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Together, the Northern and Southern lights could be considered one of the most beautiful sites in nature.
南北极光堪称自然界最美的景观之一。
But these giant curtains streaking across the sky aren't the only type of aurora out there.
但像大型幕布一样布满天空的极光并不是南北极唯一一种极光。
There's also the much more common, but less brilliant pulsating auroras.
也有更为稀松平常、只不过没那么绚烂的脉动极光。
These can happen at any time, but we haven't been able to actually observe what causes them.
极光随时都有可能发生，不过人类一直未能观测到极光发生的原因。
Until now. Thanks to research published in Nature last week, we think we finally know how the process works.
直到最近有了发现。多亏了《自然》期刊上周的发表的研究，我们觉得自己终于知道了极光发生的原理。
Active auroras, like the dazzling shows you see at the poles, have one continuous arc of light.
活跃极光，比如我们在南北两极看到的炫目极光，是有一种持续弧光的极光。
But pulsating auroras get their name because they, pulse.
但脉动极光之所以叫这个名字，是因为他们会产生脉动。
Distinct patches of sky vary in brightness over several seconds, and they're generally less striking than the active auroras.
脉动极光就在那么几块独特的天空区域里产生，并且亮度在几秒钟之内不断变化。总体而言，脉动极光没有活跃极光那么炫目。
These different appearances are caused by the auroras' related, but different origin stories.
这些不同的表征是由与脉动极光相关，但起源不同的成因造成的。
They're both caused by charged particles, usually electrons, traveling down into the atmosphere and colliding with molecules there.
这两种极光都是由带电粒子（通常是电子）引起的：带电粒子进入大气层，与那里的分子发生对撞。
The molecules' electrons gain energy from those collisions, and then release light as they relax back to their usual state.
分子中的电子从对撞中获得能量，并在回复到正常状态的过程中发光。
Now, the electrons that spawn active auroras come from dense waves of solar material colliding with the Earth's magnetic field.
产生活跃极光的电子来自于太阳物质密度波和地球磁场的撞击。
Ones that create pulsating auroras are a bit more complex, but according to this new paper, we might have figured it out.
而产生脉动极光的电子则更为复杂些，不过根据《自然》期刊发表的这篇论文，我们或许能解开这些电子形成的原理。
Basically, when the Earth's magnetic field rearranges itself, which happens all the time, it releases a bunch of stored up energy.
基本上，地球磁场始终在不停地重新排列，过程中会释放一些储存的能量。
That energy triggers the creation of plasma waves, or waves of charged particles.
这部分能量会激发等离子体波的产生，或者说带电粒子波的产生。
Specifically, these ones called chorus waves.
尤其是称之为合声波。
They form high in the Earth's atmosphere near the equator, then move north and south to more extreme latitudes.
合声波在赤道附近的地球的高空大气形成，随后向南北极的高纬度移动。
As they go, they scatter electrons that would otherwise just bounce around in the magnetic field.
移动过程中，他们会散布电子，电子会在磁场中四处移动。
Some of those electrons get jostled around, and ultimately rain down in batches into the upper atmosphere.
其中一些电子相互撞击，最后大批电子去往高空大气。
And that, finally, creates pulsating auroras.
这最终就产生了脉动极光。
This hypothesis has actually been around for over half a century, but it's only just been proven.
这种假说已经有半个多世纪的历史了，但最近才得以证实。
We had to wait until we had sensitive enough equipment to observe a clear interaction between chorus wave plasma and the electrons causing the auroras.
我们现在还没有足够灵敏的设备，所以无法清晰地观测到合声波等离子体与引起极光的电子之间的相互作用。
Still, thanks to the Japanese ERG satellite and some aurora measurements from last March, scientists pulled it off.
不过，去年3月，通过日本ERG卫星以及一些极光测量的帮助，科学家观测到了它们之间的相互作用。
The authors still acknowledge that there could be holes in their findings,
发文的科学家依然承认他们的发现会有漏洞，
like that these results may not be the same across different distances from Earth and geomagnetic activity.
比如在距离地球和地磁活跃性不同距离的地方，结果可能不同。
But this new research will help scientists better understand how these interactions affect planets' atmospheres.
但这项新研究将助力科学家更好地理解这些相互作用是如何影响各个行星的大气层的。
This applies to any planet with a magnetic field, too, including Jupiter and Saturn, where we've already detected chorus waves.
该研究也可以应用于任何有磁场的行星中，包括人类已经探测到合声波的木星和土星。
So this is only the beginning. In more wide-reaching news, literally,
所以，一切才刚刚开始，还有涉及面更广的消息称，
a group of astronomers have calculated that the neighboring Andromeda Galaxy might be a lot less massive than we thought.
一些天文学家通过计算得出，临近的仙女座星系可能比我们想象的小很多。
Our galactic big sister seems more like a twin.
