Constants
& Units
Physical Constants
$\quad\rlap{a_0}$
$\qquad\rlap{\rm Bohr~radius}$ $\hskip150pt 5.29177 \times 10^{-9} {\rm
~cm}$
$\quad\rlap{c}$ $\qquad\rlap{\rm
speed~of~light~in~vacuum}$ $\hskip150pt 2.99792 \times 10^{10}{\rm
~cm~s}^{-1}$
$\quad\rlap{e}$ $\qquad\rlap{\rm electron~charge}$ $\hskip140pt
-4.80325
\times 10^{-10} {\rm ~ESU}$
$\quad\rlap{\rm eV}$
$\qquad\rlap{\rm electron~volt}$ $\hskip150pt 1.60218 \times 10^{-12}
{\rm ~erg}$
$\quad\rlap{G}$ $\qquad\rlap{\rm
gravitational~constant}$ $\hskip150pt 6.6742 \hphantom{0~} \times
10^{-8} {\rm
~dyne~cm}^2 {\rm ~g}^{-2}$
$\quad\rlap{h}$ $\qquad\rlap{\rm
Planck's~constant}$ $\hskip150pt 6.62607 \times 10^{-27} {\rm ~erg~s}$
$\quad\rlap{k}$ $\qquad\rlap{\rm
Boltzmann's~constant}$ $\hskip150pt 1.38062 \times 10^{-16} {\rm
~erg~K}^{-1}$
$\quad\rlap{m_{\rm e}}$
$\qquad\rlap{\rm electron~mass}$ $\hskip150pt 9.10956 \times 10^{-28}
{\rm ~g}$
$\quad\rlap{m_{\rm
p}}$ $\qquad\rlap{\rm proton~mass}$ $\hskip150pt 1.67262 \times
10^{-24} {\rm ~g}$
$\quad\rlap{R_\infty}$
$\qquad\rlap{\rm Rydberg~constant}$ $\hskip150pt 1.09737 \times 10^5
{\rm ~cm}^{-1}$
$\quad\rlap{R_\infty c}$
$\qquad\rlap{\rm Rydberg~frequency}$ $\hskip150pt 3.28984 \times
10^{15} {\rm ~s}^{-1}$
$\quad\rlap{\sigma}$
$\qquad\rlap{\rm Stefan-Boltzmann~constant}$ $\hskip150pt 5.66961
\times 10^{-5} {\rm ~erg~cm}^{-2}{\rm ~s}^{-1} {\rm ~K}^{-4}$
$\quad\rlap{\sigma_{\rm
T}}$ $\qquad\rlap{\rm Thomson~cross~section}$ $\hskip150pt 6.65245
\times 10^{-25} {\rm ~cm}^2$
$\quad\rlap{u}$ $\qquad\rlap{\rm
atomic~mass~unit}$ $\hskip150pt 1.66054 \times 10^{-24} {\rm ~g}$
Astronomical Constants
$\quad\rlap{\rm au}$
$\qquad\rlap{\rm astronomical~unit}$ $\hskip150pt 1.49598 \times
10^{13} {\rm ~cm}$
$\quad\rlap{H_0}$ $\qquad\rlap{\rm Hubble~constant}$ $\hskip150pt
72 {\rm ~km~s}^{-1}{\rm ~Mpc}^{-1}$
$\quad\rlap{\rm kpc}$
$\qquad\rlap{\rm kiloparsec}$ $\hskip150pt 10^3 {\rm ~pc}$
$\quad\rlap{L_\odot}$
$\qquad\rlap{\rm solar~bolometric~luminosity}$ $\hskip150pt 3.826
\hphantom{01~} \times 10^{33} {\rm ~erg~s}^{-1}$
$\quad\rlap{\rm ly}$
$\qquad\rlap{\rm light~year}$ $\hskip150pt 9.4605 \hphantom{0~}\times
10^{17} {\rm ~cm}$
$\quad\rlap{M_\odot}$
$\qquad\rlap{\rm solar~mass}$ $\hskip150pt 1.989 \hphantom{01~} \times
10^{33} {\rm ~g}$
$\quad\rlap{\rm Mpc}$
$\qquad\rlap{\rm megaparsec}$ $\hskip150pt 10^6 {\rm ~pc}$
$\quad\rlap{\rm pc}$
$\qquad\rlap{\rm parsec}$ $\hskip150pt 3.0856 \hphantom{0~} \times
10^{18} {\rm ~cm}$
$\quad\rlap{R_\odot}$
$\qquad\rlap{\rm solar~radius}$ $\hskip150pt 6.9598 \hphantom{0~}
\times 10^{10} {\rm ~cm}$
$\quad\rlap{\rm yr}$
$\qquad\rlap{\rm year}$ $\hskip150pt 3.156 \times 10^7 \approx 10^{7.5}
{\rm ~s}$
MKS and Gaussian CGS Units
$\quad\rlap {\rm Type}$ $\hskip60pt
\rlap{\rm mks~unit}$ $\hskip50pt \rlap{\rm cgs~unit}$ $\hskip60pt {\rm
conversion}$
$\quad\rlap {\rm mass}$ $\hskip60pt
