# Binary Star

## Types of Binary Star Systems

• Visual binary
• A binary star system where both stars can be seen with a telescope
• Spectroscopic binary

## Masses of Binary Star Systems

• Can be estimated using Isaac Newton's reformulation of Johannes Kepler's third law of planetary motion
• If two objects are in mutual revolution, then the period ($P$) with which they go around each other is related to the semi-major axis ($D$) of the orbit of one with the respect to the other
• $D^3 = (M_1 + M_2)P^2$
• $D$ is in astronomical units
• $P$ is measured in years
• $M_1 + M_2$ is the sum of the masses of the two stars in units of the Sun's mass