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  • Professor Dave again, let's kill some stars.

  • We've learned about what happened for the first billion years or so in the history of

  • the universe, which leaves us with lots of stars and galaxies, and we are now equipped

  • with the terminology needed to describe and categorize these stars.

  • But we still haven't talked about all the other elements on the periodic table, we've

  • only mentioned hydrogen and helium so far, so where did the rest come from?

  • And what about all the planets and moons?

  • How did those get here?

  • The answer to all of these questions will make sense once we learn more about what goes

  • on inside a star, from the moment they are born, to the time of their death.

  • That's right, stars actually die, so to speak, and the type of death, along with what's

  • left over, will be one of a variety of possibilities, depending entirely on the mass of the star.

  • So let's go through the lifetime of a few different kinds of stars, so that we are ready

  • to understand the next 13 billion years of development in the universe.

  • The life cycle of any star, from birth to death, and all the stages in between, will

  • span millions or even billions of years.

  • This is why stars don't seem to change at all, because a human lifetime is a snippet

  • of a fraction of a blink of an eye to these behemoths.

  • The path that will be followed by a particular star depends mainly on its mass, or how much

  • gas collected and collapsed to form the star, because that material will serve as the star's fuel.

  • As we may remember from physics and chemistry, when nuclei collide with enough energy so

  • as to overcome the electromagnetic repulsion between them, the strong nuclear force takes

  • over, and they fuse, with a small fraction of their mass converting into huge amounts

  • of pure energy, as dictated by E equals mc squared.

  • Therefore, only by colliding nuclei together and fusing them in its ultra-hot core can

  • a star release enough outward energy to counter the effects of gravity relentlessly crushing

  • inward.

  • This means that the amount of matter that forms the star determines the amount of fuel,

  • and through a variety of other factors, the lifetime and eventual fate of the star.

  • Given that mass is the key factor here, let's start with a low-mass star.

  • This would range from the smallest that stars can be, meaning the smallest amount of material

  • that can sufficiently trigger nuclear fusion so as to qualify as a star, which is about

  • thirteen Jupiter masses, to a star somehwere in the ballpark of our sun's mass.

  • As we already know, any star will begin as a cloud of gas and dust at least a few light

  • years across.

  • In the earliest era of star formation, this material was almost exclusively hydrogen and

  • helium, as this was what remained after the brief seventeen minutes of nucleosynthesis

  • soon after the Big Bang.

  • This matter collects due to gravity, pushing increasingly inward as it contracts, until

  • things get so hot over a few million years that nuclear fusion eventually begins, establishing

  • an equilibrium, and generating a yellow or red main sequence star that glows with all

  • the energy released from the collisions happening inside.

  • These fusion reactions begin with two protons fusing, followed by subsequent betay decay,

  • to get a proton and a neutron, and we call this a deuteron, which is a nucleus of heavy

  • hydrogen.

  • Then deuterons are involved in reactions that make helium, which has two protons and two

  • neutrons.

  • Such a star will continue in this manner for billions of years, slowly fusing all of the

  • hydrogen in its core into helium, and maintaining a relatively steady size, temperature, and

  • luminosity as it does so, until almost all of the hydrogen is gone.

  • At this point, things really begin to change.

  • The core of the star will shrink and get hotter, which makes the remaining hydrogen burn even

  • faster, and all of that extra energy being generated will radiate outwards and push the

  • outer layers away from the core.

  • As the outer layers expand, they cool, and thus become more and more red, and the star

  • climbs up the red giant branch until we have a red giant star.

  • The star can maintain this new status for a little while longer, around a billion years,

  • but after almost all the hydrogen is gone, the core gets even smaller and even hotter.

  • At this stage, a phase called helium flash, things are so hot that the star is able to

  • fuse these heavier helium nuclei into larger nuclei like carbon, and then oxygen, through

  • something called the triple-alpha process, and this means that the star has a whole new

  • source of fuel in all the helium it has been making for billions of years.

