Astronomy 110 PRINT Name   __________________________
Fall 2005   Section 006  

Homework 1 : How Many ?

(Due Thursday, Sep 1, 2005)

Use scientific (exponential) notation wherever possible.

  1. How many atoms ?

    There are about 1057 atoms in the Sun. It turns out that the Sun is a pretty average sort of star, and there are about a hundred billion stars in our galaxy, and about a hundred billion galaxies in the "observable" Universe. Can you put all these numbers together to estimate the number of atoms we can observe?

    At a rate of one digit per second, how long would it take you to write down this number in its long form (with all the zeroes)? Could light travel between the Sun and the Earth in this time? (Check with Figure 1.3 on page 6 of the textbook.)

    At a rate of one per second, how long would it take you to count all these atoms? Express this in terms of the lifetime of the Universe (14 billion years). Could light travel across the observable Universe in this time? Explain.


  2. How many lifetimes ?

    Express a "typical" human lifetime in years in scientific (exponential) notation, to the nearest power of ten. We think the Big Bang happened about 14 billion years ago. How many human lifetimes is this?

    Humans have been around for about a million years (very approximately). If we represent all the time since the Big Bang as a year, how long have humans been around?


  3. How many galaxies ?

    Study Figure 1.3 on page 6 of the textbook. Calculate in the most direct way how many times our galaxy, the Milky Way, would fit in the distance between the Milky Way and its nearest neighbor galaxy, Andromeda.

    Now find the radius of the Sun in kilometers in the textbook (close to the beginning of Chapter 13). Calculate in the most direct way how many times the Sun would fit in the distance between the Sun and its nearest neighbor, Proxima Centauri. (What extra step do you have to make, compared to the previous calculation?)

    Relative to their size, which are more closely packed, stars or galaxies?