The birth of the galaxy in 60 seconds - Scott Hershberger

翻译人员: Tianyun Liu 校对人员: Gia Hwang

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

在银河系内，地球位于 一条螺旋臂的内侧边缘，

But this isn’t our permanent address.

但这不是我们的永久住所。

When most dinosaurs went extinct,

当大多数恐龙灭绝时，

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

我们的太阳系位于不同的螺旋臂上。

The Milky Way never stops changing;

银河系永远不会停止变化，

even its structure continues to evolve.

甚至它的结构也在继续演变。

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

要了解原因，让我们 从银河系的起源开始说起。

13 billion years ago,

130 亿年前，

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

在超新星和早期宇宙混乱的推动下，

the gas and dust particles that eventually became our Milky Way

最终成为银河系的气体和尘埃粒子

were whizzing around in every direction.

在各个方向呼啸而过。

Each particle had a certain angular momentum—

每个粒子都有一定的角动量，

the magnitude and direction of its rotation

即围绕新出现的星系中心

around the emerging galaxy’s center.

进行旋转的幅度和方向。

Just as the new galaxy’s total mass

正如新银河系的总质量

was the sum of the individual particles’ masses,

是各粒子质量总和一样，

the galaxy also had an angular momentum

银河系也有一个角动量，

that was the sum of the particles’ angular momenta.

它是这些粒子角动量的总和。

So, despite the wild motion of its parts,

所以，尽管各部分运动狂野不羁，

the galaxy as a whole was rotating about an axis.

整个银河系依然围绕一个轴在旋转。

Meanwhile, the gas and dust particles frequently collided,

同时，气体和尘埃颗粒频繁碰撞，

losing some energy to heat.

损失的能量变成了热量。

Because they slowed down,

由于它们减速，

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

重力能够将它们拉向银河系的中心。

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

那为何银河系像多数星系一样变得扁平，

and not spherical like stars and planets?

而不是像恒星和行星那样呈球形呢？

The answer lies in its angular momentum.

答案在于它的角动量。

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

物理定律规定，在没有外力的情况下，

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

物体或系统的总角动量 在一段时间内必须保持不变。

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

在恒星或行星中，旋转的物质非常密集，

that the outward pressure partners with gravity

以至于向外的压力与重力配合

to create a mostly spherical shape.

形成了大致球形的结构。

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

但新出现的星系往往密度低、角动量大，

meaning that the spinning motion is a more significant factor

这意味着旋转运动

是一个比向外压力更显著的因素。

than the outward pressure.

Indeed, if a galaxy has a lower angular momentum,

实际上，如果星系的角动量较低，

an egg shape can form.

则可以形成一个蛋形结构。

In galaxies like the Milky Way,

在银河系这样的星系中，

particles, instead of falling directly towards the center,

粒子并不会直接落向中心，

tended to fall parallel to the axis of rotation,

而是趋向于沿着旋转轴平行下落，

as to keep the larger total angular momentum constant.

这样可以保持较大的总角动量不变。

Over billions of years the cloud of particles gradually fell,

经过数十亿年，粒子云逐渐下落，

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

在轨道上加速，形成旋转的盘状结构。

Now what about the arms?

那么，螺旋臂是怎么回事呢？

Spiral arms, like the one Earth is in,

像地球所在的螺旋臂，

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

是恒星和气体更加密集的区域。

But the arms aren't static structures.

但这些螺旋臂并不是静止的结构，

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

它们是由以波浪形式 穿过银河系的压缩区域引起的。

Just as a whirlpool has multiple peaks and troughs,

就像漩涡有多个峰谷一样，

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

星系中的密度波也有多个高密度区域，

the bright spirals— separated by regions of lower density.

即明亮的螺旋被密度较低的区域隔开。

And as the wave travels,

随着波浪的传播，

different stars are constantly entering and leaving the spirals.

不同恒星不断进出这些螺旋臂。

Density waves can form in several ways.

密度波可以通过几种方式形成。

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

对某些星系来说，附近的伴星系会搅局，

Its gravitational pull breaks the disc’s symmetry,

其引力打破了星系盘的对称性，

generating a wave that could last for a billion years.

产生可以持续十亿年的波浪。

For other galaxies,

对于其他星系来说，

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

星系内存在的一个由恒星 和气体组成的紧密的小星团

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

也能产生涟漪效应，

自发地产生波浪。

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

这被认为是银河系螺旋臂的成因。

In both scenarios,

在这两种情况下，

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

星系的整体旋转 将高密度区域弯曲成螺旋形，

which rotate around the galaxy’s center.

这些螺旋围绕星系中心旋转。

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

我们的太阳系绕银河系中心的 轨道速度比螺旋臂快，

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

在未来的几百万年中， 我们的太阳系将深入当前的螺旋臂，

before eventually putting it in our rearview mirror.

然后最终将其抛在身后。

And recent observations have added another wrinkle to the picture.

而最近的观察又为这一景象 增添了另一抹神秘色彩。

Rather than just one density wave,

典型的螺旋星系可能不只有一个密度波，

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

而是有两个或多个相互重叠，

and travel at different speeds.

并以不同速度传播。

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

结果是螺旋臂可能会持续 数千万年或数亿年，

before breaking apart and re-forming.

然后分裂并重新形成。

This may be happening in the Milky Way,

这可能正发生在银河系，

meaning that when the Earth formed 4.5 billion years ago,

意味着当地球在 45 亿年前形成时，

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

螺旋本身可能看起来与今天完全不同。

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

无论如何，我们的螺旋不会永远存在。

About 5 billion years from now,

大约 50 亿年后，

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

银河系将开始与仙女座星系融合，

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

打破角动量的平衡，形成蛋形——

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

我们银河系历史上一个新时代由此诞生。