# The Life of a Star — Study Notes

A one-page summary for revision. From lifeofastar.com.au

## The big idea
How a star lives and dies is decided mostly by ONE thing: **how much mass it starts with.**
- Low/medium mass (up to ~8 Suns) → quiet ending (white dwarf)
- High mass (more than ~8 Suns) → explosive ending (neutron star, magnetar, or black hole)

## The stages (in order)
1. **Nebula** – a giant cloud of gas and dust. Stellar nurseries form here.
2. **Protostar** – gravity pulls gas into a hot ball; fusion hasn't started yet.
3. **Main sequence** – fusion of hydrogen → helium switches on. The long, stable adult life. (The Sun is here.)

Then the path splits by mass:

### Road A — small & medium stars
4. **Red giant** – core runs low on hydrogen, star swells and cools.
5. **Planetary nebula** – outer layers gently thrown off (nothing to do with planets!).
6. **White dwarf** – Earth-sized hot core, slowly cooling for billions of years. *(The Sun's fate.)*

### Road B — massive stars
4. **Supergiant** – fuses heavier elements up to **iron**. Iron is the dead end (fusing it uses energy).
5. **Supernova** – core collapses in under a second and rebounds in a giant explosion. Makes heavy elements (gold, etc.).
6. **Neutron star / magnetar** *or* **Black hole** – depending on how much mass is left.

## Key objects
- **Neutron star:** ~20 km across but heavier than the Sun. A teaspoon ≈ a billion tonnes.
- **Pulsar:** a spinning neutron star whose beam sweeps past us like a lighthouse.
- **Magnetar:** a neutron star with the STRONGEST magnetic field known (~10^15 gauss; Earth ≈ 0.5 gauss).
  - Powered by its decaying magnetic field (pulsars are powered by spin).
  - **Soft gamma repeaters (SGRs):** magnetars that burst gamma rays when the crust cracks (starquakes).
  - **Giant flares:** rare, huge energy releases. The 2004 SGR 1806−20 flare disturbed Earth's atmosphere from 50,000 light-years away.
- **Black hole:** gravity so strong not even light escapes. From the most massive stars.

## Numbers worth remembering
- Sun fuses ~600 million tonnes of hydrogen per second.
- Sun's age ≈ 4.6 billion years, with ~5 billion to go.
- Light-year = distance light travels in 1 year ≈ 9.5 trillion km (it's a distance, not a time!).
- Nearest star (Proxima Centauri) ≈ 4.2 light-years away.

## One sentence to take away
We are made of star stuff — the calcium in your bones and the iron in your blood were forged inside stars that died before the Sun was born.

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*Made by a Year 8 student. Facts checked against NASA and ESA. Free to use for study and teaching.*