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# 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

Name Value
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

Name Value
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