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CU-Boulder ASTR 1120 - ASTR 1120 Problem Set 1

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Fall 2005 ASTR 1120-001 General Astronomy: Stars & Galaxies.Problem Set 1. Due Th 1 SepYour name:Your ID:Please show your working on this sheet, and write your answers on this sheet. Attachextra sheets if you need them. Express your answers in scientific notation. By the way, if youcan’t do any particular question, then guess a reasonable answer (and say you guessed), anduse your guess for subsequent answers. The grader will not penalize you for getting a wronganswer because you miscalculated in a previous question — (s)he’ll compensate. Mind you,if you make a ridiculous guess, (s)he might p enalise you for that.In each case the question asks you to decide whether to figure the answer exactly, orjust to estimate it. Don’t forget to explain why! Please note that you will get marked downif you give too many significant figures when only a rough estimate is justified. You mightestimate an answer as, for example, 3 × 1011, or just 1011; anything more accurate would beunjustified.Here are some cool exact numbers:Speed of light = 299,792,458 m/s (meters per second, not miles per second!)1 year = 31,556,930 seconds1 AU (Astronomical Unit) = 149,597,870 km1 pc (parsec) = 648,000/π AU1. Lightyears in a parsecHow many meters are there in a parsec? How many meters are there in a lightyear?Therefore, how many lightyears are there in a parcsec? Are these numbers you can figureout exactly, or should you just estimate them? Why?1 parsec = meters.1 lightyear = meters.1 parsec = lightyears.12. Radius of Observable UniverseFrom measurements of the rate of expansion of the Universe (the “Hubble constant”), weestimate that the age of the Universe is about 14 gigayears (1 Gyr = 109yr = a billion years).Light can travel only a finite distance during the age of the Universe, so the observable radiusof the Universe is approximately the age of the Universe times the speed of light (not exactly,because the Universe may have decelerated or accelerated a bit). What is the radius of theUniverse, first in lightyears, then in megaparsecs (1 Mpc = 106pc). Are these numbers youcan figure out exactly, or should you just estimate them? Why?The radius of the observable Universe is about lightyears.The radius of the observable Universe is about Mpc.23. Number of Galaxies in UniverseFrom question 2, what is the volume of the observable Universe, in Mpc3? As a roughaverage, the distance between galaxies is around 1 Mpc, so that there is about 1 galaxyper cubic Megaparsec in the Universe. What is the number of galaxies in the observableUniverse? Are these numbers you can figure out exactly, or should you just estimate them?Why? [Hint: Even though the observable Universe is really spherical, approximate it asa cubical box (why not?). You want to fill this cubical Universe with little cubes calledgalaxies. How many galaxies wide is the Universe? It’s a box, so you have to cub e that toget the number of little boxes in the big box, don’t you?]The volume of the observable Universe is about Mpc3.The number of galaxies in the observable Universe is about .4. Number of Stars in the GalaxyThe typical distance between stars is about 1 parsec. Our Galaxy, the Milky Way, is adisk of stars about 3 × 104parsecs in diameter, and about 300 parsecs thick. How manystars there are in the Galaxy? Is this a number you can figure out exactly, or should youjust estimate it? Why? [Hint: This is a bit like the previous problem, except the box — theGalaxy — looks more like a briefcase than a cubical box.]The number of stars in our Galaxy is about .35. Atoms in the UniverseEach star contains roughly 1057atoms. How many atoms are there in the observableUniverse? Is this a number you can figure out exactly, or should you just estimate it? Why?The number of atoms in the observable Universe is about .6. Distance between atoms in the UniverseSuppose all the atoms in the observable Universe were uniformly spread out through it.From the volume of the Universe in question 3, and the number of atoms in the Universefrom question 5, what would be the volume occupied by each atom? Express your answerin cubic meters. Therefore, what would be the average distance between atoms, in meters?Are these numbers you can figure out exactly, or should you just estimate them? Why?The average volume occupied by each atom would be about m3.The average distance between atoms would be about


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