Units of length in astronomy
Inside the Solar System distances are measured in astronomical units (1 AU ≈ 149.6 million km), but between stars larger units are needed. A light-year is the distance light travels in a year: 1 light-year ≈ 9.46 · 10¹² km ≈ 63,240 AU. A parsec (pc) is the distance to a star whose annual parallax is 1″: 1 pc ≈ 206,265 AU ≈ 3.26 light-years ≈ 3.09 · 10¹³ km. On the scale of the Galaxy the kiloparsec (1 kpc = 1000 pc) is used, and for the Universe the megaparsec (1 Mpc = 10⁶ pc). For example, the nearest star to the Sun, Proxima Centauri, is ≈ 4.24 light-years (≈ 1.3 pc) away; the Milky Way is ≈ 100,000 light-years across (≈ 30 kpc); the Andromeda galaxy is ≈ 2.5 million light-years (≈ 0.78 Mpc) away. “X light-years away” means the light takes X years to reach us, so we see the object as it was X years ago.
A simplified model: take 1 light-year = 1 m and mark along the classroom wall the nearest star (≈ 4.2 m) and Sirius (≈ 8.6 m) using a tape measure (nothing sharp).