Constants (astropy.constants
)¶
Introduction¶
astropy.constants
contains a number of physical constants useful in
Astronomy. Constants are Quantity
objects with
additional metadata describing their provenance and uncertainties.
Getting Started¶
To use the constants in International System of Units (SI 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 of the constants you need, you can do:
>>> from astropy import constants as const
and then subsequently use, for example, const.G
. Constants are fully-fledged
Quantity
objects, so you can conveniently 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 2014
>>> print(const.c.to('km/s'))
299792.458 km / s
>>> print(const.c.to('pc/yr'))
0.306601393788 pc / yr
You can then use them in conjunction with unit and other nonconstant
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))
0.3675671602160826 N
It is possible to convert most constants to Centimeter-Gram-Second (CGS) units using, for example:
>>> const.c.cgs
<Quantity 2.99792458e+10 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>
Collections of Constants (and Prior Versions)¶
Constants are organized into version modules. The constants for
astropy
1.3 can be accessed in the astropyconst13
module.
For example:
>>> from astropy.constants import astropyconst13 as const
>>> print(const.e)
Name = Electron charge
Value = 1.602176565e-19
Uncertainty = 3.5e-27
Unit = C
Reference = CODATA 2010
Physical CODATA constants are in modules with names like codata2010
or
codata2014
:
>>> from astropy.constants import codata2010 as const
>>> print(const.h)
Name = Planck constant
Value = 6.62606957e-34
Uncertainty = 2.9e-41
Unit = J s
Reference = CODATA 2010
Astronomical constants defined (primarily) by the International Astronomical
Union (IAU) are collected in modules with names like iau2012
or iau2015
:
>>> from astropy.constants import iau2012 as const
>>> print(const.L_sun)
Name = Solar luminosity
Value = 3.846e+26
Uncertainty = 5e+22
Unit = W
Reference = Allen's Astrophysical Quantities 4th Ed.
>>> from astropy.constants import iau2015 as const
>>> print(const.L_sun)
Name = Nominal solar luminosity
Value = 3.828e+26
Uncertainty = 0.0
Unit = W
Reference = IAU 2015 Resolution B 3
The astronomical and physical constants are combined into modules with
names like astropyconst13
and astropyconst20
. To temporarily set
constants to an older version (e.g., for regression testing), a context
manager is available as follows:
>>> from astropy import constants as const
>>> with const.set_enabled_constants('astropyconst13'):
... print(const.h)
Name = Planck constant
Value = 6.62606957e-34
Uncertainty = 2.9e-41
Unit = J s
Reference = CODATA 2010
>>> print(const.h)
Name = Planck constant
Value = 6.62607004e-34
Uncertainty = 8.1e-42
Unit = J s
Reference = CODATA 2014
Warning
Units such as u.M_sun
will use the current version of the
corresponding constant. When using prior versions of the constants,
quantities should be constructed with constants instead of units.
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 |
6.67408e-11 |
m3 / (kg s2) |
Gravitational constant |
N_A |
6.02214086e+23 |
1 / (mol) |
Avogadro’s number |
R |
8.3144598 |
J / (K mol) |
Gas constant |
Ryd |
10973731.6 |
1 / (m) |
Rydberg constant |
a0 |
5.29177211e-11 |
m |
Bohr radius |
alpha |
0.00729735257 |
Fine-structure constant |
|
atm |
101325 |
Pa |
Standard atmosphere |
b_wien |
0.0028977729 |
m K |
Wien wavelength displacement law constant |
c |
299792458 |
m / (s) |
Speed of light in vacuum |
e |
1.60217662e-19 |
C |
Electron charge |
eps0 |
8.85418782e-12 |
F/m |
Electric constant |
g0 |
9.80665 |
m / s2 |
Standard acceleration of gravity |
h |
6.62607004e-34 |
J s |
Planck constant |
hbar |
1.0545718e-34 |
J s |
Reduced Planck constant |
k_B |
1.38064852e-23 |
J / (K) |
Boltzmann constant |
m_e |
9.10938356e-31 |
kg |
Electron mass |
m_n |
1.67492747e-27 |
kg |
Neutron mass |
m_p |
1.6726219e-27 |
kg |
Proton mass |
mu0 |
1.25663706e-06 |
N/A2 |
Magnetic constant |
muB |
9.27400999e-24 |
J/T |
Bohr magneton |
sigma_T |
6.65245872e-29 |
m2 |
Thomson scattering cross-section |
sigma_sb |
5.670367e-08 |
W / (K4 m2) |
Stefan-Boltzmann constant |
u |
1.66053904e-27 |
kg |
Atomic mass |
GM_earth |
3.986004e+14 |
m3 / (s2) |
Nominal Earth mass parameter |
GM_jup |
1.2668653e+17 |
m3 / (s2) |
Nominal Jupiter mass parameter |
GM_sun |
1.3271244e+20 |
m3 / (s2) |
Nominal solar mass parameter |
L_bol0 |
3.0128e+28 |
W |
Luminosity for absolute bolometric magnitude 0 |
L_sun |
3.828e+26 |
W |
Nominal solar luminosity |
M_earth |
5.97236473e+24 |
kg |
Earth mass |
M_jup |
1.89818717e+27 |
kg |
Jupiter mass |
M_sun |
1.98847542e+30 |
kg |
Solar mass |
R_earth |
6378100 |
m |
Nominal Earth equatorial radius |
R_jup |
71492000 |
m |
Nominal Jupiter equatorial radius |
R_sun |
695700000 |
m |
Nominal solar radius |
au |
1.49597871e+11 |
m |
Astronomical Unit |
kpc |
3.08567758e+19 |
m |
Kiloparsec |
pc |
3.08567758e+16 |
m |
Parsec |
Functions¶
|
Context manager to temporarily set values in the |
Classes¶
A physical or astronomical constant. |
|
An electromagnetic constant. |