# The Sun and the Life Cycle of Stars

> Physics · CIE IGCSE 0625
> Source: https://www.owlsprep.com/study/cie-0625-u6-the-sun-and-the-life/

This guide covers the Core content on the Sun as a star, galaxies, the Milky Way and light-years, plus the Extended (Supplement) content on nuclear fusion and the life cycles of Sun-like and massive stars, aligned to CIE IGCSE Physics 0625 outcomes 6.2.1 and 6.2.2.

**Prerequisites:** [The Solar System (the Sun, planets and orbits)](https://www.owlsprep.com/study/cie-0625-u6-the-solar-system/)

## Learning objectives

- Describe the Sun as a medium-sized star made mostly of hydrogen and helium that radiates in the infrared, visible and ultraviolet (Core)
- State that galaxies contain many billions of stars, that the Sun is a star in the Milky Way, and that distances are measured in light-years (Core)
- (Extended) Describe the life cycle of a star the same mass as the Sun: nebula, protostar, main-sequence star, red giant, planetary nebula, white dwarf
- (Extended) Describe the life cycle of stars much more massive than the Sun, including red supergiant, supernova, neutron star and black hole
- (Extended) Know that stars are powered by nuclear reactions, and that in stable stars these are the fusion of hydrogen into helium

## The Sun as a Star (Core)

The Sun is a **star** of **medium size**. Like other stars, it is a huge ball of hot gas made up **mostly of hydrogen and helium**.

The Sun radiates most of its energy in three regions of the electromagnetic spectrum:

- Infrared
- Visible light
- Ultraviolet

> **tip**
>
> At Core level you are not expected to know how the Sun produces its energy. The idea that stars are powered by the fusion of hydrogen into helium is Extended (Supplement) content.

**Worked example:** Describe the Sun. In your answer, state its size compared with other stars, what it is made mostly of, and the regions of the electromagnetic spectrum in which it radiates most of its energy.

1. Size: the Sun is a star of medium size (a medium-sized star).
2. Composition: it is made mostly of hydrogen and helium.
3. Radiation: it radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.

> **Exam tip:** For a Core answer about the Sun, make three points only: it is a medium-sized star, it is made mostly of hydrogen and helium, and it radiates most of its energy in the infrared, visible and ultraviolet. You do not need fusion at Core.

## Galaxies, the Milky Way and Light-Years (Core)

**Galaxy** — A collection of many billions of stars.

Our Sun is just one star in a galaxy called the **Milky Way**. The other stars that make up the Milky Way are **much, much further away** from the Earth than the Sun is.

Because these distances are so large, astronomers measure them in **light-years**. One light-year is the **distance** travelled by light (in the vacuum of space) in **one year**. A light-year is therefore a unit of distance, not a unit of time.

> **Extended only**
>
> Extended (Paper 2/4) candidates must also know the value of a light-year: 1 light-year = 9.5 x 10^15 m. Core candidates can skip this figure.

**Worked example:** State what is meant by a light-year, and explain why astronomers use light-years rather than metres to measure the distances to other stars.

1. Definition: a light-year is the distance travelled by light in one year.
2. Reason: the distances between stars are extremely large, so measuring them in metres would give inconveniently huge numbers; the light-year is a much more manageable unit for such distances.

> **Exam tip:** Learn the qualitative definition of a light-year (a distance, not a time). The value 1 light-year = 9.5 x 10^15 m is only needed at Extended.

## Extended Only: The Life Cycle of Sun-like Stars

> **Extended Only Content**
>
> This section is only required for students taking Extended tier Papers 2 and 4. Core tier students can skip it.

A star is formed from **interstellar clouds of gas and dust that contain hydrogen** (a nebula). Its own **internal gravitational attraction** pulls the cloud together, so it collapses and its temperature increases, forming a **protostar**. The protostar becomes a **stable star** when the inward force of gravitational attraction is balanced by an outward force due to the high temperature at the centre of the star. This stable stage is the **main sequence**, the longest stage of a star's life.

All stars eventually **run out of hydrogen** as fuel for the nuclear reaction. When most of the hydrogen in the centre of a less massive star (such as the Sun) has been converted to helium, the star expands to form a **red giant**. A red giant from a less massive star then ejects its outer layers as a **planetary nebula**, leaving a hot, dense **white dwarf** star at its centre.

**Worked example:** Put the following stages of the Sun's life cycle in the correct order, starting from the earliest: red giant, nebula, main sequence, white dwarf, protostar, planetary nebula.

1. The star is formed from a nebula (cloud of gas and dust containing hydrogen).
2. Gravity pulls the cloud together, forming a protostar that heats up.
3. When gravity is balanced by the outward force from the hot centre, it becomes a stable main sequence star.
4. When hydrogen in the centre runs out, the star expands to form a red giant.
5. The red giant ejects its outer layers as a planetary nebula, leaving a white dwarf.
6. Correct order: nebula -> protostar -> main sequence -> red giant -> planetary nebula -> white dwarf

> **Exam tip:** Writing the stages of the Sun's life cycle in order is a common Extended question. Note the whole life cycle is Supplement content — Core (Paper 1/3) candidates are not assessed on it.

