A light year is a distance, not a time
This is the most common misconception about the unit, and it is worth being precise: a light year is the distance light travels in one Julian year of 365.25 days.
1 ly = 9.4607 × 10¹² km = 63,241 AU = 3.26156 parsecs
The light travel time follows from dividing by the speed of light, c = 299,792.458 km/s. So a light year is also 31,557,600 seconds — but that is a consequence, not the definition.
The neighbourhood, in these units
- Proxima Centauri — 4.2465 ly. Its light, leaving in 2026, arrives in 2031.
- Sirius — 8.6 ly. Light seen now left in 2017.
- Betelgeuse — about 550 ly. A supernova there would be visible for roughly 1,400 years after it happened.
- Galactic centre — 26,000 ly. The 4.1-second radio delay from Sagittarius A* is a direct consequence of that distance.
- Andromeda — 2.5 million ly. The most distant thing routinely visible to the naked eye, and the reason it looks like a fuzzy patch rather than a star.
Why parsecs exist at all
A parsec is defined by measurement rather than by a number of years: the distance at which a star shows a parallax of one arcsecond against the background stars. Because the Earth's orbit provides a baseline of 2 AU, geometry gives the parsec directly, and it is the unit the astronomical community actually uses for distances within the galaxy.
The relation is exact and worth remembering:
distance (parsec) = 1 ÷ p (arcseconds)
So a parallax of 0.1 arcsec means 10 pc, and 0.05 means 20 pc. The star distance calculator works through this.
Beyond the parsec
Parsecs stop being convenient past a few kiloparsecs, and distances then get quoted in kiloparsecs (kpc) or megaparsecs (Mpc). Andromeda is about 0.78 Mpc. The nearest large galaxy cluster, the Virgo Cluster, sits near 16 Mpc, and the observable universe extends to roughly 14,000 Mpc — which is why most of the conversion examples you find online stay in light years or parsecs.
There is a further complication in the deep field. Redshift is not a straightforward conversion: a galaxy at redshift z = 1 has a light travel time of about 7.8 billion years, but the universe was 3.8 billion years old when the light left it. Distances there depend on which cosmological model you assume, and quoting a single "distance" without saying which is why professional sources prefer the lookback time.