Thread by @anisotropela

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Why does the Carina Nebula that #JamesWebb recently showed us have such a sharp edge?

A thread:

🔵The top part of the picture is full of very hot stars emitting intense UV radiation.

🟤The bottom part is a dense gas cloud of dust, molecules, and atoms, mostly hydrogen.

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In neutral gas, UV photons can't travel very far before hitting an atom.
This 𝘪𝘰𝘯𝘪𝘻𝘦𝘴 the atom into a proton ⊕ and an electron ⊖

So the top part is ionized, while the bottom part is neutral.

Ionized gas is largely transparent, while neutral gas is largely opaque.

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Molecules and also dust grains are similarly destroyed.
In this way, the hot stars carve out an ionized bubble in the gas cloud from which they were born.

In zoology, this behavior of eating your mother is called "matriphagy". Do not google it before bedtime!

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But atoms can recombine!
When a proton and an electron meet, they stick together.

⊕ + ⊖ → ☉ + ⤳

(⅔ of these recombination result in the emission of a #LymanAlpha photon, but that's another beautiful story).

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Therefore, the size of the bubble is given by the battle between ionizations and recombinations.

If we know the brightness of the stars, and the density of the gas (and some physics), we can calculate the size of the ionized region. More photons ⇒ larger bubble.

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On the other hand, the transition from the ionized the the neutral region is given by the density and "size" of the atoms. The size is given by quantum mechanics (and temperature).

So we can also calculate how far the UV photons penetrate into the neutral region.

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And it turns out that, for typical densities, temperatures, & UV photon outputs,

➡️ ionized region: ~10 lightyears,
and
➡️transition region: ~0.01 lightyear.

That why the edge looks so sharp!

I end with a picture of the full Carina Nebula. #JamesWebb's in the bottom right.