Universe Today

Tycho's Supernova Remnant. Credit: Spitzer, Chandra and Calar Alto Telescopes.

Universe Today has a Guide to Space article on supernovae, one titled supernova, and one called exploding star, a common synonym for supernova (click the links to get to them).

Here, I'll introduce some of the most famous, and not so famous, supernovas.

Supernovas have, no doubt, been seen by humans as far back as there were humans; after all, one is bright enough to be seen in daylight about once every millennium (if not more often). And they have been recorded, in contemporary records, since at least 185 AD; prior to the Renaissance, historical records (mostly Chinese, but some Japanese, Korean, and Arabic) pretty unambiguously identify five supernovas (with another half dozen or so candidates, not yet ruled in or out) – SN 185, SN 393, SN 1006, SN 1054, and SN 1181. Why is it so hard to be certain? Not only must ancient texts be properly understood (some were very cryptic, deliberately so), not only must the record be unambiguously interpreted as being of a new star in the sky, but the position must be sufficiently precise (or description sufficiently detailed) to permit reliable associations to be drawn between what we see there today and the historical record (supernovas leave very distinct remnants; novae, comets, and other celestial transients do not).

Before telescopes were used for astronomy, there were two more certain historical supernovas, SN 1572, and SN 1604.

SN 185 is the earliest, certain historical supernova; its remnant is (almost certainly) RCW 86.

SN 1006 is probably the brightest supernova ever seen (and recorded); this is the sole supernova unambiguously recorded in documents from Europe, prior to the Renaissance; the remnant was only identified in 1965 (though it had been catalogued, as PKS 1459-41, by radio astronomers earlier).

SN 1054 left us the most iconic supernova remnant, M1, the Crab Nebula; it is famous for quite a lot of things, in the history of modern astronomy (e.g. the first optical pulsar).

SN 1572, Tycho's supernova; it played an important part in the scientific revolution which moved the Sun to the center of the universe (not the Earth). The remnant also goes by the name of G120.1+01.4.

SN 1885A, or S Andromedae, is the only supernova yet seen in M31, and the first seen outside the Milky Way (perhaps this is the most famous supernova you've never heard of!).

SN 1987A is the only supernova most people reading this have ever seen, with their own eyes … even without binoculars or a telescope! Sadly, for those of you who were not able to travel far enough, it was visible only in the southern hemisphere (and the tropics), because the star which exploded was in the Large Magellanic Cloud. Among many firsts, this was the first supernova whose neutrino emissions were detected (and it remains the only such, today).

Yes, there are Universe Today stories on all these supernovas, more than one on most! Here is a sample: Astronomers Locate High Energy Emissions from the Crab Nebula, Hubble Sees a Ring of Pearls Around 1987 Supernova, 1000 Year-Old Supernova Remnant, and Survivor Found From Tycho's Supernova.

You'll find more on some of the supernovas I've mentioned here in the Astronomy Cast episodes Famous Stars, Nebulae, and Neutron Stars and their Exotic Cousins.

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Supernova 1054 AD

How bright are supernovae? Most are as bright as – or brighter than – all the other stars in the galaxy in which they explode … and this is so in all regions (wavebands) of the electromagnetic spectrum! In other words, as bright as a billion Suns. The historical supernovae were all in our own galaxy, the Milky Way, and were bright enough to be easily visible during the day, despite the fact that they were all many thousand light-years away (thank goodness!).

There are two different ways a star can go supernova: its core can collapse (this happens only to very massive stars), or the entire star can detonate (this happens only to some white dwarf stars).

In a core collapse supernova, all elements up to iron have been burned, and there's no more fuel to generate the heat to keep the star from collapsing in on itself. When the inevitable collapse begins, exceedingly violent nuclear reactions are triggered (basically all the remaining nuclear fuel burns at once), and the star is blown apart, leaving a neutron star and cloud of glowing gas. Here's a fun fact: the energy core collapse supernovae emit is almost all in the form of neutrinos; the light we see it trivial by comparison.

