To receive full credit, you must give the correct answer **and **show that you understand it. This requires writing your explanations
in full, complete English sentences, clearly labeling all figures and graphs,
showing us how you did arithmetic, and being explicit about the units of all
numbers given. All relevant
mathematical symbols should be explicitly defined. And please use your best
handwriting; if I can't read it, I can't give you credit for it!

In this class you'll see some problems to which a good answer
is basically verbal, some for which a fairly precise calculation is needed, and
some for which ``order-of-magnitude" accuracy is all that's
appropriate. It's usually up to
you to figure out which is called for.
As you will see, astronomers and mathematicians have rather different
approaches to numbers. In
particular, it is **wrong** to simply
write down all the digits that your calculator spits out; if the numbers going
into a given calculation are imprecise, the result will have a similar
imprecision. Indeed, do the
problems without a calculator whenever possible.

*Feel free to work with your classmates on this homework, but your
write-up should be your own. Answer all questions. *

**1. How many stars? (60
points) **

**In this problem, we
will get a sense of both how big, and how empty, the universe is. **

(a) (10
points). The Milky Way galaxy in
which we live is shaped like an enormous circular flattened disk. It has a radius of 40,000 light years,
and a thickness of about 1000 light years. The mean distance between stars is about 4 light years. Calculate how many stars there are in
the Galaxy. *Hint: Think about how much volume the typical star, plus the empty space around it, occupies.*

(b) (10 points). The Milky Way is only one of many similar galaxies in the universe. The mean distance between luminous galaxies is about 5 million light years. The visible universe has a radius of about 40 billion light years. Calculate how many galaxies there are in the visible universe.

(c) (5
points). Using the results of
parts (a) and (b), calculate the total number of stars in the visible
universe. Express your answer both
in exponential notation, and for fun, in English (See http://mathworld.wolfram.com/LargeNumber.html
for a discussion of the names of * really* large numbers).

(d) (10 points) Lets compare this number to the number
of human cells on the planet. *Look
at Guesstimation p 57 for an estimate of the number of cells in the human body.*

(e) (10 points). Let's go back to that number of four light years between stars.

That is a seriously big number. Spacecraft in the solar system travel at roughly the same speed that the Earth travels around the Sun. First, calculate the latter speed (the Earth goes around the Sun, a circle of radius 1 Astronomical Unit = 150 million kilometers, in one year), and express your answer in kilometers per second. Second, knowing that light travels at 300,000 kilometers per second, calculate how many kilometers there are in four light years. Finally, calculate how long it would take for a spacecraft to reach the nearest stars.

Express your answer in years.

(f) (10 points). Space is really empty as well. Calculate the ratio of the diameter of stars to the distance between them. Now imagine scaling the whole problem down: imagine the Sun as a basketball. At that scale, how far away is the nearest star? Express your answer in kilometers.

(g) (5 points). Let us now calculate the density of matter
in the Milky Way. The sun has a
mass of 2 x 10^{33} grams. Calculate the total mass of the Milky Way
(assuming, of course, that all stars have the same mass), and then calculate
its density (i.e., the total mass divided by the total volume; see part a), in
units of grams per cubic centimeter.
Compare with the density of the most extreme laboratory vacuum

(10^{-20} g/cm^{3 }and comment.

**2. Trying to see
planets**

**Here we will start to
see why taking pictures of planets around other stars is a difficult matter...**

(a) (15 points). Consider a planet orbiting 1 Astronomical Units from its parent star, which is 10 light years from Earth. Using the small angle formula, calculate the angular distance on the sky between the planet and the star. Express your answer in arcseconds. (For simplicity, assume that the orbit of the planet is in the plane of the sky).

(b) (5 points). Assume that the planet is the same size as the Earth Calculate its angular size in arcseconds. The best telescopes on the ground can resolve (i.e., measure the angular size of) objects no less than 0.3 arcseconds in angular size. Compare this number to the two numbers you've just calculated and comment.

(c) (15
points). Assume that planet is
perfectly reflective so that all of the light from the star that hits it is
radiated towards us. Compute the
ratio of the energy radiated by the star to the energy reflected by the planet.

(d) (15
points). You will find above that
the star is *enormously* more luminous
than is the planet. Compare this to ratio of the brightness of a firefly to the
energy emitted by the lights in Time Square in NY (for the later, you can make
very rough estimates).