Binary Star

• Two stars that orbit each other
  • Bounds together by gravity
  • Both stars orbit between a point, the center of mass
• Mass can be calculated from measurements of their orbits

Types of Binary Star Systems

• Visual binary
  • A binary star system where both stars can be seen with a telescope
• Spectroscopic binary
  • Appears as a single star when photographed or seen through a telescope but spectroscopy shows a binary star system

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

Star Astronomy