IGCSE Physics Space physics notes

Core — every candidate

• 6.1.1Know that the Earth is a planet that rotates on its axis, which is tilted, once in approximately 24 hours, and use this to explain observations of the apparent daily motion of the Sun and the periodic cycle of day and night.
• 6.1.1Know that the Earth orbits the Sun once in approximately 365 days and use this to explain the periodic nature of the seasons.
• 6.1.1Know that it takes approximately one month for the Moon to orbit the Earth and use this to explain the periodic nature of the Moon’s cycle of phases.
• 6.1.2Describe the Solar System as containing one star (the Sun), the eight named planets in order from the Sun, minor planets including dwarf planets such as Pluto and asteroids in the asteroid belt, moons orbiting the planets, and smaller Solar System bodies including comets and natural satellites.
• 6.1.2Know that the four planets nearest the Sun are rocky and small and the four planets furthest from the Sun are gaseous and large, and explain this difference by referring to an accretion model for Solar System formation, including the model’s dependence on gravity, the presence of many elements in interstellar clouds of gas and dust, and the rotation of material in the cloud and the formation of an accretion disc.
• 6.1.2Know that the strength of the gravitational field (a) at the surface of a planet depends on the mass of the planet, and (b) around a planet decreases as the distance from the planet increases.
• 6.1.2Calculate the time it takes light to travel a significant distance such as between objects in the Solar System.
• 6.1.2Know that the Sun contains most of the mass of the Solar System and this explains why the planets orbit the Sun.
• 6.1.2Know that the force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun.
• 6.2.1Know that the Sun is a star of medium size, consisting mostly of hydrogen and helium, and that it radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum.
• 6.2.2State that galaxies are each made up of many billions of stars, that the Sun is a star in the galaxy known as the Milky Way, that other stars of the Milky Way are much further away from the Earth than the Sun, and that astronomical distances can be measured in light-years (the distance travelled in space by light in one year).
• 6.2.3Know that the Milky Way is one of many billions of galaxies making up the Universe, and that the diameter of the Milky Way is approximately 100 000 light-years.
• 6.2.3Describe redshift as an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies.
• 6.2.3Know that the light emitted from distant galaxies appears redshifted in comparison with light emitted on the Earth.
• 6.2.3Know that redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory.

Supplement — Extended candidates only

• 6.1.1Define average orbital speed from the equation v = 2πr/T; recall and use this equation.
• 6.1.2Know that planets, minor planets and comets have elliptical orbits, and recall that the Sun is not at the centre of the elliptical orbit, except when the orbit is approximately circular.
• 6.1.2Analyse and interpret planetary data about orbital distance, orbital duration, density, surface temperature and uniform gravitational field strength at the planet’s surface.
• 6.1.2Know that the strength of the Sun’s gravitational field decreases and that the orbital speeds of the planets decrease as the distance from the Sun increases.
• 6.1.2Know that an object in an elliptical orbit travels faster when closer to the Sun and explain this using the conservation of energy.
• 6.2.1Know that stars are powered by nuclear reactions that release energy and that in stable stars the nuclear reactions involve the fusion of hydrogen into helium.
• 6.2.2Know that one light-year is equal to 9.5 × 10¹⁵ m.
• 6.2.2Describe the life cycle of a star: formation from interstellar clouds of gas and dust containing hydrogen; protostar as a collapsing cloud heating by its internal gravitational attraction; stability when inward gravitational force is balanced by outward force due to the high temperature; eventual running out of hydrogen as fuel; expansion into red giants (or red supergiants for more massive stars); a red giant forming a planetary nebula with a white dwarf at its centre; a red supergiant exploding as a supernova forming a nebula with hydrogen and new heavier elements and leaving a neutron star or black hole; the supernova nebula possibly forming new stars with orbiting planets.
• 6.2.3Know that microwave radiation of a specific frequency is observed at all points in space around us, known as cosmic microwave background radiation (CMBR).
• 6.2.3Explain that the CMBR was produced shortly after the Universe was formed and that this radiation has been expanded into the microwave region of the electromagnetic spectrum as the Universe expanded.
• 6.2.3Know that the speed v at which a galaxy is moving away from the Earth can be found from the change in wavelength of the galaxy’s starlight due to redshift.
• 6.2.3Know that the distance d of a far galaxy can be determined using the brightness of a supernova in that galaxy.
• 6.2.3Define the Hubble constant H₀ as the ratio of the speed at which a galaxy is receding to its distance; recall and use H₀ = v/d.
• 6.2.3Know that the current estimate for H₀ is 2.2 × 10⁻¹⁸ per second.
• 6.2.3Know that the equation d/v = 1/H₀ represents an estimate for the age of the Universe and that this is evidence for the idea that all the matter in the Universe was present at a single point.