Supernovae: Cosmic Catastrophes
this next song i'm going to play is about one of these pop festivals that they've been having around the world lately in april 1970 folk singer johnny mitchell released a song named woodstock about a festival which she didn't go to in the chorus there is a lyric which has a particular relevance to our topic today we are stardust it sounds like a cliche but as it happens from a scientific perspective she was actually correct we are indeed stardust as in the cosmic dust that was present in the cloud that formed the solar system everything and everyone on this planet is made from atoms which were once floating around in space and in the case of the solar system something woke the cloud and seeded it with life-giving elements so what was that thing well it was an explosion our galaxy and all galaxies in space have a particular chaotic mechanism which drives the evolution of their stars a cosmic pollinator if you would for enriching the galaxy one which can shape worlds and destroy them alike a supernova the term supernova was coined in the 1930s it is an extension of the word nova which means new in latin a nova is a type of stellar event in which a new luminous object is created by binary stars which fades away slowly after a supernova follows a similar pattern in luminosity a brief bright peak followed by an extended dimming over months or years the difference however is that a supernova is caused by a catastrophic explosion one which destroys the object initiating it rather than creating one supernovae occur either when a very massive star has reached the end of its energy generating cycle or when the interactions of binary stars lead to unmitigated thermal nuclear reactions either way an apocalyptic explosion ensues in which an incredible amount of radiation is released from radio waves to x-rays to destructive gamma rays which pulverize the star's surroundings wreaking havoc on planetary systems within about 50 light-years of the epicenter the good news is that lone dwarf stars like the sun are not massive enough to explode in this way only stars with a mass about eight times that of the sun will be able to initiate the extreme gravitational and nuclear reactions needed when a massive star can no longer keep itself held up its layers collapse at tremendous speeds resulting in an explosion billions of times more luminous than the sun lighting up its surrounding galaxy and shining for millions of light years across space considerable amounts of the dying star's material equivalent to at least five times the mass of our sun are ejected outwards with unshakable force creating a shock wave that carries material for light years into its surrounding interstellar space invariably leaving behind a bright and beautiful nebula this material floods the interstellar medium gas clouds between stars with heavy elements released from the departed star's core creating a richer more fertile environment for new generations of stars than the one that the star in question was born into and supernovae were known long before we understood their underlying causes there have been many cases throughout history of naked eye supernovae which caught the attention of ancient observers perhaps the most significant came in the year 1006 when an awesome supernova dubbed sn1006 dazzled in the sky for a few weeks at its peak it would have been a hundred times brighter than the planet to venus in the sky strong enough to shine during the day and cast shadows at night thought to be the brightest supernova from earth in recorded history it was greeted with astonishment and anxiety alike by observers all over the northern hemisphere the japanese described it as white and blue in appearance while the chinese referred to it as a guest star astronomers have since scoured historical records to ascertain the location of this supernova tracing it to somewhere within the constellation of lupus and in that region we do indeed find a nebulous remnant which appears to be around a thousand years old another significant supernova came around half a century later when a massive star in the horn of the constellation taurus exploded it wasn't nearly as bright as sn1006 being estimated to be only as bright as the planet's jupiter in the night sky but unlike our neighboring planet the supernova remained visible during the day attracting the attention of chinese korean and possibly even central american observers the remnants of that explosion are well known to us today as one of the most studied objects in our local neighborhood of space the crab nebula a glowing cloud of supernova ejector lying around 6 000 light years from the earth initially it was thought that these events were the birthing of new stars hence the name meaning new in latin but by the 1930s it was understood that these transient phenomena were the result of stellar explosions the class supernova was coined in lectures and papers to refer to a new class of nova in which the star was destroyed in the process our understanding of these cosmic catastrophes has only improved from there but there is a problem supernovae are quite rare on human timescales at least it is estimated that they should occur between every 50 to 100 years within our galaxy but sadly we haven't managed to catch one occurring within the milky way galaxy since the invention of the telescope