Double & Multiple Stars — Spring Semester

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ASTR 110L Name: Spring 2006 Double & Multiple Stars — Spring Semester 1. (7 pts.) For each of the double (or multiple) star systems that you are able to observe, sketch and/or write the following characteristics: a. Relative brightness between the two (or more) members (when sketching, use heavier vs. lighter dots) b. Apparent separation of the two (or more) members (sketch separation wider or closer relative to the other systems seen tonight) c. Colors of the two (or more) members (note in writing) d. Any other distinguishing features that you notice. 2. a. (2 pts.) What is a true binary, and how is it different from a purely visual double (also known as an optical double)? b. (1 pt.) What is a spectroscopic binary? 3. a. (1 pt.) Suppose there is a binary system whose orbital plane is exactly “edge-on” as viewed from Earth. How will its appearance vary over the course of one orbit? (You may use simple sketches to illustrate your answer.) b. (1 pt.) Suppose there is a second binary system whose orbital plane is exactly “face-on” as viewed from Earth. How will its appearance vary over the course of one orbit? (You may use simple sketches to illustrate your answer.) c. (1 pt.) The above two situations are special cases. How would you expect the appearance of most binary systems to change over the course of their orbits? (You may use simple sketches to illustrate your answer.) d. (1 pt.) Eclipsing binaries belong to which one of the above special cases? 4. Suppose there are two binary systems, binary system I and binary system II. If they both have the same total mass rd (MA + MB), then Kepler’s 3 Law can be written: 2 2 PI PII 3 = 3 (aA + aB)I (aA + aB)II where: P = orbital period of binary system (both stars orbit each other with the same period!) aA = semi-major axis of star A’s elliptical orbit (if the orbit is exactly circular, this is the radius of the orbit) a. (2 pts.) The two stars of the α C!en tauri binary system (named “α Centauri A” and “α Centauri B”) are separated by 24 AU, and they take 79.9 years to complete one orbit of each other. Suppose that we could magically increase the separation of the two stars to 240 AU. What would happen to their orbital period: would it become shorter or longer? Justify your answer (although you do not need to calculate the new orbital period). STELLAR SYSTEM STATS σ (sigma) Orionis d = 1150 ly β (beta) Monocerotis d = 690 ly Castor = α (alpha) Geminorum d = 52 ly true triple T = 480 yr for closest pair γ (gamma) Leonis d = 126 ly true binary T = 600 yr γ (gamma) Virginis d = 39 ly true binary T = 169 yr, closest in 2005 Alcor & Mizar = ζ (zeta) Ursae Majoris dMizar = 78 ly dAlcor = 81 ly probably visual double; Mizar is itself a binary: TMizar > 5000 yr AMizar = 500 AU Polaris = α Ursae Minoris d = 431 ly true binary (actually triple: one of the two stars is actually a very close spectroscopic binary, T = 30 yr) .

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