The unit in which physical parameters are expressed is a fundamental parameter which becomes especially important when data are to be compared, or used outside a specialized field of science — most physicists have never heard of astronomy-specific units.
Special care has been taken to try to use the standard SI units, and to convert if necessary the unit to such standards: for instance, we use 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, which is not used outside our discipline.
Only simple power functions of physical units are accepted, which means that e.g. solMass3/2 (solar mass at a 3/2 power) cannot be a valid unit. Some values are however traditionnally written in a decimal logarithmic scale, and we introduced bracketed units like [solMass] to indicate logM☉ unit, i.e. a value representing solar masses expressed on a logarithmic scale.
The standard adopted here differs from the OGIP ones ( OGIP memo 93-001 about ``Specification of Physical Units within OGIP FITS files" by Ian M. George & Lorella Angelini, August 1993) for the syntax of composite units (operator symbols), and in the usage of math functions (only the square bracketted units, representing the log function is accepted here) and of obsolete CGS units which are not avoided here; the basic symbols however agree.
The numerical factor may include the letter x for the multiplication, as 1.5x10+11 to express the number 1.5×1011
Among several possible expressions of a unit, it is preferable to choose the shortest one; this leads also to prefer the division (/) to the multiplication of the inverse, e.g. prefer km/s to km.s-1.
| 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 (°) | π/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 | (π/6.48)×10–8 rad |
| (a) | min | minute of time (sideral if appropriate) | |
| (b) | mol | mole | |
| N | Newton (force) | kg⋅m/s2 | |
| Ohm | (Ω) 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) (2πe2/hc)2mec2 = 13.60583 eV |
| (b) | s | second of time | |
| S | Siemens (electric conductance) | A/V | |
| (c) | solLum | Solar luminosity | 3.826×1026 W |
| (c) | solMass | Solar mass | 1.989×1030 kg |
| (c) | solRad | Solar radius | 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 |
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See the Unit Calculator
/srv/httpd/Pages/doc/catstd-3.2.htx