nevertheless we have managed to see a great many supernovae occurring in other galaxies however there is no meaningful way to predict where one might occur next and their vast cosmic distances make their intrinsic properties difficult to constrain however all of this changed in 1987 when a supernova occurred barely a stone's throw away from the milky way galaxy on a cosmic scale anyway the large magellanic cloud is a torrid gaseous dwarf galaxy lying a mere 160 000 light years from the earth in a fairly unobstructed position of the sky as such many of its hundreds of thousands of massive supergiant stars are a great deal more visible than some of those in our own galaxy and so when a supernova illuminated the cloud in the late 80s astronomers were able to get their closest and best look at the phenomenon deducing a great deal of new information about the properties of these explosions what's more is that thanks to previous efforts to study this satellite galaxy we knew what kind of star sn1987a was before it went supernova specifically a luminous blue supergiant this helped us to combine our theories with actual observations allowing for a much deeper understanding of both the prerequisites and aftermath of these explosions and sn1987a remains to this day the last supernova that was visible from earth with the unaided eye finding and classifying supernovae used to be difficult and tedious we relied almost entirely on amateur astronomers and volunteers to sieve through contrasting photographs of space looking for any new luminous sources that may constitute a supernova thankfully automated computer systems and improved surveying technologies have since removed a lot of the legwork we can now survey millions of stars and billions of galaxies notwithstanding the fact that we haven't seen a supernova occurring within the milky way for over four centuries we are now discovering hundreds of new extra galactic supernovae every year some which lie deep into the observable universe at a first glance these explosions all seem fairly similar but if we take spectroscopic measurements of the explosion while it's happening the presence of certain elements can be determined that's how we classify supernovae based on their prevalence of hydrogen the lightest and most common element in the universe type 1 supernovae do not show any hydrogen in their spectrums whereas type 2 supernovae do show hydrogen we can also break these two classes down further based on the behavior of the supernova's light curve or by the presence of certain heavy elements this system of classification was introduced in the 1940s by physicists fritz vicki and rudolf minkowski back then they had nothing to work with inside our galaxy the only supernovae they could classify were ultra bright extra galactic supernovae and thus our first type type ones are actually the more luminous and rare type whereas type 2s are your classic massive star collapsing under the weight of its own gravity therefore we will begin with type 2 supernovae explosions caused by stars at the end of their lives which show hydrogen in their spectra as we mentioned only stars more than eight times the mass of the sun are heavy enough to explode below this threshold a star will not be able to compress its core to the level required to initiate a supernova stars like the sun fuse their core hydrogen fuel they are born with into helium and the helium into carbon but they won't proceed much further beyond that more massive stars however continue to work their way up the fusion chain gradually fusing heavier elements at ever increasing temperatures each time an elemental fuel source is exhausted the star briefly contracts making its nuclear furnace hotter and denser eventually the carbon supply in the core is fused into oxygen neon and magnesium once carbon is exhausted the process repeats and the star contracts again this time fusing the oxygen and other elements into heavier elements still like silicon sulfur calcium and argon then at an even greater level of compression these elements can be fused to form iron and this is where the energy generating phase of the star's cycle ends up until now the fusion of elements within the star's core has always produced energy atoms give up a little bit of their binding energy each time they are crushed into a stabler more tightly bound element and this energy maintains the star's tremendous size however iron is the most tightly bound and stable nuclei there is and so any attempt to change this element's nuclear structure will require energy from somewhere instead of releasing it thus any nuclear reactions beyond the fusion of iron will remove energy from the star's core as the level of iron at the center builds up the energy released by fusion plummets and the star becomes incapable of keeping its enormous layers puffed up these layers then collapse suddenly and rapidly under gravity's grip flying towards the core at speeds of around 23 to the speed of light in a matter of seconds a star dozens of times the diameter of the sun recedes into a ball of crushed subatomic particles the size of a city however these subatomic particles don't want to be crushed together the free roaming electrons within the increasingly compressed layers of the star begin exerting a force which pushes back against the collapse but this alone will not be enough to stop