Astronomy Units: Light Years and Parsecs

The Light Year (ly)

A light year is the distance light travels in a vacuum in one year.

• Speed of Light: ~300,000 km/s
• 1 Light Year: ~9.46 trillion kilometers (5.88 trillion miles)

The Astronomical Unit (AU)

Used for distances within our solar system. It's the average distance from the Earth to the Sun.

• 1 AU ≈ 150 million kilometers

The Parsec (pc)

Short for "parallax second." This is the preferred unit for professional astronomers.

• 1 Parsec ≈ 3.26 Light Years

Key Takeaways

• Proxima Centauri (nearest star) is 4.2 ly away
• The Milky Way is about 100,000 ly across
• The observable universe is 93 billion ly across