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This episode is sponsored by The Ridge.
本期节目由The Ridge赞助
Go to ridge.com/scishow and use promo code “scishow” to get 10% off your next order.
登录ridge.com/scishow 使用“scishow”优惠码 下次下单可享受10%的折扣
When astronomers observe distant galaxies, some of them are contemplating a murder mystery.
当天文学家在观察遥远的星系时，他们中的一些人正在思考一个谋杀之谜。
Our Milky Way is alive and well, producing new stars all the time.
我们的银河系生机勃勃，一直都在产生新的恒星。
But there’s another group of galaxies out there, populated only by venerable red dwarf stars -- the young stars are nowhere to be seen.
但那里还有另一组星系，里面只有古老的红矮星——年轻的恒星不见了。
These are called quiescent galaxies because at some point in the galaxy’s life, its star formation stopped.
它们被称为静止星系，因为在星系生命的某个时刻，它的恒星形成停止了。
In effect, these galaxies are... dead. But, how?
实际上，这些星系…死了。但是,为什么会这样呢?
There are no witnesses to these murders -- the distances and time scales are simply too vast for us to observe.
这些谋杀没有目击者——距离和时间尺度太大，我们根本无法观察到。
So scientists have proposed a few different ideas for how it could happen.
所以科学家们提出了一些不同的观点来解释它是如何发生的。
The first scenario has an appropriately whodunit-y name: strangulation.
第一种情况有一个很像侦探小说的名字:勒杀。
We know a galaxy can live for many billions of years, as new stars are constantly being created from cold hydrogen gas that’s hanging around in interstellar space.
我们知道一个星系可以存活几十亿年，因为新的恒星不断地由徘徊在星际空间的冷氢气产生。
And normally, new gas flows into the galaxy from the intergalactic medium to refresh that supply.
通常情况下，新的气体会从星系间介质流进星系来补充能量。
But sometimes that inflow of gas stops -- and we’re not sure why.
但有时流入的气体会停止——我们不知道为什么。
It might be because of interactions with other, neighboring galaxies, which compete for intergalactic gas supplies.
这可能是由于与其他邻近星系的相互作用，相互竞争星系间的气体供应。
Or perhaps the nearest clouds of gas just... run out.
又或者是最近的气体云…已经耗尽了。
Either way, when the inflow stops, all that’s left is the galaxy’s own stars, and the gas they expel at the end of their lives.
不管怎样，当气体流入停止时，就只剩下星系内的恒星，以及它们在生命结束时释放的气体了。
And since stars fuse hydrogen into heavier elements, the overall proportion of hydrogen in the galaxy decreases over time, replaced by heavy metals.
由于恒星将氢聚变为更重的元素，星系中氢的总体比例会随着时间的推移而减少，并被重金属所取代。
This continues until there’s not enough hydrogen for new stars to form.
这种情况会一直持续到没有足够的氢来形成新的恒星。
Without external input, the galaxy is slowly strangled to death, suffocated by its own waste products.
没有了外部输入，星系就会被自己排出的废物窒息而死。
To test the strangulation hypothesis, in 2015 researchers from the University of Cambridge and the Royal Observatory of Edinburgh surveyed the light spectra of local galaxies.
2015年，剑桥大学和爱丁堡皇家天文台的研究人员对当地星系的光谱进行了调查，以验证这一假说。
Their autopsy revealed the characteristic signs of strangulation: the quiescent galaxies had much higher levels of heavy metals than the living, star-forming galaxies.
他们的验尸结果揭示了勒杀的特征：静止星系的重金属含量比有生命的、正在形成恒星的星系要高得多。
They also showed that the dead galaxies were on average 4 billion years older than the living ones -- which makes sense, because it takes time for a galaxy to die by strangulation.
而且，死去的星系平均比活着的星系老40亿年——这是有道理的，因为一个星系被勒杀是需要时间的。
Based on their results, the researchers concluded that this was the primary cause of death for galaxies up to about twice the size of the Milky Way.
根据这些结果，研究人员得出结论，这是导致大约两倍于银河系大小的星系死亡的主要原因。
But strangulation doesn’t explain all galaxy deaths.
但勒杀不能解释所有星系的死亡。
Some galaxies still have plenty of hydrogen-rich gas, but for some reason they just stop forming stars.
一些星系中仍然有大量的富氢气体，但由于某种原因，它们不再形成恒星。
This could happen if the gas is too hot and energetic for it to condense into new stars -- a mechanism called suppression.
如果气体温度过高，能量过大，无法凝结成新的恒星，这种情况就会发生——这种机制被称为抑制。
Scientists think that black holes could be the perps in this case.
科学家们认为黑洞可能是这种情况的罪魁祸首。
In 2016, researchers made observations of a galaxy nicknamed Akira, which has a supermassive black hole at its center.
2016年，研究人员对一个绰号叫Akira的星系进行了观测，该星系中心有一个超大质量黑洞。
As supermassive black holes like these grow, they’re thought to occasionally emit powerful winds.
人们认为随着超大质量黑洞的成长，它们偶尔会释放出强风。
In the 2016 paper, researchers found evidence for those blasts of gas.
