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IGCSE Physics 0625 · Chapter 12 notes · ~12 min read

Space physics

Day, night, seasons and the Moon, then the Solar System, the life of stars, galaxies, redshift and the Big Bang.

Earth, Sun & MoonSeasons & phasesSolar SystemOrbitsStars & life cycleLight-yearsRedshiftBig Bang & Hubble constant
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • The Earth spins once a day (day and night) and orbits the Sun once a year. Its tilted axis causes the seasons.
  • The Moon orbits the Earth about monthly; its phases show how much of the lit half we see.
  • Eight planets: the inner four are small and rocky; the outer four are large and gaseous.
  • A light-year is a distance. Our galaxy, the Milky Way, is about 100 000 light-years across.
  • Distant galaxies are redshifted, so the Universe is expanding and began in a Big Bang.

By the end you can

  • Explain day, night, seasons and Moon phases.
  • Name the planets in order and explain why the inner and outer planets differ.
  • Calculate light travel time and orbital speed (v = 2πr ÷ T). EXT
  • Describe the Sun as a star and the life cycle of a star. EXT
  • Explain redshift, the Big Bang, CMBR and the Hubble constant. EXT

1The Earth, Sun and Moon

Day, night and seasons

  • The Earth spins on its axis once every 24 hours. This gives day and night, and makes the Sun seem to rise in the east and set in the west.
  • The Earth orbits the Sun once every 365 days.
  • The axis is tilted at 23.5° and always points the same way in space.
SunNNNNJUNEN summerDECEMBERN winterequinox (March / Sept)equinox (March / Sept)the tilt (23.5°) always points the same way in space, so the hemisphereleaning towards the Sun changes as the Earth orbits it
In June the northern hemisphere leans towards the Sun: more direct sunlight and longer days (summer). Six months later it leans away (winter).
Common mistake"It is summer when the Earth is closer to the Sun." No — the seasons come from the tilt of the axis, not the distance.

The Moon

Esunlightnewwaxing crescentfirst quarterwaxing gibbousfullwaning gibbouslast quarterwaning crescentthe Moon is always half lit; phases = how much of the lit half we can see
The Moon is always half lit. The phases are how much of the lit half we can see from the Earth.
  • The Moon orbits the Earth about once a month (about 28 days). We see it by reflected sunlight.
Common mistake"The phases are caused by the Earth's shadow." No — they come from our changing view of the lit half. (The Earth's shadow causes a lunar eclipse.)

Orbital speedEXTENDED

v = 2πrT
average orbital speed (m/s) = circumference ÷ time for one orbit · r = orbital radius (m), T = orbital period (s)
Worked example

EXT The Earth orbits the Sun at a radius of 1.5 × 10¹¹ m in 365 days. Find its average orbital speed.

  1. T = 365 × 24 × 3600 = 3.15 × 10⁷ s
  2. v = 2πr ÷ T = 2π × 1.5 × 10¹¹ ÷ 3.15 × 10⁷
  3. = 3.0 × 10⁴ m/s (30 km/s)

2The Solar System

What is in it

SunMercuryVenusEarthMarsJupiterSaturnUranusNeptunerocky planetsasteroid beltgas & ice giantsnot to scale · each planet orbits the Sun; moons orbit planets
The planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Memory trickMy Very Educated Mother Just Served Us Noodles — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
  • The Solar System has one star (the Sun), eight planets, dwarf planets (such as Pluto), asteroids (mostly between Mars and Jupiter), moons, and smaller bodies such as comets.
  • The Sun contains most of the mass, so its gravity keeps everything in orbit.
  • The inner four planets are small and rocky. The outer four are large and gaseous.

Why are the inner and outer planets different?

  • This is the accretion model. A cloud of gas and dust spun into a rotating disc and gravity pulled the material together.
  • Close to the young Sun it was too hot for gases to stay, so only heavier elements (rock and metals) gathered: small, rocky planets.
  • Far from the Sun it was cold, so huge amounts of hydrogen and helium gathered: large, gaseous planets.

