Stars & Galaxies

What is a Star?

A star is a massive, luminous ball of plasma held together by gravity and powered by nuclear fusion at its core. Stars are the fundamental building blocks of galaxies and the source of nearly all the energy in the observable universe.

In a star's core, extreme temperature and pressure force hydrogen nuclei to fuse into helium, releasing energy according to Einstein's equation E = mc squared. This energy radiates outward as light and heat.

Stellar Classification

Stars are classified by their surface temperature, which determines their colour:

• Blue/white — hottest (above 10,000°C); e.g., Rigel
• Yellow — intermediate (~5,500°C); e.g., our Sun
• Orange/Red — coolest (below 3,500°C); e.g., Betelgeuse

Stars are also classified by their luminosity (total energy output) and plotted on the Hertzsprung-Russell (H-R) diagram, which shows the relationship between temperature and brightness.

The Life Cycle of Stars

All stars form from a nebula — a cloud of gas (mostly hydrogen) and dust. Gravity pulls the material together, forming a protostar. As the protostar contracts, temperature and pressure increase until nuclear fusion begins, and the star enters the main sequence.

Stars Like Our Sun (low to medium mass)

Nebula → Protostar → Main Sequence (billions of years) → Red Giant → Planetary Nebula → White Dwarf → (eventually) Black Dwarf

• Main sequence: hydrogen fuses to helium for billions of years; gravity inward = radiation pressure outward (equilibrium)
• Red giant: hydrogen in core exhausted; outer layers expand enormously; core contracts and heats; shell fusion begins; Sun will expand to engulf Mercury, Venus, and possibly Earth
• Planetary nebula: outer layers expelled as a glowing shell of gas
• White dwarf: hot, dense core remains; no longer fusing; gradually cools over billions of years
• Black dwarf: theoretical final stage — a cold, dark remnant (none yet exist; universe is too young)

Very Massive Stars (more than ~8 solar masses)

Nebula → Protostar → Main Sequence → Red Supergiant → Supernova → Neutron Star or Black Hole

• Red supergiant: fuses progressively heavier elements (helium, carbon, oxygen, silicon, iron)
• Supernova: iron cannot be fused to release energy; core collapses catastrophically in seconds; outer layers explode in the most powerful explosion in the universe; briefly outshines an entire galaxy
• Neutron star: if the remaining core is 1.4–3 solar masses, it collapses to a sphere of neutrons only ~20 km across but with enormous density
• Black hole: if remaining core is more than ~3 solar masses, gravity overcomes all other forces; spacetime curves so severely that not even light can escape the event horizon

Supernovae are crucial: they forge and scatter heavy elements (carbon, oxygen, iron, gold) throughout space — all atoms heavier than iron in your body were made in supernovae.

The Main Sequence

Stars spend the majority (~90%) of their lives on the main sequence — the stable phase where gravity pulling inward is balanced by radiation pressure from nuclear fusion pushing outward.

• Our Sun has been on the main sequence for ~4.6 billion years and has ~5 billion years remaining
• More massive stars burn fuel faster and have shorter main sequence lifetimes
• The most massive stars live only a few million years; the least massive may live trillions of years

Galaxies

A galaxy is a vast gravitationally bound system of stars, stellar remnants, gas, dust, and dark matter.

Types of galaxies:

• Spiral galaxies — flat disc with spiral arms; active star formation; e.g., Milky Way, Andromeda
• Elliptical galaxies — roughly spherical or elliptical; older stars; little gas or dust; little new star formation
• Irregular galaxies — no regular shape; often distorted by gravitational interactions; e.g., Large and Small Magellanic Clouds

Our galaxy — the Milky Way:

• A barred spiral galaxy
• Contains approximately 200–400 billion stars
• Diameter: approximately 100,000 light-years
• Our Solar System is located about 26,000 light-years from the galactic centre
• The Milky Way takes approximately 225 million years to complete one rotation (a "galactic year")
• Contains a supermassive black hole (Sagittarius A*, mass ~4 million solar masses) at its centre

Nearest large galaxy: Andromeda (M31), approximately 2.5 million light-years away. It is approaching the Milky Way and will collide in about 4.5 billion years.

Scale of the Universe

Distance	Value
Earth to Moon	384,400 km
Earth to Sun	~150 million km (1 AU)
Sun to nearest star (Proxima Centauri)	~4.24 light-years
Diameter of Milky Way	~100,000 light-years
Distance to Andromeda galaxy	~2.5 million light-years
Observable universe diameter	~93 billion light-years

1 light-year = 9.46 trillion km — the distance light travels in one year at 300,000 km/s.

The Big Bang and Expanding Universe

Current evidence supports the Big Bang model — the universe began from an extremely hot, dense state approximately 13.8 billion years ago and has been expanding ever since.

Evidence:

• Redshift of distant galaxies — galaxies are moving away from us; the further away, the faster they recede (Hubble's Law)
• Cosmic Microwave Background Radiation — the afterglow of the Big Bang, uniformly detectable in all directions
• The observed abundance of hydrogen and helium matches Big Bang nucleosynthesis predictions

Key Terms

• Nebula — cloud of gas and dust from which stars form
• Protostar — early stage of star formation before nuclear fusion begins
• Main sequence — stable phase of a star's life where hydrogen fusion occurs
• Red giant / Red supergiant — expanded stage when core hydrogen is exhausted
• Supernova — catastrophic explosion of a massive star at the end of its life
• White dwarf — dense remnant of a sun-like star after it expels its outer layers
• Black hole — region of space where gravity is so strong that nothing, including light, can escape
• Galaxy — vast system of billions of stars bound together by gravity
• Light-year — the distance light travels in one year (~9.46 trillion km)

Common Mistakes

• Saying a sun-like star ends as a supernova — only massive stars (more than ~8 solar masses) end as supernovae
• Confusing light-year as a unit of time — it is a unit of distance
• Thinking black holes "suck" everything in — they only attract matter that comes within the event horizon; distant objects are not affected more than by a regular star of the same mass
• Saying our Sun is a large star — it is actually medium-sized compared to the full range of stellar sizes

Tips and Tricks

• Sun-like star path: Nebula → Protostar → Main Sequence → Red Giant → Planetary Nebula → White Dwarf
• Massive star path: the same start, then Red Supergiant → Supernova → Neutron Star or Black Hole
• More massive star = shorter life (burns fuel faster)
• Colour tells you temperature: blue = hot, red = cool (opposite of what feels intuitive from everyday life)