When stars die, sometimes after billions of years of creating energy and light in their cores, they can bloat up into swollen red giants, enormous and luminous objects. Silicon and carbon in their upper layers can blow out into the space around them, cooling and condensing into grains of material astronomers call
dust.
The galaxy is littered with dense clouds of dust, blocking our view of stars behind them due to their opacity. They can appear brown or black, like holes in space, dark and forbidding.
But dust also appears in clouds where stars are being born, and sometimes these infant stars are massive and blue, sending out vast amounts of energy into their birth nebula. And when they do the dust around them reflects that light, sending some of it Earthward, and we see those clouds not as voids but as lustrous, cerulean, even gossamer cosmic threads.