SP7 · Astronomy · Topic 7 — Astronomy | GCSE Edexcel Separate Sciences

Revise the key ideas

Our Solar System contains the Sun, eight planets and their natural satellites, dwarf planets, asteroids and comets. The Sun is a star; the Moon is Earth’s natural satellite, not a planet.
In increasing distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto is classified as a dwarf planet, not one of the eight planets.
Read the inner column from top to bottom, then the outer column. Distances and sizes are not represented.
Asteroids are mostly small rocky bodies. Comets contain ice and dust and often have long, oval-shaped (elliptical) orbits. Near the Sun, a comet develops a tail that points generally away from the Sun, not necessarily behind the direction it is moving.
Mass is the amount of matter and stays the same when an object moves from Earth to the Moon. Weight = mass × local gravitational field strength, so weight and g differ between celestial bodies.
Early geocentric models placed Earth at the centre. Heliocentric models put the Sun at the centre of planetary motion; observations such as Venus’s phases and Jupiter’s moons helped challenge simple Earth-centred models.
Improved telescopes, repeated observations and mathematical models changed scientific ideas. Accepting a new model depends on evidence and explanatory power, not only on who proposes it.

Gravity provides the inward force that maintains an orbit. In a circular orbit, speed can be constant while velocity changes continuously because its direction changes; the body is accelerating towards the centre.
On a circular-orbit diagram, draw gravity towards the central body. Draw velocity along a tangent: a line touching the circle at that point, at right angles to the radius. Gravity points inwards, not along the direction of motion.
Speed may stay constant while direction changes, so velocity is changing.
Planets orbit the Sun, moons orbit planets, and artificial satellites can orbit Earth. An orbit does not have to be circular. Comets often have more stretched-out elliptical orbits.
For stable circular orbits around the same central body, a smaller orbital radius corresponds to a higher speed, and a larger radius to a lower speed. Changing the orbital speed generally requires a change in stable orbital radius.
An orbiting satellite is continually falling under gravity while moving sideways fast enough to keep missing the surface. It does not need gravity to vanish; weightlessness inside an orbiting craft arises from shared free fall.
For a circular orbit, average orbital speed = 2πr ÷ orbital period. Use radius from the central body's centre, not height above the surface, and convert the period to the required time unit.

Stars form from a nebula, a cloud of gas and dust. Gravity draws matter together; temperature and pressure rise until fusion can sustain a star.
During the main-sequence stage, outward pressure associated with energy from fusion balances inward gravitational attraction. This balance keeps the star approximately stable over a long period.
When a Sun-like star uses up the hydrogen in its core, it expands into a red giant. It eventually loses its outer layers, leaving a hot, dense white dwarf. The white dwarf cools over time.
The final remnant of a massive star depends on its mass; these are alternative paths.
A star much more massive than the Sun can become a red supergiant and end in a supernova. The remnant may be a neutron star or, for sufficiently massive remnants, a black hole.
Mass strongly affects a star’s evolution: more massive stars generally use their fuel faster and have shorter main-sequence lifetimes despite containing more fuel.
Gravity and thermal pressure play different roles through the life cycle. A white dwarf is not simply a smaller ordinary main-sequence star burning hydrogen in the same way.

When a wave source moves relative to an observer, the observed frequency and wavelength can change: this is the Doppler effect. A source moving away gives a longer wavelength and lower frequency. A source moving towards the observer gives a shorter wavelength and higher frequency.
Spectral lines from many distant galaxies are shifted towards longer wavelengths. More distant galaxies generally show a greater red-shift, supporting expansion of space on large scales; not every nearby galaxy must be receding.
The Big Bang model describes a Universe expanding from an early hot, dense state. It is not an explosion from one ordinary location into pre-existing empty space.
The Steady State model also allowed expansion but proposed continual creation of matter to keep average density constant and no overall beginning. Red-shift alone therefore did not distinguish the two models.
Cosmic microwave background radiation is detected across the sky and fits the cooled remnant of the hot early Universe. Together with red-shift and other evidence, it supports the Big Bang rather than the Steady State model.
The Big Bang is the currently accepted model because the evidence supports it more strongly. Scientific models can be refined as new evidence appears; acceptance does not mean every question about the Universe is answered.

Observations progressed from naked-eye records to optical telescopes, photography, electronic detectors, radio astronomy and space-based instruments. Different wavelengths reveal different processes and objects.
Earth’s atmosphere absorbs some electromagnetic wavelengths, clouds obscure visible light, and atmospheric motion can blur images. Telescopes above the atmosphere avoid many of these limitations, although launch and maintenance are expensive.
Different observing methods provide complementary evidence.
Ground telescopes still provide valuable observations, including visible and radio windows. Choose an observing method for the wavelength, required resolution, practical constraints and scientific question.