In a detonating white dwarf, matter dumped onto its surface by its close companion ignites and heats the rest of the star. Because a white dwarf is pretty much the same throughout (density, temperature, composition), just below its surface, if the surface spark is hot enough, the whole star blows up (all the carbon, nitrogen, and oxygen fuses, within a second or so), leaving nothing but a rapidly expanding cloud of very hot plasma (most of the time, a surface ignition of hydrogen is not enough to detonate the core, and we see a nova, or dwarf nova).

Further reading: Supernovae (NASA's Imagine the Universe), List of supernovae, (Harvard Smithsonian Center for Astrophysics), and What are Supernovae? (Caltech).

Things which go bang! usually get lots of press, so you'd expect Universe Today to have lots of articles on supernovae. Well, you'd be right! Here is a (very) small sample: Supernova Simulations Point to White Dwarf Mergers, Shapes Reveal Supernovae History, and Hunt for Supernovae with Galaxy Zoo.

Nucleosynthesis: Elements from Stars, Nebulae, and How Amateurs Can Contribute to Astronomy are three Astronomy Cast episodes in which supernovae play a star role; check them out!

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Crab Nebula

Astronomers have classified supernovae into two broad classifications: Type I and Type II. Type I supernovae occur in binary systems, where one star pulls off mass from a second star until it reaches a certain amount of mass. This causes it to explode as a supernova. Type II supernovae are the explosions of massive stars which have reached the end of their lives.

All of the elements heavier than iron were created in supernova explosions. As a massive star runs out of hydrogen fuel, it starts to fuse together heavier and heavier elements. Helium into carbon and oxygen. And then oxygen into heavier elements. It goes up the periodic table this way, fusing heavier elements until it reaches iron. Once a star reaches iron, it's no longer able to extract energy from the fusion process. The core collapses down into a black hole, and the material around it is fused together into the elements heavier than iron. If you're wearing any gold jewelry, that was created in a supernova.

In 1054 Chinese astronomers saw a supernova explosion that was so bright it was visible in the middle of the day. The explosion of gas and dust is now visible as the Crab Nebula (that's the picture at the top of this article). The most recent powerful supernova explosion occurred in 1987, when a star exploded in the Large Magellanic Cloud.

Astronomers use Type I supernovae to judge distances in the Universe. This is because they always explode with approximately the same amount of energy. When a white dwarf star collected approximately 1.4 times the mass of the Sun, it can't support its mass and collapses. This amount is called the Chandrasekhar Limit. When an astronomer sees a Type I supernova, they know how bright it is, and so they can measure how far away it is.

We've written many articles about supernovae for Universe Today. Here's an article about a slow motion supernova, and here's an article about a theoretical supernova that was actually found to exist.

If you'd like to see a gallery of supernova photographs, check out this section of the Hubble Space Telescope site, and here's NASA's World Book on supernovae.

We've also recorded several episodes of Astronomy Cast about supernovas. Check out this one, Episode 14: We're All Made of Supernovae.

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Images of the afterglow of GRB 090423 taken (left to right) with the Y, J, H and K filters. The absence of any flux in the Y filter is a strong indication that the GRB is very high redshift (Credit: A. J. Levan & N. R. Tanvir)

Although they were first observed in the late 1960s, by the US's Vela satellites, they remained top secret for nearly a decade (the Vela satellites were intended to detect secret nuclear weapons tests, and their capabilities, and findings, were classified), when a paper on them was published in the Astrophysical Journal (in 1973).

GRBs made astronomers intensely excited, and frustrated … it took until 1997 for the origin to be pinned down. Why? Because, first, it was very hard to locate them precisely enough on the sky (the 'error circles' were generally so big as to encompass dozens, if not hundreds, of possible sources); second, because when they were sufficiently well-localized, there was nothing there! Finally, BeppoSAX observed the x-ray afterglow of GRB 970228 fast enough (and precisely enough) for ground-based telescopes to detect (and localize) the optical afterglow … and later to discover there is a very, very faint galaxy there, one with a redshift of 0.695 (in the meantime, another GRB's host galaxy was detected, and as it is brighter, its redshift (0.835) was measured first).

