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Space physics

IGCSE Physics 0625 · Unit 6 revision notes

Days, seasons and phases, the Solar System and orbits, the Sun and the life of stars, and the redshift evidence for an expanding Universe.

24 definitions to learn word for word, 8 equations in their own column, 12 diagrams and graphs. Read it, cover it, test yourself.

  1. 6.1a The Earth, the Sun and the Moon
  2. 6.1b The Solar System
  3. 6.1c Gravity and orbits
  4. 6.2a The Sun as a star
  5. 6.2b Stars and galaxies
  6. 6.2c The Universe
Definitions (mark-scheme wording)EquationsExam trapsExtended = Extended tier only

6.1aThe Earth, the Sun and the Moon

Definitions

Day
the time for the Earth to rotate once on its axis (about 24 hours).
Year
the time for the Earth to orbit the Sun once (about 365 days).
Month
the approximate time for the Moon to orbit the Earth once (about one month, 27 to 30 days).
Average orbital speed
Extended the distance travelled round the orbit divided by the orbital period, v = 2πr / T.

Key points

  • The Earth spins on a tilted axis: this gives day and night and the Sun’s apparent daily motion (rises in the east, sets in the west).
  • The Earth orbits the Sun in about 365 days.
  • The axis is tilted (about 23°) and always points the same way, so each hemisphere leans towards the Sun for part of the year: seasons.
  • Leaning towards the Sun: longer days and more concentrated sunlight, so summer.
  • The Moon shines by reflected sunlight; half of it is always lit.
  • Phases of the Moon: as it orbits, we see different amounts of its lit half (new, crescent, quarter, gibbous, full).
Sun Earth Moon day = 1 spin year = 365 days month = Moon orbit
Earth orbits the Sun once a year; the Moon orbits the Earth about once a month; the Earth spins once a day.
Sun NN N summer N winter axis always tilted the same way
Seasons: the axis keeps the same tilt, so the northern hemisphere leans towards the Sun in one half of the orbit (summer) and away in the other (winter).
sunlight Earth new full quarterquarter
Phases: half the Moon is always lit by the Sun; from Earth we see all of the lit half (full), none (new) or part (quarter).

Watch out!

  • Seasons are caused by the tilt of the axis, not by the Earth being closer to the Sun in summer.
  • Phases are not caused by the Earth’s shadow; that is a lunar eclipse.

6.1bThe Solar System

Definitions

Planet
a large body that orbits the Sun.
Moon (natural satellite)
a natural body that orbits a planet.
Minor planets
smaller bodies orbiting the Sun, including dwarf planets (e.g. Pluto) and asteroids.
Comet
a small body of ice and dust in a highly elliptical orbit around the Sun.
Accretion model
the Solar System formed from a rotating interstellar cloud of gas and dust, containing many elements, that collapsed under gravity into an accretion disc; material clumped together to form the Sun and planets.

Key points

  • Order from the Sun: Mercury, Venus, Earth, Mars (asteroid belt) Jupiter, Saturn, Uranus, Neptune.
  • Inner four: small, rocky. Outer four: large, gaseous (gas giants).
  • In the collapsing disc, only rocky material survived near the hot young Sun; light gases gathered further out.
  • The Sun contains most of the mass of the Solar System, so everything orbits the Sun.
  • Light takes about 8 minutes to reach the Earth from the Sun.
  • The gravitational field strength at a planet’s surface depends on the planet’s mass; around a planet it decreases with distance.
  • Extended analyse planetary data (orbital distance and period, density, surface temperature, surface g): e.g. planets further from the Sun are colder and have longer orbital periods.
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune rocky, inner asteroids gas giants, outer
The eight planets in order (sizes schematic, distances not to scale); the asteroid belt lies between Mars and Jupiter.
gas + dustcloud spinningdisc Sun + planetsby accretion gravity pulls the material together
Accretion: a rotating gas and dust cloud collapses into a spinning disc; the Sun forms at the centre and planets clump together from the disc.

Watch out!

  • The asteroid belt is between Mars and Jupiter.
  • Distances in space are huge: convert km to m before using t = d / c.

6.1cGravity and orbits

Definitions

Orbit
the curved path of a body around a more massive body, held by gravitational attraction.
Orbital period (T)
the time taken for one complete orbit.

Key points

  • The gravitational attraction of the Sun keeps planets and comets in orbit; a planet’s gravity keeps its moons and satellites in orbit.
  • Extended the Sun’s gravitational field gets weaker with distance, so outer planets orbit more slowly and have longer periods.
  • Extended planets, minor planets and comets have elliptical orbits; the Sun is not at the centre unless the orbit is nearly circular (as most planetary orbits are).
  • Extended a comet speeds up near the Sun (GPE → KE) and slows far away (KE → GPE).
  • Extended its total energy stays constant (conservation of energy).
Sun v gravity r v = 2πr / T
Extended: gravity acts towards the Sun; the velocity is along the tangent. One orbit is 2πr in time T.
Sun fastslow slow: KE low, GPE high fast: KE high, GPE low
Extended: a comet on an elliptical orbit is fastest nearest the Sun and slowest furthest away; total energy is constant.