it once the core's density exceeds several hundred billion times the density of water negatively charged electrons begin being crushed into atomic nuclei where they merge with positively charged protons to create non-charged neutrons the more electrons are forced into atoms the less electron pressure there is to counter the collapse which results in more electrons succumbing to pressure in a snowballing process eventually with the density of the core equivalent to that of an atomic nucleus atoms are so bursting with neutrons that they begin to spill out and escape like toothpaste being forced out of a tube much like electrons free roaming neutrons really don't want to be compressed and they exert a much stronger repelling force than the electrons which abruptly halts the collapse of the star when the core is between 12 and 25 kilometers wide at this point we have a neutron star if it is particularly heavy then it will continue to collapse into a black hole but either way the force exerted by the neutrons does manage to stop the collapse of the star at a certain stage and it is at this point that the chain reaction of a supernova begins the collapsing layers of the star are abruptly halted jolting the material and sending a powerful shock wave cascading in the opposite direction but this alone is not enough to cause the level of explosion we see there is another stronger force at work neutrinos neutrinos are a kind of subatomic particle which barely interact with matter and light and so you can think of them like little ghost particles floating around in the background of space these tiny units of energy are created every time protons and electrons merge into neutrons and the power they carry off with them is off the charts greater than the power of all the stars in a billion galaxies though these neutrinos do not ordinarily interact with mata under conditions this extreme with lighter matter coupled together they do interact slightly within the star depositing a small amount of their energy into the collapsed layers even a negligible portion of this total energy is enough to unleash a cosmic nuke resulting in a massive energy input that reverses the collapse driving the layers outwards with unbelievable force the resulting explosion is a type 2 supernova also known as a core collapse supernova which releases billions of sun's worth of energy in a matter of days freeing the heavy elements fused inside the star's core in the process elements vital for life like oxygen carbon and iron and even heavier elements than that as the explosion ensues swathes of newly formed neutrons flood through the expanding oceans of atomic nuclei and are captured via the r process synthesizing elements heavier than iron like gold and silver in the star's dying breaths type 2 supernovae can be broken down into a few classes which are classified based on the behavior of their light curves after the explosion exploding stars between 8 and 15 times the mass of our sun will result in a type 2p supernova p for plateau in this case due to the fact that this kind exhibits a period of sustained slow dimming before dropping off it is by far the most common class of type 2 accounting for more than half of all observed core collapse supernovae somewhat rarer is the type 2 linear supernova where stars greater than 15 times the sun's mass follow a steady linear decline after their initial peak in luminosity there's also the type 2b which shows a single weak hydrogen emission line owing to an old star's depleted hydrogen shell similarly there's the type 2 n n being for narrow in this case where thin or intermediate with hydrogen emission lines appear in the star's spectrum these are thought to be the result of luminous blue variable stars exploding within dense clouds of their own material next we come to type 1 supernovae supernovae which do not show hydrogen in their spectrums they come in three varieties the type 1a 1b and 1c much like type 2s type 1b and 1c are thought to be the result of core collapse but with an absence of hydrogen emissions therefore we refer to this kind as stripped core collapse supernovae both subgroups are believed to be the result of extremely massive unstable and eruptive wolf rayet stars volatile stars with high metallicities producing strong stellar winds which have blown away their lightest elements a type 1b supernova lacks hydrogen in its spectrum but shows helium whereas a type 1c supernova lacks both hydrogen and helium owing to the fact that it is a wolf rayet star at a more evolved stage where it has lost more of its light elements but broadly speaking these two types occur and proceed in a similar way to the type 2 supernovae of the type ones it is the first and foremost the type 1a where things really start to get interesting unlike the other kinds discussed type 1a supernovae are not the result of core collapse rather they occur in binary star systems with one or more white dwarf stars white dwarfs are another way in which stars end their lives as mentioned stars less than eight times our sun's mass won't end in a supernova they are not massive enough to work their way up the fusion chain instead fusion will subside around the production of carbon into oxygen and with no more energy to keep itself held together rather than exploding the outer layers of the star will peacefully stream away and disperse