Space Physics
Space physics is a Physics-only GCSE topic, not included in Combined Science: Trilogy, covering the structure of the Solar System, how stars form and change over their lifetime, why objects stay in orbit, and the evidence that the universe is expanding. The Solar System consists of the Sun and the planets, moons, asteroids and comets that orbit it, held in orbit by the Sun's gravitational pull.
Method
- Check first whether space physics is on your specification: it is examined only in the separate GCSE Physics course, not in Combined Science: Trilogy, so make sure it is actually on your course before revising it in depth.
- Learn the structure of the Solar System: one star (the Sun) at the centre, eight planets (and dwarf planets) orbiting it, natural satellites (moons) orbiting some of the planets, and smaller bodies (asteroids, mostly found in the asteroid belt, and comets, which have highly elongated orbits) also orbiting the Sun.
- Learn the life cycle of a star as a sequence: a star forms from a nebula, a cloud of dust and gas, pulled together by gravity; once the core is hot and dense enough, hydrogen nuclei fuse together to release energy, and the star becomes a stable main sequence star for the majority of its life, with the outward pressure from this energy release balancing the inward pull of gravity.
- Learn what happens after the main sequence, which depends on the star's initial mass: a star with a mass similar to the Sun swells into a red giant, then loses its outer layers to leave a white dwarf, which then cools; a star with a much larger initial mass swells into a red super giant, then explodes as a supernova, leaving behind either a neutron star or, for the most massive stars, a black hole.
- Remember that a supernova explosion is how elements heavier than iron are formed, and that the explosion also scatters all the elements a star has produced, both the lighter elements formed by fusion during its life and the heavier ones formed in the explosion, out into space, where they can go on to form new stars, planets and, eventually, living things.
- For orbit questions, explain a stable orbit using forces: gravity provides the centripetal force that constantly changes an orbiting object's direction, and therefore its velocity, since velocity is a vector, keeping it moving in a roughly circular path without changing its speed.
- Learn how orbital speed and orbital radius are related for a stable orbit: an object in a larger orbit, further from the body it is orbiting, needs a slower orbital speed to stay in a stable orbit than an object in a smaller, closer orbit, which is why planets further from the Sun take longer to complete one orbit.
- Distinguish natural satellites (moons, which orbit a planet) from artificial satellites (human-made objects such as communication or weather satellites, launched into orbit around the Earth).
Worked example
A star has approximately the same mass as the Sun. Describe and explain the sequence of stages this star will pass through, from formation to its final state.
- Identify the star's mass category first, since this determines its life cycle: a mass similar to the Sun's means it follows the lower-mass pathway, not the pathway for a much more massive star.
- Start of the sequence: the star forms from a nebula, a cloud of dust and gas pulled together by gravity, and as it contracts, the core heats up.
- Main sequence: once the core is hot and dense enough for hydrogen nuclei to fuse into helium, the star releases energy and becomes a stable main sequence star, staying in this stable, long-lasting phase as long as the outward pressure produced by fusion balances the inward pull of gravity.
- Red giant: once the hydrogen in the core starts to run out, the balance is lost, the core contracts and heats further while the outer layers expand and cool, and the star swells into a red giant.
- White dwarf: the red giant then loses its outer layers, leaving behind a hot, dense core called a white dwarf, which has no further fuel for fusion and simply cools and dims over a very long time.
- Final answer: for a star of about the Sun's mass, the sequence is nebula, main sequence star, red giant, white dwarf (and eventually a cold, dark black dwarf), because its mass is too low for it ever to explode as a supernova.
Practice questions
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Q1Name the force that keeps the planets in orbit around the Sun.Show answer
Answer: Gravity (the Sun's gravitational force/attraction).
Q2State the difference between a natural satellite and an artificial satellite, and give one example of each.Show answer
Answer: A natural satellite forms naturally and orbits a planet, for example the Moon orbiting the Earth; an artificial satellite is human-made and launched into orbit, for example a communications or weather satellite.
Q3Name the region of the Solar System where most asteroids are found.Show answer
Answer: The asteroid belt, between the orbits of Mars and Jupiter.
Q4State what a nebula is, and explain its role in the life cycle of a star.Show answer
Answer: A nebula is a cloud of dust and gas in space; gravity pulls the dust and gas in a nebula together, and this is how a star begins to form.
Q5Explain, in terms of forces, why an orbiting satellite does not travel off in a straight line.Show answer
Answer: Gravity acts on the satellite as a centripetal force, constantly pulling it towards the object it is orbiting; this continuously changes the satellite's direction, and so its velocity, which keeps it moving on a roughly circular path rather than a straight line.
Q6State what happens to the orbital speed needed for a stable orbit as the orbital radius increases.Show answer
Answer: The orbital speed needed decreases: an object in a larger, more distant orbit needs to move more slowly than an object in a smaller, closer orbit.
Q7State the eventual fate of a star with a much greater mass than the Sun, after it explodes as a supernova.Show answer
Answer: It leaves behind either a neutron star, or, if the remaining core is massive enough, a black hole.
Q8Explain how elements heavier than iron, such as gold, are created and distributed through the universe.Show answer
Answer: Elements heavier than iron are formed during a supernova explosion, the death of a massive star; the explosion also scatters these newly formed heavy elements, along with the lighter elements formed earlier in the star's life, out into space.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Comets and planets both orbit the Sun, but their orbits are different shapes. Describe the shape of a typical comet's orbit compared with a planet's orbit, and state what this means for how a comet's distance from the Sun, and its speed, changes during one orbit.
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A star has an initial mass around 20 times the mass of the Sun. Describe and explain the life cycle this star will go through, from its formation to its final possible outcomes, and explain how this life cycle differs from that of a star with a mass similar to the Sun.
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Free printable worksheet
Want more practice on paper? Download the space physics worksheet pack - 12 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 8 of GCSE Physics Workbook, the whole course as one free printable PDF.
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