在2016年的论文中，研究人员发现了这些气体爆炸的证据。
They believe this wind may be powerful enough to heat up the gas in the galaxy -- enough to prevent it from condensing into stars.
他们认为，这种风的威力可能足以加热星系中的气体——从而阻止它凝聚形成恒星。
And the heat blasts keep coming, which keeps the gas hot and suppresses the formation of new stars long term.
热风不断袭来，使气体持续处于高温状态，长期抑制新恒星的形成。
Which supports the idea that galaxies like Akira could die by suppression -- and that black holes are the killers.
这一现象支持了这种观点，即像Akira这样的星系可能会因抑制而死亡，而黑洞就是凶手。
Finally, some galaxies don’t fit into either of these categories.
最后，还有一些星系不属于这两种类型中的任何一种。
They don’t have much hydrogen gas to go around, but they don’t have the heavy metal signature of strangled galaxies either.
它们没有足够的氢气环绕，但也没有被勒杀的星系的重金属特征。
They seem to have died before their time, by having their life-giving gas physically stripped out in a process called ram pressure stripping.
它们似乎是提前死亡了，通过一种叫做冲压力剥离的过程，它们赋予生命的气体被物理地剥离出来。
Although galaxies might appear still to us here on Earth, they’re actually moving through space at millions of kilometers per hour.
虽然在地球上看星系是静止的，但它们实际上以每小时数百万公里的速度在太空中移动。
In part, they’re being dragged by the gravity of invisible dark matter.
在某种程度上，它们是被无形的暗物质的引力所拖动的。
And as they move, scientists think that they pass through clouds of intergalactic charged plasma.
当它们移动时，科学家们认为它们穿过了星系间的带电等离子体云。
This plasma acts like an atmosphere in space, applying drag to the galaxy, and stripping it of its lightweight gas.
这些等离子体就像太空中的大气层，对星系施加阻力，并将其轻质气体剥离。
Amazingly, observations of a galaxy falling into the Virgo cluster actually seem to show this process in action.
令人惊讶的是，观测到的一个落入室女座星系团的星系好像展示了这整个过程。
The galaxy is curved back at its edges from the pressure of the intergalactic wind.
星系间风的压力让星系的边缘出现了弯曲。
The light from the disk reveals streamers of gas and dust trailing behind, pushed out by the force of the intergalactic plasma.
从盘状星云发出的光来看，在星系间等离子体的力量的推动下，气体和尘埃都尾随其后。
A bit like a dog sticking its head out of a car window.
有点像一只狗把头伸出车窗外。
Ram pressure stripping has been thought to affect galaxies inside clusters since the 1970s.
自20世纪70年代以来，人们就认为冲压力剥离会影响星系团内的星系。
We now know that’s because these clusters have huge amounts of dark matter in them -- enough to put some serious hurt on the galaxies within.
现在我们知道，这是因为这些星系团中有大量的暗物质——足以对其中的星系造成严重伤害。
But a study in 2016, which surveyed more than ten thousand relatively isolated galaxies, found that this could also be a cause of death for those loners.
但是2016年的一项研究，调查了1万多个相对孤立的星系，发现这也可能是某些少量星系死亡的原因。
They saw that the dark matter that surrounds a single galaxy could be enough to tug on neighboring galaxies, pulling them along and stripping out their gas.
他们发现，围绕着单个星系的暗物质也许足以拖曳邻近星系，把它们往前拉，并剥离出它们的气体。
It seems that no single mechanism can explain this murder mystery.
好像没有哪个机制能够解释这个谋杀谜团。
Strangulation, suppression, or stripping are all capable of putting a stop to star formation.
勒杀、抑制或剥离都能阻止恒星的形成。
But the cause of death will depend on what’s going on inside and around an individual galaxy.
但星系的死亡原因将取决于单个星系内部和周围的情况。
So we’ll have to wait and see what lies in store for our own Milky Way.
所以我们必须等着，看看我们的银河系未来会发生什么。
The good news is, we've probably got a couple billion years before the killer strikes, so we’ll all be dead anyway.
好消息是，在凶手袭击之前，我们可能还有几十亿年的时间，那时候我们早就已经死翘翘了。
Thanks for watching this optimistic episode of SciShow Space.
感谢收看本期《太空科学秀》。
And thanks to The Ridge for sponsoring this episode.
感谢The Ridge对本期节目的赞助。
The Ridge makes wallets that are light, sleek, and industrial, and designed to hold everything you need -- and only what you need.
The Ridge制造的钱包既光滑又轻便，它的设计能装下你需要的所有东西——而且只装你需要的东西。
And they’re so confident that you’ll like it, that they’ll let you test drive it for 45 days -- and you can send it back for a full refund if you’re not totally starstruck.
他们相信你一定会喜欢，而且同意你试用45天——如果你并不喜欢它，也可以寄回并申请全额退款。
You can get 10% off today — with free worldwide shipping — by going to ridge.com/SCISHOW and using the code “SCISHOW”.
今天，登录ridge.com/SCISHOW并使用“SCISHOW”代码，就可以享受九折优惠——全球免费送货。
If you’re interested, there’s a link in the description.
如果你感兴趣，描述栏中有链接哦。
