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^{10}m), since the Angström is a nonstandard unit. Another example is mW/m2 , the milliwatt per m^{2}, which is identical to the CGS erg/cm^{2}/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 kilometerpersecond is coded km/s, or km.s1, but never km / s or km s1.  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 1.5×10^{11}  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 m^{2} or 10+21 for 10^{21}.
Note that + or  signs are not operators, but represent the leading sign of numeric values.
 Rule
4: a simple (noncompound) 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.s1.
Only simple power functions of physical units are accepted, which means that e.g. solMass^{3/2} (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 available here.
Symbol  Explanation  Definition  

(c)    Unitless value  
(c)  %  Unitless value, in percent  10^{2} 
(a)  a  year (also yr)  365.25d = 31.5576×10^{6}s 
(b)  A  Ampere  
(a)  AU  astronomical unit  1.49598×10^{11}m 
(a)  arcmin  minute of arc  1/60 ° 
(a)  arcsec  second of arc  1/60 arcmin 
(e)  barn  barn (crosssection)  10^{28} m^{2} 
(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 = 86.4×10^{3}s 
(a)  deg  degree of arc (°)  {pi}/180 rad 
(e)  eV  electronVolt  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/m^{2}/Hz 
(b)  K  Kelvin  
lm  lumen (luminous flux)  cd·sr  
lx  lux (illuminance)  lm/m^{2}  
(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/s^{2}  
Ohm  ({Omega}) Ohm (electric resistance)  V/A  
Pa  Pascal (pressure)  N/m^{2}  
(a)  pc  parsec  3.0857 ×10^{16}m 
(c)  pix  pixel (image element)  
(b)  rad  radian (angle)  
(e)  Ry  Rydberg (energy)  (1/2) (2{pi}e^{2}/hc)^{2}m_{e}c^{2} = 13.60583 eV 
(b)  s  second of time  
S  Siemens (electric conductance)  A/V  
(c)  solLum  Solar luminosity  3.826×10^{26} W 
(c)  solMass  Solar mass  1.989×10^{30} kg 
(c)  solRad  Solar radius  6.9599×10^{8} m 
(c)  Sun  Unit referring to the Sun (e.g. abundances)  
sr  steradian (solid angle)  
T  Tesla (magnetic field intensity)  Wb/m^{2}  
V  Volts (electric potential)  W/A  
W  Watt (power)  J/s  
Wb  Weber (magnetic flux)  V·s  
(c)  yr  year (also a)  365.25d = 31.5576×10^{6}s 
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

