Stellar EvolutionEdexcel IGCSE Physics: Revision notes

Stars can be classified according to their colour, which is directly related to their surface temperature.

• Blue and white stars have the highest surface temperatures
• Yellow stars (like the Sun) have intermediate surface temperatures
• Orange and red stars have the lowest surface temperatures

So colour is a quick visual indicator of how hot a star's surface is: blue = hottest, red = coolest.

Stars with a mass similar to the Sun go through a well-defined life cycle:

1. Nebula — a cloud of dust and gas that contracts under gravity; as it heats up and nuclear fusion begins, a new star forms
2. Main sequence star — a stable star (like the Sun currently) that fuses hydrogen into helium in its core, releasing energy for billions of years
3. Red giant — once hydrogen fuel runs low, the core contracts and the outer layers expand and cool, turning the star into a large, cooler, reddish star
4. White dwarf — the red giant's outer layers are shed, leaving behind a small, extremely dense, hot core that slowly cools over a very long time

Stars with a mass much greater than the Sun follow a more dramatic life cycle:

1. Nebula — contracts to form a massive main sequence star
2. Main sequence star — fuses hydrogen, but burns through fuel much faster than a Sun-like star
3. Red supergiant — expands into an enormous, cool, red star once hydrogen fuel is depleted
4. Supernova — the star undergoes a massive, explosive collapse and explosion
5. Neutron star or black hole — depending on the remaining mass, the collapsed core becomes either an extremely dense neutron star, or (if massive enough) a black hole

Absolute magnitude is a measure of a star's true brightness, found by imagining all stars placed at the same standard distance from an observer. This allows fair comparison of how luminous stars actually are, since a star's apparent brightness in the sky also depends on how far away it is.

• A star that looks faint in the sky could actually be very luminous but very far away
• Absolute magnitude removes the effect of distance, so it reflects only the star's true output of light

The Hertzsprung–Russell (HR) diagram is a chart used to classify stars, plotting:

• Temperature (or colour) on the horizontal axis, usually decreasing from left to right
• Luminosity/absolute magnitude (brightness) on the vertical axis, increasing upwards

On the HR diagram, stars form recognisable groups:

• Main sequence — a diagonal band running from hot/bright (top left) to cool/dim (bottom right), where most stars, including the Sun, spend most of their lives
• Red giants/supergiants — found towards the top right (cool but very bright due to large size)
• White dwarfs — found towards the bottom left (hot but dim due to small size)