Describe the life cycle of a star: (a) a star is formed from interstellar clouds of gas and dust that contain hydrogen (b) a protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attract

Life Cycle of a Star

Understanding how a star forms, evolves and ends its life is essential for the Cambridge IGCSE Physics (0625) syllabus. The sequence below follows the key points (a)–(h) specified in the learning objective.

1. Formation from Interstellar Clouds (a)

Stars begin in giant interstellar clouds, also called nebulae, which consist mainly of hydrogen (≈ 90 %) with smaller amounts of helium and dust.

2. Collapse to a Protostar (b)

Gravitational attraction causes the cloud to contract. As the cloud contracts:

• The density increases.
• Potential energy is converted into thermal energy, raising the temperature.
• When the central temperature reaches a few thousand kelvin, the object is called a protostar.

3. Arrival on the Main Sequence – Stable Star (c)

When the inward pull of gravity is exactly balanced by the outward pressure of the hot gas (hydrostatic equilibrium), the protostar becomes a stable star on the main sequence.

The core temperature is now high enough for nuclear fusion of hydrogen:

\$4p \rightarrow {}^{4}\text{He} + 2e^{+} + 2\nu + 26.7\text{ MeV}\$

This reaction provides the outward pressure that counteracts gravity.

4. Exhaustion of Hydrogen Fuel (d)

Over millions to billions of years the star converts hydrogen into helium in its core. When the hydrogen supply is depleted, the balance described in (c) can no longer be maintained.

5. Expansion to Red Giants or Red Supergiants (e)

Two pathways depend on the star’s initial mass:

• Low‑ to intermediate‑mass stars (≤ 8 M☉) swell into red giants.
• High‑mass stars (> 8 M☉) become red supergiants.

In both cases the outer layers expand and cool, giving the characteristic red colour, while the core contracts and heats further.

6. End‑states of Low‑Mass Stars – Planetary Nebula & White Dwarf (f)

When a red giant’s outer envelope is expelled, a glowing shell of ionised gas – a planetary nebula – remains. The hot, dense remnant at the centre is a white dwarf, composed mainly of carbon and oxygen and supported by electron degeneracy pressure.

7. End‑states of High‑Mass Stars – Supernova, Neutron Star or Black Hole (g)

Red supergiants undergo core collapse once iron builds up in the centre. The collapse triggers a supernova explosion, ejecting a nebula enriched with heavy elements.

Depending on the remaining core mass:

• If the core mass is ≤ 3 M☉, it becomes a neutron star, supported by neutron degeneracy pressure.
• If the core mass exceeds this limit, gravity overcomes all known forces and a black hole forms.

8. Recycling of Material – New Stars and Planets (h)

The supernova nebula mixes freshly forged elements (e.g., carbon, oxygen, iron) with the surrounding interstellar medium. Over time, this enriched material can collapse again to form new stars and planetary systems, completing the cosmic cycle.

Summary Table