The Star Life Cycle

Stars are born, change over time, and end in different ways. The Sun is a medium-sized star. A star’s life depends mainly on its mass (how much matter it has).

1) Birth in a Nebula

Interstellar clouds of gas and dust (called nebulae), mostly hydrogen, slowly clump together due to gravity. As the cloud collapses, the centre heats up. This hot, dense growing ball is a protostar.

2) Stable Star (Main Sequence)

When the inward pull of gravity is balanced by the outward push from the hot, high-pressure core, the star becomes stable. This balance is like a steady tug-of-war that ends in a draw.

In the core, hydrogen nuclei fuse to form helium and release energy. This energy makes the star shine:

$$4\,^1\mathrm{H} \;\rightarrow\; ^4\mathrm{He} + \text{energy}$$

The Sun is in this long, stable stage.

3) Running Out of Hydrogen

Over time, hydrogen in the core is used up. Gravity squeezes the core smaller and hotter, while the outer layers expand.

4) Two Pathways (Set by Mass)

Less massive stars (like the Sun)

• Red giant: the star swells and cools at the surface.
• Planetary nebula: outer layers drift away, forming a glowing shell of gas.
• White dwarf: the hot, dense core remains and slowly cools.

More massive stars

• Red supergiant: a much larger swelling stage.
• Supernova: a rapid core collapse and a huge explosion that makes and scatters heavier elements.
• Neutron star or black hole: an extreme, compact remnant at the centre.

5) Recycling the Universe

Gas and dust from planetary nebulae and supernovae mix into space. New nebulae form, and new stars (with planets) can be born from this material. Many elements in our bodies were made inside stars.

Common Misconceptions

• A planetary nebula is not a planet; it is gas that a red giant has shed.
• A white dwarf is not a planet; it is a hot, dense stellar core with no fusion.
• Supernovae come from very massive stars; stars like the Sun do not explode as supernovae.