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UT AST 301 - The Milky Way Galaxy

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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) much in class, but readit—there will probably be a question or a few on it.In following lecture outline, numbers refer to the Figure numbers in yourtextbook.A basic theme in this chapter is how it was gradually discoveredthat our Galaxy is not the whole universe, but that instead, if we couldview it from the outside, our Galaxy would look something like:Think about the problem of getting a characterization of the natureof the surface of the earth while standing within a forest. In the Galaxycase the trees are dust grains, preventing us from seeing outside our localneighborhood (at visible wavelengths or smaller). But even using lightof longer wavelengths, there is a big problem: how do we get accuratedistances to the objects too distant to use trigonometric, or evenspectroscopic, parallaxes?One thing we can tell by just looking at the pattern of stars and gasin the sky: our Galaxy must be flat. Look at the images of the sky atdifferent wavelengths (similar to an image in your book):But how do we know that this disk doesn’t extend for a thousand, or abillion, parsecs?What about applying spectroscopic parallax (remember?) to all the Oand B stars we can see (think: why use these stars?). Here is a schematicof the result:In hindsight, we interpret this as seeing parts of the spiral arms of ourgalaxy, but until around 1950-1960 the calibration wasn’t good enoughto see this, and even when we could, there are still these importantquestions:1. How far does this disk extend? Is it round, elongated, … ?2. Is our Galaxy made of bands in a disk, or is there more? What is theshape of the young and old stars?3. Our there other galaxies like ours?Answers to these questions require distances using the next“standard candle”: variable (pulsating) stars.Finding our position in the Milky Way (MW)—Counting stars indifferent directions very misleading (23.4). Instead the breakthroughcame from using RR Lyrae stars to get the distances to globular clusters(23.9).There are two kinds of variable stars (their apparent brightnessvaries periodically because they are pulsating) that are used as “standardcandles” for distance estimates:a. RR Lyrae stars—all have similar light curves, periods 0.5 to 1day (23.5), and all have approximately the same luminosity! (Thinkabout how handy this is—see 23.6.). Only low-mass metal-poor starsbecome RR Lyrae stars, so these gave distances to globular clusters(think: old, metal-poor), showing that we weren’t located at the center ofour Galaxy, and that the Galaxy has a roughly spherical “halo.” (23.9)b. Cepheid variables—periods 1-100 days; show very tight period-luminosity relation (23.7), which can be used to get their distances.Important because they are much brighter than RR Lyrae stars, so canget distances to the nearest galaxies using Cepheids.These two methods form the next rung of the ladder of “standardcandles” of distance indicators that we will eventually extend to map thestructure of the whole universe. Make sure you understand the relationbetween needing distances to things in order to make a map!Our galaxy’s “stellar populations”: Disk, halo, and bulge. Properties todiscuss and understand (see also table 23.1):Spatial distribution (23.10)ColorAgeMetal abundancesOrbits (23.13)The illustration below shows how the disk and halo stellar populationsof our Galaxy are distinct in their spectra, with the halo stars havingweaker spectral lines than disk stars of the same temperature (spectraltype).The following illustration shows the different orbital characteristics ofdisk and halo stars.These population characteristics suggest a consistent picture for theformation and evolution of our galaxy (Fig.23.14)—halo forms first innearly spherical shape, rest of gas collapsed to disk which has formedstars continuously since that time. (Think about how above propertiessuggest this.)More recently it was discovered that our Galaxy has a weak butdetectable bar structure in the bulge. This rotating bar is important,because it keeps things “stirred up” through its gravity, and might evendrive density waves (see below).Mapping the disk—can’t use stars except nearby (why?); must use radioHI and CO spectral lines for more distant regions. Result from radialvelocities and distances: differential galactic rotation (23.12). Innerparts rotating faster than outer parts (at least at our distance from thecenter—see below). Note that this is just for the disk—the halo stars aremoving differently (see 23.13).Here is an illustrative HI map of our Galaxy.Spiral structure – These maps show evidence for spiral arms (althoughthese can be seen more clearly in other galaxies, e.g. 23.3). How canspiral structure persist? If they were material structures, differentialrotation would wind them up very tightly (23.17).Two theories, probably both contribute:a. Spiral density waves—spirals are only wave patterns movingthrough the disk. (Think of sound waves—the gas itself doesn’t movefrom one place to another.) Gas passing through wave is slowed downand compressed, get enhanced star formation (23.18 and Discovery 23-2).b. Self-propagating star formation---Star formation at one locationcauses explosions that compress the gas some distance away, causingmore star formation, and the process repeats, spreading through thegalaxy. Makes spiral pattern in differentially rotating disk. (23.19)The origin and maintainance of the spiral is only a problem for a),not b). But a plausible answer for (a) is: waves are “excited” bygravitational interactions with galactic neighbors, or by a “bar” withinthe bulge of our Galaxy and others.Another possibility, not mentioned in the book, is that the armscould be generated by the bar that is located in the central regions of thegalaxy. Here is a simulated IR map of our galaxy showing the bar andthe ring it definitely produces.Mass of our Galaxy: Use rotational speeds of stars around galacticcenter to infer how much mass is inside of that orbit (23.20):Total mass = (orbit size )3/(period)2this is just Kepler’s 3rd law—remember?)This use of orbital speeds is called the rotation curve of a galaxy(23.21). We expected speeds to decrease as you get further from centerof galaxy (like planets in the solar system, so called “Keplerian rotationcurve”). Instead the rotation


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UT AST 301 - The Milky Way Galaxy

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