon as a newly born star brighter than a crescent moon:

#Stephenson, F. R. (2010): SN 1006: the brightest supernova. Astronomy & Geophysics, Vol. 51 (5)
https://academic.oup.com/astrogeo/article/51/5/5.27/206961
Quote: “ ‘I shall now describe a spectacle which I saw at the beginning of my studies. This spectacle appeared in the zodiacal sign of Scorpio, in opposition to the Sun. The Sun on that day was 15 deg in Taurus and the spectacle in the 15th deg of Scorpio. This spectacle was a large circular body, 2½ to 3 times as large as Venus. The sky was shining because of its light. The intensity of its light was as bright as that of the Moon a little more than one quarter illuminated. It remained fixed, moving daily with its zodiacal sign until the Sun was in sextile with it in Virgo, when it disappeared suddenly…’ (Ali ibn Ridwan: Commentary on the Tetrabiblos of Ptolemy)”

This is how the remnant of SN 1006 looks today. It has a diameter of about 60 light years and it is still expanding at about 3000 km/s.

#Hubblesite (2008): SN 1006 Supernova Remnant
https://hubblesite.org/contents/media/images/2008/22/2351-Image.html

– Things begin to get a tiny bit icky once a supernova occurs around 300 lightyears away. We can expect one this close to us every few million years: a single star giving the night sky an eerie glow like twilight. And while this is far and dim enough not to do harm to us, they can affect the earth. At these distances it is like being hit by the last weak waves of the star tsunami. Not strong enough to do real damage but still noticeable.

There is now evidence that supernovae explosions within 300 light years (about 100 parsecs) of earth have happened in the last million years:

#Fields, B. D. et al. (2019): Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities. Astro2020 Science White Paper submitted to the 2020 Decadal Survey on Astronomy and Astrophysics
https://arxiv.org/abs/1903.04589
Quote: “supernova explosions within 100 pc of Earth are expected to have occurred every few Myr. The Local Bubble surrounding the Sun implies nearby events within 2 Myr [24]. These would probably not have caused a mass extinction, but may have perturbed the biosphere and left a detectable radioisotope signature.”

How would we perceive such a supernova from Earth? For the (hypothetical but possible) supernova explosion of Betelgeuse, a red supergiant located at about 600 light year from earth, researchers have estimated an apparent magnitude close to that of the full moon at visible wavelengths, and possibly outshining the moon when considering all wavelengths.

#Goldberg, J. A. et al. (2020): Apparent Magnitude of Betelgeuse as a Type IIP Supernova. Research Notes of the American Astronomical Society, Vol. 4 (3)
https://iopscience.iop.org/article/10.3847/2515-5172/ab7c68
Quote: “Assuming Betelgeuse explodes as a typical Type IIP Supernova at a distance of 197 pc (Harper et al. 2008), for a reasonable range of explosion energies and nickel masses, we expect the plateau brightness to reach ∼10% of the brightness of the full moon in optical (UBVRI) luminosity (or up to ≈50% for a particularly luminous IIP), and potentially outshine the full moon in bolometric luminosity for the first few days after shock breakout.”

If instead of being at 600 light years the distance would be 300 light years (i.e. 2 times closer), the radiant flux on earth (i.e. the energy per unit area and unit time) will be 2² = 4 times larger because of the inverse-square law. Depending on the details of the supernova, this could easily be brighter than the full moon in visible wavelengths (the flux/brightness factor is the vertical right scale in the figures above)

– In fact, we know that over the past 10 million years multiple supernovae have struck Earth from these distances because we can find radioactive isotopes of iron deep in the rocks and sediments at the bottom of the ocean.

A line of evidence pointing to past nearby SN explosions comes from the radioactive iron ⁶⁰Fe found in earth sediments and lunar regolith, pointing to a nearby supernova about 2-3 Myr ago, and maybe to another one about 8 Myr ago.

#Fields, B. D. et al. (2019): Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities. Astro2020 Science White Paper, submitted to the 2020 Decadal Survey on Astronomy and Astrophysics
https://arxiv.org/abs/1903.04589
Quote: “The time series of these ⁶⁰Fe measurements are shown in Fig. 1. There is a clear peak at 2 to 3 Myr ago, pointing to at least one nearby supernova at that epoch. Also, there are hints of a second peak around 8 Myr ago.”

