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Homeworks 1

Homeworks 1 About this Assignment Topics Chapters 1 - 3 Lectures 1 - 5

Homeworks 1 is the attempted solution to our second task in the course Introduction to Astrophysics. This assignment is due on Wednesday 09/09/09 and was assigned on 09/01/09.

Problem 1

List in order of increasing size and give the approximate size of the following objects: An atom, a biological cell, a cluster of galaxies, the Earth, a galaxy, the Local Group of galaxies, a neutron, a neutron star, a person, the Solar System, our sun. Note: you may have to look in other books besides your textbook to get all this information.

1. Neutron = $3quarks\;$ (or $r\approx 1fm=10^{-25}m\;$ )
2. Atom = $r\approx 62-520pm=62-520\times 10^{-12}m\;$
3. Biological Cell
4. Person
5. Earth
6. Sun
7. Neutron Star
8. Solar System
9. Galaxy
10. Local Group of Galaxies
11. Cluster of Galaxies

Problem 2

The nearest star outside the solar system is about 4 light years away.

1. How far away is the star in kilometers?
2. Suppose you travel to the nearest star in a rocket ship moving at 100 km per hour (100 km/hr is

about 62 mi/hr, a typical automobile speed on a Florida highway). How many years will it take you to get to the star?

1. Suppose you travel to the star at 10 km per second (the speed of a rocket in orbit around the Earth). How many years will it take you to get to the star?

Problem 3

Use the size of the Astronomical Unit in kilometers and the length of the year in seconds to calculate how fast the Earth moves in its orbit in kilometers/second.

Problem 4

Describe the essential differences between the Ptolemaic, Copernican, and Keplerian descriptions of planetary motion.

Problem 5

Use Newton’s laws to show that the orbits of planets are ellipses.

References

• B.W. Carroll & D. A. Ostlie (2007). An Introduction to Modern Astrophysics. Addison Wesley. ISBN 0-8053-0402-9