Space physics

Revision notes

The Solar System contains the Sun, planets and their moons, dwarf planets, asteroids and comets. Gravity provides the inward force needed for orbital motion. An orbiting object can have changing velocity even if its speed is constant because its direction changes. For a stable circular orbit, a smaller orbital radius requires a higher speed.

Stars form when gravity pulls gas and dust together. As a protostar contracts, its temperature rises until hydrogen fusion can begin. A main-sequence star remains stable while the inward effect of gravity is balanced by pressure associated with the hot stellar material. Lower-mass stars eventually become red giants, shed outer layers and leave white dwarfs. More massive stars can become red supergiants, explode as supernovae and leave neutron stars or black holes.

Light from many distant galaxies is shifted toward longer wavelengths. This red-shift is consistent with those galaxies receding, and more distant galaxies generally show greater recession. Together with other astronomical observations, this supports the model of an expanding Universe.

The Solar System and gravity

The Solar System contains the Sun, planets, dwarf planets, moons, asteroids and comets. Gravity provides the centripetal force that keeps planets and satellites in orbit. An orbiting object is continually accelerating because its velocity direction changes even if its speed is constant.

For a stable circular orbit, a smaller orbital radius requires a higher speed: if the speed changes, the radius of the orbit must change too. Do not describe an orbiting astronaut as being beyond gravity; apparent weightlessness occurs because the astronaut and spacecraft are in continuous free fall together.

Stars and stellar evolution

Stars form when regions of gas and dust collapse under gravity. As material contracts, gravitational potential energy is transferred and temperature rises. When the core becomes hot and dense enough, nuclear fusion of hydrogen into helium begins. A stable main-sequence star exists while inward gravitational effects are balanced by outward pressure from the energy released by fusion.

A Sun-like star expands into a red giant when hydrogen fuel in the core becomes depleted, later ejecting outer layers and leaving a white dwarf. More massive stars can become red supergiants and undergo supernova explosions, leaving neutron stars or, for sufficiently massive remnants, black holes. The exact route depends strongly on initial mass. Fusion in stars produces elements up to iron; elements heavier than iron are made in supernovae, which spread them through space.

Evidence about the Universe

Light from distant galaxies contains spectral features. If those features are shifted to longer wavelengths compared with laboratory measurements, the light is red-shifted. The general observation that more distant galaxies show greater recession is evidence that the Universe is expanding.

The Big Bang model describes an early Universe that was much hotter and denser than today and has expanded and cooled. Evidence includes cosmological red-shift and the cosmic microwave background radiation. Observations of galaxies also suggest there is much that is not yet understood, such as dark matter and dark energy. Scientific models are supported because they explain multiple independent observations, not because a single observation 'proves' them absolutely.

Satellites and communication

Artificial satellites have different orbits depending on purpose. A geostationary satellite orbits above the equator with the same period and direction as Earth's rotation, so it appears fixed over one point. Lower orbits have shorter periods and are used for many observation and communication applications.

Exam technique

Separate observations from interpretations. 'Spectral lines are red-shifted' is an observation; 'the galaxy is receding' is the interpretation. In stellar-evolution questions, start with the star's mass because that determines the later pathway.