Stars and the Universe

A star is a vast ball of hydrogen and helium held together by its own gravity. At its core the pressure and temperature are so extreme — millions of degrees — that hydrogen nuclei are forced together and fuse to form helium, releasing enormous quantities of energy.

The process is nuclear fusion, and it is worth being precise about the word, because fusion and fission are opposites and confusing them loses every mark in the question. Fusion joins light nuclei together. Fission splits heavy ones apart. Stars run on fusion; nuclear power stations on Earth run on fission.

A star is stable for billions of years because two effects are in balance. Gravity pulls all its material inwards. The outward pressure generated by the energy released in the core pushes outwards. While the two are equal, the star neither collapses nor expands — it simply shines.

Stars form from vast clouds of dust and gas called nebulae. Gravity pulls the material together; as it contracts it heats up, and when the core becomes hot enough for fusion to begin, a star is born. This stage is called a protostar.

The star then settles into its long stable phase, the main sequence, where it will spend most of its life. Our Sun is about halfway through a main-sequence lifetime of roughly ten billion years.

When the hydrogen in the core runs out, the balance fails. The core contracts and heats further while the outer layers expand and cool, turning the star into a red giant — red because a cooler surface glows red rather than white.

What happens next depends entirely on mass. A star like the Sun sheds its outer layers and leaves the hot, dense core behind as a white dwarf, which slowly cools over billions of years. A star much more massive than the Sun becomes a red supergiant and ends in a colossal explosion, a supernova, leaving behind either a neutron star or, if massive enough, a black hole.

Step through the stages, then switch to the heavier star and step through again. Both begin in a nebula; only the mass decides whether the story ends quietly as a white dwarf or violently as a supernova.

Stars are not spread evenly through space. They are gathered into galaxies, each containing many billions of stars bound together by gravity. Our Sun is one star among a few hundred billion in the Milky Way.

The distances involved defeat ordinary units, so astronomers use the light-year: the distance light travels in one year, about 9.5 × 10¹⁵ m. It is a unit of distance, not of time — a point examiners like to test.

The nearest star beyond the Sun is roughly four light-years away, meaning its light left it four years ago. The Andromeda galaxy is about 2.5 million light-years away, so we see it as it was 2.5 million years ago. Looking further into space is looking further back in time.

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speed of lightm s⁻¹

t

times

When light from a distant galaxy is analysed, the pattern of dark lines in its spectrum is found in the right order but shifted towards the red — the longer-wavelength — end. This is red-shift, and it means the galaxy is moving away from us.

The crucial observation is not merely that galaxies are receding, but that the more distant a galaxy is, the greater its red-shift and so the faster it is moving away. This is a systematic relationship, not a coincidence, and it holds in every direction we look.

The only sensible interpretation is that space itself is expanding, carrying the galaxies apart. It is not that we sit at the centre of an explosion — an observer in any galaxy would see exactly the same thing.

Run that expansion backwards and everything converges. At some point in the past the whole universe must have been concentrated in an extremely small, hot, dense state, from which it has been expanding ever since. That is the Big Bang theory, and red-shift is its principal evidence. Around 13.8 billion years is the current best estimate for how long ago it began.