Miscellaneous "Numbers"

Two units which are often used to express surface brightness and sky background measurements are related by

$\displaystyle 1~\mathrm{MJy/sr}=\frac{\pi^2}{0.419904}~\mu\mathrm{Jy/arcsec^2}\simeq23.5~\mu\mathrm{Jy/arcsec^2}~.$ (30)

Given the privileged role of the Sun in shaping our view of the Universe, the Solar Luminosity, Mass and Radius, are often used to express the luminosities, masses and radii of stars and/or galaxies. Currently accepted values are
$\displaystyle L_\odot$ $\displaystyle =$ $\displaystyle 3.8478\cdot10^{26}~\mathrm{J/s} = 3.8478\cdot10^{33}~\mathrm{erg/s}$ (31)
$\displaystyle M_\odot$ $\displaystyle =$ $\displaystyle 1.9891\cdot10^{30}~\mathrm{kg} = 1.9891\cdot10^{33}~\mathrm{g}$ (32)
$\displaystyle R_\odot$ $\displaystyle =$ $\displaystyle 6.960\cdot10^{8}~\mathrm{m} = 6.960\cdot10^{10}~\mathrm{cm}$ (33)

In astronomy, distances within the Solar System are generally expressed in

$\displaystyle 1~\textrm{Astronomical Unit}=1~\textrm{A.U.}=1.4960~10^{11}~\mathrm{m}~,$

while distances outside the Solar System are generally expressed in

$\displaystyle 1~\textrm{Parsec}=1~\textrm{pc}=3.0857~10^{16}~\mathrm{m}~.$