Constants (astropy.constants)


astropy.constants contains a number of physical constants useful in Astronomy. Constants are Quantity objects with additional meta-data describing their provenance and uncertainties.

Getting Started

To use the constants in S.I. units, you can import the constants directly from the astropy.constants sub-package:

>>> from astropy.constants import G

or, if you want to avoid having to explicitly import all the constants you need, you can simply do:

>>> from astropy import constants as const

and then subsequently use for example const.G. Constants are fully-fledged Quantity objects, so you can easily convert them to different units for example:

>>> print(const.c)
  Name   = Speed of light in vacuum
  Value  = 299792458.0
  Uncertainty  = 0.0
  Unit  = m / s
  Reference = CODATA 2014

>>> print('km/s'))
299792.458 km / s

>>> print('pc/yr'))  
0.306601393788 pc / yr

and you can use them in conjunction with unit and other non-constant Quantity objects:

>>> from astropy import units as u
>>> F = (const.G * 3. * const.M_sun * 100 * / (2.2 * ** 2
>>> print(  
0.3675671602160826 N

It is possible to convert most constants to cgs using e.g.:

>>> const.c.cgs
<Quantity 29979245800.0 cm / s>

However, some constants are defined with different physical dimensions in cgs and cannot be directly converted. Because of this ambiguity, such constants cannot be used in expressions without specifying a system:

>>> 100 * const.e
Traceback (most recent call last):
TypeError: Constant u'e' does not have physically compatible units
across all systems of units and cannot be combined with other
values without specifying a system (eg. e.emu)
>>> 100 * const.e.esu  
<Quantity 4.8032045057134676e-08 Fr>

Collections of constants (and prior versions)

Constants are organized into version modules. The constants for Astropy 1.3 can be accessed in the astropyconst13 module. For example:

>>> from astropy.constants import astropyconst13 as const
>>> print(const.e)
  Name   = Electron charge
  Value  = 1.602176565e-19
  Uncertainty  = 3.5e-27
  Unit  = C
  Reference = CODATA 2010

Physical CODATA constants are in modules with names like codata2010 or codata2014:

>>> from astropy.constants import codata2010 as const
>>> print(const.h)
  Name   = Planck constant
  Value  = 6.62606957e-34
  Uncertainty  = 2.9e-41
  Unit  = J s
  Reference = CODATA 2010

Astronomical constants defined (primarily) by the IAU are collected in modules with names like iau2012 or iau2015:

>>> from astropy.constants import iau2012 as const
>>> print(const.L_sun)
  Name   = Solar luminosity
  Value  = 3.846e+26
  Uncertainty  = 5e+22
  Unit  = W
  Reference = Allen's Astrophysical Quantities 4th Ed.
>>> from astropy.constants import iau2015 as const
>>> print(const.L_sun)
  Name   = Nominal solar luminosity
  Value  = 3.828e+26
  Uncertainty  = 0.0
  Unit  = W
  Reference = IAU 2015 Resolution B 3

The astronomical and physical constants are combined into modules with names like astropyconst13 and astropyconst20.


Units such as u.M_sun will use the current version of the corresponding constant. When using prior versions of the constants, quantities should be constructed with constants instead of units.


astropy.constants Package

Contains astronomical and physical constants for use in Astropy or other places.

A typical use case might be:

>>> from astropy.constants import c, m_e
>>> # ... define the mass of something you want the rest energy of as m ...
>>> m = m_e
>>> E = m * c**2
<Quantity 0.510998927603161 MeV>

The following constants are available:

Name Value Unit Description
G 6.67408e-11 m3 / (kg s2) Gravitational constant
N_A 6.02214086e+23 1 / (mol) Avogadro’s number
R 8.3144598 J / (K mol) Gas constant
Ryd 10973731.6 1 / (m) Rydberg constant
a0 5.29177211e-11 m Bohr radius
alpha 0.00729735257   Fine-structure constant
atm 101325 Pa Standard atmosphere
b_wien 0.0028977729 m K Wien wavelength displacement law constant
c 299792458 m / (s) Speed of light in vacuum
e 1.60217662e-19 C Electron charge
eps0 8.85418782e-12 F/m Electric constant
g0 9.80665 m / s2 Standard acceleration of gravity
h 6.62607004e-34 J s Planck constant
hbar 1.0545718e-34 J s Reduced Planck constant
k_B 1.38064852e-23 J / (K) Boltzmann constant
m_e 9.10938356e-31 kg Electron mass
m_n 1.67492747e-27 kg Neutron mass
m_p 1.6726219e-27 kg Proton mass
mu0 1.25663706e-06 N/A2 Magnetic constant
muB 9.27400999e-24 J/T Bohr magneton
sigma_T 6.65245872e-29 m2 Thomson scattering cross-section
sigma_sb 5.670367e-08 W / (K4 m2) Stefan-Boltzmann constant
u 1.66053904e-27 kg Atomic mass
GM_earth 3.986004e+14 m3 / (s2) Nominal Earth mass parameter
GM_jup 1.2668653e+17 m3 / (s2) Nominal Jupiter mass parameter
GM_sun 1.3271244e+20 m3 / (s2) Nominal solar mass parameter
L_bol0 3.0128e+28 W Luminosity for absolute bolometric magnitude 0
L_sun 3.828e+26 W Nominal solar luminosity
M_earth 5.97236473e+24 kg Earth mass
M_jup 1.89818717e+27 kg Jupiter mass
M_sun 1.98847542e+30 kg Solar mass
R_earth 6356800 m Nominal Earth equatorial radius
R_jup 71492000 m Nominal Jupiter equatorial radius
R_sun 695700000 m Nominal solar radius
au 1.49597871e+11 m Astronomical Unit
kpc 3.08567758e+19 m Kiloparsec
pc 3.08567758e+16 m Parsec


Constant A physical or astronomical constant.
EMConstant An electromagnetic constant.

Class Inheritance Diagram

Inheritance diagram of astropy.constants.constant.Constant, astropy.constants.constant.EMConstant