  • The star begins pulsating as it runs through its final energy reserve, entering what we

  • call the horizontal branch, and in this time it becomes smaller, hotter, and bluer, until

  • at last much of the helium has been fused into larger nuclei.

  • Once the core is predominately carbon and oxygen, with just a shell of helium around

  • it, and a shell of hydrogen around that, the star has very little material left to burn,

  • so the core will collapse and the star enters the asymptotic giant branch.

  • This means it will grow rapidly and become a giant star again, until the last bursts

  • of energy eject the outer layer, pushing it away from the core and back into the interstellar

  • medium, leaving only a tiny, very hot, bare core behind, about the size of Earth.

  • This will gradually cool, as it has no more fuel to burn, not being hot enough to fuse

  • carbon or oxygen nuclei, and it will contract further until we are left with a white dwarf star.

  • The ejected shell is called a planetary nebula, which is misleading, since it is not a planet

  • and did not come from a planet, but the name originated from confusion upon its discovery,

  • and it stuck.

  • The material in a planetary nebula will then become available to join more gas particles

  • to form yet another star.

  • Now for a high-mass star, ones much more massive than our sun, things are quite different.

  • Their demise will not be so quiet.

  • Big stars go out with a bang.

  • Things start out normally, with a gas cloud collecting under the influence of gravity.

  • It is simply that this cloud will be much larger than those that form low-mass stars,

  • so it will contain much more mass.

  • More mass means more gravity, which means the force pushing inward is much stronger,

  • and the star gets much hotter.

  • A hotter temperature means faster fusion, which generates greater outward pressure to

  • counteract the greater inward pull of gravity.

  • This will result in a main-sequence star that is hot, big, bright, and blue.

  • This is where things start to go differently from low-mass stars.

  • Whereas low-mass stars take billions of years to use up all their fuel, high-mass stars

  • are much hotter and burn their fuel much faster.

  • That means they use up all the hydrogen in their cores in around just a fleeting hundred

  • million years, or even ten million if big enough.

  • As the fuel starts running out, the core contracts and heats, producing more energy, so the star

  • will swell up into a giant star, just like we saw for low-mass stars.

  • But while the core of a high-mass star continues to compress, it gets much hotter than the

  • core of a low-mass star, and it becomes able to fuse helium nuclei to form carbon, and

  • then oxygen, and then neon, and then silicon, each heavier nucleus being relegated to a

  • smaller and smaller region of the core that is hot enough to fuse it.

  • All the way at the center sits the heaviest element that can be fused within a star, iron.

  • As this occurs in these different layers, each performing a particular type of fusion

  • until no fuel remains, the star is left with a core of iron nuclei that are so stable that

  • further fusion can release no more energy.

  • At this point, gravity wins the fight, and the star collapses within a single second,

  • the outer layers bouncing off the core and triggering an explosion, thus ejecting all

  • of the heavy nuclei the star has created, out into space.

  • This awesome event, one of the most violent and energetic phenomena in the universe, is

  • called a supernova.

  • A supernova generates such an unbelievable burst of energy that in this brief moment,

  • dozens of elements heavier than iron can also be synthesized.

  • Nickel, copper, zinc, silver, gold, any element with an atomic number greater than twenty-six,

  • is made either in a supernova, or a rare event like the collision of two neutron stars, or

  • a neutron star and a black hole, which are objects we will discuss in a moment.

  • That's why these heavy elements are so rare compared to elements like carbon and oxygen,

  • because stars can't synthesize them the way they can synthesize all the elements up

  • to iron throughout their long lives.

  • Nature only makes these rare elements during the death of a high-mass star, or in certain

  • exotic collision events.

  • Supernovae are also so bright that they are brighter than the entire galaxy they belong

  • to when viewed through telescopes, and if in our own galaxy, they can even be visible

  • with the naked eye, like the one that generated the famous Crab Nebula, which was recorded

  • by a variety of civilizations in 1054.