\rlap{\rm kg}$ $\hskip50pt \rlap{\rm g}$ $\hskip60pt 10^3 {\rm ~g} = 1
{\rm ~kg}$
$\quad\rlap {\rm length}$
$\hskip60pt \rlap{\rm m}$ $\hskip50pt \rlap{\rm cm}$ $\hskip60pt 10^2
{\rm ~cm} = 1 {\rm ~m}$
$\quad\rlap {\rm time}$ $\hskip60pt \rlap{\rm s}$ $\hskip50pt \rlap{\rm
s}$
$\quad\rlap {\rm frequency}$ $\hskip60pt \rlap{\rm Hz}$ $\hskip50pt
\rlap{\rm Hz}$ $\hskip60pt 1 {\rm ~Hz} = 1 {\rm ~s}^{-1}$
$\quad\rlap {\rm charge}$ $\hskip60pt \rlap{\rm coulomb}$ $\hskip50pt
\rlap{\rm statcoulomb}$ $\hskip60pt 3
\times 10^9 {\rm ~statcouloumb} = 1 {\rm ~coulomb}$
$\quad\rlap {\rm current}$
$\hskip60pt \rlap{\rm ampere}$ $\hskip50pt \rlap{\rm statampere}$
$\hskip60pt 3 \times 10^9 {\rm ~statamp} = 1 {\rm ~amp} = 1 {\rm
~coulomb~s}^{-1}$
$\quad\rlap {\rm electric~field}$
$\hskip60pt \rlap{{\rm v~m}^{-1}}$ $\hskip50pt \rlap{{\rm
statvolt~cm}^{-1}}$ $\hskip60pt (1/3) \times 10^{-4}{\rm
~statvolt~cm}^{-1} = 1 {\rm ~v~m}^{-1}$
$\quad\rlap {\rm energy}$
$\hskip60pt \rlap{\rm joule}$ $\hskip50pt \rlap{\rm erg}$ $\hskip60pt
10^7 {\rm ~erg} = 1 {\rm ~joule}$
$\quad\rlap {\rm force}$
$\hskip60pt \rlap{\rm newton}$ $\hskip50pt \rlap{\rm dyne}$ $\hskip60pt
10^5 {\rm ~dyne} = 1 {\rm ~newton}$
$\quad\rlap {\rm magnetic~field}$
$\hskip60pt \rlap{\rm tesla}$ $\hskip50pt \rlap{\rm gauss}$ $\hskip60pt
10^4 {\rm ~gauss} = 1 {\rm ~tesla}$
$\quad\rlap{\rm resistance}$
$\hskip60pt \rlap{\rm Ohm}$ $\hskip50pt \rlap{{\rm sec~cm}^{-1}}$
$\hskip60pt (1/9) \times 10^{-11} {\rm ~s~cm}^{-1} = 1 {\rm ~Ohm}$
$\quad\rlap{\rm temperature}$
$\hskip60pt \rlap{\rm Kelvin}$ $\hskip50pt {\rm Kelvin}$
Other Constants and Units
$\quad\rlap {\rm arcmin}$
$\hskip60pt 1/60 {\rm ~deg}$
$\quad\rlap {\rm arcsec}$ $\hskip60pt 1/60 {\rm ~arcmin}$
$\quad\rlap {\rm Angstrom}$
$\hskip60pt 10^{-10} {\rm ~m}$
$\quad\rlap {\rm dB}$ $\hskip60pt 0.1\log_{10}(P_1/P_2)$
$\quad\rlap {\rm e}$ $\hskip60pt
2.71828\dots$
$\quad\rlap {\rm GHz}$
$\hskip60pt 10^9 {\rm ~Hz}$
$\quad\rlap {\rm Jy}$ $\hskip60pt
10^{-26} {\rm ~W~m}^{-2} {\rm ~Hz}^{-1} \quad {\rm (1~mJy} = 10^{-3}
{\rm ~Jy,~1~}\mu{\rm Jy} = 10^{-6} {\rm ~Jy)}$
$\quad\rlap {\rm MHz}$
$\hskip60pt 10^6 {\rm ~Hz}$
$\quad\rlap {\mu{\rm m}}$
$\hskip60pt 10^{-6} {\rm ~m}$
$\quad\rlap \pi$ $\hskip60pt
3.14159\dots$
$\quad\rlap {\rm radian}$
$\hskip60pt (180/\pi) {\rm ~deg} \approx 206264.8 {\rm ~arcsec}$
Engineers and physicists prefer mks
(meter, kilogram,
second) units, so most radio astronomers use mks units to describe
their equipment and the results of their observations. Most
astrophysicists prefer Gaussian cgs (centimeter, gram, second) units to
describe astronomical sources. Thus you have to deal with both
systems
of units and be able to convert between them in order to do
astronomy. J. D. Jackson's Classical
Electrodynamics has an
appendix explaining the different
systems in detail.