Astrophysics (Index)About

dex

(order-of-magnitude, decimal exponent)
(a number or ratio's log base 10)

The term dex (for decimal exponent) was coined within astrophysics as a convenient unit indicating any number or ratio's order-of-magnitude, i.e., its (rounded) base-10 logarithm. For example, 100 could be described as 2 dex, or two numbers that differ by a factor of 1000 could be said to differ by 3 dex. (The phrase, order-of-magnitude is used slightly differently: differing by 3 orders of magnitude is synonymous with differing by 3 dex, but indicating a number to be close to 100 would often be stated as having an order-of-magnitude of ten to the two). As such a unit, a dex is sometimes cited with fractions (such as tenths, making it the value's more-precise log base 10) whereas a cited order-of-magnitude is commonly rounded to an integer. The term dex is used in astrophysics but order-of-magnitude may well be more common.

The term decimal exponent describes the exponent in the typical scientific notation of numbers:

2.40×107
 aka
2.40e7

This number's decimal exponent is 7 and the number could be roughly described as "7 dex" or "having an order-of-magnitude of 10 to the 7". It could also be described as "7.38 dex", which is the number's log base 10 not rounded to an integer. Example of a ratio as a dex: "Jupiter's and Earth's masses differ by 2.5 dex" (i.e., log10 of 318, their ratio). This statement is a short way of saying "the base-10 logs of Jupiter's and Earth's masses differ by 2.5".


(mathematics,terminology,unit)
Further reading:
https://en.wiktionary.org/wiki/dex
https://ui.adsabs.harvard.edu/abs/1951Obs....71..157A/abstract
https://joe-antognini.github.io/astronomy/what-is-a-dex
https://astronomy.stackexchange.com/questions/17911/understanding-dex

Referenced by pages:
astronomical quantities
AT2018cow
axion (A0)
back-of-the-envelope calculation
barrier
CMB Stage-4 (CMB-S4)
cosmic dust
dark energy (Λ)
dense core mass function (DCMF)
event horizon (EH)
fast blue optical transient (FBOT)
Fly's Eye
free-fall time
fuzzy dark matter (FDM)
giant elliptical galaxy
gravitational wave strain (h)
H-R diagram (HRD)
hyperfine structure
inflation
ionizing radiation
IRAS 13224-3809
jansky (Jy)
Kelvin-Helmholtz timescale (KH timescale)
Lee-Weinberg bound
M-type star (M)
metallicity (Z)
N-body problem
numerical methods
Oh-My-God Particle
parts per million (PPM)
planet formation
projected separation
SI
speed of light (c)
stellar dynamics
stellar halo
sub-GeV dark matter
superluminous supernova (SLSN)
supermassive black hole (SMBH)
task-based parallelism (TBP)
time standard
valley of beta stability
white dwarf (WD)

Index