# 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
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>
```

## Reference/API¶

### astropy.constants Module¶

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
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

#### Classes¶

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