Studying GRBs was also exciting, and frustrating, because their light curves (plot of intensity, of gamma rays, against time) are all over the place … some have many peaks, some only one; some rise quickly and fall just as quickly, others rise (or fall) slowly); etc, etc, etc. However, two classes of GRBs have been identified – short (and hard) and long (and soft), referring to the average duration (and the average spectral energy distribution; 'hard' means more energy in the shorter wavelength gammas than 'soft'), and only one GRB mystery was solved in the late 1990s (long GRBs were localized, but no short ones were). It wasn't until 2005 that a short GRB was localized well enough for a likely source to be identified … it too was a distant galaxy.

GRBs, especially the long kind, appear so bright because we are looking right into a narrow beam; long GRBs are most likely a particular kind of core-collapse supernova, in which two intense jets rip through the poles of the doomed star … we see the supernova itself only some time later (if at all). Short GRBs are still not well understood, but are most likely the merger of a pair of neutron stars (a close binary) or a black hole and a neutron star (also a close binary).

GRBs are the ultimate death ray … a GRB almost anywhere in our own galaxy, were its jets to be pointed at us, would instantly kill all life in the solar system, and, if close enough, vaporize the entire Earth (this scenario is one of the most interesting chapters in Phil Plait's book, Death from the Skies).

NASA has a simple intro to GRBs: Gamma Ray Bursts: Introduction to a Mystery.

There are LOTS of Universe Today articles on GRBs; here are a couple: More Observations of GRB 090423, the Most Distant Known Object in the Universe, and Closer, Dimmer Gamma Ray Burst Spotted.

Astronomy Cast has an episode dedicated to GRBs, Gamma-Ray Bursts; it's well worth a listen.

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A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A. Credit: NASA/CXC

Supernova remnant Cassiopeia A (Cas A) has always been an enigma. While the explosion that created this supernova was obviously a powerful event, the visual brightness of the outburst that occurred over 300 years ago was much less than a normal supernova, — and in fact, was overlooked in the 1600's — and astronomers don't know why. Another mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. But in 1999, astronomers discovered an unknown bright object at the core of Cas A. Now, new observations with the Chandra X-Ray Observatory show this object is a neutron star. But the enigmas don't end there: this neutron star has a carbon atmosphere. This is the first time this type of atmosphere has been detected around such a small, dense object.
Read more…

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Supernova 1054 AD

First, there's no possible way that the Sun will ever explode. It might seem huge to us, but the Sun is a relatively low mass star compared to some of the enormous high mass stars out there in the Universe. When our Sun runs out of hydrogen fuel, it will expand up as a red giant, puff off its outer layers, and then settle down as a compact white dwarf star; slowly cooling down for trillions of years.

But let's say that our Sun has about 10 times as much mass. Now we're talking explosion. When this super massive Sun runs out of hydrogen fuel in its core, it switches over to converting atoms of helium, and then atoms of carbon. It keeps consuming heavier and heavier fuel in concentric layers, like an onion. Each layer takes a shorter period of time, all the way up to nickel, which might take a mere day to burn through.

Then iron starts to build up in the core of the star. And iron doesn't give off any energy when it undergoes nuclear fusion. Because of this, the star has no more outward pressure in its core stopping it from collapsing inward. When about 1.38 times the mass of the Sun in iron collects at the core, it catastrophically implodes, releasing an enormous amount of energy.

Within 8 minutes, the amount of time it takes for light to travel from the Sun to Earth, an incomprehensible amount of energy would sweep past the Earth, and destroy everything in the Solar System. Supernovae can briefly shine more than an entire galaxy. A new nebula, like the Crab Nebula, would be visible to nearby star systems, expanding outward for thousands of years.

