Constants (astropy.constants
)¶
Introduction¶
astropy.constants
contains a number of physical constants useful in
Astronomy. Constants are Quantity
objects with
additional metadata describing their provenance and uncertainties.
Getting Started¶
To use the constants in International System of Units (SI units), you can
import the constants directly from the astropy.constants
subpackage:
>>> from astropy.constants import G
Or, if you want to avoid having to explicitly import all of the constants you need, you can do:
>>> from astropy import constants as const
and then subsequently use, for example, const.G
. Constants are fullyfledged
Quantity
objects, so you can conveniently convert them to
different units.
Example¶
To convert constants to different units:
>>> print(const.c)
Name = Speed of light in vacuum
Value = 299792458.0
Uncertainty = 0.0
Unit = m / s
Reference = CODATA 2018
>>> print(const.c.to('km/s'))
299792.458 km / s
>>> print(const.c.to('pc/yr'))
0.306601393788 pc / yr
You can then use them in conjunction with unit and other nonconstant
Quantity
objects:
>>> from astropy import units as u
>>> F = (const.G * 3. * const.M_sun * 100 * u.kg) / (2.2 * u.au) ** 2
>>> print(F.to(u.N))
0.3675671602160826 N
It is possible to convert most constants to CentimeterGramSecond (CGS) units using, for example:
>>> const.c.cgs
<Quantity 2.99792458e+10 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 4.8032045057134676e08 Fr>
Collections of Constants (and Prior Versions)¶
Constants are organized into version modules. The constants for
astropy
2.0 can be accessed in the astropyconst20
module.
For example:
>>> from astropy.constants import astropyconst20 as const
>>> print(const.e)
Name = Electron charge
Value = 1.6021766208e19
Uncertainty = 9.8e28
Unit = C
Reference = CODATA 2014
Physical CODATA constants are in modules with names like codata2010
,
codata2014
, or codata2018
:
>>> from astropy.constants import codata2014 as const
>>> print(const.h)
Name = Planck constant
Value = 6.62607004e34
Uncertainty = 8.1e42
Unit = J s
Reference = CODATA 2014
Astronomical constants defined (primarily) by the International Astronomical
Union (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
, astropyconst20
, and astropyconst40
for different versions.
However, importing these prior version modules directly will lead to
inconsistencies with other subpackages that have already imported
astropy.constants
. Notably, astropy.units
will have already used
the default version of constants. When using prior versions of the constants
in this manner, quantities should be constructed with constants instead of units.
To ensure consistent use of a prior version of constants in other Astropy
packages (such as astropy.units
) that import constants
, the physical and
astronomical constants versions should be set via ScienceState
classes.
These must be set before the first import of either astropy.constants
or
astropy.units
. For example, you can use the CODATA2010 physical constants and the
IAU 2012 astronomical constants:
>>> from astropy import physical_constants, astronomical_constants
>>> physical_constants.set('codata2010')
<ScienceState physical_constants: 'codata2010'>
>>> physical_constants.get()
'codata2010'
>>> astronomical_constants.set('iau2012')
<ScienceState astronomical_constants: 'iau2012'>
>>> astronomical_constants.get()
'iau2012'
Then all other packages that import astropy.constants
will selfconsistently
initialize with that prior version of constants.
The versions may also be set using values referring to the version modules:
>>> from astropy import physical_constants, astronomical_constants
>>> physical_constants.set('astropyconst13')
<ScienceState physical_constants: 'codata2010'>
>>> physical_constants.get()
'codata2010'
>>> astronomical_constants.set('astropyconst13')
<ScienceState astronomical_constants: 'iau2012'>
>>> astronomical_constants.get()
'iau2012'
If either astropy.constants
or astropy.units
have already been imported, a
RuntimeError
will be raised.
>>> import astropy.units
>>> from astropy import physical_constants, astronomical_constants
>>> astronomical_constants.set('astropyconst13')
Traceback (most recent call last):
...
RuntimeError: astropy.units is already imported
Reference/API¶
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
>>> E.to('MeV')
<Quantity 0.510998927603161 MeV>
The following constants are available:
Name 
Value 
Unit 
Description 

G 
6.6743e11 
m3 / (kg s2) 
Gravitational constant 
N_A 
6.02214076e+23 
1 / (mol) 
Avogadro’s number 
R 
8.31446262 
J / (K mol) 
Gas constant 
Ryd 
10973731.6 
1 / (m) 
Rydberg constant 
a0 
5.29177211e11 
m 
Bohr radius 
alpha 
0.00729735257 
Finestructure constant 

atm 
101325 
Pa 
Standard atmosphere 
b_wien 
0.00289777196 
m K 
Wien wavelength displacement law constant 
c 
299792458 
m / (s) 
Speed of light in vacuum 
e 
1.60217663e19 
C 
Electron charge 
eps0 
8.85418781e12 
F/m 
Vacuum electric permittivity 
g0 
9.80665 
m / s2 
Standard acceleration of gravity 
h 
6.62607015e34 
J s 
Planck constant 
hbar 
1.05457182e34 
J s 
Reduced Planck constant 
k_B 
1.380649e23 
J / (K) 
Boltzmann constant 
m_e 
9.1093837e31 
kg 
Electron mass 
m_n 
1.6749275e27 
kg 
Neutron mass 
m_p 
1.67262192e27 
kg 
Proton mass 
mu0 
1.25663706e06 
N/A2 
Vacuum magnetic permeability 
muB 
9.27401008e24 
J/T 
Bohr magneton 
sigma_T 
6.65245873e29 
m2 
Thomson scattering crosssection 
sigma_sb 
5.67037442e08 
W / (K4 m2) 
StefanBoltzmann constant 
u 
1.66053907e27 
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 
5.97216787e+24 
kg 
Earth mass 
M_jup 
1.8981246e+27 
kg 
Jupiter mass 
M_sun 
1.98840987e+30 
kg 
Solar mass 
R_earth 
6378100 
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 
Functions¶

Deprecated since version 4.0. 
Classes¶
A physical or astronomical constant. 

An electromagnetic constant. 