The birth of the galaxy in 60 seconds - Scott Hershberger

譯者: Esther Lam 審譯者: Amanda Zhu

Within the Milky Way, the Earth resides on the inner edge of a spiral arm.

地球位於一個銀河系旋臂的內緣。

But this isn’t our permanent address.

但這不是我們的永久地址。

When most dinosaurs went extinct,

當大多數恐龍滅絕時，

our solar system was in a different arm than it is today!

我們的太陽系是在另一個的旋臂上！

The Milky Way never stops changing;

銀河系永遠在改變，

even its structure continues to evolve.

連它的結構都不斷在演變。

To see why, let's start at our galaxy's very beginning.

要了解為什麼，就要 從我們的星系起源開始。

13 billion years ago,

130 億年前，

propelled by supernovae and the general chaos of the early universe,

在超新星和宇宙早期 混亂的能量推動下，

the gas and dust particles that eventually became our Milky Way

我們銀河系在形成之前 都是氣體和塵埃顆粒，

were whizzing around in every direction.

它們到處亂飛。

Each particle had a certain angular momentum—

每一個顆粒都有一定的角動量，

the magnitude and direction of its rotation

就是它圍繞著成形中的銀河中心 而旋轉的幅度和方向。

around the emerging galaxy’s center.

Just as the new galaxy’s total mass

就像新星系的總質量

was the sum of the individual particles’ masses,

是個別粒子質量的總和一樣，

the galaxy also had an angular momentum

星系具有的角動量

that was the sum of the particles’ angular momenta.

是個別粒子角動量的總和。

So, despite the wild motion of its parts,

因此，儘管星系各部分混亂地運動，

the galaxy as a whole was rotating about an axis.

整個星系是繞著一根軸旋轉。

Meanwhile, the gas and dust particles frequently collided,

同時，氣體、灰塵顆粒等 經常相互碰撞，

losing some energy to heat.

因而讓一些能量轉換成熱而散失，

Because they slowed down,

速度也因此變慢，

gravity was able to pull them closer to the galactic center.

此時重力就能夠將它們 拉近星系中心。

So, why did the Milky Way, like most galaxies, become flat,

那麼，銀河和大多數星系 為什麼是扁平的，

and not spherical like stars and planets?

而不像恆星和行星一樣是圓球形？

The answer lies in its angular momentum.

答案在於它的角動量。

The laws of physics dictate that in the absence of external forces,

根據物理定律， 在沒有外力的情況下，

the total angular momentum of an object or system has to stay the same over time.

旋轉的物體或系統， 其總角動量會永遠保持不變。

In a star or planet, the spinning material is so dense

以恆星或行星來說， 旋轉材料密度很高，

that the outward pressure partners with gravity

向外的壓力與向內的重力抵銷，

to create a mostly spherical shape.

形成近球體的形狀。

But emerging galaxies tend to have low densities and high angular momenta,

但是新興的星系往往 具有低密度和高角動量，

meaning that the spinning motion is a more significant factor

這意味著旋轉運動的離心作用

than the outward pressure.

比其他向外壓力更大。

Indeed, if a galaxy has a lower angular momentum,

所以如果星系的角動量較低，

an egg shape can form.

則可能會形成蛋形。

In galaxies like the Milky Way,

在銀河系這樣的星系中，

particles, instead of falling directly towards the center,

粒子不朝中心點直接掉落，

tended to fall parallel to the axis of rotation,

而是與旋轉軸平行掉落，

as to keep the larger total angular momentum constant.

這樣才能夠保持角動量的總和不變。

Over billions of years the cloud of particles gradually fell,

數十億年來，粒子雲逐漸掉落，

sped up in their orbits, and created a spinning disc.

在它們的軌道上加速， 並形成了一個旋轉的圓盤。

Now what about the arms?

那麼旋臂呢？

Spiral arms, like the one Earth is in,

就像地球所在的旋臂，

are regions where stars and gas are packed more tightly together.

是星星和氣體較密集的區域。

But the arms aren't static structures.

但旋臂不是靜態結構。

They are caused by zones of compression that travel through the galaxy as waves.

它們是由星系中分布的壓縮區形成。

Just as a whirlpool has multiple peaks and troughs,

就像漩渦有很多波峰和波谷一樣，

a density wave in a galaxy has multiple regions of high density—

星系中的密度波具有多個 高密度的壓縮區，也就是明亮的螺旋，

the bright spirals— separated by regions of lower density.

並被低密度區域分隔開。

And as the wave travels,

密度波移動在移動當中，

different stars are constantly entering and leaving the spirals.

不同的星星不斷進出旋臂。

Density waves can form in several ways.

密度波可以多種方式形成。

For some galaxies, a nearby companion galaxy stirs the pot.

對於某些星系來說， 附近的星系也會造成影響，

Its gravitational pull breaks the disc’s symmetry,

附近星系的重力 破壞了星系盤狀的對稱性，

generating a wave that could last for a billion years.

形成可能持續十億年的波，

For other galaxies,

對於有些星系而言，

the presence of a small clump of tightly packed stars and gas within the galaxy

如果有一小堆星星和氣體擠在一起，

can have a ripple effect, spontaneously giving rise to a wave.

就可能會產生漣漪效應， 自發性地形成波，

This is thought to be the cause of the Milky Way's spirals.

一般認為銀河系的螺旋是這樣來的。

In both scenarios,

在這兩種情況下，

the galaxy’s overall rotation bends the dense region into spirals,

整個星系的旋轉 將密集區域彎曲為螺旋狀，

which rotate around the galaxy’s center.

圍繞星系的中心旋轉。

Our solar system is orbiting the galactic center faster than the spiral arms.

環繞銀河中心旋轉時，太陽系的運行 比旋臂繞行的速度更快，

We’ll be moving deeper into our current arm for millions of years

所以在接下來的數百萬年中， 我們將繼續朝目前旋臂的深處運行，

before eventually putting it in our rearview mirror.

直到脫離這個旋臂。

And recent observations have added another wrinkle to the picture.

最近的一個觀察為這些現象 增加了另一個變數。

Rather than just one density wave,

典型的螺旋星系不僅有一個密度波，

a typical spiral galaxy likely has two or more waves that overlap with each other

可能有兩個或更多的波彼此重疊

and travel at different speeds.

並以不同的速度移動。

The result would be spiral arms that last for tens or hundreds of millions of years

結果是旋臂會持續數千萬或數億年，

before breaking apart and re-forming.

才會崩解並重新形成，

This may be happening in the Milky Way,

這可能正在銀河系中發生。

meaning that when the Earth formed 4.5 billion years ago,

這意味著在 45 億年前 地球形成時，

the spirals themselves may have looked entirely different than they do today.

那些螺旋看起來可能 與今天完全不同。

In any case, our spirals won't last forever.

無論如何，我們的銀河系 不會永遠存在。

About 5 billion years from now,

大約 50 億年後，

the Milky Way will start to merge with the Andromeda galaxy,

銀河系將開始與仙女座星系合併，

throwing off the balance of angular momentum and creating an egg shape—

從而破壞角度動量的平衡 並創造出像雞蛋的形狀，

the birth of a new era in our galaxy’s history.

這是我們銀河系歷史中 一個新時代的誕生。