All that would remain of the Sun would be a rapidly spinning neutron star, or maybe even a stellar black hole.

But remember… the Sun has too little mass to ever explode. It won't explode, so don't worry.

We have written many articles about exploding stars for Universe Today. Here's an article about a white dwarf that's very close to exploding as a supernova. And here's an article about a supernova story.

You can see how this question has already been answered by NASA's Ask an Astronomer.

We have recorded many episodes of Astronomy Cast about exploding stars. Here's one, Episode 14: We're all Made of Supernovae.

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Crab Nebula. Credit: NASA/ESA

The supernova which gave rise to the Crab Nebula was seen widely here on Earth in 1054 (and so it's called SN 1054 by astronomers); it is perhaps the most famous of the historical supernovae. It is certainly one of the brightest (estimated to be –7 at peak), partly because it is so close (only 6,300 light-years away), and partly because it's not hidden by dust clouds. The expansion of the nebula – as in seen-to-be-getting-bigger, rather than the-gas-is-moving-very-fast – was first confirmed in 1930.

As it was a core collapse supernova (a massive star which ran out of fuel), it left behind a neutron star; by chance, we are in line with its 'lighthouse beam', so we see it as a pulsar (all young neutron stars are pulsars, but not all of them have beams which point to us in one part of the cycle). It's a pretty fast pulsar; the neutron star rotates once every 33 milliseconds. Because it's so young and so close, the Crab Nebula pulsar was the first to be detected in the visual waveband, and also in x-rays and gamma rays. Being the source of the tremendous output of energy, from both the pulsar wind nebula and the pulsar itself, and as energy is conserved, the pulsar is slowing down, at a rate of 15 microseconds per year.

The inner part of the Crab Nebula, the pulsar wind nebula, contains lots of really hot ('relativistic') electrons spiraling around magnetic fields; this creates the eerie blue glow … synchrotron radiation. This makes the Crab Nebula one of the brightest objects in the x-ray and gamma ray region of the electromagnetic spectrum, and as it is a relatively steady source (unlike most high energy objects) it has given its name to a new astronomical unit, the Crab. For example, a new x-ray source may be 2 mCrab (milli-Crab), meaning 0.002 times as strong an x-ray source as the Crab Nebula.

This SEDS page has a lot more information on the Crab Nebula, both historical and contemporary.

Such an intensively studied object, no wonder there are lots of Universe Today stories on it; for example Nearly a Thousand Years After the Death of a Star, Giant Hubble Mosaic of the Crab Nebula, The Peculiar Pulsar in the Crab Nebula, Astronomers Locate High Energy Emissions from the Crab Nebula, and Evidence of Supernovae Found in Ice Core Sample.

Astronomy Cast's Neutron Stars and Their Exotic Cousins has more on pulsars, and Nebulae more on nebulae.

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SN 1987A Credit: Hubble Space Telescope (2006)

Ian Shelton, then a research assistant with the University of Toronto, working at the university's Las Campanas station, and Oscar Duhalde, a telescope operator at Las Campanas Observatory, were the first to spot it, on the night of 23/24 February 1987 (around midnight actually); over the next 24 hours several others also independently discovered it.

The IAU's CBAT went wild! That's the International Astronomical Union's Central Bureau for Astronomical Telegrams, the clearing house for astronomers for breaking news. You can read the historic IAUC (C for Circular) 4416 here.

Once the discovery of SN 1987A became known, physicists examined the records from various neutrino detectors … and found three, independent, clear signals of a burst of neutrinos several hours before visual discovery, just as predicted by astrophysical models! Champagne flowed.

Not long afterwards, the star which blew up so spectacularly – the progenitor – was identified as Sanduleak -69° 202a, a blue supergiant. This was not what was expected for a Type II supernova (the models said red supergiants), but an explanation was quickly found (Sanduleak -69° 202a had a lower-than-modelled oxygen abundance, affecting the transparency of its outer envelope).