Taking into account the deposition times and other characteristics, the ⁶⁰Fe-signal is believed to indicate multiple supernova events in the past 10 million years:

#Wallner, A. et al. (2016): Recent near-Earth supernovae probed by global deposition of interstellar radioactive ⁶⁰Fe. Nature, Vol. 532 (7597)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4892339/
Quote: “The rate of supernovae (SNe) in our local galactic neighborhood within a distance of ~100 parsec from Earth (1 parsec (pc)=3.26 light years) is estimated at 1 SN every 2-4 million years (Myr), based on the total SN-rate in the Milky Way (2.0±0.7 per century^1,2). [...] Here we report that the ⁶⁰Fe signal observed previously in deep-sea crusts^10,11, is global, extended in time and of interstellar origin from multiple events. Deep-sea archives from all major oceans were analyzed for ⁶⁰Fe deposition via accretion of interstellar dust particles.Our results, based on ⁶⁰Fe atom-counting at state-of-the-art sensitivity⁸, reveal ⁶⁰Fe interstellar influxes onto Earth 1.7–3.2 Myr and 6.5–8.7 Myr ago. The measured signal implies that a few percent of fresh ⁶⁰Fe was captured in dust and deposited on Earth. Our findings indicate multiple supernova and massive-star events during the last ~10 Myr at nearby distances ≤100 pc.”

– Amazingly, these supernovae around the solar system have cleared a 1000 light-year wide pocket of space that is called the 'Local Bubble'. They blew away the interstellar gas and dust, creating a lumpy wall of gas that is now a cradle for star formation.

The Local Bubble is a cavity in the interstellar medium surrounding the sun. With a diameter of about 1000 light-years, it is characterized by a low-density plasma whose origin scientists trace back to the "tsunami" of past supernova explosions. The most recent and complete study was published in 2022 in Nature, where the authors estimated an amount of 15 SN explosions in the last 14 million years.

#Zucker, C. et al. (2022): Star formation near the Sun is driven by expansion of the Local Bubble. Nature, Vol. 601 (334-337)
https://arxiv.org/abs/2201.05124
Quote: “Tracebacks of these young stars’ motions support a scenario where the origin of the Local Bubble was a burst of stellar birth and then death (supernovae) taking place near the bubble’s center beginning ~14 Myr ago. The expansion of the Local Bubble created by the supernovae swept up the ambient interstellar medium into an extended shell that has now fragmented and collapsed into the most prominent nearby molecular clouds, in turn providing robust observational support for the theory of supernova-driven star formation.”

Quote: “Based on the amount of momentum injection required by supernovae to sweep up the total mass of the shell (1.4 −0.62 +0.65 × 10⁶ M☉) given its present-day expansion velocity (6.7 −0.4 +0.5 km/s), we estimate that 15 −7 +11 supernovae were required to form the Local Bubble”

The researchers have set up a web page with visuals to explore the properties of the Local Bubble.

#The Local Bubble (retrieved 2022): Star Formation near the Sun is driven by expansion of the Local Bubble.
https://faun.rc.fas.harvard.edu/czucker/Paper_Figures/Interactive_Figure1.html

As a consequence of the supernova “tsunami” that swept the interstellar medium, star formation near the solar system occurs mainly on the surface of the Local Bubble:

#The Local Bubble (retrieved 2022): Visuals. Interactive Figures
https://sites.google.com/cfa.harvard.edu/local-bubble-star-formation/visuals?authuser=0

These findings support the importance of supernova explosions in the star formation process: the (old) theory that shock waves sent by the supernovae stir the interstellar medium, triggering the gravitational collapse of the molecular clouds.

#Zucker, C. et al. (2022): Star formation near the Sun is driven by expansion of the Local Bubble. Nature, Vol. 601 (334-337)
https://arxiv.org/abs/2201.05124
Quote: “Regardless of UCL and LCC’s potential origins, we find 6D (3D position and 3D velocity) observational support for the theory of supernova-driven star formation in the interstellar medium^23–26 — a long-invoked theoretical pathway for molecular cloud formation seen in numerical simulations.²⁷ ”

– Stars have extremely powerful magnetic fields. When they die, the tsunami of dead star actually retains a lot of this magnetic energy, woven through the shockwave that expands outwards.