Watch out!

  • Extended: in an elliptical orbit the speed is greatest nearest the Sun, not constant.
  • Extended: use r in metres and T in seconds to get v in m/s (1 year = 3.2 × 107 s).

6.2aThe Sun as a star

Definitions

Star
a large ball of hot gas that radiates energy.
The Sun
a medium-sized star, consisting mostly of hydrogen and helium.

Key points

  • The Sun radiates most of its energy as infrared, visible light and ultraviolet.
  • It is the nearest star to Earth and the source of almost all our energy resources.
  • Extended stars like the Sun are powered by nuclear fusion of hydrogen into helium in the core.
  • Extended the Sun is stable because the outward pressure from fusion balances the inward pull of gravity.
H → He mostly H and He infrared visible light ultraviolet fusion in the core
The Sun: a medium-sized star of hydrogen and helium, radiating mainly infrared, visible and ultraviolet.

Watch out!

  • The Sun is a medium-sized star, not a large one.
  • Extended: fusion joins hydrogen into helium; it is not burning (no chemical reaction, no oxygen).

6.2bStars and galaxies

Definitions

Galaxy
a collection of many billions of stars held together by gravity.
Milky Way
the galaxy that contains the Sun.
Light-year
the distance travelled in (the vacuum of) space by light in one year.
Protostar
Extended a hot, dense ball of gas forming from a collapsing interstellar cloud (nebula).
Stable (main-sequence) star
Extended a stable star fusing hydrogen into helium, with outward pressure balancing gravity.
Supernova
Extended the explosion of a red supergiant, forming a nebula containing heavy elements.

Key points

  • The Sun is one of many billions of stars in the Milky Way; other stars in it are much further from the Earth than the Sun is.
  • Distances between stars and galaxies are measured in light-years.
  • Extended stars form from interstellar clouds of gas and dust (nebulae) collapsing under gravity to form a protostar.
  • Extended when hydrogen runs out, a star expands: a red giant (low mass) or red supergiant (high mass).
  • Extended a red giant loses its outer layers as a planetary nebula, leaving a white dwarf.
  • Extended a red supergiant explodes as a supernova, leaving a neutron star or, if very massive, a black hole.
  • Extended supernova nebulae contain heavy elements, which may form new stars and planets.
nebula protostar main sequence Sun-sizedmassive red giant planetary nebula white dwarf red supergiant supernova neutron staror black hole
Extended: life cycle of a star. The path after the main sequence depends on the star’s mass.
H → He gravity pulls in fusion pressure pushes out
Extended: a main-sequence star is stable because inward gravity is balanced by outward pressure from fusion.

Watch out!

  • A light-year is a distance, not a time.
  • Extended: a white dwarf comes from a Sun-sized star; neutron stars and black holes come from massive stars.

6.2cThe Universe

Definitions

Universe
all of space and everything in it, including many billions of galaxies.
Redshift
the increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies.
Big Bang theory
the Universe began from a single point about 13.8 billion years ago and has been expanding ever since.
Cosmic microwave background radiation (CMBR)
Extended microwave radiation of a specific frequency observed from all directions, produced shortly after the Big Bang and stretched by expansion.
Hubble constant (H0)
Extended the ratio of the speed at which a galaxy is moving away from the Earth to its distance from the Earth.

Key points

  • The Milky Way is one of many billions of galaxies; its diameter is about 100 000 light-years.
  • Light from distant galaxies is redshifted compared with the same light emitted on Earth.
  • Redshift shows galaxies are moving away: the Universe is expanding, supporting the Big Bang theory.
  • Extended more distant galaxies show greater redshift: they recede faster.
  • Extended the CMBR is further evidence for the Big Bang.
  • Extended recession speed v comes from the redshift; distance d from the brightness of a supernova in that galaxy.
  • Extended H0 = 2.2 × 10-18 per second; d / v = 1 / H0 estimates the age of the Universe.
spectrum in the lab light from a distant galaxy blue endred end longer wavelength: galaxy receding
Redshift: the galaxy’s spectral lines keep their pattern but all move towards the red (longer wavelength) end.
recession speed vdistance d gradient = H0
Extended: recession speed is proportional to distance; the gradient of the line is the Hubble constant.

Watch out!

  • Redshift means the wavelength gets longer (towards red), not that the galaxy turns red.
  • Extended: 1/H0 gives the age in seconds; convert to years if asked.
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