OxfordAQA International GCSE Physics: Life cycle of a star (9203) – Revision Notes

These revision notes cover section 3.8.1 (Life cycle of a star, statements a to i) of the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. The qualification is untiered. For full explanations and the longer worked examples, read the study guide, then test yourself with the practice questions. Related pages: the OxfordAQA IGCSE Physics hub, the printable checklist and the 10-minute diagnostics.

Key facts at a glance

Spec	Fact to recall
a	Gravity pulls dust and gas (mainly hydrogen and helium) together to form a star. Smaller masses can become planets, or even stars.
b	Main sequence: hydrogen nuclei fuse to helium nuclei in the core. Forces inside the star are balanced, so it is stable.
c	The core has the greatest temperature and density. Most fusion happens there.
d	More massive star means hotter core and heavier nuclei made by fusion.
e, f	Life cycle is set by mass. When hydrogen runs out, the next stage depends on mass.
g	Larger star: red supergiant, supernova, then neutron star or black hole.
h	Smaller star like the Sun: red giant, white dwarf, black dwarf.
i	Fusion makes all the natural elements. Supernovae spread them. Elements heavier than iron form in a supernova.

The two routes

Gas and dust --gravity--> star on the main sequence
Smaller star (like the Sun):  red giant -> white dwarf -> black dwarf
Larger star:  red supergiant -> supernova -> neutron star OR black hole

• Mass decides the route. Both routes start the same way and end differently.
• The supernova belongs only to the larger-star route.
• The white dwarf and black dwarf belong only to the smaller-star route.

Main sequence: balanced forces

Effect	Direction	Cause
Gravity	Inwards	Mass of the star pulling itself together
Outward push	Outwards	Very hot gas produced by fusion in the core

Balanced means the star neither shrinks nor swells. If the two were not balanced, the star would change size. Fusion has not stopped; it is going on in the core all the time.

Method: explaining why a star is stable (steps)

1. State that hydrogen nuclei fuse to helium nuclei in the core, releasing energy.
2. State that gravity pulls inwards.
3. State that the outward push from the hot gas acts against it.
4. Conclude that the forces are balanced, so the size stays steady.

Mass and the core

• Larger mass gives a stronger gravitational squeeze, so a hotter and denser core.
• A hotter core fuses faster and can make heavier nuclei.
• A larger star therefore uses its fuel faster and has a shorter life.

Worked reminder: fuel versus lifetime

A star with 10 times the Sun’s mass has 10 times the fuel. If it lasts 25 million years while the Sun lasts 10 000 million years, the lifetime ratio is 10 000 ÷ 25 = 400. Rate of use = fuel ratio × lifetime ratio = 10 × 400 = 4000 times faster than the Sun.

Why stars last millions of years

• Fusion releases a very large amount of energy per kilogram of hydrogen.
• The star holds a vast mass of hydrogen.
• The balance of forces keeps fusion at a steady rate.

Reminder from the study guide: the Sun fuses about 5.9 × 10¹¹ kg of hydrogen every second, yet its fuel would last for billions of years.

Where the elements come from

1. The early universe contained only hydrogen (spec wording).
2. Stars formed. Fusion in their cores made helium, then heavier nuclei.
3. Red giants and red supergiants fuse helium to carbon, then make nuclei such as nitrogen and oxygen.
4. Any element heavier than iron comes from a supernova.
5. The explosion of a massive star distributes the elements through the universe.

Formation: points to hit

• Clouds of dust and gas, mainly hydrogen and helium, in space.
• Gravitational attraction pulls the material together.
• The material gets denser and hotter as it falls inwards.
• Smaller masses can form and be attracted to a larger mass, becoming planets, or even stars.

Stage-by-stage summary of each route

Stage	What happens	Route
Main sequence	Hydrogen fuses to helium in the core; forces balanced	Both
Hydrogen runs out	Next stage depends on mass	Both
Red giant	Star expands; sheds outer layers of gas	Smaller star
White dwarf	Exposed core	Smaller star
Black dwarf	White dwarf has cooled	Smaller star
Red supergiant	Expands, cools, turns red; helium fuses to carbon, then heavier nuclei such as nitrogen and oxygen	Larger star
Supernova	Outer layers blasted away; core collapses	Larger star
Neutron star or black hole	Which one depends on mass	Larger star

Worked reminder: energy over time

Data supplied: a star gives out 1.9 × 10²⁷ J every second, and 1 kg of hydrogen fused releases 6.4 × 10¹⁴ J.

