Two cities endure a series of fires. In one, the wind carries ash far beyond the outskirts, leaving the sky clear. In the other, ash settles as a dense veil, painting sunsets crimson. Roughly the same thing, just on a galactic scale, happens with supernova explosions, the final flares of massive stars.
When supernovae ignite one after another in a tight cluster of young stars, their shock waves merge into a powerful flow. This flow can push dust, microscopic grains of heavy elements, out of the stellar cloud and even punch through the galaxy's entire gas disk. If the galaxy is compact and gas-rich, the dust is almost completely swept away. Such a galaxy shines with bright blue light, and astronomers call it a 'blue monster.' If the stellar mass is large, the dust lingers, shrouding the galaxy and absorbing blue rays. Then it becomes a 'red monster.'
The decisive moment is whether the shock wave crosses two barriers: the home molecular cloud where the stars were born, and the entire gas layer of the galaxy. If it does, we see a blue giant without dust. If not, a red one wrapped in a dusty blanket. This same mechanism explains why, by the sixth to seventh billion years after the Big Bang, massive dusty galaxies become commonplace: they accumulate enough stars to hold onto their dust.
So the two faces of the early universe, dazzling blue and somber red galaxies, turn out not to be different evolutionary paths but two outcomes of the same process. It all came down to whether the stellar ash could fly away.
Dust grains in galaxies are mostly made of carbon and silicon, and each one is about the size of a particle of tobacco smoke. Despite their tiny size, they efficiently absorb blue light, like a thick curtain.