The early universe contained only the lightest elements: hydrogen and helium. Heavier elements, such as carbon, were produced only when stars nucleated, and emitted when the stars exploded, and existed in the subsequently formed gas clouds. Gas clouds contained carbon 4 billion years ago, when there were only stars in the universe no more than 2 billion years old. It can be inferred from this:. ['The carbon in the gas cloud has since formed part of the star.', 'As of now, no star is as old as a gas cloud.', 'Gas clouds also contain both hydrogen and helium.', 'The formation of carbon in this gas cloud does not exceed 6 billion years.']