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 2010

>>> 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))  
0.367669392028 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>

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	6.67384e-11	m3 / (kg s2)	Gravitational constant
L_sun	3.846e+26	W	Solar luminosity
M_earth	5.9742e+24	kg	Earth mass
M_jup	1.8987e+27	kg	Jupiter mass
M_sun	1.9891e+30	kg	Solar mass
N_A	6.02214129e+23	1 / (mol)	Avogadro’s number
R	8.3144621	J / (K mol)	Gas constant
R_earth	6378136	m	Earth equatorial radius
R_jup	71492000	m	Jupiter equatorial radius
R_sun	695508000	m	Solar radius
Ryd	10973731.6	1 / (m)	Rydberg constant
a0	5.29177211e-11	m	Bohr radius
alpha	0.00729735257		Fine-structure constant
atmosphere	101325	Pa	Atmosphere
au	1.49597871e+11	m	Astronomical Unit
b_wien	0.0028977721	m K	Wien wavelength displacement law constant
c	299792458	m / (s)	Speed of light in vacuum
e	1.60217657e-19	C	Electron charge
eps0	8.85418782e-12	F/m	Electric constant
g0	9.80665	m / s2	Standard acceleration of gravity
h	6.62606957e-34	J s	Planck constant
hbar	1.05457173e-34	J s	Reduced Planck constant
k_B	1.3806488e-23	J / (K)	Boltzmann constant
kpc	3.08567758e+19	m	Kiloparsec
m_e	9.10938291e-31	kg	Electron mass
m_n	1.67492735e-27	kg	Neutron mass
m_p	1.67262178e-27	kg	Proton mass
mu0	1.25663706e-06	N/A2	Magnetic constant
muB	9.27400968e-24	J/T	Bohr magneton
pc	3.08567758e+16	m	Parsec
sigma_T	6.65245873e-29	m2	Thomson scattering cross-section
sigma_sb	5.670373e-08	W / (K4 m2)	Stefan-Boltzmann constant
u	1.66053892e-27	kg	Atomic mass

Classes

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

Class Inheritance Diagram