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P6.2.1P6. Space Physics

The Sun as a star — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 6 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P6.2.1 The Sun as a star — COMBINED SCIENCE notes: Describe the Sun's composition, energy output and the nuclear fusion that powers it.Star type — a medium-sized star at the centre of the Solar SystemComposition — it consists mostly of hydrogen and heliumRadiation — it radiates m…

  • Visible light
  • Ultraviolet
  • Possible reaction
  • High temperature and pressure
  • Balance of forces
  • Light-year

Part 1 of 7The Sun

The Sun
Star type
  • Star type a medium-sized star at the centre of the Solar System
Composition
  • Composition it consists mostly of hydrogen and helium
Radiation
  • Radiation it radiates most of its energy in the infrared, visible and ultraviolet parts of the electromagnetic spectrum

Part 2 of 7Energy from the Sun

Travel
  • Travel the energy reaches the Earth as electromagnetic radiation at 3.0 × 108 m/s through the vacuum of space
Infrared
  • Infrared warms the Earth
Visible light
  • Visible light used for photosynthesis and vision
Ultraviolet
  • Ultraviolet can cause sunburn and skin cancer

Part 3 of 7Nuclear fusion

Fusion
Fusion hydrogen nuclei join to form helium nuclei in the centre of a star, releasing a huge amount of energy
  • Stars are huge balls of mostly hydrogen gas.
  • All stable stars are powered by nuclear fusion.
Possible reaction
  • 21H + 31H → 42He + 10n + energy
  • Deuterium and tritium are both isotopes of hydrogen.
  • They are formed through other fusion reactions in the star.

Part 4 of 7Conditions for fusion

  • High temperature and pressure
    • needed so that the positive nuclei can overcome their electrostatic repulsion

Part 5 of 7Stability of the Sun

  • Balance of forces
    • the Sun is stable because the inward pull of gravity is balanced by the outward pressure from the energy released in the core
    • Gravitational forces act inwards and pull the outer layers towards the core.
    • Thermal pressure from the hot expanding gas acts outwards.

Part 6 of 7Distances

  • Light-year = the distance travelled by light in one year
    • One light-year is about 9.5 × 1015 m, or 9.5 × 1012 km.
    • Light takes about 8 minutes to reach the Earth from the Sun.

Part 7 of 7More from the specification

1
Solar spectroscopy as an astrophysical means
  • Solar spectroscopy as an astrophysical means analyzing Fraunhofer dark absorption lines provides an astrophysical means to determine the Sun's elemental composition.
2
Stellar Formation in a Nebula
  • The Sun and surrounding solar system originated when a vast interstellar cloud of gas and dust (nebula) collapsed under gravitational force to ignite nuclear fusion.
3
Solar Life Cycle Evolution
  • The Sun is currently on the main sequence;
    • once core hydrogen is exhausted, it will expand into a red giant before shedding outer layers into a planetary nebula with a white dwarf core.
4
Galactic Redshift Evidence
  • Absorption lines in light from distant galaxies are shifted toward longer wavelengths (redshift), showing that galaxies are moving away from our galaxy.
5
Expanding Universe and Hubble's Law
  • Hubble established that galactic recession velocity is proportional to distance, providing direct observational evidence that the entire universe is expanding.
6
White dwarf
  • The cooling collapsed core left after a low-mass star ejects its outer layers.
7
Background radiation
  • The radiation that exists around us all the time from natural and man-made sources.
8
Planetary nebula
  • Shell of gas shed by a red giant.
9
Universe
  • Everything that exists.
10
Galaxy
  • A huge group of stars, gas and dust held by gravity.
11
Redshift
  • An increase in the observed wavelength of light from a receding source.
12
Wavelength
  • The distance between the same points on two neighbouring waves.
13
Nebula
  • A cloud of dust and gas in space.
14
v = H0d
What the symbols mean: v = recession speed of the galaxy (km s−1) · H0 = Hubble constant (km s−1 Mpc−1) · d = distance (Mpc)

Key terms

TermMeaning
StarA huge ball of gas, mostly hydrogen, that releases energy by nuclear fusion.
Nuclear fusionThe joining of light nuclei to form heavier nuclei, releasing energy.
Light-yearThe distance light travels in one year, about 9.5 × 1015 m.
Equilibrium (stable star)The state in which inward gravitational force balances outward thermal pressure.
Electromagnetic spectrumThe family of waves that includes infrared, visible light and ultraviolet.

Avoid these mistakes

1
Saying the Sun burns hydrogen like a fire — it releases energy by nuclear fusion
2
Saying a light-year is a unit of time — it is a unit of distance
3
Saying fusion happens at low temperatures — it needs very high temperature and pressure
4
Saying ultraviolet is used for photosynthesis — visible light is used and ultraviolet can harm skin

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