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

我們已經了解了在宇宙歷史上 的第一個十億年左右發生了什麼

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

給我們留下了很多恆星和星系

with the terminology needed to describe and categorize these stars.

而我們現在具備了描述和分類這些恆星所需要的術語。

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

但我們還沒有談到所有在元素週期表上的其他元素

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

我們到目前為止僅提到氫和氦， 那麼剩下的來自哪裡呢？

And what about all the planets and moons?

那所有的行星和衛星呢？

How did those get here?

它們是如何產生的？

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

所有這些問題的答案都將得到解答

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

當我們更了解關於恆星內部的知識，從它們出生那一刻，到他們死亡之時。

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

那是對的，恆星真的死了。 可以這麼說，恆星死亡的方式

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

以及什麼將遺留下來，完全取決於恆星的質量。

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

讓我們來看看不同種類的恆星的一生

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

讓我們做好準備以了解未來130億年宇宙的發展。

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

任何一顆恆星的生命週期，從出生到出生 死亡，以及介於兩者之間的所有階段

span millions or even billions of years.

跨越數百萬甚至數十億年。

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

這就是恆星似乎完全沒有改變的原因

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

因為人的一生對於這樣的龐然大物來說就是一眨眼之間

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

特定恆星將遵循的路徑主要取決於其質量

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

或者收集和坍塌以形成恆星的氣體量，因為該材料將作為恆星的燃料。

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

正如我們可能從物理學和化學中記得的那樣 當原子核碰撞時有足夠的能量

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

以克服他們之間的電磁排斥

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

強核力接管，並將它們融合

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

他們的一小部分質量大規模轉換成巨額的純粹能量，透過E=mc2公式。

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

因此，只能通過將原子核碰撞在一起 並將它們融合在超熱核心中

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

一顆恆星才能釋放出足夠的外向能量 來對抗無情地向內塌縮的重力的影響

inward.

這意味著形成恆星物質的數量將決定燃料量

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

並通過各種其他因素決定恆星的終身和最終命運。

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

鑑於質量是這裡的關鍵因素 讓我們從一個低質量的恆星開始。

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

這是恆星範圍的底線 意味著可以充分觸發核聚變的最小量的材料

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

這樣才有資格成為恆星

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

從十三個木星質量 到大約一個太陽質量

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

正如我們已經知道的那樣 任何一顆恆星都將以至少幾光年寬的氣體和塵埃雲開始。

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

在最早的恆星形成時代 這些材料幾乎全是氫和氦

years across.

因為這是大霹靂後不久的17分鐘短暫太初核合成剩下的

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

這個物質由於重力而聚集，在收縮時逐漸向內壓縮

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

直到在幾百萬年裡變得足夠熱 致使核融合最終開始建立一個平衡

soon after the Big Bang.

並產生黃色或紅色的主序星

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

從內部碰撞釋放的所有能量中發出光芒

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

這些聚變反應以兩個質子開始融合

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

並在隨後的負β衰變中，轉變為質子和中子，我們稱之為氘核

the energy released from the collisions happening inside.

，是一個較重的氫同位素核心。

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

然後氘核參與了反應並製造氦 它有兩個質子和兩個中子

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

這樣的恆星將以這種方式(質子-質子鏈反應)持續數十億年

hydrogen.

慢慢融合所有的氫在其核心成氦 並保持相對穩定的尺寸、溫度和光度

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

直到幾乎全部氫都耗盡了。

neutrons.

在這一點上，事情真的開始發生變化。

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

恆星的核心將縮小並變得更熱， 這使得剩餘的氫更快燃燒

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

並且所有產生的額外能量將向外輻射並將外層推離核心。

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

隨著外層膨脹，它們會冷卻， 並且因此變得越來越紅

At this point, things really begin to change.

而且這顆恆星爬上紅巨星分支 ，直到我們擁有一顆紅巨星。

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

恆星可以保持這種新的地位一段時間 ，大約十億年

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

但幾乎​​所有的氫都耗盡了之後 核心變得更小更熱。

outer layers away from the core.

