21st Century Astronomy
21st Century Astronomy
6th Edition
ISBN: 9780393428063
Author: Kay
Publisher: NORTON
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Chapter 23, Problem 38QP
To determine

The time taken in merging the elliptical galaxies to form spiral galaxies.

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1. The current (critical) density of our universe is pe = 10-26kg/m³. Assume the universe is filled with cubes with equal size that each contain one person of m = 100kg. What would the length of the side of such a cube have to be in order to give the correct critical density? How many hydrogen atoms would you need in a box of 1 m³ to reach the critical density? The matter we know, which consists mostly of hydrogen, constitutes only 4.8% of the current critical energy density of our universe. So how many hydrogen atoms are actually in a box of 1 m3 in our universe? Deep space is very empty and a much better vacuum than we can obtain on earth in a laboratory.
2. A galaxy cluster has a galaxy behind it whose image we see as being smeared out and curved, with an angular radius of curvature on the sky of θE.  The background galaxy is at redshift zgal and the cluster is at zclust.  What is the mass of the cluster in solar masses?  Give your response in scientific notation with one decimal place.  (The Hubble constant is of course 70 km/s/Mpc, and you can assume the Hubble law for these low redshifts).   Values: zgal = 0.11 zclust = 0.07 θE = 117.4 arcseconds
Astronomers frequently say that "there are more stars in the universe than there are grains of sand on all the beaches on the earth". Given that a typical grain of sand is about 0.5 – 1.0 mm in diameter, estimate the number of grains of sand on all the earth's beaches. The diameter of the Earth is 12,742 km. a) About 1011 b) About 1016 c) About 1021. 6. Assume that a typical galaxy contains about 200 billion stars and that there are more than 150 billion galaxies in the known universe. Estimate the total number of stars in the universe. b) About 1022 a) About 1010 c) About 1016. 7. Compare the values of the number of grains of sand in all earth's beaches (from problem 5) with the number of stars in the universe (from problem 6) – which is greater? a) Number of sand grains b) number of stars c) they are about the same.
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