  • Now, a supernova does not leave behind a white dwarf.

  • Lower-mass stars that begin with less than about eight solar masses leave behind white

  • dwarfs, because once reduced to its lighter earth-sized core, there is not enough gravity

  • to overcome electron degeneracy pressure.

  • In other words, a white dwarf will become kind of like one gigantic metallic solid,

  • with the electron clouds around the nuclei pushing against each other and preventing

  • further collapse.

  • Even still, this object is very dense, with one teaspoon weighing around fifteen tons.

  • So below around 1.4 solar masses, the maximum mass of a white dwarf, which is also known

  • as the Chandrasekhar limit, this is the fate of the core of a star.

  • But for a high-mass star, where upon its death the core of the star is above the Chandrasekhar

  • limit, which means it is massive enough for a supernova to occur, one of two things will

  • be left behind.

  • If the core is between around 1.4 and 3 solar masses, having been generated by a star that

  • was originally somewhere in the ballpark of ten to forty solar masses, the core will not

  • be able to support itself against gravity, and it will collapse with such tremendous

  • force that all the electrons get squeezed into protons such that they combine to form

  • neutrons, and the shockwave from this event is what triggers the supernova.

  • The object that remains is a ball of neutrons bunched up together, like one huge atomic

  • nucleus the size of New York City, containing all of the mass originally within the core

  • of the star.

  • A teaspoon of a neutron star would weigh a whopping ten million tons!

  • But even more miraculously, if the core of the star is above around three solar masses,

  • even the outward pressure of neutrons pressing right up against each other, or neutron degeneracy

  • pressure, is not enough to stop the immense gravity, and the neutrons will be crushed

  • together as the remaining mass collapses into a single point of infinite density.

  • The entire mass of the star's core, contained within zero volume.

  • This object is called a black hole.

  • The outer layers of the star that have been ejected, full of heavy nuclei fused during

  • the lifetime of the star, and the additional even heavier ones formed during the supernova,

  • will leave behind a colorful nebula.

  • But the singularity that is left behind is anything but colorful.

  • A black hole, given its infinite density, warps spacetime so much that not even light

  • can escape.

  • Whatever a black hole might look like, if that can even mean anything, we will probably

  • never find out, because it is impossible for photons to leave it and reach our eyes, which

  • is how we see things.

  • As incredible as this may sound, this is how nature works, and black holes do indeed exist

  • all over the universe, as the remnants of huge dead stars.

  • Black holes are so fascinating that they will require a whole chapter unto themselves, which

  • we will get to in a moment.

  • For now, let's review what we just learned about the lifetime of a star.

  • When a star forms from a gas cloud of some mass, which is almost always between a tenth

  • of a solar mass and around thirty solar masses, a star is produced somewhere along the main

  • sequence.

  • As the fuel in the core begins to run out, it contracts, which raises the pressure around

  • the core and pushes the outer layers outward, where they will then cool, producing a red giant.

  • So all stars will have a red giant phase when their fuel is almost gone, regardless of their mass.

  • Then finally, when the star can no longer perform sufficient nuclear fusion so as to

  • counter the effects of gravity, the star will collapse, leaving a white dwarf if it is of

  • low mass, a neutron star if it is of intermediate mass, and a black hole if it is of especially

  • high mass.

  • As we mentioned, black holes are among the most fascinating objects in the universe,

  • and they are a popular area of study amongst astronomers and theoretical physics alike,

  • because there is so much that we still don't understand about these strange creatures.

  • Let's move forward and learn a little more about black holes.

Professor Dave again, let's kill some stars.

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星の生と死白色矮星、超新星、中性子星、ブラックホール (The Life and Death of Stars: White Dwarfs, Supernovae, Neutron Stars, and Black Holes)

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    劉源清 に公開 2021 年 01 月 14 日
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