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From this photo it's easy to see that not all parts of a flame are made the same,
从这张照片很容易看出，火焰的各个部分并不都是一样的，
they can be brighter around the edges and darker in the middle.
边缘更亮，而在中心更暗。
Which is weird, because a flame is normally brightest in the middle,
这很奇怪，因为火焰通常在中间是最亮的，
since that's where you're looking at the most hot burning gas.
因为你看到高温气体在燃烧。
I mean, Look at the sun – actually, don't do that without a solar filter,
我的意思是，看看太阳-事实上，没有戴滤光镜的话还是别这么做吧。
but, it's a big ball of hot glowing gas, and it's definitely brightest in the middle.
但是，它是一个巨大的球体，里面充满着炽热气体，而且它的中心绝对是最明亮的。
In fact, not only does it get darker towards the edges, it gets redder.
事实上，它不仅在边缘变深，而且变得更红。
These dark, red edges of the sun are called "limbs," since "limbus" is latin for "edge".
这些深红色的太阳边缘被称为“肢端”，因为“肢端”在拉丁语中是“边缘”的意思。
And the limbs of the sun get darker and redder near the edge for three simple reasons:
太阳的边缘变得越来越暗红的原因很简单：
First, the sun is cooler closer to its "surface" and hotter deeper in.
首先，太阳的表层温度较低，而在更深层的地方温度则更高。
Second, hotter stuff glows brighter and yellower,
其次，温度越高的东西会发出更亮黄的光，
which you've probably seen with a hot poker or candle flame.
这你可能见过一块烧红的烙铁或蜡烛吧，就是那个样子。
And third, if you look across the edge of a sphere,
第三，如果你从一个球体的边缘看过去，
you have to look through more of the sphere for your line of sight
你的视线必须透过球体的更多部分，
to reach a given depth than when you look directly into the middle.
才能让你的视线达到给定的深度，而不是直接看中间。
If the sphere is only partially transparent (like the sun),
如果球体仅仅是部分透明的（像太阳一样），
when looking at the edge, you won't see as deep into the depths before your vision is blocked.
当你看太阳的边缘时，在你的视线被遮挡住之前，你看不了很深。
So, when you look directly – through a solar filter – into the middle of the sun,
所以，当你戴着滤光镜直视太阳中心时，
you see deeper in to hot gas which glows bright and yellow.
你可以看到更深处的炽热气体发出的亮黄色光芒。
And when you look at the edge of the sun, you see shallower, cooler, gas which glows darker and redder.
当你看着太阳的边缘，你会看到表层低温气体会发出的暗红光。
The same effect in reverse explains why some flames have bright edges and darker centers,
同样的效果反过来解释了为什么有些火焰边缘明亮而中心较为灰暗，
these flames are clearly hotter and brighter near their surfaces,
显而易见，这些火焰在其表面附近更热更亮，
likely because that's where the best mixture of oxygen and fuel is,
可能是因为那里恰好有生成氧气和燃料混合物的最佳条件，
so when we look at their edges,
因此，当我们看火焰的边缘时，
our line of sight passes through more hot light-emitting gases than when we look straight-on.
我们的视线穿过的发光气体比我们直视时要多。
Take a candelabra and turn it side-on, and you'll see exactly what's going on.
拿一把枝状大烛台，侧过来观察，你就会知道到底是怎么回事了。
The optical effect I describe in the video is called limb darkening or limb brightening.
我在视频中描述的光学效果被称为外缘变暗或外缘变亮。
It also explains the theory the appearance observe nebulae which are actually hollow expanding shell of glowing gases.
它也解释了这个理论，即观察到的星云实际上是由不断膨胀的发亮气体组成的。
