# 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_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. |