Stars have powerful and intricate magnetic fields:

#NASA (2017): Understanding the Magnetic Sun
https://www.nasa.gov/feature/goddard/2016/understanding-the-magnetic-sun/
Quote: “This comparison shows the relative complexity of the solar magnetic field between January 2011 (left) and July 2014. In January 2011, three years after solar minimum, the field is still relatively simple, with open field lines concentrated near the poles. At solar maximum, in July 2014, the structure is much more complex, with closed and open field lines poking out all over – ideal conditions for solar explosions."

And supernova remnants host magnetic fields, too. Some magnetic maps of SNRs can be found e.g. here:

#Reynolds, S. P. et al. (2011): Magnetic Fields in Supernova Remnants and Pulsar-Wind Nebulae. Space Science Reviews, Vol. 160
https://arxiv.org/abs/1104.4047

– In this highly magnetized cloud, we get conditions like in a huge particle accelerator that is accelerating charged particles like protons, nuclei, and electrons to immense speeds. Which means we have an expanding cloud that is shooting deadly radiation in all directions, long after the bright light from the initial explosion has faded away.

A significant fraction of the kinetic energy released in a supernova explosion is invested in accelerating particles (mostly protons and other atomic nuclei) to almost the speed of light. Once accelerated, these particles become what we call “cosmic rays” – a constant flux of radiation traveling through the galaxy. Cosmic rays were discovered about one century ago, when they were found to arrive at the upper atmosphere from random directions, but their precise origin is still largely uncertain in many respects. Supernovae are thought to be one of the main sources of cosmic rays in the galaxy:

#Max Planck Institute for Nuclear Physics (2012): Astrophysics with H.E.S.S.
https://www.mpi-hd.mpg.de/hfm/HESS/pages/about/physics/
Quote: “Demonstrated or suspected sites of particle acceleration and particle interaction in the Cosmos include: Supernovae – The shock wave launched into the circumstellar medium after the collapse of a star, that has burnt its nuclear fuel, can very efficiently accelerate particles. Models predict that 10% or more of the kinetic energy of the explosion is transferred to high-energy particles. Supernovae might be responsible for the bulk of the cosmic rays in the Galaxy, at least up to energies of 10¹⁵ eV.”

#Vink, J. (2014): Supernova remnants and the origin of cosmic rays. Proceedings of the International Astronomical Union, Vol. 9 (S296)
https://www.cambridge.org/core/journals/proceedings-of-the-international-astronomical-union/article/supernova-remnants-and-the-origin-of-cosmic-rays/A654A9AB5E72001B47726CE5F4476F56
Quote: “SNRs have for a long time been considered the dominant sources of Galactic cosmic rays. If true, SNRs must be able to accelerate particles up to at least 3 × 10¹⁵ eV and have an efficiency of around 10% in transferring energy to accelerated particles. The rapid development in X-ray and gamma-ray astronomy have helped to strengthen the case for cosmic ray acceleration by SNRs. The presence of X-ray synchrotron emission and the narrowness of the X-ray synchrotron emitting regions indicate that magnetic fields are amplified near young SNRs, which helps to create the right conditions for accelerating protons up to 3 × 10¹⁵ eV.”

#NASA (2017): Cosmic rays
https://imagine.gsfc.nasa.gov/science/toolbox/cosmic_rays1.html
Quote: “About 90% of the cosmic ray nuclei are hydrogen (protons), about 9% are helium (alpha particles), and all of the rest of the elements make up only 1%. Even in this one percent there are very rare elements and isotopes.”

– If a supernova happens too close by, waves of these ‘cosmic rays’ will wash over the solar system for thousands of years. While we’re mostly protected on earth’s surface by the atmosphere and ozone layer, the influx of extra radiation will still increase cancer and mutation rates. Not enough to cause a mass extinction but it will be noticeable.