Gravity and orbits

  • Gravity at the surface of a planet depends on the planet's mass. It decreases with distance from the planet.
  • EXT Planets and comets move in ellipses. The Sun is not at the centre unless the orbit is almost circular.
red arrow = velocity · green arrow = gravitySunfastesthereslowest heregravity pulls the comet towards the Sun; it speeds up as it falls closer
EXT A comet speeds up as it falls towards the Sun and slows as it moves away. Gravity (green) pulls it in; velocity is red.
  • EXT Objects move faster when they are closer to the Sun, because gravitational potential energy is converted into kinetic energy. The Sun's gravity and the orbital speeds of the planets get smaller further from the Sun.

Planetary dataEXTENDED

PlanetDistance from Sun (million km)Orbit timeSurface temp. (°C)Surface gravity (N/kg)
Mercury5888 days1673.7
Venus108225 days4648.9
Earth150365 days159.8
Mars228687 days−653.7
Jupiter77811.9 years−11024.8
Neptune4500165 years−20011.2
  • Further from the Sun means a longer year and (usually) a colder planet. Venus is hottest because its thick atmosphere traps heat.
  • Surface gravity depends on the planet's mass, not on its distance from the Sun.

Light travel time

Worked example

How long does light take to travel from the Sun to Neptune (4.5 × 10¹² m)? Use 3.0 × 10⁸ m/s.

  1. t = distance ÷ speed = 4.5 × 10¹² ÷ 3.0 × 10⁸
  2. = 15 000 s
  3. = 4.2 hours

3Stars and galaxies

The Sun as a star

  • The Sun is a medium-sized star, made mostly of hydrogen and helium. It gives out mostly infrared, visible and ultraviolet radiation.
  • EXT Stars are powered by nuclear fusion of hydrogen into helium (see Nuclear physics).

Light-years and galaxies

Light-year — the distance light travels in one year. It is a unit of distance, not time. EXT 1 light-year = 9.5 × 10¹⁵ m.

  • Other stars are much further away than the Sun. The nearest, Proxima Centauri, is about 4.2 light-years away.
  • A galaxy is a group of billions of stars held together by gravity. The Sun is in the Milky Way, which is about 100 000 light-years across. The Universe has billions of galaxies.
our Sunthe Milky Way~100 000 light-yearsacross, 100 billion+starsbillions of galaxies make up the Universe
The Milky Way is a spiral galaxy about 100 000 light-years across, with over 100 billion stars. Our Sun is out in one of the spiral arms.
Worked example

EXT Proxima Centauri is 4.2 light-years away. Find the distance in metres.

  1. distance = 4.2 × 9.5 × 10¹⁵
  2. = 4.0 × 10¹⁶ m

Life cycle of a starEXTENDED

Nebula (gas + dust)ProtostarMain sequence starSun-sizedmuch more massiveRed giantWhite dwarfBlack dwarfRed supergiantSupernovaNeutron star orblack hole
A star's path depends on its mass. A Sun-sized star ends as a white dwarf; a much more massive star ends in a supernova.
  • A star begins as a nebula (a cloud of gas and dust). Gravity pulls it together to make a protostar, which heats up until hydrogen fusion starts.
  • It then stays stable for billions of years, while the inward pull of gravity balances the outward push of the energy from fusion.
  • A Sun-sized star then expands into a red giant, sheds its outer layers, and shrinks into a white dwarf.
  • A much more massive star becomes a red supergiant and explodes as a supernova, leaving a neutron star or a black hole.
  • The supernova scatters heavy elements into space. Later new stars and planets form from this gas and dust.

4The Universe

Redshift and the Big Bang

violetredlab (source at rest)nearby galaxyvery distant galaxyspectral lines shifted towards red = the galaxy is moving AWAYthe bigger the shift, the faster it is receding
Light from a galaxy moving away is stretched to a longer wavelength — shifted towards the red end of the spectrum. The faster it moves away, the bigger the shift.

Redshift — an increase in the observed wavelength of light from a source that is moving away from us.

