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Journal 1/26/17 What do you think life would be like if we had more than one sun? Objective Tonight’s Homework To learn about star systems with more than 1 star Read “Binary Stars” on page 436 and 437. Do review 4, 7, 20 on page 442. Binary Star Systems There are 4 kinds of binary stars, and we classify them by how we observe them. Visual binaries are when we can see both stars in the binary system visually with a telescope. These are the hardest to see. The brighter star is called the primary. Example: Sirius A Sirius B Binary Star Systems Visual binaries are rare. We can’t often distinguish between multiple stars in single images. There are a few examples, though. Binary Star Systems We see spectroscopic binaries by looking at emission spectra lines. We look for small red and blue shifts that tell us stars are moving around each other. Binary Star Systems These are also fairly hard to observe. We have to take very detailed spectra to see the shift. Binary Star Systems We see eclipsing binaries by looking at stars in the sky and seeing if they change brightness. This would mean the stars are going in front and behind each other, blocking out some of the light. We call what we see a “light curve”. What it would look like What astronomers observe Binary Star Systems We see astrometric binaries when we look at a star and can see it wobbling, but can’t see any other stars near it. Maybe the other star is too dim, but we can’t see it. We can only see it’s effect from gravity making the star we can see wobble. Binary Star Systems Some stars are in groups of 3, 4, or more, even up to thousands of stars. These stars all have complex orbits and can be very hard to predict orbits. The large groups of thousands have special names: open clusters and globular clusters. Binary Star Systems Open clusters are one of the most common types of multiple star systems. Open clusters usually contain thousands of stars that all formed at about the same time and are all orbit around each other. The average density in an open cluster is 500 times greater than the density of where we are. Imagine 500 more times stars in the sky! Binary Star Systems Globular clusters are extremely tight clusters with tens or hundreds of thousands of stars all orbiting each other. They’re believed to be some of the oldest things in the universe. Globular clusters have very high densities. Stars are packed in at least twice as dense as in an open cluster (1,000 times denser than where we are!) Exit Question Which kind of binary star system is the hardest to detect? a) Visual binaries b) Spectroscopic binary c) Eclipsing binary d) Astrometric binary