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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

MV,☉ = Mbol,☉ = 4.8

M = 2 x 1030 kg

R = 6.96 x 105 km

V☉,relative = 30 km/s

T = 5780 K

RSun-GC = 8 kpc

tgalactic orbit = 250 x 106 yr

Vgalactic 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

MEarth = 5.974 x 1024 kg

REarth = 6371 km

ρEarth = 5.5 g/cm3

>> Milky Way / Galaxy

bulgeradius ~ 5 kpc

diskradius ~ 15 kpc

haloradius > 75 kpc (perhaps 100 kpc)

dust ± 300 pc from disk level

stars ± 1000 pc from disk level

MMW = -20.5 ( 25 mag more than MSun, 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)]