into a planetary nebula what remains of the star is its now exposed core an earth-sized sphere of crystallizing carbon and oxygen matter and thus we have a white dwarf white dwarfs are by themselves incredibly stable when our sun evolves into one it will stay luminous for another quadrillion years or so before it finally goes out however white dwarves don't always exist in isolation many evolve in old binary star systems where they have a massive companion then you have the ingredients for a type 1a supernova while stable by themselves all white dwarfs share a deadly hidden property above a certain mass known as the chandrasekhar limit a runaway thermal nuclear process will take hold that causes the white dwarf star to explode ejecting its superluminous carbon and oxygen matter along with silicon and iron which appears in its spectrum ordinarily white's dwarves don't usually gain the mass required to reach this level but with a late-stage stellar companion ejecting material nearby some of it will inevitably fall onto the white dwarf star and grow its mass then when it reaches this deadly threshold about 1.4 times the mass of the sun the star explodes in what is the brightest supernova there is so bright that they can outshine an entire galaxy from billions of light years across space because type 1a supernovae always occur at the same critical mass point they share a common peak luminosity this makes them a very useful indicator of cosmic distances if we take something with a known luminosity we can use it as a marker known as a standard candle to work out how far away that marker is by measuring its dimness and redshift many projects probed the depths of the universe using this technique throughout the 90s and early 2000s in an attempt to measure its rate of expansion previously scientists have predicted that the universe's expansion would slow down over time due to the gravity of the galaxies within pulling it back but when we started hunting for distant type 1a supernovae we found evidence of the opposite the most remote type 1a supernovae seem to be considerably farther away and dimmer than they should be in a universe which is slowing down rather it's as if the universe has expanded more in the recent half of its life and that its expansion is accelerating a far cry from previous theories type 1a supernovae led to the revelation that the universe is filled with a mysterious anti-gravity force known as dark energy which is crucial to our current ideas on the behavior of the large-scale universe fritz spicky did propose further classes of supernova the type 3 type 4 and type 5 based on certain individual instances of these explosions which did not fit the agreed characteristics of the first two however it is thought that these classes are actually the result of certain imposter nova events which mimic supernova traits generally our type ones and our type 2s are our main classes understood via the science of nuclear physics and evidenced by observations but with that said there is one more kind of stellar explosion that we've yet to discuss one that is not as bright or long-lasting as a standard supernova but still constitutes an apocalyptic yet fruitful explosion nonetheless a cataclysm caused not by a collapsing star but by a pair of rotating neutron stars two dead super dense stellar cores left behind by supernovae if these two extreme objects manage to remain intertwined after their creation the consequence can be an even wilder kind of explosion as the neutron stars circle each other their vice-like counter-balancing influences will cause their orbits to decay the closer they draw the faster they will spin until they are traveling at more than 60 percent the speed of light rippling the very fabric of space and sending reverberations across the entire universe in the form of gravitational waves once these two neutron stars reach a critical distance their tidal stresses become so strong that their up until now indestructible iron shells will shatter exposing both of their interiors the atomic nucleus density matter on the inside suddenly goes from being around half a trillion times denser than water to no compression whatsoever in the near vacuum of space this results in a brief but spectacular explosion one not as bright as a supernova but one that is detectable from multi-billion light-year distances thanks to its gravitational wave precursor this kind of explosion warrants its own class of nova known as a macro nova and better known as a kilonova they released enormous quantities of radiation including their own kind of gamma-ray burst and like supernovae they also produce abundances of heavy elements via the r process neutrons flood through atomic nuclei in the matter under captured resulting in the production of gold silver europium and strontium among other elements this type of explosion is so fruitful in fact that some propose colliding neutron stars to be the real source of the universe's heavy elements but it is not known whether such explosions are common enough in space for this to be true nevertheless supernovae and macronute explosions are the only way we know of in which materials like gold and silver can be created so next time you buy some jewellery containing one of those two materials just remember that you are wearing the products of a cosmic catastrophe you
2021-11-24 23:30
