Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.2.2Space Physics

The Scale of the Universe — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 18 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

6.2.2 The Scale of the Universe — IGCSE Physics Notes — IGCSE PHYSICS notes: Describe the structure of the Universe from the Solar System to galaxies and compare the sizes and distances involved.Universe — a large collection of billions of galaxies, and also the name for the whol…

  • Order of size
  • Other stars
  • Planets elsewhere
  • Local Group
  • Large-scale structure
  • Galaxy types

Part 1 of 8Structure of the Universe

Structure of the Universe
Universe
  • Universe a large collection of billions of galaxies, and also the name for the whole of space.
Galaxy
  • Galaxy a large collection of billions of stars, gas and dust held together by gravity.
Star
  • Star a large astronomical object such as the Sun.
Solar System
  • Solar System a star with its planets, dwarf planets, asteroids and comets.
Order of size
  • Order of size planets and moons, then the Solar System, then a galaxy, then groups and clusters of galaxies, then the Universe.

Part 2 of 8Our place in the Universe

Milky Way
  • Milky Way the galaxy that contains the Solar System, one of many billions of galaxies.
The Sun
  • The Sun one of many billions of stars in the Milky Way, and the star at the centre of the Solar System.
Other stars
  • Other stars they are much further from the Earth than the Sun is.
Planets elsewhere
  • Planets elsewhere some of these stars also have planets that orbit them.

Part 3 of 8Groups of galaxies

Groups of galaxies
Cluster
  • Cluster a group of tens to thousands of galaxies bound together by gravity.
Local Group
  • Local Group the Milky Way belongs to a group of about 50 galaxies.
Large-scale structure
  • Large-scale structure galaxies are gathered into groups and clusters, with giant empty voids between them.
Galaxy types
  • Galaxy types spirals have arms of stars winding round a central bulge, ellipticals are smooth and round, and some are irregular.

Part 4 of 8Astronomical distances

1Problem
  • Problem distances between stars and galaxies are extremely large.
2Unit
  • Unit astronomers use the light-year, the distance travelled by light in one year.
3Value
  • Value one light-year is 9.5 × 1012 km, or 9.5 × 1015 m.
4Light-year is a distance
  • Light-year is a distance it is not a unit of time.

Part 5 of 8Size of the Milky Way

Diameter
  • Diameter about 100 000 light-years.
Light crossing time
  • Light crossing time light would take about 100 000 years to travel from one side to the other.
Number of stars
  • Number of stars about 100 billion.

Part 6 of 8Typical distances

Typical distances
Sun
  • Sun light takes about 8 minutes to reach the Earth.
Nearest star after the Sun
  • Proxima Centauri is 4.2 light-years away.
Nearest large galaxy
  • Andromeda is about 2.5 million light-years away.
Looking back in time
  • Looking back in time light from an object left it long ago, so the further away it is the further back in time we see.
Space between stars
  • Space between stars almost entirely empty, with stars separated by millions of times their diameters.

Part 7 of 8Comparing sizes

ObjectApproximate size or distance
Earth (diameter)107 m
Solar System1013 m
Milky Way1021 m
Observable Universe1026 m
Each step is a factor of a million or more.
Observable Universe
  • Observable Universe only the part from which light has had time to reach us.

Part 8 of 8More from the specification

More from the specification
Cosmic distance ladder
  • Cosmic distance ladder combining radar ranging, stellar parallax,
    • and standard candles provides a systematic means to map vast extragalactic distances.
Cosmological Redshift and Galactic Recession
  • Light emitted by glowing hydrogen gas in distant galaxies shows an increase in observed wavelength toward the red end of the spectrum, proving that distant galaxies are moving away from Earth.
Hubble's Law and Expanding Universe
  • Hubble's law states that the recession speed of a galaxy is proportional to its distance, confirming universal cosmological expansion that originated in the Big Bang.
Cosmic Microwave Background and Stellar Fusion
  • Cosmic Microwave Background (CMB) radiation fills space as the cooled remnant radiation of the Big Bang;
    • within galaxies, stars power their life cycle by thermonuclear fusion of hydrogen into helium, releasing radiant energy.
Cosmological redshift
  • Cosmological redshift spectral absorption lines from distant galaxies shift to longer wavelengths, demonstrating expanding spacetime and receding galactic motion.
Hubble constant law
  • Hubble constant law recessional velocity of distant receding galaxies is directly proportional to cosmological distance from Earth.
Cosmic microwave background radiation
  • Cosmic microwave background radiation remnant thermal radiation from the early hot Universe, now stretched into microwave wavelengths.
Spectrum
  • Range of colours in white light.
Stable
  • Returns to its original position when tilted slightly.
Galactic redshift
  • Shift of galaxy spectral lines to longer wavelength due to recession.
White dwarf
  • Small, hot, dense core left by a low-mass star.
Background radiation
  • Radiation that is always present in the environment.
Redshift
  • Increase in the wavelength of light from a source moving away from the observer.
Fusion
  • Joining of light nuclei into a heavier one.
Radiation
  • Transfer by electromagnetic waves, requiring no medium
Wavelength
  • Distance between a point on one wave and the same point on the next wave
Reaction
  • The partner force acting on the other object.
Nebula
  • Cloud of gas and dust in which stars form.
Helium
  • Product of the fusion of hydrogen in stars.
Constant
  • Not changing.
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
UniverseAll of space, containing billions of galaxies.
GalaxyA large collection of billions of stars held together by gravity.
Milky WayThe galaxy that contains the Solar System.
Light-yearThe distance light travels in one year, 9.5 × 10^{15} m.
Cluster of galaxiesA group of galaxies bound together by gravity.

Avoid these mistakes

1
Confusing the Solar System with the galaxy — the Solar System has one star, whereas a galaxy has billions.
2
Using the light-year as a unit of time — it is a unit of distance.
3
Thinking stars in a galaxy are close together — they are extremely far apart compared with their size.
4
Believing the Milky Way is the only galaxy — it is one of billions.

Last updated:

\n