The initial light from the supernova explosion won’t cause particular damage on earth. But over the next few thousands of years, the expanding shell of the supernova remnant will generate higher rates of cosmic radiation. This will have effects on the ozone layer, increasing the amount of ionizing radiation on earth’s surface:

#Fields, B. D. et al. (2019): Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities. Astro2020 Science White Paper, submitted to the 2020 Decadal Survey on Astronomy and Astrophysics
https://arxiv.org/abs/1903.04589
Quote: “The spectacular optical display of a nearby supernova would not be very dangerous to life if the explosion is ∼ 100 pc away. The outburst would also bring higher-energy ionizing radiation, including extreme UV, X-rays and gamma rays, yet even these are not catastrophically harmful from ∼ 100 pc. However, charged cosmic rays would arrive later with the supernova blast that accelerates them, and linger for many thousands of years. They would deplete the Earth’s ozone layer, which would in turn allow more solar UVB radiation to reach the Earth’s surface and upper ocean layers for an extended period [16, 52]. Increases in ionizing radiation can damage DNA, harm tissues in animals, and degrade photosynthesis in plants.”

– Spaceflight would become impossible in the solar system, as astronauts would not survive the waves of radiation for long. We don’t know exactly how bad this would be, but a supernova that is close enough may trap our species on earth for generations, maybe thousands of years. It only gets worse from here.

One of the main dangers of space travel comes from the exposure to cosmic rays. Even with the current flux of galactic radiation, it is not clear if a human trip to e.g. Mars would be safe for the astronauts:

#Parihar, V. K. et al. (2015): What happens to your brain on the way to Mars. Science Advances, Vol. 1 (4)
https://www.science.org/doi/full/10.1126/sciadv.1400256
Quote: “As NASA prepares for the first manned spaceflight to Mars, questions have surfaced concerning the potential for increased risks associated with exposure to the spectrum of highly energetic nuclei that comprise galactic cosmic rays. [...] Our data indicate an unexpected and unique susceptibility of the central nervous system to space radiation exposure, and argue that the underlying radiation sensitivity of delicate neuronal structure may well predispose astronauts to unintended mission-critical performance decrements and/or longer-term neurocognitive sequelae.”

Given the fact that the current levels of cosmic rays might imply considerable health risks during a space trip to our neighbor planet, the extra bath of high-energy particles coming from a nearby supernova could well make space travel impossible.

– First, the high energy photons arrive from the explosion, followed by many decades of radiation from the radioactive tsunami, both of which seriously damage the ozone layer, earth’s shield against harmful radiation. The ozone layer absorbs ultraviolet radiation by breaking apart ozone, O3, into O2 and a free oxygen atom, which later reforms back into another ozone molecule.

Ozone depletion from a nearby SN explosion would be caused by two distinct types of radiation: gamma rays, which would last a few hundred days, and cosmic rays, which would last for decades:

#Gehrels, N. et al. (2003): Ozone Depletion from Nearby Supernovae. The Astrophysical Journal, Vol. 585 (2)
https://iopscience.iop.org/article/10.1086/346127/meta
Quote: “We model the effects of a core-collapse SN on the Earth’s ozone by inputting the expected cosmic and gamma irradiation into an atmospheric model. We consider (1) the relatively short-lived (~ 100 days) gamma rays from the initial blast and (2) the longer lived (> 10 yr) cosmic rays accelerated in the SN blast wave”

The atmospheric effects of a nearby SN blast were considered in this foundational article, where the effects of cosmic radiation were estimated to last for several decades:

#Ruderman, M. A. (1974): Possible Consequences of Nearby Supernova Explosions for Atmospheric Ozone and Terrestrial Life, Science, Vol. 184 (4141)
https://www.science.org/doi/abs/10.1126/science.184.4141.1079
Quote: “At 50 light-years a 10⁵⁰-erg cosmic-ray burst would then give 40 times the present cosmic-ray flux for 80 years”

Ozone is a (generally unstable, but long-lived) molecule composed by three atoms of oxygen, O₃. Ordinary molecular oxygen has two atoms: O₂. The basic chemistry responsible for the ozone-shielding from UV radiation is well known:

#NASA (2004): Ozone: What is it, and why do we care about it?
https://www.nasa.gov/audience/foreducators/postsecondary/features/F_Ozone.html
Quote: “Ozone is extremely valuable since it absorbs a range of ultraviolet energy. When an ozone molecule absorbs even low-energy ultraviolet radiation, it splits into an ordinary oxygen molecule and a free oxygen atom. Usually this free oxygen atom quickly re-joins with an oxygen molecule to form another ozone molecule. Because of this "ozone-oxygen cycle," harmful ultraviolet radiation is continuously converted into heat.”

