Stars, galaxies and the Big Bang

⭐How a star works

A star begins in a nebula, a vast cloud of hydrogen gas and dust. Gravity pulls part of the cloud together; as it collapses it heats up and becomes a protostar. When the core reaches about 10 million °C, hydrogen nuclei begin to fuse into helium, releasing enormous amounts of energy. The star is born.

For most of its life a star is a main-sequence star, stable because two effects balance: gravity pulling inwards, and the outward pressure of the hot gas heated by fusion. The Sun has been in this stage for about 4.6 billion years and has roughly 5 billion to go.

🔄The life cycle of stars

• Sun-like stars: when hydrogen in the core runs out, the star swells into a red giant, fusing helium into carbon. Its outer layers drift away as a planetary nebula and the hot core is left as a small, dense white dwarf, which slowly cools.
• Massive stars (more than about eight times the Sun’s mass) become red supergiants, fusing heavier and heavier elements up to iron. Then the core collapses and the star explodes as a supernova, briefly outshining a whole galaxy. The remnant is a neutron star or, for the most massive, a black hole, whose gravity is so strong that not even light escapes.

Elements heavier than iron are made in supernovae and in collisions of neutron stars, then scattered into space where they become part of new stars and planets. The iron in your blood and the calcium in your bones were made inside stars that died before the Sun was born.

🌌The scale of the universe

The Sun is one of 100–400 billion stars in the Milky Way, a spiral galaxy about 100 000 light-years across. A light-year is a distance: how far light travels in one year,

The nearest star after the Sun, Proxima Centauri, is 4.2 light-years away, so we see it as it was 4.2 years ago. Looking at a galaxy millions of light-years away means looking millions of years into the past.

🔴Red-shift and the expanding universe

Each element absorbs light at particular wavelengths, giving dark lines in a star’s spectrum — a fingerprint. In the light from distant galaxies these lines are shifted towards the red (longer-wavelength) end. This red-shift means the galaxies are moving away from us, stretching the light as it travels.

In 1929 Edwin Hubble found that the further away a galaxy is, the greater its red-shift, so the faster it is receding. The simplest explanation is that space itself is expanding, carrying galaxies apart, like dots on a balloon being blown up: every dot moves away from every other, and distant dots separate fastest.

💥The Big Bang

If galaxies are moving apart now, they were closer together in the past. Running the expansion backwards leads to the Big Bang theory: about 13.8 billion years ago the universe was extremely hot and dense and has been expanding and cooling ever since. The main evidence:

1. Red-shift of distant galaxies, increasing with distance.
2. The cosmic microwave background (CMB): a faint glow of microwaves reaching us from every direction, discovered by accident in 1965. It is the heat left over from the early universe, stretched by expansion into microwaves.
3. The observed proportions of hydrogen and helium in the universe match the amounts predicted to form in the first few minutes.

A scientific theory is accepted because it explains the evidence and makes predictions that are later confirmed; the CMB was predicted before it was found. Open questions remain — what dark matter and dark energy are, and what happened “before” — which is why cosmology is still an active field.