Constants

jaff.physics.constants

Physical and astronomical constants exposed as module-level astropy.units.Quantity objects. Each constant carries both a value and a unit, sourced from astropy.constants (CODATA 2018) where available, and computed from those primitives otherwise. Constant names follow the astropy.constants spelling (k_B, m_p, M_sun, N_A, u, R, ...).

Unit systems

There are no longer separate cgs / si / gaussian / natural tables. Because every constant is a unit-aware Quantity, the unit system is selected per quantity at the call site:

from jaff.physics import constants as c

c.c.cgs            # speed of light in CGS -> <Quantity 2.99792458e+10 cm / s>
c.c.si             # ... in SI             -> <Quantity 2.99792458e+08 m / s>
c.k_B.to("eV / K") # ... in any unit       -> <Quantity 8.617e-05 eV / K>
float(c.m_p.cgs.value)  # bare float (grams) when a number is required

The elementary charge keeps astropy's electromagnetic-unit views:

c.e.esu   # Gaussian / CGS-ESU (Franklin) -> <Quantity 4.803e-10 Fr>
c.e.si    # SI (Coulomb)                   -> <Quantity 1.602e-19 C>

Migration from the old Constants dataclass

Old	New
`constants.cgs.c`	`constants.c.cgs`
`constants.cgs.k_b`	`constants.k_B.cgs`
`constants.si.m_e`	`constants.m_e.si`
`constants.cgs.ev_to_erg`	`(1 * u.eV).to("erg")`
`constants.cgs.kb_ev`	`constants.k_B.to("eV / K")`

The `gaussian` and `natural` systems and the trivial conversion factors
(`ev_to_erg`, `kb_ev`, `Ry_hc`) have been removed; derive them on demand
with `.to(...)`.

Constants

Fundamental constants

Name	Description
`c`	Speed of light in vacuum
`h`	Planck constant
`hbar`	Reduced Planck constant (h / 2π)
`G`	Newtonian gravitational constant
`k_B`	Boltzmann constant
`e`	Elementary charge (use `.esu` or `.si`)
`m_e`	Electron rest mass
`m_p`	Proton rest mass
`m_n`	Neutron rest mass
`u`	Atomic mass unit (1 u)
`me_mp`	Electron-to-proton mass ratio (dimensionless)

Astronomical constants

Name	Description
`M_sun`	Solar mass
`R_sun`	Solar radius
`L_sun`	Solar luminosity
`pc`	Parsec
`kpc`	Kiloparsec
`Mpc`	Megaparsec
`au`	Astronomical unit
`ly`	Light-year
`H0`	Hubble constant (67.4 km/s/Mpc)

Radiation and electromagnetism

Name	Description
`sigma_sb`	Stefan-Boltzmann constant
`a_rad`	Radiation constant (a = 4σ/c)
`alpha`	Fine-structure constant (dimensionless, ≈ 1/137)
`sigma_T`	Thomson scattering cross section
`lambda_C`	Compton wavelength of the electron
`a0`	Bohr radius
`Ry`	Rydberg energy (≈ 13.6 eV)

Gas/plasma astrophysics

Name	Description
`r_e`	Classical electron radius (α² · a0)
`gyro_coeff`	Electron gyromagnetic coefficient (e / m_e) in CGS-ESU (Fr/g)

Astrochemistry

Name	Description
`mu_H`	Mean mass of a hydrogen atom (≈ proton mass)
`mu_H2`	Mean mass of a hydrogen molecule (≈ 2 proton masses)
`nH_ref`	Reference hydrogen number per unit mass (1 / m_p)
`T_cmb`	CMB temperature (2.725 K)
`T_1ev`	Temperature equivalent of 1 eV

Cross sections

Name	Description
`barn`	1 barn (1e-24 cm²)
`mbarn`	1 megabarn (1e6 barn)

Reference

Name	Description
`N_A`	Avogadro constant
`R`	Molar gas constant (k_B × N_A)