Model of a methane molecule. Hydrocarbons are molecules made out of hydrogen atoms and carbon atoms. The simplest hydrocarbon molecule is methane, which is one carbon atom with four hydrogen atoms stuck to it.Because both carbon and hydrogen were common in nebulae, hydrocarbon molecules are very common in space.Many of these molecules are small, like methane, but others are very big, involving hundreds of atoms.he simplest hydrocarbon molecule is methane, which is one carbon atom with four hydrogen atoms stuck to it. Because both carbon and hydrogen were common in nebulae, hydrocarbon molecules are very common in space. Many of these molecules are small, like methane, but others are very big, involving hundreds of atoms.