The iconic Hubble Space Telescope image (above) of SN 1987A shows the inner ring, where the debris from the explosion is colliding with matter expelled from the progenitor about 20,000 years ago; more from the Hubble here.

AAVSO (American Association of Variable Star Observers) has a nice write-up of SN 1987A.

No wonder, then, that SN 1987A features so often in Universe Today stories; for example Supernova Shockwave Slams into Stellar Bubble, XMM-Newton's View of Supernova 1987A, Supernova Left No Core Behind, and Hubble Sees a Ring of Pearls Around 1987 Supernova.

SN 1987A figures prominently in Astronomy Cast The Search for Neutrinos, and in Nebulae.

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Subrahmanyan Chandrasekhar (credit: University of Chicago Press)

White dwarfs are the end of the road for most stars; once they have used up all their available hydrogen 'fuel', low mass stars shed their outermost shells to form planetary nebulae, leaving a high density core of carbon, oxygen, and nitrogen (that's a summary, it's actually a bit more complicated). The star can't collapse further because of electron degeneracy pressure, a quantum effect that comes from the fact that electrons are fermions (technically, only two fermions can occupy a given energy state, one spin up and one spin down).

So what happens in the core of a massive star, one whose core weighs in at more than 1.4 sols? As long as the star is still 'burning' nuclear fuel – helium, then carbon etc, then neon, then … – the core will not collapse because it is very hot (electron degeneracy pressure won't hold it up 'cause it's too massive). But once the core gets to iron, no more burning is possible, and the core will collapse, spectacularly, producing a core collapse supernova.

There is a way a white dwarf can go out with a bang rather than a whimper; by getting a little help from a friend. If the white dwarf has a close binary companion, and if that companion is a giant star, some of the hydrogen in its outer shell may end up on the white dwarf's surface (there are several ways this can happen). The white dwarf thus adds mass, and every so often the thin hydrogen envelope blows up, and we see a nova. One day, though, the extra mass may put it over the limit, the Chandrasekhar limit … the temperature in its center gets high enough that the carbon 'ignites', the 'flame' spreads throughout the star, and it becomes a special kind of supernova, a Ia supernova.

For more technical details of the Chandrasekhar limit, Richard Fitzpatrick of the University of Texas at Austin has an online Thermodynamics & Statistical Mechanics course, which includes a page on the Chandrasekhar limit.

Supernovae are very important to astronomy, so you won't be surprised to learn that there are lots of Universe Today stories on the Chandrasekhar limit! Some examples: White Dwarf Theories Get More Proof, White Dwarf "Close" to Exploding as Supernova, and Colliding White Dwarfs Caused a Powerful Supernova.

Astronomy Cast Episode 90 (The Scientific Method) includes a look at how Chandrasekhar worked out the limit that now bears his name, and Where Do Stars Go When They Die? also covers this topic.

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An exploding star, also known as a supernova, is one of the most majestic events in the entire universe. At the climax of this event, it can outshine even an entire galaxy.

In some cases, this cataclysmic event (including its fading) can last up to many years.

There are two main types of supernovae: Type I and Type II. These two are then subdivided into subclasses like Type Ia, Ib, and Ic (for Type I) or Type IIP and Type IIL (for Type II).

A Type I supernova can happen to some binary star systems (star systems made up of two stars). On the other hand, a Type II can happen to a very massive star, typically much more massive than the Sun.

Some exploding stars, like the type Ib, Ic and II supernovae, are accompanied by a burst of high energy neutrinos. These neutrinos travel through space and may be detected here on Earth. It is for this reason that observatories like the Super-Kamiokande and the Sudbury Neutrino Observatory were constructed.

When exactly a supernova can occur is hard to predict. Because of this, scientists can only start studying them when they have already occurred. This is where neutrinos play an important role. Detection of neutrinos in the observatories above may indicate the presence of an exploding star somewhere in the Universe.