## Extended Only: The Life Cycle of Massive Stars

> **Extended Only Content**
>
> This section is only required for students taking Extended tier Papers 2 and 4. Core tier students can skip it.

**Supernova** — The explosion of a red supergiant, which forms a nebula containing hydrogen and new heavier elements.

A more massive star follows the same early stages as the Sun (nebula, protostar, main sequence), but because it is more massive it expands to form a **red supergiant** rather than a red giant when most of the hydrogen in its centre has been converted to helium.

A red supergiant explodes as a **supernova**, forming a nebula that contains hydrogen and new heavier elements and leaving behind a **neutron star** or, for the most massive stars, a **black hole** at its centre. A black hole has gravity so strong that not even light can escape. The nebula thrown out by a supernova may go on to form **new stars with orbiting planets**.

**Worked example:** State two differences between the life cycle of a star much more massive than the Sun and the life cycle of the Sun.

1. Difference 1: the massive star explodes as a supernova, whereas the Sun ejects its outer layers as a planetary nebula (no explosion).
2. Difference 2: the massive star leaves behind a neutron star or a black hole, whereas the Sun leaves behind a white dwarf.

> **Exam tip:** Extended questions often ask you to compare the life cycles of Sun-like and massive stars. State the differences in the final stages clearly to gain full marks.

## Extended Only: How Stars Release Energy (Nuclear Fusion)

> **Extended Only Content**
>
> That stars are powered by nuclear reactions is Supplement content. Core tier students can skip this section.

Stars are powered by **nuclear reactions that release energy**. In a stable star, these reactions are the **fusion of hydrogen into helium**. The energy released by fusion at the centre of the star provides the outward force that balances the inward pull of gravity, keeping the star stable throughout its main sequence stage.

> **tip**
>
> You do not need to recall any fusion equations, and you do not need E=mc² or to calculate the energy released. You only need to state that hydrogen fuses into helium, releasing energy.

**Check your understanding**

1. What is the energy source of a stable star such as the Sun?

   *Why:* Remember that fusion powers stars, not fission, which is used in nuclear power stations on Earth.

2. What two forces balance to keep a main sequence star stable?

   *Why:* If this balance breaks, the star will contract or expand, moving to the next stage of its life cycle.

> **Exam tip:** State the reaction (hydrogen fuses into helium) and that it releases energy. No equations, mass defect or E=mc² are required at IGCSE.

## Common pitfalls

- **Wrong:** Treating the Sun's life cycle (or fusion) as Core content
  - Why it fails: In CIE 0625, the whole star life cycle and the fact that stars are powered by fusion of hydrogen into helium are Supplement outcomes; Core only requires the Sun as a star plus galaxies and light-years
  - Correct: Only revise the life cycle and fusion if you are sitting the Extended tier (Papers 2 and 4)
- **Wrong:** Treating a light-year as a length of time
  - Why it fails: A light-year is a distance (how far light travels in one year), not a duration
  - Correct: Define a light-year as the distance travelled by light in one year
- **Wrong:** (Extended) Including supernova, neutron star or black hole in the Sun's life cycle
  - Why it fails: The Sun is not massive enough to form a supernova; these stages only apply to stars much more massive than the Sun
  - Correct: Use only red giant, planetary nebula and white dwarf when describing the Sun's future
- **Wrong:** (Extended) Saying a protostar or white dwarf produces energy by fusion
  - Why it fails: A protostar has not yet started fusion; a white dwarf is a leftover core in which no fusion occurs
  - Correct: State that fusion begins only when a protostar becomes a stable main sequence star, and that a white dwarf no longer undergoes fusion

## Cheatsheet

| Item | Tier | What you must know |
| --- | --- | --- |
| The Sun | Core | Medium-sized star; mostly hydrogen and helium; radiates mainly infrared, visible and ultraviolet |
| Galaxy / Milky Way | Core | A galaxy is many billions of stars; the Sun is a star in the Milky Way; other stars are much further than the Sun |
| Light-year | Core (value Extended) | Distance light travels in one year; value 1 ly = 9.5 x 10^15 m is Extended only |
| Star's energy source | Extended | Nuclear reactions; in stable stars, fusion of hydrogen into helium releases energy |
| Sun-like life cycle | Extended | Nebula -> protostar -> main sequence -> red giant -> planetary nebula -> white dwarf |
| Massive-star life cycle | Extended | Nebula -> protostar -> main sequence -> red supergiant -> supernova -> neutron star or black hole |

## What's next

You have now covered the Sun as a star, galaxies and light-years (Core), and the Extended star life cycles and nuclear fusion. Next, move on to the Universe, where you will meet redshift and the Big Bang theory (Core) and, for Extended, the cosmic microwave background radiation and the Hubble constant. Core candidates should make sure they can describe the Sun and define a light-year; Extended candidates should practise writing out both the Sun-like and massive-star life cycles in order.

- [The Universe: Redshift, Big Bang and Hubble's Law](https://www.owlsprep.com/study/cie-0625-u6-the-universe-redshift-big-bang/)

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