– But the supernova radiation breaks up Nitrogen molecules that gobble up the free oxygen, breaking the cycle and depleting the ozone layer quickly:

Cosmic radiation is known to damage the ozone layer by breaking N₂ molecules and interrupting the ozone-oxygen cycle. In the case of a steady source of increased radiation caused by a nearby supernova, the depletion of the global ozone layer would amount to 25% on average, dwarfing the human-induced ozone loss of the 20th century:

#Melott, A. L. (2018): Terrestrial effects of moderately nearby supernovae. Lethaia, Vol. 51 (3)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6027649/
Quote: “The cosmic rays break up the N₂ molecule, and the isolated N become available to react – usually with the first thing they encounter. As much of the atmosphere is oxygen, more oxides of nitrogen are produced. [...] The nitrogen dioxide acts as a catalyst and converts ozone (O₃) back to ordinary oxygen (O₂) (Thomas et al. 2005). It is recycled in this process. It takes 5–10 years for the NO_x, as they are called, to be removed from the atmosphere, primarily by being rained out after a burst event. [...] However, with a relatively steady source, such as the flux of cosmic rays from a nearby supernova, they are constantly produced and reach a steady state. The ozone depletion does too, and in our recent example, computations showed a nearly steady mean 25% depletion in stratospheric ozone from the 150 light year supernova. This can be compared with a 3%–5% maximum global average ozone loss in the late 20th century from refrigerants.”

– Without a radiation shield everybody living on the surface is exposed to very high levels of UV radiation from our sun – cancer rates would skyrocket and just going outside during the day could be life threatening.

Even a supernova at 150 light years could increase cancer rates by 50%:

#Melott, A. L. (2018): Terrestrial effects of moderately nearby supernovae. Lethaia, Vol. 51 (3)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6027649/
Quote: “The reason that stratospheric ozone depletion is serious has to do with ultraviolet radiation. The ozone layer in the stratosphere absorbs in a band called UVB which is responsible for skin cancer and cataracts and is dangerous for unicellular organisms. [...] In the most realistic case presented in Melott et al. (2017), O₃ column density is reduced by 15%–40% depending on latitude . Under these conditions, UVB irradiance is increased by about 50% on average (locally higher or lower), increasing skin damage (sunburn, known as ‘erythema’) and skin cancer risk by a similar factor.”

– The extra radiation would also kill a lot, if not most of the plankton in the oceans that live near the surface and are the basis for the marine food chains – leading to a mass extinction.

The effects of radiation on plankton are expected to appear at distances of 150-300 light years and should be significantly worse at shorter distances.

#Melott, A. L. (2018): Terrestrial effects of moderately nearby supernovae. Lethaia, Vol. 51 (3)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6027649/
Quote: “Marine phytoplankton productivity would be reduced, but the magnitude varies widely depending on species. For species commonly found in today’s Antarctic ocean, reductions of 5–20% can be expected.”

– Worse still supernova radiation would ionize gas in the atmosphere, which means that it would punch through molecules and knock electrons off nuclei, leaving them charged. These charged nuclei then act as seeds for water vapor to gather and form massive global clouds.

Cosmic rays and their ionizing effects have been long thought to increase the amount of “cloud condensation nuclei” (the “seeds” of clouds). The general principle is explained here:

#Svensmark, H. et al. (2017): Increased ionization supports growth of aerosols into cloud condensation nuclei. Nature Communications, Vol. 8 (2199)
https://www.nature.com/articles/s41467-017-02082-2
Quote: “Ions produced by cosmic rays have been thought to influence aerosols and clouds. In this study, the effect of ionization on the growth of aerosols into cloud condensation nuclei is investigated theoretically and experimentally. We show that the mass-flux of small ions can constitute an important addition to the growth caused by condensation of neutral molecules. [...] Ion-induced condensation should be of importance not just in Earth’s present day atmosphere for the growth of aerosols into cloud condensation nuclei under pristine marine conditions, but also under elevated atmospheric ionization caused by increased supernova activity.”

