8.1.2a The life cycle of a star

A star's life cycle depends on its mass

1. Every star goes through a life cycle, a sequence of stages from formation to its final state.
2. The path a star follows is decided by its mass, so there are two pathways to learn: a star about the size of the Sun, and a star much more massive than the Sun.
3. All stars begin the same way, forming from a nebula, a cloud of dust and gas in space.
4. Gravitational attraction pulls the dust and gas together, so the material becomes denser and hotter, forming a protostar.
5. When the protostar's core is hot enough, nuclear fusion begins and releases energy.
6. The star then becomes a stable main sequence star, where it spends most of its life.
7. A main sequence star is stable because the inward pull of gravity is balanced by the outward push from the energy released by fusion.

The life cycle of a star about the size of the Sun

1. A star about the size of the Sun follows the path: a nebula, then a protostar, then a main sequence star, then a red giant, then a white dwarf, and finally a black dwarf.
2. It stays on the main sequence while gravity and fusion pressure stay balanced.
3. When the core runs low on fuel, the balance changes and the star swells into a red giant, which is larger and cooler.
4. The star then sheds its outer layers, leaving a hot, dense core called a white dwarf.
5. Over an extremely long time the white dwarf cools and fades into a black dwarf.
6. A star this size does not explode as a supernova.

The life cycle of a star much more massive than the Sun

1. A much more massive star starts the same way: a nebula, then a protostar, then a main sequence star.
2. Because it has far more mass, its later stages are different.
3. After the main sequence, it swells into a red supergiant, even larger than a red giant.
4. The red supergiant then explodes as a supernova, a huge and sudden explosion.
5. The core left behind becomes either a neutron star or, if enough mass remains, a black hole.
6. Which one forms depends on the mass of the remaining core.

8.1.2b Formation of the elements in stars

Fusion in stars forms new elements

1. An element is identified by the number of protons in its nucleus.
2. When nuclei fuse, they make a new nucleus with a different number of protons, so fusion can produce a new element.
3. For example, hydrogen nuclei fuse to form helium, and further fusion builds up increasingly heavy elements.
4. Fusion happens in stars because their cores are at extremely high temperature and pressure, which push the positively charged nuclei close enough to join.
5. In this way, fusion processes in stars produce all of the naturally occurring elements up to iron.

Elements heavier than iron form in a supernova

1. Fusion during a star's life builds up elements only as far as iron.
2. Elements heavier than iron are produced in a supernova, the explosion of a massive star.
3. The conditions during the explosion are extreme enough to build these heavier nuclei; you do not need to describe the detailed nuclear processes.

Supernovae spread the elements through the universe

1. A supernova does more than make heavier elements; the explosion also throws the star's material outwards.
2. This distributes the elements throughout the universe, so they are not left trapped inside the star.
3. The scattered material can later become part of new stars, planets and everything on them.