仙女座星系这个我们在银河系里庞大的姐妹更像是我们的双胞胎妹妹。
There are a lot of ways to calculate a galaxy's mass, and they all yield slightly different results.
有很多方法可以计算一个星系的质量，得出的计算结果都略微不同。
Most astronomers have treated our galactic neighbor as 2 to 3 times more massive than the Milky Way, because that's what pops up in a lot of studies.
很多天文学家假定仙女座星系的质量是银河系的2-3倍，因为这是很多研究使用的基础数据。
But its mass is still far from certain. It's hard to measure the mass of a whole galaxy, okay?
不过，对于仙女座星系的质量，我们还无法给出较为精确的数值，毕竟测量一整个星系的质量不是件容易的事儿，不是吗？
But new research, published last week in the Monthly Notices of the Royal Astronomical Society,
不过，上周，英国皇家天文学会在其月报中发表了一项新研究，
attempts to better pin down that mass by using a completely different technique.
该研究试图通过一种完全不同的方法来更精确地得出仙女座星系的质量。
It requires measuring how fast an object needs to travel to escape a galaxy's gravity; basically, its escape velocity.
测量某个星系的质量，首先需要测量物体脱离该星系引力的最低速度，也就是逃逸速度。
Galaxies with more mass will have more gravity, so you need to travel faster to get away from them.
质量越大的星系，引力就越大，也就需要越大的逃逸速度。
You'll also need to have a higher escape velocity if you're near the galaxy's center of mass, as opposed to at its edge.
在星系质心的物体比在星系两极的物体需要更大的逃逸速度。
Scientists can use all this data to work backwards.
科学家可以通过这些数据来回推。
By calculating escape velocities at different locations, they can do some math to figure out a galaxy's mass.
通过在不同地点计算逃逸速度，科学家就可以通过数学计算来得出某个星系的质量。
In this study, the team of astronomers used velocities of 86 speedy planetary nebulas, or the remnants of certain stars, in the Andromeda Galaxy.
在这项研究中，这组天文学家用到了仙女座星云中86个高速运动的行星状星云（某些恒星消亡后的残余物）的速度。
By doing some math and making some estimates, they were able to use these nebulas to calculate the escape velocity of Andromeda from different locations.
通过数学计算和估测，他们就能使用这些星云来计算仙女座星云不同地方的逃逸速度。
For example, at about 49,000 light-years from the galaxy's center, that escape velocity is around 470 kilometers per second.
比如，在距离星系中心大约4.9万光年的地方，逃逸速度大约是470km/s。
Then, from those escape velocity values, they were able to derive the mass of the galaxy itself.
接着，通过逃逸速度的绝对值，他们就能得出星系本身的质量。
And it came out to be around 800 billion times the mass of our Sun, which is roughly the same mass as the Milky Way,
计算结果是：仙女座星云的质量是太阳质量的近8000亿倍，跟银河系质量差不多，
not twice as massive or larger, like we used to suspect.
而并非我们过去想的：银河系质量的两倍甚至以上。
Still, the authors did note that they had to make certain educated assumptions in their calculations,
不过，该文作者确实强调，在计算中他们不得不做一些合情合理的假设，
so this mass value isn't 100% certain, just like all the ones that came before.
所以该质量的计算结果并非100%准确，这一点与以前的各种研究一样。
There's also a chance that, although they were moving really quickly,
还有一种可能性：虽然研究中涉及的行星状星云移动速度几块，
none of the planetary nebulas they studied were going anywhere close to the real escape velocity.
但它们的速度离真实的逃逸速度还差很远。
That would have also affected the mass estimate. So we'll have to run a few more models before we're totally certain.
这也可能会影响到质量的估值，所以还要对一些模型进行测试，才能完全确定。
One way or another, pinning down the deets on Andromeda is crucial for our understanding of galactic evolution, and the ultimate fate of the Milky Way.
不管怎样，对仙女座星云质量的准确估测对于我们理解银河系演化以及银河系最终走向来说至关重要。
After all, our galaxy and Andromeda are on their way to a collision in several billion years.
毕竟，银河系和仙女座星系在几十亿年后就会发生碰撞。
But when that happens and how it'll look depend on, surprise, how massive they are.
不过具体何时撞击以及撞击的形势如何都惊人地取决于它们的质量。
Luckily, we have, you know, some time before we need to get those measurements done precisely.
幸运的是，在撞击发生之前，我们有时间做精确的测量。
Thanks for watching this episode of SciShow Space!
感谢收看本期的《太空科学秀》！
If you would like to keep learning about the universe with us, you can do that at youtube.com/scishowspace,
如果您想继续与我们一起了解宇宙的话，您可以登录youtube.com/scishowspace，
where we have hundreds of episodes that are all very good, and also, we just keep uploading new ones every week.
这里有很多期节目都很棒，而且每周我们会上传新的节目。
