The Sun, Stars and the Life Cycle of Stars

This lesson explains The Sun, Stars and the Life Cycle of Stars for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. The goal is to explain the idea accurately and apply it in an unfamiliar context.

Core: what is the Sun?

The Sun is a star of medium size. It consists mostly of hydrogen and helium and radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.

The Sun is one of many billions of stars in the Milky Way galaxy. Other stars in the Milky Way are much farther from Earth than the Sun is.

Core: what is a light-year?

A light-year is the distance travelled by light through a vacuum in one year. It is a unit of distance, not time.

Light-years are useful because distances between stars and galaxies are extremely large when written in metres.

A Core answer should define the unit in words. The numerical conversion belongs to the Supplement requirement below.

Core original question

(a) State two facts about the composition of the Sun. [2]

The Sun consists mostly of hydrogen and helium.

(b) State the three main regions of the electromagnetic spectrum in which the Sun radiates most of its energy. [3]

Infrared, visible and ultraviolet.

(c) Define a light-year. [1]

The distance travelled by light through a vacuum in one year.

Supplement: how do stars release energy?

Stars release energy through nuclear fusion. In a main-sequence star, hydrogen nuclei fuse to form helium nuclei, releasing energy.

A main-sequence star remains stable because two effects balance:

• inward gravitational attraction
• outward pressure associated with the hot material and energy produced by fusion

If the two effects balance, the star does not collapse or expand rapidly.

Supplement: how is a star formed?

A star begins in a cloud of gas and dust called a nebula.

1. Gravity pulls gas and dust together.
2. The material becomes denser and hotter, forming a protostar.
3. When the core becomes hot enough, nuclear fusion begins.
4. The object becomes a stable main-sequence star.

Supplement: what happens after the main-sequence stage?

The later stages depend on the initial mass of the star.

A star with a mass similar to the Sun

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

The white dwarf cools as it radiates energy.

A star much more massive than the Sun

$\text{main sequence} \rightarrow \text{red supergiant} \rightarrow \text{supernova} \rightarrow \text{neutron star or black hole}$

A supernova can eject material into space. That material may become part of new stars and planets.

Supplement: the numerical value of a light-year

The syllabus gives:

$1\text{ light-year} = 9.5 \times 10^{15}\text{ m}$

This value can be used to convert a distance in light-years into metres.

Supplement worked question

A star is 4.0 light-years from Earth. Calculate its distance from Earth in metres. [2]

$d = 4.0 \times 9.5 \times 10^{15}$

$d = 3.8 \times 10^{16}\text{ m}$

Original marking guidance

• one mark for multiplying by $9.5 \times 10^{15}$
• one mark for $3.8 \times 10^{16}$ m

Supplement original explanation question

A main-sequence star remains stable for a long period.

Explain why the star does not collapse. [2]

The inward gravitational attraction is balanced by an outward pressure maintained by the high temperature and energy released through nuclear fusion.

Original marking guidance

• one mark for inward gravitational attraction
• one mark for balancing outward pressure linked to the hot core or fusion

Common mistakes

• Telling Core candidates to skip the entire page. The Sun, galaxies and the definition of a light-year contain Core outcomes.
• Calling a light-year a unit of time. It is a distance.
• Saying stars burn hydrogen chemically. Stars release energy through nuclear fusion.
• Mixing the two life-cycle branches. A Sun-like star becomes a red giant and white dwarf; a much more massive star becomes a red supergiant and then a supernova.
• Skipping the protostar stage. The formation sequence includes nebula, protostar and main sequence.
• Giving only a black hole as the massive-star remnant. The remnant can be a neutron star or, for a sufficiently massive core, a black hole.
• Writing only that pressure equals gravity. Name the inward gravitational attraction and the outward pressure associated with the hot, fusing star.

Exam technique

For a sequence question, write one stage per line or use arrows. For a stability question, use a balanced-force explanation. For a Core light-year definition, give the distance travelled by light in a vacuum in one year without adding unnecessary numerical detail.

How this is examined

Core Papers 1 and 3 can test the Sun’s composition, its main emitted regions, the Milky Way and the definition of a light-year. Extended Papers 2 and 4 can additionally test fusion, the light-year conversion, stellar stability and both life-cycle branches. Practical papers do not centre on stellar experiments, although astronomical data can be used to assess graph or calculation skills.