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Total number of stars
3 X 1023
are the possible number of stars in the universe !
( a number equivalent to the total number of human cells, as mentioned by Conroy)
(source, based on Pieter G. van Dokkum & Charlie Conroy, 2010, Nature, 468,940–942)
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)]
