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when an 8 – 20 solar mass star ends itslife, it does so with a bang: a supernova. and when it’s all over, there’s a coupleof octillion tons of superheated plasma expanding away from the explosion site at a fractionof the speed of light, a whole mess of energy released in the form of light and neutrinos,and a bizarre little ball of quantum nastiness in the center composed almost entirely ofneutrons. the properties of this neutron star are almostas bizarre as things get in the universe. and if it all seems rather alien to you, well,that’s ok. for a little while, astronomers wondered if aliens really were behind whatthey were seeing. now i’m not sayin’ aliens.
when we last left the core of a high massstar, it was in a bad way: milliseconds ago it was fusing silicon into iron, but now it’s collapsingunder its own immensely powerful gravity. the collapse takes a fraction of a second,but a lot happens in that fraction of a second. in lower mass stars, the core supports itselfvia electron degeneracy pressure, the result of a rule in quantum mechanics that says electronsvehemently resist being squeezed together. but even electron degeneracy fails to stopthe collapse if the core has a mass more than about 1.4 times the mass of the sun. that’s justtoo much of a load to bear, and the collapse continues. under these huge pressures, a funny thinghappens: protons, electrons, and other subatomic particles get smashed together, and they mergeto form neutrons. and this happens to almost
all of them. when the core collapses downto about 20 km in diameter, it’s essentially a ball of neutrons with some protons and electronshere and there that survived, and a crust of normal but highly compressed matter ontop. when this happens yet another effect comesinto play: neutron degeneracy. like electrons, neutrons resist being squeezed too tightlytogether, but this time the strength of the pressure is far, far stronger than for electrons.if the core is less than about 2.8 times the sun’s mass, the collapse runs into a wall.it stops. this generates a huge shock wave, which, alongwith a flood of energetic subatomic particles called neutrinos, blasts outwards, blowingup the star.
what’s left of the core after the metaphoricalsmoke clears is a neutron star, one of the most bizarre objects in the universe. such a star would be extremely weird. or really,just extreme. its mass would be more than that of the entire sun, all packed into asphere maybe 20 km across. now let’s just stop there for a sec andlet that sink in. the sun has a mass 300,000 times the earth. imagine packing that allinto a ball the size of a small city. too mind boggling? ok, think of it this way:you are mostly empty space. every atom in your body has a nucleus made of tightly packedneutrons and protons, and electrons whizzing around outside them. if you could magnifyan atom to be 100 meters across, the nucleus
would be roughly the size of a marble. imagineall that empty space between the nucleus and the electrons. that’s a normal atom. but in a neutron star, all of that space wouldbe filled with neutrons. all of it. every nook and cranny inside the neutron star hasmatter in it, all the way down to the scale of an atomic nucleus. this is what gives aneutron star its mind-crushing properties. i’m now going to barrage you with very large numbers.so, take a deep breath, and you might wanna sit down. neutron stars are ridiculously dense. a singlecubic centimeter of neutronium, as neutron star stuff is usually called, has a mass ofabout 400 million tons. want some perspective on that number? well, very roughly, that’sthe total mass of every single car and truck
in the united states. imagine a couple ofhundred million vehicles, crushed down until they could all fit inside this six-sided die.that’s neutronium. it’s so dense that, as far as it’s concerned,normal matter is a slightly polluted vacuum. if you set it on the ground it would fallright through the earth. now, anything that dense has a huge gravitationalpull. if you were on the surface of a neutron star… well, you’d be very dead, obviously-- like immediately, flattened down to a thickness of just a few atoms. and that’s becausea typical neutron star has a surface gravity 100 billion times stronger than earth’s.i have a mass of about 77 kilos, and here on earth i weigh about 170 pounds. on a neutronstar, i’d weigh 17 trillion pounds. that’s
23,000 times the weight of the empire statebuilding. but wait! there’s more! in our introduction to the solar system imentioned that when you take a spinning object and shrink it the spin will increase—theusual example is an ice skater drawing in his arms, increasing his rotation until he’sa blur. the same is true for the star’s core whenit collapses into a neutron star. it may have had a very slow spin before the supernova,maybe even taking weeks to spin once. but when it shrinks down to just 20 km acrossand becomes a neutron star, that rotation will increase by a huge factor. a freshly mintedneutron star might spin several times per second.