在這個階段，一個叫做氦閃的階段， 溫度熱到以至於恆星能夠

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

將這些較重的氦核融合成較大的核如碳、然後是氧

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

通過一個反應稱為三α過程(或三氦過程)

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

這意味著這顆恆星有一個它已經製造了數十億年的所有氦作為全新的燃料來源。

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

這顆恆星在消耗其最終能量儲備時開始脈動

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

進入我們所謂的水平分支 ，在此時恆星會變得更小、更熱、更藍

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

直到最後，大部分氦被融合成更重的元素。

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

一旦核心主要是碳和氧 ，周圍只有一層氦圍繞

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

以及氦殼周圍的氫殼圍繞 恆星只有很少的材料可以燃燒，

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

所以核心將崩潰 ，且恆星進入漸近巨星分支。

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

這意味著它將迅速成長並再次成為巨星

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

，直到最後一陣能量彈出外層 ，將它推離核心並返回到星際介質中

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

，只留下一個微小的，非常熱 ，大約相當於地球大小的裸露核心。

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

這個裸露核心將逐漸冷卻 ，因為它已經沒有燃料燃燒

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

由於沒有足夠的熱量來融合碳核或氧核， 它會進一步收縮，直到留下一顆白矮星。

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

彈射的外殼稱為行星狀星雲 ，這是誤導性的，因為它不是一個行星

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

而且也並非來自一個行星 ，而是這個名字源於對其發現的困惑，

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

，然後它的名字就固定了

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

行星狀星雲中的物質隨後可以加入更多的氣體粒子 ，形成另一顆恆星。

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

現在對於一個高質量的恆星來說 ，比我們的太陽大得多，事情就完全不同了。

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

它們的死亡不會那麼安靜。

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

巨星死亡伴隨著一聲巨響。

and it stuck.

事情正常開始 ，在重力的影響下一片氣體雲聚集。

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

只是，簡單地說 ，這個雲將比形成低質量恆星的雲大得多

to form yet another star.

，所以它將包含更多的質量。

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

更多質量意味著更多的重力 ，這意味著向內塌縮的力量要大得多

Their demise will not be so quiet.

，而且這個恆星將變得更熱。

Big stars go out with a bang.

溫度越高意味著融合越快 ，這會產生更大的外向壓力以抵消更大的內在引力。

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

這將產生一個很熱、很大、很亮的藍色主序星。

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

這是事情開始和低質量恆星變得不同的地方。

so it will contain much more mass.

低質量恆星需要數十億年耗盡他們所有的燃料

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

，高質量的恆星更熱，燃燒更快。

and the star gets much hotter.

這意味著他們將在短短幾億年左右耗盡核心中所有的氫氣

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

，如果足夠大，這些氫氣在一千萬年內就已消耗殆盡。

counteract the greater inward pull of gravity.

隨著燃料開始耗盡，核心開始收縮和加熱，以產生更多的能量

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

，所以恆星會膨脹成一顆巨星 ，就像我們看到的低質量恆星一樣。

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

但是當高質量恆星的核心繼續壓縮時 ，它會比低質量恆星的核心熱得多

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

，並且能夠將氦原子核融合成碳 ，然後是氧，再來是氖，接著是矽，

are much hotter and burn their fuel much faster.

每個較重的核被降級到核心的越來越小的區域 ，該區域熱到足以融合它。

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

直到在恆星中心可以融合的最重的元素，鐵。

million years, or even ten million if big enough.

由於這發生在這些不同的層中， 每個都進行特定類型的融合

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

直到沒有剩餘燃料

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

，這顆恆星留下了鐵核質的核心是如此穩定 ，以至於進一步的融合無法釋放出更多的能量

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

在這一點上，重力贏得了戰鬥 ，並且這顆恆星在一秒內坍塌，

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

外層從核心彈回並引發爆炸，

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

從而將恆星產生的所有重核噴射到太空中。

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

這個令人敬畏的事件 ，是宇宙中最暴力和最具活力的現象之一

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

，被稱為超新星。

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

超新星在這短暫的瞬間產生了令人難以置信的能量爆發，

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

比鐵還重的幾十個元素也可以 合成。

further fusion can release no more energy.

鎳、銅、鋅、銀、金，任何元素 原子序數大於二十六，

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

都是在超新星或兩個中子星碰撞或一顆中子星和一個黑洞碰撞之類的

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

我們將在稍後討論的稀有事件中製造的。

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

這就是為什麼與碳和氧等元素相比 ，這些重元素如此罕見

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

，因為恆星無法以它們合成元素到鐵的方式合成這些重元素

called a supernova.

