# Astronomy

• The study of the objects that lie beyond Earth and the process by which these objects interact with one another
• My definition: The study of celestial objects beyond Earth and how they interact with one another.
• An observational science
• Any hypotheses must be tested by observing many samples of the objects being studies
• We note how different samples vary

## Useful Constants

### Physical Constants

NameValue
speed of light ($c$)$2.9979 \times 10^8 m/s$
gravitational constant ($G$)$6.674 \times 10^{-11} m3/(kg s^2)$
Planck’s constant ($h$)$6.626 \times 10^{−34} J-s$
mass of a hydrogen atom ($M_H$)$1.673 \times 10^{−27} kg$
mass of an electron ($M_e$)$9.109 \times 10^{−31} kg$
Rydberg constant ($R \inf$)$1.0974 \times 10^7 m^{−1}$
Stefan-Boltzmann constant (σ)$5.670 \times 10^{−8} J/(s \times m^2 deg^4)$
Wien’s law constant ($\lambda \max T$)$2.898 \times 10^{−3} m K$
electron volt (energy) ($eV$)$1.602 \times 10^{−19} J$
energy equivalent of 1 ton TNT$4.2 \times 10^9 J$

### Astronomical Constants

NameValue
astronomical unit ($AU$)$1.496 \times 10^{11} m$
Light-year ($ly$)$9.461 \times 10^{15} m$
parsec ($pc$)$3.086 \times 10^{16} m = 3.262 light-years$
sidereal year ($y$)$3.156 \times 10^7 s$
mass of Earth ($M_{Earth}$)$5.974 \times 10^{24} kg$
equatorial radius of Earth ($R_{Earth}$)$6.378 \times 10^{6} m$
obliquity of ecliptic$23° 26’$
surface gravity of Earth ($g$)$9.807 m/s^2$
escape velocity of Earth ($v_{Earth}$)$1.119 \times 10^4 m/s$
mass of Sun ($M_{Sun}$)$1.989 \times 10^{30} kg$
equatorial radius of Sun ($R_{Sun}$)$6.960 \times 10^8 m$
luminosity of Sun ($L_{Sun}$)$3.85 \times 10^{26} W$
solar constant (flux of energy received at Earth) ($S$)$1.368 \times 10^3 W/m^2$
Hubble constant ($H_0$)approximately 20 km/s per million light-years, or approximately 70 km/s per megaparsec

Space