The lives of stars
Stars are born, live for millions to trillions of years, and die. Their life story is determined almost entirely by one property: mass.
Birth: from nebula to protostar
- A nebula, a vast cloud of hydrogen, helium and dust, is disturbed (perhaps by a nearby supernova).
- Gravity pulls denser regions together. They contract and heat up.
- A spinning protostar forms, surrounded by a disc of material that may form planets.
- When the core reaches about 10 million °C, hydrogen fusion ignites. A star is born.
The Orion Nebula, visible with binoculars, is a stellar nursery about 1300 light-years away.
Life: the main sequence
On the main sequence, a star fuses hydrogen into helium in its core:
4 ¹H → ⁴He + energy (through several steps)
The outward pressure from fusion balances the inward pull of gravity: hydrostatic equilibrium. This balance keeps the star stable for most of its life.
| Star mass (Sun = 1) | Surface colour | Main-sequence lifetime |
|---|---|---|
| 0.1 (red dwarf) | Red | Trillions of years |
| 1 (the Sun) | Yellow | About 10 billion years |
| 10 | Blue-white | About 20–30 million years |
| 50 | Blue | A few million years |
Massive stars burn fuel much faster, so they live much shorter lives.
Death of low- and medium-mass stars (like the Sun)
- Hydrogen in the core runs out. The core contracts and heats; the outer layers swell into a red giant. In about 5 billion years, the Sun will expand to roughly the size of Earth’s orbit.
- The core fuses helium into carbon and oxygen.
- The outer layers drift away as a glowing planetary nebula.
- The exposed core becomes a white dwarf, about Earth-sized but incredibly dense (a teaspoon would weigh tonnes). It slowly cools.
Death of massive stars
- The star becomes a red supergiant, fusing heavier elements in shells: helium, carbon, oxygen, neon, silicon, up to iron.
- Fusing iron absorbs energy instead of releasing it. Fusion stops, and the core collapses in less than a second.
- The collapse rebounds in a supernova, briefly outshining an entire galaxy. Elements heavier than iron form in the explosion and in neutron star collisions.
- The remaining core becomes:
- a neutron star (about 20 km wide, so dense a teaspoon would weigh about a billion tonnes), sometimes seen as a spinning pulsar; or
- a black hole, if the core is massive enough (more than about 3 solar masses).
Black holes
A black hole has gravity so strong that not even light can escape past its event horizon. Black holes can’t be seen directly, but we detect them by:
- their effect on nearby stars
- glowing hot gas swirling around them
- gravitational waves from black hole mergers, first detected by LIGO in 2015
In 2019, the Event Horizon Telescope released the first image of a black hole’s shadow, in galaxy M87.
Stardust
The Big Bang made almost only hydrogen and helium (and a trace of lithium). Nearly every other element, including the carbon in your cells, the oxygen you breathe and the iron in your blood, was forged in stars and scattered by stellar explosions.
The Big Bang
The Big Bang theory states that the universe began about 13.8 billion years ago in an extremely hot, dense state and has been expanding and cooling ever since. It wasn’t an explosion in space; it was the expansion of space itself.
Timeline of the early universe
| Time after Big Bang | Event |
|---|---|
| Tiny fraction of a second | Rapid expansion (inflation); fundamental forces separate |
| About 1 second | Protons and neutrons form |
| About 3 minutes | Hydrogen and helium nuclei form |
| About 380 000 years | Atoms form; the universe becomes transparent; the cosmic microwave background is released |
| About 100–200 million years | First stars ignite |
| About 9 billion years | Solar system forms (4.6 billion years ago) |
Evidence for the Big Bang
| Evidence | What it shows |
|---|---|
| Redshift and Hubble’s law | In 1929, Edwin Hubble found that distant galaxies’ light is shifted toward red, and the farther away they are, the faster they recede. The universe is expanding. |
| Cosmic microwave background (CMB) | Discovered accidentally in 1965 by Arno Penzias and Robert Wilson; a faint glow at about 2.7 K coming from every direction, the cooled afterglow of the early universe |
| Abundance of light elements | Observed amounts of hydrogen (about 75%) and helium (about 25%) match Big Bang predictions |
| Galaxy evolution | Distant (younger) galaxies look different from nearby ones, showing the universe has changed over time |
Redshift
Like the Doppler effect for sound, light from objects moving away is stretched to longer (redder) wavelengths. Hubble’s law:
v = H₀ × d
H₀, the Hubble constant, is about 70 km/s per megaparsec. (Scientists are still debating its exact value because two measurement methods give slightly different answers.)
Dark matter and dark energy
Ordinary matter makes up only about 5% of the universe.
- Dark matter (about 27%) doesn’t emit light but its gravity holds galaxies together.
- Dark energy (about 68%) is causing the expansion of the universe to accelerate, discovered in 1998.
What they actually are remains one of the biggest open questions in science.
Common mistakes
- Thinking the Sun will become a black hole. It isn’t massive enough; it will become a white dwarf.
- Thinking the Big Bang happened at a point in space. It happened everywhere; space itself expanded.
- Thinking black holes suck everything in. At a distance, their gravity is like any object of the same mass.
Try it yourself
- Draw two flowcharts showing the life cycle of a Sun-like star and a star 20 times the Sun’s mass.
- Draw dots on a balloon, then inflate it. Measure distances between dots before and after. How does this model the expanding universe? What are its limits?
- A galaxy is 100 megaparsecs away. Using H₀ = 70 km/s/Mpc, how fast is it receding?
(Answer to 3: 7000 km/s)
Quick recap
- Stars form in nebulae and spend most of their lives on the main sequence fusing hydrogen.
- Mass determines lifetime and fate: white dwarf, neutron star or black hole.
- Supernovas create and scatter heavy elements; we are made of stardust.
- The Big Bang theory: the universe began 13.8 billion years ago and has been expanding.
- Evidence includes redshift, the cosmic microwave background and light-element abundances.