And scientists think that in the past, some nearby supernova might have increased the cloud cover by the process explained above, cooling the earth’s surface:

#Clery, D. (2016): Earth barraged by supernovae millions of years ago, debris found on moon. Science
https://www.science.org/content/article/earth-barraged-supernovae-millions-years-ago-debris-found-moon
Quote: “Some have suggested that particles from supernovae could have increased cloud cover, cooling the Earth's surface.”

– In the worst case they would reflect enough sunlight to trigger an ice age. In fact, it's thought that the ice age 2.5 million years ago was caused by a supernova.

Not all scientists agree on the actual reasons for the Pleistocene glaciation 2.6 million years ago. However, some astrophysicists have linked the nearby supernova events revealed by the deposits of radioactive iron ⁶⁰Fe to the start of that glaciation, as it was argued in this article in Nature a few years ago:

#Wallner, A. et al. (2016): Recent near-Earth supernovae probed by global deposition of interstellar radioactive ⁶⁰Fe. Nature, Vol. 532 (7597)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4892339/
Quote: “Our broad and global ⁶⁰Fe-influx on Earth demonstrates recent (<10 Myr) and wide-spread massive-star ejections in our near galactic neighborhood (<100pc), most likely from SN-explosions. Interestingly, the older event coincides with a strong increase in ³He and temperature change ~8 Myr BP³⁰ [before present], while the more recent activity starting ~3 Myr BP occurred at the same time as Earth’s temperature started to decrease during the Plio-Pleistocene transition.”

– Some scientists even think that a supernova about 60 light years away might have been the cause for the Devonian mass extinction 350 million years ago.

The Late Devonian extinction, about 350 million years ago, was one of the five major mass extinctions that have impacted the history of life on our planet. The cause of the Devonian extinction is not completely clear, but some findings have pointed at a significant depletion of the ozone layer. This in turn has been recently linked to supernova explosions happening at about 60 light years.

#Fields, B. et al. (2020): Supernova triggers for end-Devonian extinctions. Proceedings of the National Academy of Sciences of the United States of America, Vol. 117 (35)
https://www.pnas.org/doi/abs/10.1073/pnas.2013774117
Quote: “The Late Devonian was a protracted period of low speciation resulting in biodiversity decline, culminating in extinction events near the Devonian–Carboniferous boundary. Recent evidence indicates that the final extinction event may have coincided with a dramatic drop in stratospheric ozone, possibly due to a global temperature rise. Here we study an alternative possible cause for the postulated ozone drop: a nearby supernova explosion that could inflict damage by accelerating cosmic rays that can deliver ionizing radiation for up to ∼100 ky. We therefore propose that the end-Devonian extinctions were triggered by supernova explosions at ∼20pc, somewhat beyond the “kill distance” that would have precipitated a full mass extinction.”

– But wait, there is more. The electrons punched free by the radiation, form enormous electric avalanches – or in other words: lightning. Earth is hit by some of the worst thunderstorms in millions of years. The intense lightning causes global wildfires that consume forests and crops, devastate cities, disrupt our electrical grids and global supply chain. All while a decimated ozone layer leaks deadly radiation.

The possibility of SN-induced radiation leading to increased lightning and global wildfires has been put forward in this recent work:

#Melott, A. L. (2019): From Cosmic Explosions to Terrestrial Fires? The Journal of Geology, Vol. 127 (4) https://arxiv.org/abs/1903.01501
Quote: “Multiple lines of evidence point to one or more moderately nearby supernovae, with the strongest signal at ∼2.6 Ma. We build on previous work to argue for the likelihood of cosmic ray ionization of the atmosphere and electron cascades leading to more frequent lightning and therefore an increase in nitrate deposition and wildfires. [...] The wildfires would have contributed to the transition from forest to savanna in northeast Africa, long argued to have been a factor in the evolution of hominin bipedalism.”

– A supernova closer than 25 light years means that we’re in its ‘kill radius’ where a mass extinction is all but guaranteed.

The so-called “kill radius” is the distance at which a supernova explosion is expected to cause a mass extinction event. The idea was put forward almost 30 years ago in this work, and the kill radius was set at about 10 parsecs (roughly 30 light years).

