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6.2Space Physics

Stars and The Universe — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

15 min read 23 parts Lesson 13 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

6.2 Stars and The Universe (Part 1) — IGCSE PHYSICS notes: Describe the Sun as a star, the structure and scale of the Universe, how stars form and how low-mass stars evolve.Type — the Sun is a medium-sized star at the centre of the Solar System.Composition — mostly hydrogen and h…

  • Energy source (Extended)
  • Other stars
  • Light-year
  • Conversion (Extended)
  • Planetary nebula
  • White dwarf

Part 1 of 7The Sun as a star

The Sun as a star
Type
  • Type the Sun is a medium-sized star at the centre of the Solar System.
Composition
  • Composition mostly hydrogen and helium.
Radiation
  • Radiation most of its energy is radiated in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.
Energy source (Extended)
  • Energy source (Extended) in the core hydrogen nuclei fuse to form helium, releasing a huge amount of energy;
    • all stable stars are powered by fusion.
One possible reaction is hydrogen-2 + hydrogen-3 → helium-4 + a neutron, and its details are not required.

Part 2 of 7Structure of the Universe

Structure of the Universe
Universe
  • Universe a large collection of billions of galaxies; it is also the name for all of space.
Galaxy
  • Galaxy a large collection of billions of stars; the Milky Way is one of many billions of galaxies.
Our place
  • Our place the Sun is one star in the Milky Way, at the centre of the Solar System.
Other stars
  • Other stars are much further away than the Sun, and some have planets that orbit them.

Part 3 of 7Astronomical distances

1Light-year
  • Light-year the distance travelled by light in one year.
2Size
  • Size the Milky Way is about 100 000 light-years across, so light takes 100 000 years to cross it.
3Conversion (Extended)
  • 1 light-year = 9.5 × 1012 km = 9.5 × 1015 m.

Part 4 of 7Star formation (Extended)

1
A nebula, a giant cloud of gas and dust, is pulled together by gravitational attraction.
2
The particles collide more often as the density rises, so the temperature increases and a hot ball of gas called a protostar forms.
3
When the core is hot enough, hydrogen fuses to helium and the star becomes stable.
4
A stable star is in equilibrium: gravity acts inwards and thermal pressure from the hot expanding gas acts outwards, and they balance.

Part 5 of 7Life cycle of a low-mass star (Extended)

1Low-mass star
  • Low-mass star up to about eight times the mass of the Sun.
2Red giant
  • Red giant when hydrogen runs out the fusion rate drops, the core shrinks and heats, and helium fuses into carbon;
    • the outer layers expand and cool.
3Planetary nebula
  • Planetary nebula when the helium runs out the core collapses and the outer layers are ejected into space.
4White dwarf
  • White dwarf the collapsed core, which cools and emits less energy over time.
5Sequence
  • Sequence nebula → protostar → stable star → red giant → planetary nebula → white dwarf.

Part 6 of 7Why the stages change

1Fuel running out
  • Fuel running out when hydrogen in the core is used up, fusion slows, gravity wins and the core shrinks and heats.
2Helium fusion
  • Helium fusion the hotter core fuses helium into carbon, so the outer layers expand and cool, giving a red giant.
3Ending
  • Ending a low-mass star cannot fuse heavier elements, so it sheds its outer layers and the core cools as a white dwarf.

Part 7 of 7More from the specification

1
Stellar luminosity determination
  • Stellar luminosity determination combining radiant apparent brightness with measured astronomical distance provides a reliable means to calculate solar luminosity.
2
Galactic Redshift and Receding Motion
  • Light observed from distant galactic sources shows spectral lines shifted toward longer wavelengths, indicating that distant galaxies are moving away from Earth.
3
Hubble's Law and Expansion Rate
  • Hubble discovered that a galaxy's recession speed v is directly proportional to its distance d from Earth (v = H0 * d),
  • where H0 is the Hubble constant.
4
Cosmic Expansion of Space
  • The observation that redshift increases with distance indicates that the fabric of space itself is continuously expanding in all directions.
5
Stellar Life Cycles
  • Stars progress through an evolutionary cycle from interstellar gas clouds to stable hydrogen-burning stars, eventually ending as white dwarfs, neutron stars,
  • or black holes.
6
Galactic redshift
  • Shift of galaxy spectral lines to longer wavelength due to recession.
7
Background radiation
  • Radiation that is always present in the environment.
8
Redshift
  • Increase in the wavelength of light from a source moving away from the observer.
9
Wavelength
  • Distance between a point on one wave and the same point on the next wave
10
Constant
  • Not changing.
11
Recession
  • Movement away from the observer.
12
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
GalaxyA large collection of billions of stars.
UniverseAll of space, containing billions of galaxies.
Light-yearThe distance light travels in one year.
ProtostarA hot ball of gas formed as a nebula collapses, before fusion starts.
Red giantA large cool star formed when hydrogen in the core runs out.
White dwarfThe small dense core left by a low-mass star.
NebulaA cloud of gas and dust in space.

Avoid these mistakes

1
Mixing up galaxy and Universe — the Universe contains billions of galaxies.
2
Treating a light-year as a time — it is a distance.
3
Saying fusion in stars is fission — fusion joins nuclei and fission splits them.
4
Thinking the Sun will become a black hole — a low-mass star becomes a white dwarf.

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