Astronomy
Our Solar System
The Solar System contains: the Sun (a star), 8 planets, dwarf planets (e.g. Pluto), moons, asteroids (in the asteroid belt between Mars and Jupiter), comets, and meteoroids.
Inner (rocky/terrestrial) planets: Mercury, Venus, Earth, Mars. Outer (gas/ice giants): Jupiter, Saturn, Uranus, Neptune.
Orbits: planets travel in near-circular elliptical orbits around the Sun. Gravity provides the centripetal force. Closer planets move faster and have shorter orbital periods (Kepler's third law).
Comets have highly elliptical orbits — very fast near the Sun, slow far away.
Life Cycle of Stars
Stars like the Sun: Nebula → Protostar → Main sequence star (billions of years) → Red giant → Planetary nebula → White dwarf → (eventually) Black dwarf
Massive stars (much larger than the Sun): Nebula → Protostar → Main sequence star → Red supergiant → Supernova → Neutron star OR (if very massive) Black hole
Key stages:
- Nebula: cloud of gas and dust; gravity causes contraction.
- Protostar: contracting cloud heats up; nuclear fusion not yet started.
- Main sequence: nuclear fusion of hydrogen → helium in core; radiation pressure balances gravity (stable for billions of years).
- Red giant / supergiant: hydrogen exhausted in core; outer layers expand; fusion of heavier elements begins.
- Supernova: massive star explodes; heavier elements ejected into space — the source of all elements heavier than iron.
- White dwarf: remnant of a solar-mass star; cools over billions of years.
- Neutron star / black hole: remnant of a massive star; extremely dense; black hole has gravity so strong not even light can escape.
The Expanding Universe and the Big Bang
Redshift: light from distant galaxies is shifted to longer (red) wavelengths. Galaxies are moving away from us — the universe is expanding.
The further away a galaxy, the greater its redshift → the faster it is moving away (Hubble's Law).
Cosmic Microwave Background Radiation (CMBR): low-level EM radiation detected uniformly in all directions. It is the remnant heat from the Big Bang, now redshifted to microwave wavelengths.
The Big Bang Theory: the universe began as an extremely hot, dense point approximately 13.8 billion years ago and has been expanding ever since. Evidence:
- Redshift of galaxies (expanding universe).
- CMBR (remnant heat from the Big Bang).
- Abundance of light elements (hydrogen and helium).
The steady-state model (universe always existed and expands, with matter being created) has been largely abandoned due to the CMBR evidence.
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