#Ellis, J. & Schramm, D. N. (1995): Could a nearby supernova explosion have caused a mass extinction? Proceedings of the National Academy of Sciences of the United States of America, Vol. 92 (1)
https://www.pnas.org/doi/abs/10.1073/pnas.92.1.235
Quote: “We examine the possibility that a nearby supernova explosion could have caused one or more of the mass extinctions identified by paleontologists. A supernova explosion of the order of 10 pc away could be expected as often as every few hundred million years and could destroy the ozone layer for hundreds of years, letting in potentially lethal solar ultraviolet radiation. In addition to effects on land ecology, this could entail mass destruction of plankton and reef communities, with disastrous consequences for marine life as well.”

– Probably about half of the ozone layer would be destroyed, and massive climatic disruption on a scale we have never witnessed would ravage earth.

Ozone depletion from a nearby SN explosion would be caused by two distinct types of radiation: gamma rays, which would last a few hundred days, and cosmic rays, which would last for 10-20 years. The damage to the ozone layer has been modeled for different distances. After taking into account both types of radiation, it has been found that a supernova at 8 parsecs (26 light years) would destroy about 50% of the ozone layer. Such a depletion would double the amount of harmful UV radiation reaching the earth’s surface:

#Gehrels, N. et al. (2003): Ozone Depletion from Nearby Supernovae. The Astrophysical Journal, Vol. 585 (2)l
https://iopscience.iop.org/article/10.1086/346127/meta
Quote: “Our primary finding is that a core-collapse SN would need to be situated approximately 8 pc away to produce a combined ozone depletion from both gamma rays and cosmic rays of 47%, which would roughly double the globally averaged, biologically active UV reaching the ground.”

– Entire ecosystems would swiftly be wiped out by radiation, as global wildfires envelop the planet. All the things described before happen, but way more intensely and much faster. A few people might survive for years in bunkers, if they have food supplies, but the world they return to will be devastated and hostile to life for hundreds of housands of years. Human extinction is extremely likely.

The time the biosphere might need to recover after a mass extinction event is not known and very different estimates ranging from hundreds to millions of years have been published by different authors. However, a recent analysis of the ecosystems recovery after the Chicxulub impact that killed the dinosaurs has suggested recovery times of the order of a few million years.

#Alvarez, S. A. et al. (2019): Diversity decoupled from ecosystem function and resilience during mass extinction recovery. Nature, Vol. 574 (7777)
https://eprints.soton.ac.uk/434760/1/reboot_accepted.pdf
Quote: “The Chicxulub bolide impact 66 million years ago drove the near-instantaneous collapse of ocean ecosystems. [...] The absence of sufficiently detailed biotic data that span the post-extinction interval has limited our understanding of how ecosystem resilience and biochemical function was restored; estimates6,7,8 of ecosystem ‘recovery’ vary from less than 100 years to 10 million years. Here, using a 13-million-year-long nannoplankton time series, we show that post-extinction communities exhibited 1.8 million years of exceptional volatility before a more stable equilibrium-state community emerged that displayed hallmarks of resilience.”

– Being any closer to a supernova is very unlikely because space is big. But the effects would be extreme. Even from 4 light years away, the distance to Alpha Centauri, a supernova would be almost as bright as the sun in the sky.

The brightness of the sun as seen from the earth is given by the intrinsic luminosity of the sun (about 4·10³³ erg/s) divided by the surface area of the sphere defined by the earth’s orbit (R = 1.5·10¹¹ m)

Flux (Sun) = 4·10³³ erg/s / 4π(1.5·10¹¹ m)² = 1.4·10¹⁰ erg/s/m²

As explained above, a supernova explosion can have a peak luminosity between 10⁴³ and 10⁴⁵ erg/s. If we consider a typical (10⁴³ erg/s) or strong (10⁴⁴ erg/s) peak luminosity at a distance of 4 light years (3.8·10¹⁶ m), we get a brightness of

Flux (SN, 4ly) = 10^{43 – 44} erg/s / 4π(3.8·10¹⁶ m)² = 5.5·10^{8 – 9} erg/s/m²

Rounding off, this is between 2-20 times fainter than the sun depending on the strength of the supernova explosion. For any practical purposes, this basically means having a second sun in the sky (just for comparison, the full moon is about 400,000 times fainter than the sun).

– While casting two shadows could be fun for a few hours, within days the earth’s surface gets as hot as a sauna, baking the surface for weeks until the explosion fades.

