Absolute magnitude, M (A-level only)
Parsec and light-year
A light-year and a parsec are units of distance used in astronomy. Both describe how far away an object is, not how long a journey takes.
Light-year
A light-year is the distance that light travels through a vacuum in one year. Although its name contains “year”, it is a distance, not a unit of time.
Parsec
A parsec is defined using an angle. Imagine viewing a length of one astronomical unit (AU), approximately the average distance from Earth to the Sun, from far away. The farther away you are, the smaller the angle that this length appears to span. At a distance of one parsec, it spans an angle of one arcsecond, or one 3,600th of a degree.
This definition is equivalent to saying that an object one parsec away has a stellar parallax angle of one arcsecond. Parallax is the apparent shift in an object's position against a more distant background when it is viewed from different positions. The parallax angle uses a viewing-position change of one AU perpendicular to the line of sight.
One parsec is approximately 3.26 light-years: $$1\text{ pc} \approx 3.26\text{ ly}$$.
Notes for this section are being prepared.