Space physics

Learning objectives

• Explain gravity’s role in orbits.
• Distinguish stars, galaxies and stages of stellar life cycles.
• Connect redshift, the Hubble relationship and cosmic microwave background to expansion.

The big idea

Gravity changes the direction of an orbiting object’s velocity. The Solar System contains one star and bodies bound to it, while a galaxy contains vast numbers of stars.

Stars release energy through fusion and evolve differently according to their mass. Light carries information about temperature and composition. For suitable nearby galaxies, the approximate Hubble relationship connects recession speed with distance. Cosmic microwave background radiation is further evidence for a hot early Universe. Redshift concerns spectral wavelengths, not simply brightness.

Topics to connect

• Earth and the Solar System
• Orbital motion
• Stars and stellar life cycles
• The Universe
• Redshift and the Hubble relationship
• Cosmic microwave background radiation

1. Earth and the Solar System

Earth rotates about its axis, producing the daily pattern of daylight, and orbits the Sun over a year. Its axis is tilted, so seasonal changes involve solar angle and day length rather than simply Earth–Sun distance. The Solar System includes the Sun, planets, moons and smaller bodies. A moon orbits a planet; a planet orbits a star; a star produces radiation through processes in its interior.

2. Gravity and orbital motion

Gravity supplies the inward acceleration that continuously turns an orbiting body. Without sufficient sideways motion, an object falls towards the central body; with suitable speed it continually falls around it. A circular orbit has period equal to circumference divided by speed. More distant planets generally have longer periods around the same central mass. Orbits need no fuel to maintain in an ideal vacuum, although real spacecraft can need corrections.

3. Stars and their life cycles

Gravity gathers gas and dust into a protostar; rising pressure and temperature can allow fusion to begin. In a main-sequence star, outward pressure balances inward gravitational effects. Later development depends strongly on mass. A Sun-like star can form a red giant and eventually a white dwarf after shedding outer layers. Massive stars can undergo supernova explosions and leave neutron stars or black holes. Ejected matter supplies material for later stars and planets.

4. Galaxies and the expanding Universe

The Sun belongs to the Milky Way, one galaxy among many. A light-year is a distance. Identify a known spectral pattern to measure redshift: observed wavelengths longer than laboratory values indicate a shift, not merely a red-looking galaxy. For appropriate low-redshift galaxies, the Hubble relationship gives recession speed approximately H₀d. Distance estimates can use calibrated standard-candle behaviour, including certain supernovae, rather than assuming all objects share one brightness.

5. The cosmic microwave background

Microwave radiation arrives from essentially every direction and has a nearly thermal spectrum. It is interpreted as radiation released when the young Universe became transparent, stretched to longer wavelengths as space expanded. This evidence and galactic redshifts support a hotter, denser early Universe. A simple age estimate 1/H₀ is model-dependent: the expansion rate has not necessarily remained constant. These observations concern the Universe as a whole, not the ordinary gravitational orbits of nearby stars.

A method you can reuse

1. Identify whether the question concerns rotation, orbit or cosmic expansion.
2. Use distance from the central body for an orbit calculation.
3. Distinguish intrinsic luminosity from measured brightness.
4. Compare a spectral line with its reference wavelength.
5. State the range and assumptions of any Hubble-law estimate.

Equations and relationships

Investigate and explain

Use an orbit model to compare how changing orbital radius affects period while keeping the central body fixed.

Use the orbital models for bound systems and the spectrum model for temperature. None directly simulates cosmic expansion, stellar evolution or the microwave background; discuss those using observational evidence.

Open the interactive investigation

Check your understanding