MRT Unit Standards

Introduction to unit standards

As more scientific analyses and observations are interlinked via the web it becomes more important to adopt standardized units so that information can be easily compared and used outside of specialized fields. In minimize potential confusion, the AAS machine readable (MR) tables use standard SI units in most cases. For example, the string 0.1nm to express Angströms (Å=10-10m), since the Angström is a non-standard unit. Another example is mW/m2 , the milliwatt per m2, which is identical to the CGS erg/cm2/s unit and is not used outside astronomy.

Rules for expressing units

Unit syntax should abide by the following rules:

  • Rule 1: any unit is described by a single word -- no space is allowed. For instance, the Angström is coded 0.1nm, and never 0.1 nm; the kilometer-per-second is coded km/s, or km.s-1, but never km / s or km s-1.
  • Rule 2: the only allowed numerical factor is at the very beginning of the Unit string. The structure of the unit is therefore the concatenation of factor and unit_expression and we will write the ``number of pixels per Å" as 10pix/nm, and neither pix/0.1nm nor pix/(0.1nm). The numerical factor may include the letter x for the multiplication, as 1.5x10+11 to express the number 1.5×1011
  • Rule 3: The operators to express a compound unit are
    • / for the division -- as in km/s
    • . for the multiplication (the dot is however understood as a decimal point in the leading numerical factor) -- as in kW.h
    • nothing for a power -- as m2 for m2 or 10+21 for 1021.
    Note that + or - signs are not operators, but represent the leading sign of numeric values.
  • Rule 4: a simple (non-compound) unit is made of a basic unit symbol, eventually preceded by a multiple prefix.

When possible choose the shortest syntax. For example, km/s to km.s-1.

Only simple power functions of physical units are accepted, which means that e.g. solMass3/2 (solar mass at a 3/2 power) is not valid. However for units expressed on a base 10 logarithmic scale, use bracketed units such as [solMass] to indicate logM{sun}.

Allowed unit symbols

Note that a more extensive list of allowed units is provided by VizieR.

  Symbol Explanation Definition
(c) -- Unitless value
(c) % Unitless value, in percent 10-2
(a) a year (also yr) 365.25d = 31.5576×106s
(b) A Ampere
(a) AU astronomical unit 1.49598×1011m
(a) arcmin minute of arc 1/60 °
(a) arcsec second of arc 1/60 arcmin
(e) barn barn (cross-section) 10-28 m2
(c) bit binary information unit (computer storage)
(c) byte byte (computer storage) 8 bit
  C Coulomb (electric charge) A·s
(b) cd Candela (luminous intensity)
(c) ct Count (events)
  D Debye (1/3)×10-29 C·m
(a) d day 24h = 86.4×103s
(a) deg degree of arc (°) {pi}/180 rad
(e) eV electron-Volt 1.602177×10-19 J
  F Farad (electric capacitance) C/V
(b) g gram 10-3 kg
(a) h hour of time (sideral if appropriate) 3600s
  H Henry (inductance) Wb/A
  Hz Hertz (frequency) s-1
  J Joule (energy) N·m
(a) Jy Jansky 10-26 W/m2/Hz
(b) K Kelvin
  lm lumen (luminous flux) cd·sr
  lx lux (illuminance) lm/m2
(b) m metre
(a) mag magnitudes
(a) mas millisecond of arc ({pi}/6.48)×10-8 rad
(a) min minute of time (sideral if appropriate)
(b) mol mole
  N Newton (force) kg·m/s2
  Ohm ({Omega}) Ohm (electric resistance) V/A
  Pa Pascal (pressure) N/m2
(a) pc parsec 3.0857 ×1016m
(c) pix pixel (image element)
(b) rad radian (angle)
(e) Ry Rydberg (energy) (1/2) (2{pi}e2/hc)2mec2 = 13.60583 eV
(b) s second of time
  S Siemens (electric conductance) A/V
(c) solLum Solar luminosity (alt: Lsun ) 3.826×1026 W
(c) solMass Solar mass (alt: Msun ) 1.989×1030 kg
(c) solRad Solar radius (alt: Rsun ) 6.9599×108 m
(c) Sun Unit referring to the Sun (e.g. abundances)  
  sr steradian (solid angle)
  T Tesla (magnetic field intensity) Wb/m2
  V Volts (electric potential) W/A
  W Watt (power) J/s
  Wb Weber (magnetic flux) V·s
(c) yr year (also a) 365.25d = 31.5576×106s

The basic symbols listed include basic standard SI units (b), the extensions listed by the IAU style book marked (a), other frequent physical extensions (e), and a few further astronmical extensions (c).

Unit multiples

  Symbol Explanation Value
  d deci 10-1
  c centi 10-2
  m milli 10-3
  u micro (µ) 10-6
  n nano 10-9
  p pico 10-12
  f femto 10-15
  a atto 10-18
  z zepto 10-21
  y yocto 10-24
  Symbol Explanation Value
  da deca 10
  h hecto 102
  k kilo 103
  M mega 106
  G giga 109
  T tera 1012
  P peta 1015
  E exa 1018
  Z zetta 1021
  Y yotta 1024