IGCSE Physics 0625: Life Cycle of a Sun-like Star

Astronomy students are researching a star named Ophira, which has a mass very similar to the Sun's. Ophira formed inside a large, slowly-collapsing cloud of gas and dust called the Ilvenna Nebula, and is currently a stable star part-way through its life.

(a) Describe how gravity acting on the material of the Ilvenna Nebula led to the formation of Ophira as a protostar, and state what begins inside a protostar once it becomes hot and dense enough for it to become a stable main-sequence star. [3]

(b) Once Ophira eventually uses up the hydrogen fuel in its core, describe the sequence of stages it will go through, given that its mass is similar to the Sun's, ending with the compact object it leaves behind. [3]

(c) State the one property of a star that determines whether it follows this pathway, or instead follows the alternative pathway that ends in a supernova and a neutron star or black hole. [1]

Part (a): From nebula to main-sequence star

A nebula is a large cloud of gas and dust. Within the Ilvenna Nebula, gravity pulls this material together, causing regions of the cloud to clump and contract. As a clump contracts under its own gravity, it heats up and becomes denser, forming a protostar.

Once the protostar has become hot and dense enough, nuclear fusion of hydrogen begins in its core. This marks the point at which Ophira becomes a stable main-sequence star. The energy released by fusion pushes outward, balancing the inward pull of gravity, so the star’s size stays roughly constant while it steadily fuses hydrogen.

Part (b): Ophira’s fate as a Sun-like star

Because Ophira’s mass is similar to the Sun’s, once it uses up the hydrogen fuel in its core it will follow the pathway of a lower-mass star:

$\text{red giant} \rightarrow \text{planetary nebula} \rightarrow \text{white dwarf}$

Ophira will first expand into a red giant. It will then shed its outer layers into space, forming a glowing shell of gas called a planetary nebula. Left behind at the centre, exposed once the outer layers are gone, is a small, hot, extremely dense core called a white dwarf. The compact object Ophira will eventually become.

Part (c): What determines a star’s eventual fate

The property that determines which pathway a star follows after the main-sequence stage is its mass.

A star with a mass similar to the Sun’s, like Ophira, follows the red giant → planetary nebula → white dwarf pathway described in part (b).
A star with a much greater mass instead expands into a red supergiant and ends its life in a supernova, leaving behind a neutron star or a black hole.

Final answers

(a) Gravity contracts nebula material into a protostar; nuclear fusion of hydrogen starting in the core marks the start of the main-sequence stage
(b) Ophira’s sequence is red giant → planetary nebula → white dwarf
(c) A star’s mass determines which of the two pathways it follows