The birth of the most massive stars — those ten to a hundred times the mass of the Sun — has posed an astrophysical riddle for decades. Massive stars are dense enough to fuse hydrogen while they’re still gathering material from the gas cloud, so it was a mystery why their brilliant radiation does not heat the infalling gas and blow it away. New simulations show that as the gas cloud surrounding a massive star collapses, it forms dense filamentary structures that absorb the star’s radiation when it passes through them. This makes heated nebulae flicker like a candle flame.