Understanding Light Years

Study Notes

Light year is a unit of distance measurement, not time.
Light travels at approximately 186,000 miles per second (300,000 kilometers per second).
One light year equals the distance light travels in one year: about 5.87 trillion miles (approximately 9.46 trillion kilometers).

The universe originated from a hot explosion known as the Big Bang.
Post-Big Bang conditions were primarily radiation; as the universe cooled, hydrogen and helium formed.
Evidence supports the Big Bang theory:
- The universe is believed to have had a definitive beginning.
- Hubble's Law indicates that galaxies are moving away, suggesting universal expansion from a compact state.
- In 1965, Cosmic Microwave Background radiation (CMB) was discovered, measuring about 2.725 K, indicating remnants of heat from the Big Bang.
- The observed abundance of hydrogen and helium aligns with predictions of the Big Bang model.

Planets and stars form from the same primordial gas and dust cloud.
Observations of T Tauri stars show planet formation in dust disks around them.
The solar nebula, a spinning disk, was where the solar system (including the Sun) formed approximately 5 billion years ago.
The formation process involves:
- The solar nebula contracting and flattening.
- The central mass becoming the Sun and smaller masses potentially forming jovian planets.
- Dust grains acting as condensation nuclei, leading to moon-sized planetesimals.
- The expulsion of gaseous nebula by strong solar winds.
- Continued collisions among planetesimals forming larger planets with circular orbits.

The Big Bang theory discusses the creation of matter and energy in the universe.
The Solar Nebula theory addresses how that matter evolves into galaxies and solar systems.