Stars and the Universe

A star's life starts as a nebula — a vast cloud of hydrogen gas and dust. Gravity pulls the material together; the centre heats up; once temperature and pressure get high enough, hydrogen nuclei fuse into helium and a new star ignites.

For most of its life (millions to billions of years) a star sits on the main sequence, balancing inward gravity against outward radiation pressure from fusion. Our Sun is currently a main-sequence star and has been for ~4.6 billion years.

When hydrogen runs out:

Low/medium-mass stars (like the Sun):

1. Hydrogen ends → core contracts → outer layers expand → red giant.
2. Outer layers drift off as a glowing planetary nebula.
3. Core left behind is a dense, hot white dwarf — slowly cooling over billions of years.

High-mass stars (>~8 solar masses):

1. Hydrogen ends → bigger and brighter → red supergiant.
2. Fusion of heavier elements continues for a short time.
3. Iron is the end of the line — fusion stops; core collapses; outer layers blast off in a supernova.
4. What's left: a tiny neutron star (~20 km wide, made entirely of neutrons), or — for the most massive — a black hole.

Our Sun is one of ~100 billion stars in the Milky Way, a barred-spiral galaxy ~100 000 light-years across. The Milky Way itself is one of ~200 billion galaxies in the observable universe.

Galaxies come in a few main shapes:

• Spiral (including barred spirals like the Milky Way) — rotating disc with arms.
• Elliptical — football-shaped, mostly older stars.
• Irregular — no obvious shape; often the result of mergers.

The size scales are mind-bending:

• Earth to Sun: ~150 million km = 1 astronomical unit (AU)
• Sun to nearest star (Proxima Centauri): ~4.2 light-years
• Milky Way diameter: ~100 000 light-years
• Observable universe diameter: ~93 billion light-years.

A light-year is the distance light travels in one year ≈ 9.5 × 10¹⁵ m. Because light from far galaxies takes billions of years to reach us, looking at the night sky is also looking BACK IN TIME.

Light from distant galaxies arrives shifted towards LONGER wavelengths — the red end of the spectrum. The further the galaxy, the bigger the shift. This is red shift, the cosmic version of the Doppler effect.

Why the shift means moving away. As a galaxy moves away from us, the light waves it emits are stretched out, increasing their wavelength (and reducing their frequency). Red shift = motion away.

Hubble's law. The further a galaxy is, the faster it appears to be moving away. This isn't because we're at the centre — every galaxy sees the same thing from its own perspective. The universe ITSELF is expanding.

The Big Bang theory. If we run the expansion backwards in time, all matter and energy were once concentrated in a tiny region. Around 13.8 billion years ago that point expanded rapidly — the Big Bang. The universe has been expanding (and cooling) ever since.

Two main pieces of evidence:

1. Red shift of distant galaxies → universe is expanding.
2. Cosmic Microwave Background (CMB) — faint microwave radiation filling all space, the "afterglow" of the early hot universe, now cooled to ~2.7 K.

The CMB was first detected in 1964 — its discovery and properties match Big Bang predictions very closely, and is a key reason the theory is so widely accepted.