AST 248 HW Problems for Chapter 3
Ryan Richards
Briefly define and describe each of the various levels of structure illustrated in Figure 3.1.
The universe is made up of superclusters which are tightly packed clusters of groups of galaxies.
One cluster is the Local Supercluster which is about 1021 kilometers big. The Local Group we
belong to is one the outskirts of the Local Supercluster. It is two orders of magnitude smaller
than the Local Supercluster i.e., 100 times smaller. The Local Group consists of about 40
galaxies one of which is the Milky Way. The Milky Way is one of the two largest galaxies in this
group. It is about 10 times smaller than the Local Group. Our solar system is located halfway
through the disk of the Milky Way. The solar system is about 108 times smaller than the Milky
Way. Our planet Earth is the third planet from the Sun in the solar system. Relative to the solar
system, it is about ten-thousand times smaller.
Describe the solar system as it looks on the 1-10-billion scale used in the text. How far away
are the other stars on this same scale? How does this model show the difficulty of detecting
planets around other stars? What does it tell us about the challenge of interstellar travel?
On the 1-10 billion scale, the Sun is the size of a grapefruit. The Earth is the size of a ball point.
In this model, the nearest star Alpha Centauri which is 4.4 light-years away is about 440
kilometers or about the distance across the United States. The difficulty of detecting other
planets around other stars is that you can’t see the orbits of other planets that are further from the
Sun (the analogy is given on page 57). The challenge of interstellar travel is that it would take an
astronomical amount of time to reach other stars (even our nearest star) even at relativity high
speeds. There is no current technology capable of reaching other stars quickly.
What is a light-year? Is it a unit of distance or time? Explain clearly.
A light-year is a unit of distance. It is the distance light travels in one year. It is 946 trillion
kilometers.
Briefly describe the scale of the galaxy. How long would it take to count 100 billion stars?
Why is the search for extraterrestrial intelligence (SETI) primarily a search for signals
broadcast by civilizations in the past, rather than an attempt to carry out two-way radio
conversations?
The text mentions that the galaxy is too large to fit on the 1-10 billion scale with the Earth due to
the abundance of stars. To picture the galaxy, the book discusses reducing the solar by another
factor of one billion. In this scale, a light-year represents one millimeter so that the diameter of
the Milky Way is about 100 meters. As mentioned in the text, if you counted one star per second,
it would take you 100 billion seconds or about 3000 years. The reason why SETI searches for
signals is because it would take twice as long for a response if there was an attempt to carry out
two-way radio conversations. The example the book gives is a two-way conversation with Alpha
Centauri. Alpha Centauri is 4.4 light-years away. It would take 4.4 years for a radio signal to
reach there and 4.4 years for a response back. Please remember that radio waves are
electromagnetic waves and travel at the speed of light.
The Voyager 2 spaceship, launched in 1977, has just crash landed on a planet orbiting
another star.
This is hard to believe. The interstellar travel is quite difficult as mentioned above. We do not
have the technology capable of interstellar travel. Remember that it would take 100,000 years to
reach the Alpha Centauri. We should keep in mind that the distances traveled are light years
which are trillions of kilometers.
At a junior high school talent show, 14-year-old Sam Smally read off the names he had
given to each of the 100 billion stars in the Milky Way galaxy.
I find this hard to believe. As mentioned above if he takes one second per star it would take him
3000 years to finish reading the names. One might say for example, suppose he took one
nanosecond (10-9 seconds) to read each star. Then it would take him only 100 seconds or close to
two minutes to be done. Although this would be an interesting thought, nobody can speak in a
matter of nanoseconds.
SETI researchers announced today that if they receive a message from a civilization
located on the other side of the Milky Way galaxy, they plan to respond with a message
asking the aliens 20 questions about current mysteries in science.
This question is the most interesting of the three. I am slightly on the fence with this question.
The reason being is the statement only says SETI researchers plan to respond which to me seems
reasonable. However, if they sent a message to the opposite side of the Milky Way, that implies
it was sent billions of years ago which is highly unreasonable.
Remarks:
1. The last three questions are my opinions. It does not mean that they are right or wrong.
Some may argue against these opinions which is ok as long as they present suitable
evidence supporting their claims. I say this to avoid students taking my answers as face
value for opinion based questions. Otherwise, the factual questions were taken from the
textbook.
2. As for the science vs. nonscience questions in Chapter 2. Here are some things to be
considered. Remember science is a testable field. Notice that this doesn’t just apply to
physical sciences but also social sciences as well. So one can argue for example that
David Beckham is the best player of his generation is testable. Indeed one would run into
issues in the definition of best, which vary for different people. I advise you to look at the
diagram on page 34 in Chapter 2 which discusses the hallmarks of science.