Kepler's 3^rd Law           T² = (4π • R³)/(G • M)

• (M) Mass of the system. 
• (T) Period of the orbit.
• (R) Separation distance between the two objects.
• (G) Universal Gravity Constant

Flux and Magnitude

• Change in Magnitude from Flux Ratio
• Flux Ratio from Magnitudes

Small Angle Formula         α = S / D

• (α) Angle
• (D) Distance from Size and Angle
• (S) Size of astronomical object

Telescope formulas

• Resolving Power of a Telescope
• Magnification of a Telescope

Luminosity

• Luminosity from Stellar Mass

Mass formulas

• Mass from Luminosity
• Mass from Acceleration and Radius
• Mass from Speed and Separation
• Mass of extra-solar planet from mass of star, radius of star orbit and radius of exoplanet orbit
• Mass of extra-solar planet from mass and velocity of star and velocity of planet
• Black Hole Mass Calculator
• Black Hole Event Horizon Calculator

Other Formulas

• Distance from absolute and apparent magnitude
• Radius from Speed and Period
• Speed of Circular Motion
• Hubble Time
• Galaxy Recession Velocity
• Luminosity from Mass
• Hubble's Law
• Exoplanet Calculator 
• Orbital Radial Acceleration
• Distance to Barycenter
• Sakuma-Hattori Equation
• Astronomical Distance Calculator 
• Astronomical Travel Time Calculator 
• 3D Vector Calculator
• Drake Equation Calculator
• Seager Equation Calculator
• Friedman Equation Calculator
• Lagrange Points L1 and L2
• Schwarzschild Radius
• Hill Sphere Radius
• Escape Velocity
• Barycenter Distance

Relative Size Formulas

• Relative Size from Relative Temperature and Luminosity
• Relative Temperature from Relative Size and Luminosity
• Relative Luminosity from Relative Temperature and Size

Wavelength Formulas

• Velocity from shift in Wavelength and Unshifted Wavelength
• Wavelength Shift from Wavelength and Speed
• Blackbody Wavelength from Temperature
• Blackbody Temperature from Peak Wavelength
• Photon Energy from Wavelength
• Photon Wavelength from Energy
• Redshift
• Galaxy Recession Velocity