Stellar Evolution

Stars come in a wide range of colours, and the colour reflects the SURFACE TEMPERATURE. This is the same physics as a piece of metal in a furnace — warm metal glows red, hotter metal glows yellow/white, very hot metal glows blue-white.

Colour	Approx surface temperature (K)	Example
Blue	10 000-30 000	Rigel, Spica
White	~7 500-10 000	Sirius, Vega
Yellow	~5 000-6 000	Sun
Orange	~3 500-5 000	Aldebaran
Red	~2 500-3 500	Betelgeuse, Proxima Centauri

Edexcel learning point. A blue star is HOTTER than a red star, even though "red hot" is the everyday phrase for something dangerously hot. In astronomy, the order is consistent: blue/UV emission requires the highest temperatures.

Spectral classes. Astronomers use the letter sequence O B A F G K M (from hottest to coolest) — not required to memorise for 4PH1, but worth knowing the Sun is a class G2 (yellow) star.

Stars are born, live, and die. A Sun-mass star (any star with mass roughly equal to or up to a few times the Sun's mass) follows this sequence:

1. Nebula. A vast cloud of dust and hydrogen gas in interstellar space. Within a denser region, GRAVITY pulls material together. Compression heats the gas. A protostar forms.

2. Main sequence star. When the core temperature reaches ~10⁷ K, NUCLEAR FUSION of hydrogen → helium begins, releasing energy. This is the LONGEST and most STABLE phase of a star's life, lasting BILLIONS of years for Sun-mass stars. The Sun is currently about halfway through its 10-billion-year main sequence life.

The star is stable because of HYDROSTATIC EQUILIBRIUM: the outward radiation pressure from fusion balances the inward gravitational pull.

3. Red giant. When hydrogen in the core is exhausted, fusion stops there and the core CONTRACTS under gravity. Contraction heats the core further; helium begins to fuse to carbon. The increased core energy causes the OUTER LAYERS to EXPAND and cool — the star becomes a RED GIANT (large, cool, red).

When our Sun reaches this stage in ~5 Gyr, it will engulf Mercury, Venus and possibly Earth.

4. Planetary nebula. The outer layers are gently shed into space as a glowing PLANETARY NEBULA (the name is misleading — nothing to do with planets; early astronomers thought they looked like planets through small telescopes).

5. White dwarf. The leftover, exposed hot core is a WHITE DWARF — Earth-sized but EXTREMELY DENSE (a teaspoon of white-dwarf material weighs tonnes). No fusion. It SLOWLY COOLS over many billions of years and will eventually fade.

Spec wording (spec 8.9). Edexcel names four stages: nebula → star (main sequence) → red giant → white dwarf. Memorise the order.

A star much more massive than the Sun (typically ≥ 8 solar masses) follows a similar but more spectacular path. The key differences:

Faster. Massive stars burn fuel much more rapidly — millions rather than billions of years on the main sequence.

Red SUPERgiant (not just 'giant'). After main-sequence hydrogen burning, the star becomes a RED SUPERGIANT — far larger than a red giant. Successive shells in the star fuse heavier elements: helium, then carbon, oxygen, neon, silicon and finally IRON.

The iron problem. Once the core has fused all the way to iron, FURTHER FUSION RELEASES NO ENERGY. (Iron has the most stable nuclei per nucleon — fusing or splitting iron requires energy input.) The radiation pressure that has been holding up the star against gravity collapses.

Supernova. The core implodes catastrophically; outer layers REBOUND and EXPLODE outward as a SUPERNOVA. Briefly outshining an entire galaxy, the supernova:

• Disperses heavy elements made during the star's life into space (we are made of supernova material — every atom heavier than iron in your body was forged in a supernova).
• Compresses surrounding gas, triggering new star formation nearby.

Remnant. What is left of the original core depends on the original mass:

Progenitor mass	End remnant
~8-25 M☉	NEUTRON STAR (~10 km radius, almost pure neutrons, magnetic, sometimes seen as pulsars)
> ~25 M☉	BLACK HOLE (gravity so strong even light cannot escape)

Spec 8.10 wording. Edexcel lists: massive main sequence → RED SUPERGIANT → SUPERNOVA → NEUTRON STAR or BLACK HOLE. Memorise this chain; do NOT substitute 'red giant' or 'white dwarf' — those are the SUN-mass terminology.

Why do we need it? A faint-looking star in the night sky might be either:

• Genuinely dim AND nearby, or
• Genuinely bright AND far away.

Just measuring how bright a star appears (its APPARENT magnitude) doesn't tell us its TRUE brightness.

Absolute magnitude (spec 8.11P) solves this. It is defined as the APPARENT BRIGHTNESS a star WOULD have if it were placed at a STANDARD DISTANCE from Earth — specifically, 10 PARSECS (~32.6 light-years).

By imagining all stars at the same distance, we can compare their TRUE brightnesses without distance distortion.

Scale convention.

• SMALLER absolute magnitude = BRIGHTER star (the scale is inverted, from historical convention).
• The Sun: absolute magnitude ~4.8 (a fairly ordinary star).
• A bright supergiant like Rigel: absolute magnitude about −7 (intrinsically thousands of times brighter than the Sun).
• A dim red dwarf: absolute magnitude +15 or higher.

Use. Astronomers use absolute magnitude to compare star populations and to plot the HR diagram (next section).

The HERTZSPRUNG-RUSSELL (HR) diagram is a scatter plot of stars with two axes:

• y-axis: ABSOLUTE MAGNITUDE / brightness. UPWARD = brighter. (Confusing point: because of the inverted magnitude scale, smaller magnitude numbers are at the TOP.)
• x-axis: SURFACE TEMPERATURE / spectral class. Temperature DECREASES from left to right — hot blue stars on the LEFT, cool red stars on the RIGHT. (This unusual orientation is historical.)

Main features (must memorise for spec 8.12P).

1. Main sequence — a diagonal band from TOP-LEFT (hot, bright blue main-sequence stars) to BOTTOM-RIGHT (cool, dim red main-sequence stars). Most stars (~90% of those we see, including the Sun) lie on this band.

2. Red giants and supergiants — UPPER-RIGHT. They are COOL (red, far right) but BRIGHT (high in the diagram), because they are very large in surface area.

3. White dwarfs — LOWER-LEFT. They are HOT (left side, blue-white) but DIM (low in the diagram), because they are very small.

4. Sun's position. Yellow main-sequence star, roughly in the middle of the main sequence band.

Why the HR diagram matters. Stars evolve THROUGH the diagram during their lives. A Sun-mass star starts on the main sequence, moves up-right into the red giant region, then down-left to the white dwarf region. Massive stars take a higher path with a final supernova step that destroys the star (no remnant plotted, except a neutron star that lies far below the bottom of the diagram).

Edexcel exam expectations. You should be able to SKETCH the diagram with axes labelled, mark the three regions, and place the Sun. You do NOT need to know specific stars by name.