Units in Astronomy

Units of distance used in this book are the astronomical unit (AU), the parsec (pc), and the light-year (ly). The astronomical unit is defined to be the mean Earth-Sun distance, and so

1 AU = 1.496 × 10⁸ km

The parsec is defined in terms of the astronomical unit. It is the distance at which the Earth-Sun distance of 1 AU subtends one second of arc (i.e., 1/3600 of one degree), so that

1 pc = 206 265 AU = 3.086 × 10¹³ km

The light-year is defined as the distance travelled at the speed of light in a vacuum (2.9979 × 10⁸ m s⁻¹) in one year (365.25 days, or 3.1558 × 10⁷ s), so that

1 ly = 9.461 × 10¹⁵ m

and therefore

1 pc = 3.26 ly

The unit of astronomical mass is usually taken as the solar mass, M_ʘ, determined to be

1 M_ʘ = 1.989 × 10³⁰ kg

In these units, for example, the Milky Way galaxy (which is the main focus of our attention in this book) has a mass of about 10¹² M_ʘ, a diameter of about 27 kpc (kiloparsec), and the distance of the Sun from the galactic centre is about 8 kpc. Thus, distances within the Milky Way galaxy are typically measured in kpc. Distances between the Milky Way and external galaxies are typically measured in units of Mpc.