Physics Basics
The physical ideas that make sense of the universe.
Overview
The core physics behind astronomy — gravity, light, energy and motion — at an accessible level, so that the rest of the site's science follows. These are the ideas that recur on nearly every entity page in the platform.
- Gravity is the only force with unlimited range that is always attractive, which is why it organises the universe at large scales.
- Nearly everything astronomy knows arrives as electromagnetic radiation, which is why understanding light is understanding the method.
- Energy conservation constrains what stars, accretion discs and explosions can and cannot do.
- Relativity is not exotic: GPS and the orbits of Mercury both require it.
Gravity and orbits
Newton's law of universal gravitation states that every mass attracts every other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. The inverse-square dependence is what makes gravity fall off gradually enough to bind galaxies while still being weak enough locally that ordinary matter holds together.
An orbit is continuous free fall. A satellite is not beyond gravity's reach — orbital gravity is nearly as strong as at the surface — it is moving sideways fast enough that it keeps missing. The apparent weightlessness of astronauts is free fall, not absence of gravity, and this distinction resolves most popular confusion about orbital mechanics.
Kepler's laws describe the resulting motion: orbits are ellipses with the primary at one focus, a body sweeps equal areas in equal times so it moves faster when closer, and the square of the orbital period is proportional to the cube of the semi-major axis. Newton showed all three follow from the inverse-square law.
Light and the spectrum
Light is electromagnetic radiation, and visible light is a narrow band within a continuum running from radio waves through microwave, infrared, visible, ultraviolet and X-ray to gamma rays. The only physical difference between them is wavelength — and therefore photon energy.
Astronomy uses the whole range because different physical processes emit at different energies. Cold dust radiates in the infrared, hot stellar photospheres in the visible and ultraviolet, million-degree gas in X-rays, and non-thermal relativistic electrons across the radio band. Observing at only one wavelength gives a systematically incomplete picture of almost any object.
How radiation carries information
- Blackbody radiation: a hot dense object emits a characteristic continuous spectrum whose peak wavelength shifts to shorter wavelengths as temperature rises. This is why stellar colour indicates temperature.
- Spectral lines: atoms absorb and emit at specific wavelengths determined by their energy levels, so lines identify composition.
- Doppler shift: motion along the line of sight shifts observed wavelengths, giving radial velocity directly.
- Redshift at cosmological scales: the expansion of space stretches wavelengths in transit, which is a different phenomenon from ordinary Doppler motion though it produces a similar observable.
- The inverse-square law of brightness: observed flux falls as the square of distance, which is what makes standard candles work as distance indicators.
Energy, matter, and nuclear processes
Mass and energy are equivalent, related by E = mc². In stars this is not a metaphor: fusing four hydrogen nuclei into one helium nucleus leaves a small mass deficit, and that difference becomes the energy the star radiates. Roughly 0.7 percent of the hydrogen mass involved is converted, which over a stellar lifetime is an enormous quantity.
Fusion of successively heavier elements releases energy only up to iron; beyond that, fusion consumes energy rather than releasing it. This single fact explains why massive stellar cores collapse once they become iron, and therefore why core-collapse supernovae happen at all.
Accretion onto a compact object is even more efficient than fusion — a substantial fraction of infalling rest-mass energy can be radiated — which is why accreting black holes outshine entire galaxies.
Relativity, where it matters
Special relativity holds that the speed of light is the same for all observers, with the consequences that time dilates and lengths contract at high relative speeds, and that nothing carrying information exceeds light speed.
General relativity describes gravity as the curvature of spacetime by mass and energy rather than as a force. Its observable consequences include the precession of Mercury's orbit, the bending of starlight by mass — gravitational lensing, now a routine astronomical tool — the slowing of clocks in gravitational fields, and gravitational waves.
These effects are not confined to extreme environments. Satellite navigation systems must correct for both special and general relativistic time dilation to remain accurate; without those corrections positional errors would accumulate rapidly.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why do astronauts float if gravity is still strong in orbit?
- Because they are in free fall. At the International Space Station's altitude, Earth's gravity is still roughly 90 percent of its surface value — the station and everything in it are continuously falling toward Earth while moving sideways fast enough to keep missing. Everything falls together, so nothing presses against anything else, and the sensation is weightlessness.
- What does E = mc² actually mean for stars?
- That mass can be converted into energy. When hydrogen fuses into helium, the resulting nucleus is slightly less massive than the inputs, and that missing mass — about 0.7 percent — becomes radiated energy. It is a small fraction, but applied to the enormous quantity of hydrogen in a stellar core it powers the star for billions of years.
- Why can't fusion power a star past iron?
- Because iron sits at the peak of nuclear binding energy per nucleon. Fusing lighter elements toward iron releases energy; fusing anything beyond iron requires a net energy input. When a massive star's core becomes iron, fusion can no longer supply the pressure holding the core up, and it collapses — the trigger for a core-collapse supernova.
- Is relativity relevant to everyday life?
- Yes, in at least one system nearly everyone uses. Satellite navigation requires corrections for both special relativistic time dilation, from the satellites' orbital speed, and general relativistic time dilation, from their weaker gravitational field. Without those corrections, positional errors would grow to kilometres within a day.