`astropy.constants` contains a number of physical constants useful in
Astronomy. Constants are `Quantity` objects with
additional meta-data describing their provenance and uncertainties.

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
Error = 0.0
Units = m / (s)
Reference = CODATA 2010
>>> print const.c.to('km/s')
299792.458 km / (s)
>>> print const.c.to('pc/yr')
0.306594845466 pc / (yr)
```

and you can use them in conjunction with unit and other non-constant
`Quantity` objects:

```
>>> 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
ERROR: TypeError: Constant '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.esu) [astropy.constants.constant]
...
>>> 100 * const.e.esu
<Quantity 4.80320450571e-08 Fr>
```

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 0.510998927603 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 |

au | 1.49597871e+11 | m | Astronomical Unit |

c | 299792458 | m / (s) | Speed of light in vacuum |

e | 1.60217657e-19 | C | Electron charge |

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 |

pc | 3.08567758e+16 | m | Parsec |

sigma_sb | 5.670373e-08 | W / (K4 m2) | Stefan-Boltzmann constant |

Constant(abbrev, name, value, unit, ...[, ...]) |
A physical or astronomical constant. |

EMConstant(abbrev, name, value, unit, ...[, ...]) |
An electromagnetic constant. |