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Comment by naavis | Hacker News Reader

It is just about the brightness. Answer number 1 in the FAQ states:

> A fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation.

Or was your question why are some so bright? That is mainly a function of size and speed.

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Could it also be composition? eg. Mainly water vs mainly rock vs rock with particular metallic impurities?

Meteors are very small. The bright light comes mostly from the molecules of atmosphere in front of the meteor being compressed, heated, and ionized[0] - essentially from the friction of the high-speed meteor entering the atmosphere.

I guess if the meteor was a ball of pure magnesium, it might glow brighter, but there isn't so much oxygen up at that height, so my no-calc-gut-feel estimate is that this would contribute orders of magnitude less brightness than the glowing atmospheric molecules.

[0] https://www.amsmeteors.org/meteor-showers/meteor-faq/#3

To the extent it's about composition it's probably mostly about how long the meteor holds together so it can retain velocity and the surface can stay hot enough to glow. A bunch of small pieces decelerate faster than one big piece. Compositional changes would only change the color somewhat but likely not enough to be worth commenting on.

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