Space Physics

The One Thing

Gravity, nuclear fusion and the behaviour of light provide the framework for understanding the Solar System, stars and galaxies. Astronomical measurements, including parallax, brightness and redshift, allow scientists to determine distances and investigate the universe’s history and expansion.

Reactions, Processes and Experiments

Key Terms

• Solar System: The Sun and all objects that orbit it because of gravity.
• Planet: A large, nearly spherical body that orbits a star and has cleared most other objects from its orbital path.
• Dwarf planet: A nearly spherical object that orbits the Sun but has not cleared its orbital neighbourhood and is not a moon.
• Orbit: The curved path followed by an object moving under the gravitational influence of another object.
• Gravity: The attractive force between masses that keeps planets, moons and other objects in orbit.
• Asteroid: A small rocky or metallic object orbiting the Sun, found mainly in the asteroid belt between Mars and Jupiter.
• Comet: An icy body that follows an elongated orbit; when near the Sun, its ice vaporises to form a coma and sometimes a tail.
• Light-year: The distance travelled by light in one year, equal to approximately 9.46 x 10^15 metres.
• Astronomical unit: The average distance from Earth to the Sun, approximately 1.50 x 10^11 metres.
• Parallax: The apparent change in position of a nearby star when viewed from different positions as Earth orbits the Sun.
• Star: A very large, hot sphere of plasma that produces energy and light, usually by nuclear fusion.
• Nuclear fusion: The process in which light atomic nuclei combine to form heavier nuclei, releasing energy.
• Main sequence: The longest stage in a star's life, during which hydrogen nuclei fuse into helium in its core.
• Red giant: A large, cooler outer stage of a low- or medium-mass star after hydrogen in its core is used up.
• White dwarf: The hot, dense remaining core of a low- or medium-mass star after its outer layers have been expelled.
• Supernova: A powerful explosion marking the end of a massive star's life.
• Neutron star: An extremely dense remnant formed when the core of a massive star collapses after a supernova.
• Black hole: A region of space with such strong gravity that no matter or electromagnetic radiation can escape from within its event horizon.
• Galaxy: A vast system of stars, gas, dust and dark matter held together by gravity.
• Milky Way: The spiral galaxy containing the Solar System.
• Universe: All space, time, matter and energy.
• Redshift: An increase in the observed wavelength of light from a receding object; the redshift of distant galaxies provides evidence that the universe is expanding.
• Big Bang model: The scientific model that describes the universe as having begun in an extremely hot, dense state and then expanded.
• Cosmic microwave background radiation: Weak microwave radiation present throughout space, regarded as evidence of the hot early universe.

Easily Confused

• Planet and dwarf planet: A planet has cleared most objects from its orbital path; a dwarf planet has not and is not a moon.
• Asteroid and comet: An asteroid is mainly rocky or metallic, whereas a comet is icy and may form a coma and tail near the Sun.
• Light-year and astronomical unit: A light-year is the distance light travels in one year, whereas an astronomical unit is approximately the average Earth–Sun distance.
• Parallax and redshift: Parallax measures relatively nearby stellar distances through apparent positional shifts; redshift is used to study receding objects and the expansion of the universe.
• Red giant and red supergiant: A red giant is a stage of a low- or medium-mass star, whereas a red supergiant is a stage of a massive star.
• White dwarf and neutron star: A white dwarf is the remaining core of a low- or medium-mass star; a neutron star forms from the collapsed core of a massive star after a supernova.
• Neutron star and black hole: A neutron star is an extremely dense stellar remnant; a black hole has gravity strong enough that neither matter nor electromagnetic radiation can escape from within its event horizon.
• Main sequence and later stellar stages: The main sequence is when hydrogen fuses into helium in the core; later stages occur after core hydrogen is used up.
• Solar System and Milky Way: The Solar System contains the Sun and its orbiting objects; the Milky Way is the galaxy containing the Solar System.
• Solar System age and universe age: The Solar System is about 4.6 billion years old, while the universe is about 13.8 billion years old.

What Gets Asked

• Ordering the planets: Questions may require the planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Marks are lost by omitting a planet or changing the order.
• Explaining orbital motion: Students may need to state that the Sun’s gravity provides the centripetal force and changes the direction of velocity rather than continually increasing the planet’s speed.
• Using the gravitational equation: Questions may use F = Gm1m2/r^2. Marks are lost by confusing the distance between the centres with another distance or by failing to recognise the inverse-square dependence.
• Comparing astronomical units: Questions may require the values 1 light-year ≈ 9.46 x 10^15 m and 1 astronomical unit ≈ 1.50 x 10^11 m, or the definition of a parsec as the distance at which 1 astronomical unit subtends an angle of 1 arcsecond.
• Interpreting stellar evolution: A low- or medium-mass star follows the sequence red giant, planetary nebula and white dwarf; a massive star becomes a red supergiant, undergoes a supernova and may form a neutron star or black hole. Mixing these pathways loses marks.
• Explaining expansion of the universe: Questions may require redshift, Hubble’s law v = H0d and cosmic microwave background radiation as evidence for the Big Bang model. A common error is to describe redshift merely as a colour change without linking it to recession and expansion.