In space, an interstellar cloud of dust and gas collapses to form a protostar. The cloud contains hydrogen.

1. (a)Describe the energy transfers that take place as the cloud collapses and forms the protostar.[2]
2. (b)(i)The collapse results in a nuclear reaction in the protostar that involves the isotope hydrogen-3 (³H). The isotope hydrogen-3 (³₁H) is radioactive. It decays by beta particle emission. State how the nuclei of isotopes of the same element are different.[1]
3. (b)(ii)Explain why hydrogen-3 cannot decay by alpha particle emission.[1]
4. (b)(iii)Complete the equation for the decay of hydrogen-3 to an isotope of helium (He). ³₁H → ........ β + ........ He[2]
5. (b)(iv)Explain how the nuclear reaction in the protostar stops further collapse as the protostar becomes a stable star.[3]
6. (c)The cloud of dust and gas that collapses also contains atoms of the heaviest elements. State which part of the life cycle of a star is responsible for the production of the heaviest elements.[1]