21st Century Astronomy
6th Edition
ISBN: 9780393428063
Author: Kay
Publisher: NORTON
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Chapter 19, Problem 32QP
To determine
The number of galaxies astronomers would find.
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Let's say that the number density of galaxies in the universe is, on average, 3 × 10–68 galaxies/m3. If astronomers could observe all galaxies out to a distance of 1010 light-years, how many galaxies would they find? (Note that there are 1016 meters in 1 light-year.)
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.
Suppose you want to observe every galaxy within some distance. Your enterprising assistant says that instead you can observe every galaxy within double the original distance. What is the ratio of the number of galaxies you can now observe as opposed to before?
Chapter 19 Solutions
21st Century Astronomy
Ch. 19.1 - Prob. 19.1ACYUCh. 19.1 - Prob. 19.1BCYUCh. 19.2 - Prob. 19.2CYUCh. 19.3 - Prob. 19.3CYUCh. 19.4 - Prob. 19.4CYUCh. 19 - Prob. 1QPCh. 19 - Prob. 2QPCh. 19 - Prob. 3QPCh. 19 - Prob. 4QPCh. 19 - Prob. 5QP
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- Our galaxy is approximately 100,000 light years in diameter and 2,000 light years thick through the plane of the galaxy. If we were to compare the ratio of the diameter galaxy and its thickness to the ratio of the diameter of a CD and its thickness (CD has a diameter of 12 cm and thickness of 0.6 mm), what would be the factor differentiating those ratios? Put differently, if the galaxy were scaled down to the diameter of a CD, how many times thicker or thinner would the galaxy be than the CD? (For example if it would be twice as thick, you would answer 2 and if it were twice as thin you would answer 0.5 (aka 1/2))arrow_forwardAstrophysicists estimate that there are about 100 billion galaxies in the universe, and thaton average each galaxy has about 100 billion stars. Comparing this estimate to Archimedes’ estimate, which is larger: the number of stars in the universe, or the number of grains of sand needed to fill a volume the size of the Earth?arrow_forwardIn the reading, you were told that there were roughly 10,000 galaxies in the image of the Hubble Ultra Deep Field alone. The image is roughly 10 square arcminutes and there are roughly 1.5*10^8 square arcminutes composing the entire sky. With that in mind and assuming that the Hubble Ultra Deep Field represents an average part of the sky, roughly how many galaxies may exist in the observable universe? (Please include commas for every factor of 1,000; for example 2,343,567,890)arrow_forward
- Pretend that galaxies are spaced evenly, 2.0 Mpc apart, and the average mass of a galaxy is 1.0 x 1011 Mo: What is the average density (in kg/m3) of matter in the universe? (ote: The volume of a sphere is , and the mass of the sun is 2.0 x 1030 kg.) kg/m3arrow_forwardSuppose you want to observe every galaxy within some distance. Your enterprising assistant says that instead you can observe every galaxy within double the original distance. What is the ratio of the number of galaxies you can now observe as opposed to before? (For example, if you can observe twice as many: 2. If you can observe half as many: 0.5) Your answerarrow_forwardHow many galaxies are there in the universe?arrow_forward
- Astronomers recently claimed to have discovered the most distant galaxy known. How would they have determine this distance?arrow_forwardPretend that galaxies are spaced evenly, 7.0 Mpc apart, and the average mass of a galaxy is 1.0 ✕ 1011 M. What is the average density (in kg/m3) of matter in the universe? (Note: The volume of a sphere is 4/3pieR^3 and the mass of the sun is 2.0 ✕ 1030 kg.) ______ kg/m^3 Which model universe does this density value support? A: open B: flat C: closedarrow_forwardmathematician Archimedes, responding to a claim that the number of grains of sand was infinite, calculated that the number of grains of sand needed to fill the universe was on the order of 1063. Our understanding of the size of the universe has changed since then, and we now know that the observable universe alone is a sphere with a radius of 1026 m. Estimating the size of a grain of sand, A) Approximately how many grains of sand would fill the observable universe? B) How many times larger or smaller is this number than Archimedes' result?arrow_forward
- The Universe is approximately 13.8 Billion years old. What is the volume of the visible universe in m3?arrow_forwardHow long ago was that galaxy right next door to our own Galaxy if it has always been receding at its present rate? Express your answer in years. Since the universe began when all galaxies were very close together, this number is a rough estimate for the age of the universe.arrow_forwardAssume that the average galaxy contains 1011 MSun and that the average distance between galaxies is 10 million light-years. Calculate the average density of matter (mass per unit volume) in galaxies. What fraction is this of the critical density, 9.6 * 10-27?arrow_forward
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