Lecture 27 – Binary Systems and Star Clusters
Summary / Checklist of the Main Concepts to Understand and Remember

Binary Stars
	How common are binary stars? Polaris, Mizar and Sirius as examples of binary stars. (Where are those stars?)
•	What is a visual binary system?
•	What is a spectroscopic binary? How should the orbits of the stars be oriented for them to be seen as a spectroscopic binary?
•	What is an eclipsing binary? How should the orbits of the stars be oriented for them to be seen as an eclipsing binary?
•	Can a binary star be of more than one type? (Visual / Spectroscopic / Eclipsing.)
Star Masses
	How can we find out the mass of a star?
•	Why are binary stars the only ones for which we can directly find the mass? (As opposed to guessing it based on the type of star.)
	Approximately where is the Sun, in the range of possible star masses? Are most stars more massive or less massive than the Sun?
Star Clusters
•	Star clusters are common, in particular where young stars are found (stars are born in large interstellar clouds that break into fragments).
	What are open clusters? (Approximately how many stars do they contain, in what part of the galaxy are they, are they young or old?)
	What are globular clusters? (Approximately how many stars, in what part of the galaxy, are they young or old?)
•	How can we find out how old a star cluster is? Name one example of open cluster and one example of globular cluster.

Topics from the lecture page and textbook not listed above [or between square brackets] were not covered in class;
Underlined words indicate that I will expect students to remember a number or a name related to that topic.

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