Cocoon nebula (IC 5146, centre left), optical image. This emission nebula glows as the hydrogen it contains is ionised by radiation from the hot young stars within it. The stars formed from the coalescence of the gas in the nebula. Gravity drew the gas into tighter and tighter balls until it was hot and dense enough for stellar reactions to begin. The bright central star is thought to be only around 100,000 years old, compared to five billion years for our Sun. The powerful stellar wind from this star has blown a hole in the dark gas cloud surrounding it, making the glowing region visible. The nebula lies some 4000 light years from Earth in the constellation Cygnus.