Stellar Evolution
Astrophysics
All stars, regardless of how massive they end up, start the same way.
Nebula
- A nebula is a giant cloud of dust and gas (mostly hydrogen) drifting through interstellar space
- Some patches of a nebula are denser than others. The dense patches have stronger gravitational pull on the surrounding gas
Protostar
- Inside a dense patch, gravity pulls the gas and dust together. The cloud begins to collapse inwards
- As the gas gets squeezed:
- The density rises
- Particles collide more frequently, transferring kinetic energy
- The temperature rises sharply
- The hot ball of contracting gas is called a , a star not yet fully alive, because fusion has not yet begun
Main-sequence star
- Once the core of the protostar reaches about 10 million K, the hydrogen nuclei in the core start to fuse into helium, releasing huge amounts of energy
- A star that is fusing hydrogen in its core is called a
- The Sun has been a main-sequence star for about 4.6 billion years and will continue for another 5 billion or so before its hydrogen runs out
- During the main-sequence stage, the star is stable, because the outward pressure from fusion balances the inward pull of gravity. The star stays the same size for billions of years (covered in topic 21)
Every star starts the same way: → protostar → main-sequence star
Common exam question
Describing how a main sequence star forms from a nebula
Question: Describe how a main sequence star forms from a nebula, or show how the energy stores of the collapsing gas change before the star is created (3–4 marks).
Asked in 3 of the 24 papers. The credited steps form a sequence: a nebula is a cloud of dust and gas; gravity makes the cloud collapse; its temperature rises as it contracts; a protostar forms; and the star joins the main sequence when fusion starts. Any three earn the marks, so give them in order, naming both the force and the process. In the fill-in version, it is the particles' kinetic energy store that increases and raises the temperature.
The energy-store table is marked a row at a time: the gravitational potential store decreases and the thermal store increases as the gas falls inward, while the chemical store stays the same, and so does the nuclear store because fusion has not begun. A row with two ticks scores nothing.