Useful astronomical numbers

Just some useful numbers and units to remember and to have an easy access to!
[last updated: 29 Dec 2011]

>> Distance

1 AU = 150 x 10⁶ km

1 pc = 3.09 × 10¹³ km = 206260 AU = 3.26 ly

1 ly = 9.4 x 10¹² km = 64 x 10³ = 0.31 pc

>> Sun

L_☉ = 3.86 x 10²⁶ W = 3.86 x 10³³ erg / s

M_V,☉ = M_bol,☉ = 4.8

M_☉ = 2 x 10³⁰ kg

R_☉ = 6.96 x 10⁵ km

V_☉,relative = 30 km/s

T_☉ = 5780 K

R_Sun-GC = 8 kpc

t_{galactic orbit} = 250 x 10⁶ yr

V_{galactic rotation} = 200 km/s

solar wind’s energy ~ 10^-7 L_☉ (on average)

solar wind’s density (in the vicinity of Earth) ~ 5 particles / cm³

solar wind’s velocity (in the vicinity of Earth) ~ 400 km / s

mass loss ~ 10^-14 M_☉ / yr

>> Earth

M_Earth = 5.974 x 10²⁴ kg

R_Earth = 6371 km

ρ_Earth = 5.5 g/cm³

>> Milky Way / Galaxy

bulge_radius ~ 5 kpc

disk_radius ~ 15 kpc

halo_radius > 75 kpc (perhaps 100 kpc)

dust ± 300 pc from disk level

stars ± 1000 pc from disk level

M_MW = -20.5 ( 25 mag more than M_Sun, 10¹⁰ times that of Sun)

>> Ionization potential

for H = 13.6 eV

for He = 24.6 eV (one electron) + 54.4 eV (both electrons)

>> Hydrogen atom

R = 1 Å

>> Extinction

Sun’s neighbourhood: ~ 1 mag/kpc

towards GC: ~ 21 mag/kpc – 1 optical photon per 10¹² reaches us, while 1 out of 10 in IR

>> magnitude difference: 5 mags = 100 times in flux [m₁-m₂=-2.5 log₁₀(f₁/f₂)]