，它們在它們漫長的一生中所能合成的元素只到鐵。

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

大自然只會在高質量恆星死亡或某些異國碰撞事件中產生這些稀有元素。

dozens of elements heavier than iron can also be synthesized.

超新星也是如此明亮 比他們所屬的整個星系更亮

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

通過望遠鏡觀察時，如果它發生在我們自己的星系中 ，它們甚至可以被用肉眼看見

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

，就像產生那個著名的蟹狀星雲那次，被1054年的各種文明記錄下來

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

現在，超新星不會留下一顆白矮星。

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

小於大約八個太陽質量的較低質量恆星留下了白矮星

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

因為一旦減少到其較輕的地球大小的核心，就沒有足夠的重力來克服電子簡併壓力。

to iron throughout their long lives.

換句話說，白矮星會變得像一顆巨大的金屬固體，

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

原子核周圍的電子云相互推擠並防止進一步坍塌。

exotic collision events.

即便如此，這個物體也非常密集 一茶匙重約十五噸。

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

所以低於大約1.4太陽質量 ，最大白矮星的質量

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

，也是眾所周知的錢德拉塞卡極限 ，這就是低質量恆星核心的命運。

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

但對於一個高質量的恆星來說 ，它的死亡核心質量在錢德拉塞卡極限之上

by a variety of civilizations in 1054.

這意味著它足以發生超新星 ，下列兩件事之一將會發生。

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

如果核心質量在大約1.4到3個太陽質量之間

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

這顆恆星最初大約有位於十到四十個太陽質量

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

，核心將無法支持自身抵抗重力， 並且它將以如此巨大的力量崩潰

to overcome electron degeneracy pressure.

迫使所有電子被擠壓進入質子 ，使它們結合形成中子

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

，而這次事件的衝擊波是觸發超新星的原因。

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

剩下的物體是一團聚集在一起的中子球 ，就像一個像紐約市一樣大的原子核

further collapse.

，包含原本在恆星核心內的所有質量。

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

一茶匙的中子星會重達千萬噸！

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

但更奇蹟般的是 ，如果恆星的核心質量高於三個太陽質量

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

即使中子向外擠壓的向外壓力 ，或中子退化壓力，也不足以阻止巨大的引力

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

，當剩餘的質量坍縮成一個無限密度的點時，中子將被壓碎在一起。

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

恆星核心的整個質量，包含在零體積內。

be left behind.

這個對象叫做黑洞。

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

被射出的恆星的一生融合的充滿了重元素的外層，

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

超新星期間形成的更重的元素 ，將留下一個五彩繽紛的星雲。

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

但留下的奇點絕不是多彩的。

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

一個黑洞，鑑於其無限的密度， 扭曲了時空，甚至連光也無法逃脫

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

無論黑洞看起來像什麼 ，如果這甚至可能意味著什麼，

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

我們可能永遠都找不到 ，因為光子不可能離開它

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

，並以我們看待事物的方式到達我們的眼睛。

of the star.

這聽起來令人難以置信 ，這就是大自然的運作方式

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

，宇宙中確實存在著黑洞 ，就像巨大死星的殘餘一樣。

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

黑洞是如此迷人，以至於它們需要整整一章，我們將在之後得到它們。

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

現在，讓我們回顧一下我們剛剛學到的關於恆星壽命的知識。

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

當一顆恆星從某個質量的氣體雲形成時，它幾乎總是在十分之一太陽質量和大約三十個太陽質量之間

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

，在主序列的某處產生一顆恆星

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

當核心中的燃料開始耗盡時，它會收縮，從而提高核心周圍的壓力

This object is called a black hole.

，將外層向外推，然後冷卻，產生一個紅巨星。

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

因此，當他們的燃料幾乎消失時，無論質量如何，所有恆星都會有一個紅巨星階段。

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

最後，當恆星不能再進行足夠的核聚變以抵消重力的影響時，恆星會塌縮，

will leave behind a colorful nebula.

如果質量較小則留下白矮星，如果是中等質量則留下中子星，如果質量特別高則留下黑洞。

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

正如我們所提到的 ，黑洞就是宇宙中最迷人的物體之一，

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

它們是天文學家和理論物理學中一個受歡迎的研究領域，

can escape.

因為有太多我們還沒有 了解這些奇怪的生物。

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

讓我們繼續前進，再學習一點 關於黑洞。

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.