Star Formation — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 9Sequence of formation

1

A nebula, a giant interstellar cloud of gas and dust, is the starting point.

2

Gravity pulls the particles closer together into a hot ball of gas, the protostar.

3

The core becomes hot enough for hydrogen to fuse into helium.

4

The star becomes stable when forces are balanced.

Section 2 of 9Nebula

Description

• Description a giant, cold interstellar cloud of gas and dust.

Composition

• Composition mostly hydrogen with some helium and dust from older stars.

Trigger

• Trigger a denser region, perhaps after a shock wave from a nearby exploding star, attracts more gas.

Size

• Size a nebula can be several light-years across and contain enough material for thousands of stars.

Section 3 of 9Protostar

Gravity

• Gravity the gravitational attraction within the nebula pulls the particles closer together.

Energy transfer

• Energy transfer gravitational potential energy is transferred to kinetic energy and then to thermal energy.

Density

• Density it increases as the particles come closer.

Collisions

• Collisions more frequent collisions between particles raise the temperature.

Protostar

• Protostar the hot, dense ball of gas at the centre; it shines because it is hot, not because of fusion.

Disc

• Disc the cloud around it flattens into a disc in which planets may form.

Section 4 of 9Fusion starts

Temperature

• Temperature the core must reach about 10 million °C.

Fusion

• Fusion hydrogen nuclei move fast enough to overcome their electrical repulsion and fuse into helium.

Energy

• Energy every fusion reaction releases heat and light, which keeps the core hot.

Stable star

• Stable star once fusion starts the object is a stable (main-sequence) star.

Section 5 of 9Balanced forces in a stable star

Gravity

• Gravity an attractive force that acts inwards and pulls the outer layers towards the core.

Thermal pressure

• Thermal pressure acts outwards from the hot expanding gases as energy is released by fusion.

Equilibrium

• Equilibrium the inward and outward forces are balanced.

Self-correcting

• Self-correcting a star that heats up expands and cools, and one that cools shrinks and heats up.

Section 6 of 9Mass of the protostar

1Stronger gravity

• Stronger gravity a more massive protostar has a hotter core and fuses hydrogen faster.

2Brightness

• Brightness massive stars are brighter and bluer.

3Lifetime

• Lifetime massive stars use their fuel up faster and have shorter lives, and small red stars can last trillions of years.

4Later stages

• Later stages the final stages of a star's life depend on its mass.

5Brown dwarf

• Brown dwarf an object with less than about 8% of the Sun's mass never gets hot enough for fusion.

Section 7 of 9More from the specification

1

Interstellar cloud collapse

• Jeans mass gravitational instability in molecular dust clouds provides the physical means for nebular collapse into protostars.

2

Protostar formation from interstellar clouds

• Gravitational attraction draws interstellar gas and dust into collapsing nebulae, converting gravitational potential into thermal energy.

3

Main sequence hydrostatic equilibrium

• A stable main sequence star maintains hydrostatic equilibrium where outward thermal radiation pressure balances inward gravitational contraction.

4

Redshift and cosmic expansion

• Spectroscopic redshift of absorption lines from distant galaxies demonstrates universal expansion, supporting the Big Bang model.

5

Expanding Universe and redshift

• Expanding Universe and redshift light arriving at Earth from distant galaxies exhibits redshift, showing cosmological expansion across the Universe.

6

Hubble relationship

• Hubble relationship observing that more distant galaxies recede faster demonstrates that the Universe originated from an initial singular expansion.

7

Stellar life cycle

• Stellar life cycle gravitational collapse of interstellar nebulae triggers hydrogen nuclear fusion, maintaining stable main-sequence stars for billions of years.

8

Spectrum

• Range of colours in white light.

9

Galactic redshift

• Shift of galaxy spectral lines to longer wavelength due to recession.

10

White dwarf

• Small, hot, dense core left by a low-mass star.

11

Background radiation

• Radiation that is always present in the environment.

12

Universe

• All of space and everything in it.

13

Galaxy

• Huge group of stars, gas, dust and solar systems held by gravity.

14

Redshift

• Increase in the wavelength of light from a source moving away from the observer.

15

Distance

• Total length of path travelled, a scalar.

16

Radiation

• Transfer by electromagnetic waves, requiring no medium

17

Wavelength

• Distance between a point on one wave and the same point on the next wave

18

Constant

• Not changing.

19

Recession

• Movement away from the observer.

20

v = H0d

What the symbols mean: v = recession speed of the galaxy (km s⁻¹) · H₀ = Hubble constant (km s⁻¹ Mpc⁻¹) · d = distance (Mpc)

Section 8 of 9Key terms

Term	Meaning
Nebula	Giant cloud of gas and dust in space where stars form.
Protostar	Hot, dense ball of gas that has not yet started fusion.
Nuclear fusion	Joining of hydrogen nuclei to form helium, releasing energy.
Stable star	A star in which inward gravity and outward thermal pressure are balanced.
Brown dwarf	An object too small for hydrogen fusion.

Section 9 of 9Exam technique

1

Saying stars form by burning — they form by gravity and then shine by nuclear fusion.

2

Thinking fusion starts as the cloud begins to contract — it needs a core temperature of about 10 million °C.

3

Saying a bigger star lives longer — a bigger star uses its fuel faster and lives for a shorter time.

4

Mixing up protostar and main sequence — a protostar is not yet fusing hydrogen.

5

Forgetting the balance of forces — gravity pulls inwards while thermal pressure pushes outwards.