milky_way

# milky_way - Notes on Ch 23 and 24 The Milky Way Galaxy(ch...

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Notes on Ch. 23 and 24. The Milky Way Galaxy (ch. 23) [Exceptions: We won’t discuss sec. 23.7 (Galactic Center) in class, but look it over in order to just get the most basic point—I might put a question or two on the exam to make sure you have looked at this interesting material. There will be several sections that we will have to treat this way.] In following lecture outline, numbers refer to the Figure numbers in your textbook. In this draft they have not been revised to reflect the new edition. A basic theme in this chapter is how it was gradually discovered that our Galaxy is not the whole universe, but that instead, if we could view it from the outside, our Galaxy would look something like:

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Think about the problem of getting a characterization of the nature of the surface of the earth while standing within a forest. In the Galaxy case the trees are dust grains, preventing us from seeing outside our local neighborhood (at visible wavelengths or smaller). But even using light of longer wavelengths, there is a big problem: how do we get accurate distances to the objects too distant to use trigonometric, or even spectroscopic, parallaxes? [Make sure you can explain spectroscopic parallax so that you don’t forget the general approach to getting distances, which we will begin to encounter at a rapid rate.] One thing we can tell by just looking at the pattern of stars and gas in the sky: our Galaxy must be flat . Look at the images of the sky at different wavelengths (similar to an image in your book): But how do we know that this disk doesn’t extend for a thousand, or a billion, parsecs?
What about applying spectroscopic parallax to all the O and B stars we can see (think: why use these stars? I will ask you this on the exam). Here is a schematic of the result: In hindsight, we interpret this as seeing parts of the spiral arms of our galaxy, but until around 1950-1960 the calibration wasn’t good enough to see this, and even when we could, there are still these important questions: How far does this disk extend? Is it round, elongated, … ? Is our Galaxy made of bands in a disk, or is there more? What is the shape of the distribution of the young and old stars? Our there other galaxies like ours? Answers to these questions require distances using the next “standard candle”: variable (pulsating) stars.

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Finding our position in the Milky Way (MW)—Counting stars in different directions is very misleading (23.4). Instead, the breakthrough came from using RR Lyrae stars to get the distances to globular clusters (23.9). There are two kinds of variable stars (their apparent brightness varies periodically because they are pulsating ) that are used as “standard candles” for distance estimates: a. RR Lyrae stars —all have similar light curves, periods 0.5 to 1 day (23.5), and all have approximately the same luminosity! (Think about how handy this is—see 23.6.). Only low-mass metal-poor stars become RR Lyrae stars, so these
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milky_way - Notes on Ch 23 and 24 The Milky Way Galaxy(ch...

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