• Hydrogen fused per second = 1.9 × 10²⁷ ÷ 6.4 × 10¹⁴ = 3.0 × 10¹² kg.
• If 6.0 × 10²⁹ kg of hydrogen is available, time = 6.0 × 10²⁹ ÷ 2.97 × 10¹² = 2.0 × 10¹⁷ s, which is 6.4 × 10⁹ years (taking one year as 3.156 × 10⁷ s).

Method: power ÷ energy per kilogram gives mass per second; fuel ÷ mass per second gives time; divide by seconds in a year for years.

Sentence frames for 4-mark answers

• “Gravity pulls the dust and gas together, and the material gets hotter and denser until hydrogen nuclei fuse.”
• “The star is stable because the inward force of gravity is balanced by the outward push of the hot gas.”
• “The star follows a different route because its mass is different. After the main sequence, mass decides what happens.”
• “Heavier elements were made by fusion in stars, and the supernova distributed them through the universe.”

Reading and drawing charts

1. Find the common start: dust and gas, then main sequence star.
2. Find the split. A label such as “Sun-sized star” or “much more massive star” shows the mass.
3. Follow the arrows in order. Each stage should appear once.
4. Check that no stage has jumped across branches.
5. If you draw one, label the split with the word mass.

Must-know distinctions

Mix-up	Correct version
Red giant and red supergiant	Red giant: smaller star. Red supergiant: larger star.
White dwarf and neutron star	White dwarf: end of the smaller route. Neutron star: left after a supernova.
Neutron star and black hole	Both can follow a supernova. Which one forms depends on the mass of the collapsing core.
White dwarf and black dwarf	A white dwarf is the exposed core. A black dwarf is that core after it has cooled.
Stable and fusion stopped	A main sequence star is stable and fusion is still going on.

Quick self-test

1. What pulls dust and gas together to form a star?
2. Name the nuclear process that releases energy in a main sequence star.
3. Which nuclei fuse to which in the core of a main sequence star?
4. Where in a star are the temperature and density greatest?
5. State why a main sequence star is stable.
6. What decides the life cycle of a star?
7. A star has 20 times the Sun’s mass and lasts 8 million years; the Sun lasts 10 000 million years. How many times faster does it use fuel?
8. Give the order of stages for a star like the Sun after the main sequence.
9. Give the order for a much larger star after the main sequence.
10. What are the two possible end points after a supernova?
11. Where are elements heavier than iron formed?
12. What happens to the elements made in a star when it explodes?

Answers

1. Gravitational attraction.
2. Nuclear fusion.
3. Hydrogen nuclei fuse to make helium nuclei.
4. In the core (centre).
5. The forces inside it are balanced (gravity inwards, outward push from the hot gas).
6. Its mass.
7. Lifetime ratio = 10 000 ÷ 8 = 1250. Rate = 20 × 1250 = 25 000 times faster.
8. Red giant, white dwarf, black dwarf.
9. Red supergiant, supernova, then neutron star or black hole.
10. A neutron star or a black hole.
11. In a supernova.
12. They are distributed through the universe by the supernova explosion.

Where marks are usually lost

• Giving the supernova to a star like the Sun.
• Writing that fusion “stops” in a stable main sequence star. Say the forces are balanced.
• Saying “gravity balances fusion”. Name the outward effect: the push of the very hot gas.
• Writing “red giant” for a larger star, or “red supergiant” for the Sun.
• Leaving out “depends on mass” when asked why two stars follow different routes.
• Saying all elements form in supernovae. Only those heavier than iron need one.
• Giving only one of neutron star and black hole when the question asks for the possible outcomes.
• Omitting the final point in an elements question: the supernova distributes the elements.
• Describing the black dwarf as a black hole. It is a cooled white dwarf.

Official syllabus

OxfordAQA International GCSE Physics (9203), specification version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Section 3.8.1, Life cycle of a star.