Because an exploding star can really stand out in the sky (due to its extreme brightness), early astronomers were able to observe them. The earliest recorded observation of an exploding star was made by Chinese astronomers in 185 AD.

When a supernova occurs near the Earth, its gamma rays can deplete the ozone layer and subsequently permit solar and cosmic radiation to pass through. Some scientists believe its effects are so deadly that it may have caused the end Ordovician extinction wherein 60% of the living organisms found in the ocean perished.

It has been suggested that when an exploding star is located within 100 light years away, it can pose great risks to our planet. A number of stars in our own galaxy, the Milky Way, are believed to be approaching supernovae status.

Rho Cassiopeiae, Eta Carinae, RS Ophiuchi, U Scorpii, HD 179821, VY Canis Majoris, Betelgeuse, Antares, and Spica, are among those that may turn into supernovae in the near future. No need to be alarmed right now though. We actually mean a minimum of several thousands of years when we say 'near future' for the stars mentioned above.

We have some related articles here in Universe Today. Here are the links:

• Neutrino
• Computer to Simulate Exploding Star

Here are the links of two more articles from NASA:

• Supernova
• Bighter than an exploding star, it's a hypernova!

Here are two episodes at Astronomy Cast that you might want to check out as well:

• We're all Made of Supernovae
• The Search for Neutrinos

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Super-Kamiokande, a neutrino detector in Japan, holds 50,000 tons of ultrapure water surrounded by light tubes (Super-Kamiokande)

Since this was already in the 1930's the equivalence of mass and energy, more popularly known as E = mc², was already known. This prompted Wolfgang Pauli to propose that the missing energy must have been released in the form of a third particle, which was dubbed by Enrico Fermi as 'neutrinos'. In 1953, Fred Reines and Clyde Cowan finally detected neutrinos, which earned them the Nobel Prize.

Neutrinos are naturally produced as part of beta decay processes of radioactive substances in the soil, and can be found in the decay chains of Uranium-238 and Thorium-232 isotopes. Beta decay processes are those that involve the release of electrons a.k.a. beta particles. Other decay processes involve the release of alpha (Helium nuclei) and gamma (photons) particles. 

Neutrinos are also released when cosmic rays collide with nuclei found in the atmosphere. Since cosmic particles carry a great deal of energy, the bombarded nuclei become unstable and subsequently release neutrinos upon decay. Other sources include nuclear fusions in the sun, supernovae, and of course, nuclear reactors and particle accelerators.

Finding neutrinos is relatively a tall order because they interact very weakly. As such most neutrino detectors are constructed underground. This is done to reduce the possibility of detecting cosmic rays and other background radiation instead. 

The detectors are typically very large to ensure collection of a substantial amount of neutrinos, i.e., substantial enough to allow detection. Examples of such detectors are the Super Kamiokande (or Super K) of Japan and the Sudbury Neutrino Observatory (SNO) of Canada.

Neutrino detection aids scientists to probe into the depths of outer space that are unreachable by other techniques such as those using visible light or even radio waves. For example, during a supernova, the cataclysmic demise of a star, only a few particles are able to escape to tell the tale, save for the neutrinos. In fact, 99% of of a supernova's energy is released via a torrent of neutrinos. 

Thus, when a great amount of neutrinos is detected, it could mean that a star might have exploded. This phenomena has served as the motivation for projects like the Supernova Early Warning System (SNEWS), which makes use of a network of neutrino detectors. 

We've got a good collection of neutrino-related articles here in Universe Today. Care to read about solar neutrinos? Want to know how neutrino detection confirms Big Bang predictions?

There's also two more at Physics World. One's entitled Neutrinos: ghosts of matter. The other one's Neutrino mass discovered.

Of course, if your eyes are too tired to read, then you can also listen to these two episodes at Astronomy Cast:

• Hidden Fusion, the Speed of Neutrinos, and Hawking Radiation
• The Search for Neutrinos

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