The early universe contained only the lightest elements, hydrogen and nitrogen. Heavier elements, such as carbon, only formed in the astral nuclear reactions and were emitted when the stars exploded. The carbon found in a layer of recently discovered nebulae is billions Formed years ago when the universe was less than two billion years old. Can we know from this? ['Gas clouds contain hydrogen and nitrogen.', 'The earliest stars contained the only hydrogen.', 'Stars formed before the universe was two billion years old.', 'The carbon in the gas cloud later became part of the star.']