The Life of a Star

The Life of a Star

Stars are not eternal: they are born, live for billions of years, and eventually die. The Sun is a middle-aged star right now, and how a star ends depends above all on its mass. Stars form from a nebula, a giant cloud of dust and gas. Gravity slowly pulls the cloud together, and as it collapses it heats up, forming a protostar. When the protostar gets hot and dense enough, something remarkable begins.

The main sequence

The core gets hot enough for nuclear fusion: hydrogen nuclei fuse into helium, releasing huge amounts of energy. The star is now a main-sequence star. It stays stable for billions of years because the outward pressure from fusion exactly balances the inward pull of gravity.

What holds a star up?

During its main-sequence life, what stops a star collapsing under its own gravity?

• The outward pressure from nuclear fusion in its core
• Its magnetic field
• The vacuum of space pushing inwards
• Nothing: it is slowly collapsing

Order the Sun's life

Put the stages in the life of a star like our Sun in order, from birth onwards.

• Nebula: a cloud of dust and gas
• Protostar: gravity pulls it together and it heats up
• Main-sequence star: hydrogen fuses to helium, stable for billions of years
• Red giant: the core changes and the star swells
• White dwarf: the outer layers drift off, leaving a hot dense core

It all depends on mass

When a star runs low on hydrogen, it leaves the main sequence, and what happens next depends on its mass: • A star about the size of the Sun takes one path. • A star much more massive than the Sun takes a far more dramatic one.

Two ways to die

Everything up to this point is the same for every star. From here the path splits on mass, and the exam almost always asks you to follow one arm and say why it is not the other.

Label the life cycle

The stellar life cycle with its stages removed. Fill in the two shared stages, then follow each branch to its end.

Match each stage to its description

What is a supernova?

Which best describes a supernova?

• A gigantic explosion at the end of a massive star's life
• The long, stable, hydrogen-fusing phase of a star
• The cloud of gas where stars are born
• A small, cool, dead star

What is left behind?

After a supernova, a very massive star leaves one of two remnants. Pick the TWO possible outcomes.

• A neutron star
• A black hole
• A white dwarf
• A brand-new nebula only

Order the massive star's death

Put the final stages of a star much more massive than the Sun in order.

• Main sequence: the massive star fuses hydrogen
• Red supergiant: it swells to an enormous size
• Supernova: it explodes
• Neutron star or black hole: the remnant left behind

Star stuff

Fusion in stars does not just make helium. Over a star's life it builds up heavier elements, up to iron. Elements heavier than iron are made in the extreme conditions of a supernova, which scatters them into space: the raw material for new stars, planets and, ultimately, us.

The life-cycle rules

A star spends most of its life on the _____ sequence, fusing hydrogen into helium. A star the size of the Sun ends as a white _____. A much more massive star explodes as a _____.