A very rough estimate of the temperature reached on the earth’s surface can be made by modeling the earth as a black body (slightly corrected to take into account greenhouse effects) illuminated by the sun and the SN and computing its (new) equilibrium temperature. This is the standard method used to estimate the surface temperature of an exoplanet being heated by its star.

The standard calculation is easy to perform and it’s explained e.g. here:

#Mihos, C. (retrieved 2022): Equilibrium Temperatures of Planets. Department of Astronomy, Case Western Reserve University
http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/equiltemp.html

In terms of the luminosity of the sun, L_SUN, the equilibrium temperature T_EQ of the earth’s surface is given by:

T_EQ⁴ = k · L_SUN

where k is some constant. In normal circumstances, the earth’s average surface temperature is about 15 degrees Celsius, or 288 K:

#NASA (2022): Solar System Temperatures
https://solarsystem.nasa.gov/resources/681/solar-system-temperatures/

If a SN explodes so close to us that it actually looks like “another sun in the sky”, its effect can be approximated by considering a “sun” twice as luminous than ours. The new equilibrium temperature will therefore be:

T_EQ^NEW = (k · 2L_SUN)^1/4 = 2^1/4 · 288 K ≈ 342 K ≈ 68 ºC

i.e. a temperature more typical of a sauna.

– So should you worry? No! Fortunately, there are only a handful of stars that may explode within 1000 lightyears of earth and none are close enough to be a serious threat. Even better, these stars will probably not go supernova for many millions of years.

Just a few stars at distances below 1000 light years (300 parsecs) are known to be supernova candidates, and none of them is expected to pose an immediate danger to earth. The closest one is IK Pegasi, located at 150 light years (46 parsecs).

#Firestone, R. B. (2014): Observation of 23 supernovae that exploded <300 pc from Earth during the past 300 kyr. The Astrophysical Journal, Vol. 789 (1)
https://iopscience.iop.org/article/10.1088/0004-637X/789/1/29
Quote: “The explosion of a near-Earth SN in the future would not be surprising and appears to be overdue. At least six stars <300 pc from Earth may be considered as SN candidates and are listed in Table 4. IK Pegasi is a binary star system 46 pc from Earth that may evolve into a Type 1a SN. The others should all evolve into Type II SN. None of these potential near-Earth SNe are likely to cause immediate danger to life on Earth, although there appears to be a strong correlation between increased cosmic radiation and global warming that could prove important in modern times should another near-Earth SNe occur."

For IK Pegasi (the closest SN candidate) and Betelgeuse (which may be the first one to explode), the lifespans before going supernova have been estimated in about 2 billion years (IK Pegasi) and in up to 100,000 years and 2 million years (Betelgeuse).

#Beech, M. (2011): The past, present and future supernova threat to Earth’s biosphere. Astrophysics and Space Science, Vol. 336
https://link.springer.com/article/10.1007/s10509-011-0873-9
Quote: “The evolutionary model timescale indicate that IK Pegasi will become a Type Ia supernova some 1.9 Gyr from the present. [...] As noted earlier, Harper et al. (2008) have argued that Betelgeuse had an initial mass of order 20 M, and accordingly, we might well expect Betelgeuse to undergo core-collapse anytime within the next 2 million years.”

#Dolan, M. M. et al. (2016): Evolutionary tracks for Betelgeuse. The Astrophysical Journal, Vol. 819 (1)
https://iopscience.iop.org/article/10.3847/0004-637X/819/1/7
Quote: “Our best guess is that the star will supernova in less than ∼100,000 yr (even longer in the EG model)”

The approximate distances (in light-years) and locations (in equatorial coordinates) of these near-Earth supernova candidates are as follows:

Star Distance RA Declination

IK Pegasi 150 light-years 21h 26m +19º 22’

Spica 260 light-years 13h 25m –11º 09’

Alpha Lupi 460 light-years 14h 41m –47º 23’

Antares 550 light-years 16h 29m –26º 25’

Betelgeuse 640 light-years 05h 55m +07º 24’

Rigel 860 light-years 05h 14m –08º 12’

Where the celestial coordinates have been taken from this database:

#SIMBAD Astronomical Database, Centre de Données Astronomiques de bStrasbourg
http://simbad.cds.unistra.fr/simbad/sim-fbasic