Space Physics
The Solar System
The Solar System consists of the Sun (a star) and all the objects gravitationally bound to it:
- 8 planets (in order from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: "My Very Eager Mother Just Served Us Nachos."
- Moons — natural satellites of planets. Earth has one Moon.
- Asteroids — rocky bodies mostly in the Asteroid Belt between Mars and Jupiter.
- Comets — icy bodies with elongated orbits; tails point away from the Sun.
- Dwarf planets (e.g. Pluto, Eris).
Planets orbit the Sun in elliptical paths (approximately circular for inner planets). Gravitational force from the Sun provides the centripetal force keeping planets in orbit.
Units of distance
- Astronomical unit (AU): average Earth-Sun distance ≈ 1.5 × 10¹¹ m. Used within the Solar System.
- Light-year (ly): distance light travels in one year ≈ 9.5 × 10¹⁵ m. Used for distances between stars.
- Parsec (pc): ≈ 3.26 light-years. Used by professional astronomers.
Life cycle of stars
Low/medium mass stars (like the Sun):
- Nebula (cloud of gas and dust) → gravity causes collapse.
- Protostar → gravitational energy converts to heat; core heats up.
- Main sequence star → hydrogen fusion in core; stable for billions of years (Sun: ~10 billion years total).
- Red giant → hydrogen in core runs out; star expands; helium fusion begins.
- White dwarf → outer layers blown off as planetary nebula; core cools and contracts.
- Cools to a black dwarf (very long timescale).
High mass stars:
- Nebula → Protostar → Main sequence (shorter life, hotter, more luminous, blue).
- Red supergiant (larger than red giant).
- Supernova — catastrophic explosion; releases huge energy; can outshine an entire galaxy briefly.
- Core collapses to a neutron star or, if very massive, a black hole.
Supernovae are important: they synthesise heavy elements (heavier than iron) and scatter them through space. We are literally made of "star stuff."
Galaxies and the universe
A galaxy is a system of billions of stars bound by gravity. The Milky Way is our galaxy (~200–400 billion stars, ~100,000 light-years across). Galaxies are grouped into clusters; clusters into superclusters.
Redshift and the expanding universe
Redshift: light from distant galaxies is shifted towards longer (red) wavelengths. The further the galaxy, the greater the redshift. This is the Doppler effect for light — galaxies are moving away from us, so their light is stretched.
Hubble's law: recession speed v = H₀ × d, where H₀ is the Hubble constant and d is distance. More distant galaxies recede faster.
Implication: the universe is expanding. Running time backward → everything was once in the same place → the Big Bang theory.
The Big Bang theory
- The universe began approximately 13.8 billion years ago from an extremely hot, dense state (a singularity).
- The universe has been expanding and cooling ever since.
- Evidence: (1) redshift of galaxies; (2) Cosmic Microwave Background CMB radiation — faint microwave radiation coming uniformly from all directions, the "echo" of the hot early universe.
- The Big Bang was not an explosion into space but an expansion of space itself.
⚠Common mistakes
- Confusing red giant and red supergiant — the latter is from a high-mass star.
- Saying the Sun will go supernova — it will not; it is too low mass and will become a white dwarf.
- Redshift = moving away; blueshift = moving towards — all distant galaxies are redshifted (expanding universe).
- Big Bang was not an explosion INTO space — space itself expanded.
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