Constants (astropy.constants)

Introduction

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(const.c.to('km/s'))
299792.458 km / s

>>> print(const.c.to('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 * u.kg) / (2.2 * u.au) ** 2
>>> print(F.to(u.N))  
0.3675671602160826 N

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

>>> 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 4.8032045057134676e-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.

Warning

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.

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 6.67408e-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 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

Classes

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

Class Inheritance Diagram

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