ASTR 0115 Lecture 8: AstronomyNotes1013

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San Francisco State University
ASTR 115
Mc Carthy, Christopher

Notes 1013 Motion Measures Mass in a binary star system both stars orbit around the center of mass or balance point by measuring the motions of the stars (ie their orbits) we can determine their mass Keplers 3rd law and Mass Isaac Newton used Gravity to derive Keplers Laws of planetary motion. He improved Keplers 3rd law to read P^2 = a^3 (M1+M2) P= ORBITAL PERIOD (IN YEARS) a= semi major axis (AU) M1 M2 =Masses of the two bodies (in solar masses Msun) We can use this to measure the masses of stars in a binary system. We just need to measure the size of the orbit (semimajor axis,a ) and the period (P) Binary Star Example If a binary star has a period of 4 years and a semi major axis of 2 AU, what is the combined mass of the two stars. P^2 = a ^3 M1+M2) (M1+M2) = a^3P^2 (M1+M2) =a^3P^2= 2^^2=816=0.5 Msun The combined mass is the mass of the Sun. Mass and Luminosity are related We have measured the mass of many main sequence stars using binary stars We find that: More massive stars are also more luminous Much more luminous Chapter 6 Summary Distance to stars: Parallax method Luminosity of stars (Lsun) Chapter 7 The lives of the stars how stars are born how long they live why they die Where do stars come from some stars are old but others are very young they must have just formed
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