the magnetic field increases as well. a starlike the sun has an overall magnetic strength not too different from the earth’s. but whenthat core collapses, the strength of the field skyrockets, and a neutron star can easilyhave a magnetic field several trillion times stronger than the sun’s. that’s strongenough to erase your credit card from a hundred thousand kilometers away. see? ridiculous. all of these properties are brain-melting. but arethey real? could an object like this really exist? oh my, yes. the first neutron star was detectedin 1965, though not recognized for what it was at the time. a couple of years later anotherone was found, and this time was correctly
identified as a neutron star. but then things got...weird. in 1967, jocelyn bell was a graduate studenthelping build a radio telescope. there was a persistent noise in their data they couldn’tseem to fix. bell studied it night after night, finally figuring out that the pattern wasn’ta problem with their data, it was from an actual astronomical object. she had discoveredthe first known pulsar. what’s a pulsar, you ask? pulsars are neutron stars. in a nutshell,their rapid rotation coupled with their incredibly strong magnetic fields launch twin beams ofenergy away from the star, like the beams from a lighthouse. the beams sweeps aroundas the star rotates, and from earth we see
this as a pulse, a blip, of increased brightness.this pulse can be detected in visible light, radio waves, and even x-rays! the spin of a neutron star is amazingly stable,making these pulses act like a very accurately timed cosmic clock. in 1967, no one couldbelieve a natural object could do this, and this object was half-jokingly given the namelgm-1. little green men 1. now we know of over a thousand pulsars injust our galaxy alone, and we know they are the leftover cores of massive stars that exploded.some spin with periods many seconds or even minutes long. some are in binary systems;another normal star orbits them. if they’re close enough together the neutron star canrip material off the other star and feed on
it. this increases the pulsar’s spin, andwe know of a few that have incredibly rapid rotation rates; some spin hundreds of timesper second! these are called millisecond pulsars, and if they spun much faster the centrifugal forcewould rip them apart despite their tremendous gravity! even after a thousand years, a pulsar canstill be a force to reckon with. there’s a pulsar in the center of the crab nebula,the remains of a star that exploded to create that supernova remnant. a substantial fractionof the light emitted from the nebula is powered by the pulsar itself; its fierce output energizesthe nebula, causing it to glow brightly even after a millennium. i’m telling ya, thinking about neutronsstars makes the hair on the back of my neck
stand up. and i haven’t even mentioned magnetars yet. neutron stars are more than just weird littleballs of neutrons. they have a crust, probably a few centimeters thick, made of highly compressedbut more or less normal matter, squeezed into a kind of highly stiffened crystal state.the magnetic field of the star penetrates this crystalline crust and stretches out forquite a distance. in some neutron stars the magnetic field iseven stronger than usual, and can be — get this — a quadrillion times stronger thanthe sun’s. these ã¼ber-powerful neutron stars are given the name magnetars. they’rerelatively rare; maybe 10% of all neutron
stars are born as magnetars. and they haveshort lifetimes; the magnetic field is so strong it acts like the brakes on a car, slowingthe neutron star’s spin. that spin helps power the magnetic field, so the field weakensas the star slows. but while they're around, magnetars are themost magnetic objects in the universe. and with great power comes great responsibility…if your responsibility is to be one of the scariest objects in the universe.why? in a neutron star, the crust and magneticfield are locked together, so a change in one affects the other. the crust of the staris under incredible strain due to the intense gravity and rapid rotation. if the structureslips, it can snap, creating a star quake
— like an earthquake, but just a wee bitstronger. in an earthquake, huge amounts of energy are released when the earth’s crustshifts and snaps, enough to destroy buildings and quite literally move mountains. but in a neutron star this effect is multiplied.hugely. remember, the crust is phenomenally dense, and the gravity is enormous. if thecrust strains and snaps, dropping just a single centimeter, the resulting release of energyis vast beyond imagining. this energy is released as a tremendous explosionin the crust, shaking it. this also shakes the magnetic field, which reacts…poorly. when the sun’s magnetic field throws a tantrum,we get a solar flare, which can be as powerful
as billions of nuclear bombs. a magnetar flaredwarfs that into insignificance. it can be trillions of times stronger than a solar flare— in a fraction of a second, a magnetar can release as much energy as the sun givesof in a quarter of a million years. in 2004, astronomers were stunned when a hugeblast of x-rays slammed into orbiting satellites. one of these satellites, named swift, actuallyhad its detectors saturated with x-rays, even though it wasn’t even pointing at the sourceat the time! the x-rays literally came right through the side of the satellite with suchintensity that swift — which was designed to detect powerful x-ray sources — was momentarilyblinded by them. the source of this x-ray burst was quicklydetermined to be a magnetar called sgr-1806-20,
and the effects were incredible: it actuallycompressed the earth’s magnetic field, and partially ionized the earth’s upper atmosphere. oh, did i mention that this magnetar is 50,000light years away? that’s halfway across the galaxy. that’s incredible. at a distanceof 500 quadrillion km its effects were felt more strongly than a powerful flare from thesun! the good news is that there are very few magnetarslike this in the galaxy, probably fewer than a dozen. also, catastrophic explosions likethe one in 2004 are rare; if one had had happened any other time in the past 40 or so yearswe would’ve detected it. and frankly, it’s really cool that we had astronomical satellitesorbiting the earth which could study it!
we’ve come a long way in understanding neutronstars since they were first discovered, but there’s much about them we don’t understand.every time we find out more we find out they’re even weirder than we first thought. and yet, for all that, they’re not the weirdestthings in the sky. not by a long shot. that place is held pretty securely by the othertype of object created in a supernova: a black hole.stay tuned. today you learned that when a star between8 and 20 times the sun’s mass explodes, the core collapses to form a neutron star.neutrons stars are incredibly dense, spin rapidly, and have very strong magnetic fields.some of them we see as pulsars, flashing in
brightness as they spin. neutrons stars withthe strongest magnetic fields are called magnetars, and are capable of colossal bursts of energythat can be detected over vast distances. crash course astronomy is produced in associationwith pbs digital studios. head over to their youtube channel to catch even more awesomevideos. this episode was written by me, phil plait. the script was edited by blake de pastino,and our consultant is dr. michelle thaller. it was directed by nicholas jenkins, editedby nicole sweeney, the sound designer is michael aranda, and the graphics team is thought cafã©.
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