can be brighter around the edges and darker in the middle. Which is weird, because a flame

is normally *brightest* in the middle, since that's where you're looking at the most

hot burning gas.

I mean,

Look at the sun – actually, don't do that without a solar filter – but, it's a

big ball of hot glowing gas, and it's definitely brightest in the middle. In fact, not only

does it get darker towards the edges, it gets redder. These dark, red edges of the sun are

called “limbs,” since “limbus” is latin for “edge”. And the limbs of the

sun get darker and redder near the edge for three simple reasons:

First, the sun is cooler closer to its “surface” and hotter deeper in.

Second, hotter stuff glows brighter and yellower, which you've probably seen with a hot poker

or candle flame.

And third, if you look across the edge of a sphere, you have to look through more of

the sphere for your line of sight to reach a given depth than when you look directly

into the middle. If the sphere is only partially transparent (like the sun), when looking at

the edge, you won't see as deep into the depths before your vision is blocked.

So, when you look directly – through a solar filter – into the middle of the sun, you

see deeper in to hot gas which glows bright and yellow. And when you look at the edge

of the sun, you see shallower, cooler, gas which glows darker and redder.

The same effect in reverse explains why some flames have bright edges and darker centers

– these flames are clearly hotter and brighter near their surfaces, likely because that's

where the best mixture of oxygen and fuel is, so when we look at their edges, our line

of sight passes through more hot light-emitting gases than when we look straight-on. Take

a candelabra and turn it side-on, and you'll see exactly what's going on.

The optical effect I describe in the video is called limb darkening

or limb brightening.

It also explains the theory the appearance observe nebulae

which are actually hollow expanding shell of glowing gases