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- Constants (`astropy.constants`)

# 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