  • Light from distant galaxies is redshifted, so they are moving away. The more distant the galaxy, the bigger the redshift.
  • So the Universe is expanding. Running time backwards, everything was once much closer together: this is the Big Bang Theory, about 13.8 billion years ago.
NOWLATERusABCusABCarrows = speed away from usspace itself stretches: every galaxy moves away from every otherfarther galaxies recede faster (v = H₀ × d)the Big Bang: everything began ~13.8 billion years ago
Space itself is stretching. Every galaxy moves away from every other galaxy, and the more distant ones move away faster.

CMBR and the Hubble constantEXTENDED

  • Cosmic microwave background radiation (CMBR) is radiation coming from all directions in space. It was produced soon after the Big Bang. As the Universe expanded, its wavelength was stretched into the microwave region. It is strong evidence for the Big Bang.
  • The speed of a receding galaxy can be found from its redshift. The distance of a far galaxy can be found from the brightness of a supernova in it.
EXT H₀ = vd
Hubble constant = recession speed ÷ distance · H₀ ≈ 2.2 × 10⁻¹⁸ per second
EXT age of the Universe ≈ 1H₀
this is an estimate of the time since the Big Bang
Worked example

EXT A galaxy is 1.0 × 10²³ m away. Find its recession speed. (H₀ = 2.2 × 10⁻¹⁸ /s)

  1. v = H₀ × d = 2.2 × 10⁻¹⁸ × 1.0 × 10²³
  2. = 2.2 × 10⁵ m/s (220 km/s)
Worked example

EXT Estimate the age of the Universe from H₀ = 2.2 × 10⁻¹⁸ /s. Give your answer in years.

  1. age = 1 ÷ H₀ = 1 ÷ (2.2 × 10⁻¹⁸) = 4.5 × 10¹⁷ s
  2. 1 year = 3.15 × 10⁷ s
  3. age = 4.5 × 10¹⁷ ÷ 3.15 × 10⁷ = 1.4 × 10¹⁰ years (14 billion)

5Test yourself

Test yourself — cover the answers first

Quick check

  1. How long does the Earth take to spin once on its axis?
  2. What causes the seasons?
  3. Name the eight planets in order from the Sun.
  4. Why are the outer planets large and gaseous?
  5. What is a light-year?
  6. What does redshift of a galaxy tell us?

Exam-style

  1. EXT Mars orbits at a radius of 2.3 × 10¹¹ m with an orbit time of 687 days. Find its average orbital speed.
  2. EXT Explain why a comet moves faster as it gets closer to the Sun. [2]
  3. Light takes 760 s to travel from the Sun to Mars. Find the distance. (c = 3.0 × 10⁸ m/s)
  4. EXT Describe the life cycle of a star much more massive than the Sun. [4]
  5. EXT Explain how the cosmic microwave background radiation supports the Big Bang Theory. [3]
Show answers
  1. 24 hours
  2. The tilt of the Earth's axis as the Earth orbits the Sun.
  3. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
  4. It was cold enough far from the Sun for lots of hydrogen and helium to be gathered by gravity.
  5. The distance light travels in one year.
  6. The galaxy is moving away from us, and the Universe is expanding.
  7. T = 687 × 24 × 3600 = 5.94 × 10⁷ s
    v = 2πr ÷ T = 2π × 2.3 × 10¹¹ ÷ 5.94 × 10⁷ = 2.4 × 10⁴ m/s
  8. The Sun's gravitational pull does work on the comet
    so gravitational potential energy is changed into kinetic energy
  9. 3.0 × 10⁸ × 760 = 2.3 × 10¹¹ m
  10. A nebula is pulled together by gravity to form a protostar
    fusion starts and it is stable for millions of years
    it becomes a red supergiant
    it explodes as a supernova, leaving a neutron star or black hole
  11. It was produced soon after the Big Bang
    as the Universe expanded the wavelength stretched into the microwave region
    it comes from all directions in space, as predicted
Practise this chapter
Earth & the Solar SystemStars & the Universe
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