structure

# structure - Chapter 25(and end of 24 Lecture Notes These...

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Chapter 25 (and end of 24): Lecture Notes These notes cover some of the material that will appear on the last exam: Friday Dec. 7. The rest is in chapters 26 and 27. It is a lot of material—get started now! First must discuss the “Hubble law” so that you are comfortable with the idea that redshift is the same as distance is the same as how long ago. You also have to be comfortable with the “distance ladder” that we have been building throughout the semester. So we begin with basic ideas of making a map of the local, and then not-so-local, universe. (Continuation from Chap.24: Review of sec. 24.2): How are the different kinds of galaxies distributed in space? That is the main topic of the next two lectures, the key to discovering the larger-scale structure of the universe, and even its history and clues to its origin. We are trying to make a map of the entire universe, by locating all its contents. To do this we need big telescopes (to see faint objects, since they are far away), and some method to get distances to very distant objects. So we are still “climbing the distance ladder” (the pyramid your book has been constructing). Groups of galaxies — We can use Cepheid variables (from their period-luminosity relation) to make a map of our “local” galactic neighborhood (can get distances out to about 15 Mpc with this method). We find that nearby galaxies are clustered . Our Local Group —About 50 galaxies within about 1 Mpc of each other. (Look at Fig. 24.13) (1 Mpc = 1000 kpc = (roughly) 10 x size of our Galaxy. This will be our unit of distance from now on: the megaparsec.) Most of the mass of the Local Group is in the large spirals Milky Way and Andromeda (M31). Most of the number of galaxies are small dE and dIrr galaxies. Many of these are satellites of larger galaxies (e.g. 3 or more satellites of Milky Way are LMC, SMC, and Sgr dwarfs, a few others that are more distant; Andromeda has several small satellites.

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To get to larger distances, must use brighter standard candles. The next technique in the ladder of standard candles is the Tully-Fisher relation very tight relation (for disk galaxies) between rotational velocity (from broadening of galaxy’s spectral lines—see Fig. 24.11) and luminosity . (Think why this makes sense: the galaxy’s rotation is balancing its gravity, which is due to its mass, related to its luminosity…) So for a galaxy too far away to use any other method, just obtain a spectrum (21 cm neutral hydrogen line is best) and measure width of line; the Tully-Fisher relation then gives you the luminosity, so (knowing the apparent brightness) you get the distance. This method can be used out to about 200 Mpc allows us to make a map of the relatively nearby universe, but far beyond our Local Group. Before looking at the results, there is one more rung in the ladder of distance indicators or standard candles, called the Hubble relation or Hubble’s law, to consider. But it is much much than simply a technique for obtaining distances: It is our first big clue about the origin of the universe.
Hubble’s Law (Sec. 24.3)

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