Milky Way

In Classical times, there were two major theories as to the existence of the Milky Way. The Pythagoreans thought it was residue left from the sun as it traveled that path (Darn that slimy residue.) Aristotle's Peripatetics believed it was formed of suspended exhalations. (Darn that morning breath.) There was a third theory according to William Hone ( a nineteenth century author on the subject), proposed by Democritus without the aid of a telescope. It was that the Milky Way contained innumerable fixed stars which caused the white trail in the sky. Of course, Galileo gets all the credit. Just because he almost got tortured or something.

The name "Milky Way" is derived from a Greek myth. The infant Hercules, son of Zeus and one of his mortal lovers, was given to Zeus' wife, Hera, to nurse. But her milk spilled into the sky and created the Milky Way. The Greeks called it Kiklios Galaxios -- meaning the "milky circle" -- from which the word galaxy is derived. Other cultures thought of the Milky Way as a road or river on which the souls of the dead traveled through the heavens.

The Milky Way is the name given to the faint band of light that stretches accross the night sky. This light comes from stars and nubulae in our galaxy, known as the Milky Way Galaxy or simply as the Galaxy.

The galaxy is shaped like a spiral, with a dense central bulge that is encircled by four arms spiralling outwards and surrounded by a less dense halo. We are unable to see the spiral shape because the Solar System is in one of the spiral arms which is also known as the local arm.

From our position the centre of the galaxy is completely obscured by dust clouds. As a result, optical maps give only a limited view of the galaxy. However a more complete picture can be obtained by studying radio, infra-red and other radiation.

The central bulge of the galaxy is a relatively small, dense sphere that contains mainly older red and yellow stars. The halo is a less dense region in which the oldest stars are situated. Some of these stars may be as old as the galaxy itself (possibly 15 billion years). The spiral arms contain mainly hot, young blue stars as well as nebulae (clouds of dust and gas inside which stars are born).

The galaxy is about 100,000 light years across, in comparison the Solar System seems small at about 12 light hours across.

The entire galaxy is rotating in space, although the inner stars travel faster than those that are further out. The sun which is about two-thirds out from the centre, completes one lap of the Galaxy about every 220 million years.

SIDE VIEW OF OUR GALAXY:

      Disc of spiral arms containing 
      mainly young stars              Surrounding halo contains oldest stars,
              |           ___________ Central bulge containing mainly older      
              |          |            stars
              |          V            
              V        /**\
   .....oo00OOOXXXXXXX<****>XXXXXXXOOO00oo.....
                       \**/
   |                                          |
   |___________ 100,000 Light Years __________|

OVER HEAD VIEW OF OUR GALAXY:

                            Central bulge
                            containing nucleus
                                  |
                                  |       *
                                  |             *
                           *  *  *|               *
                        *         |   *             *
                      *           |      *            *
                     *         *  |*      *    *      *
                    *      *  ****|***         *      *
                   *     *   *** _V ***   *    *      *
    Perseus ---------->*   *** /    \ ****     *<----------- Sagittarius
    Arm           *   *    ***|      |***     *      *       Arm
                  *   *    ****\ __ /***     *
                  *   *     ** *******      *      *<--- Orian Arm
                   *   *     * *          *       *      (Local Arm)
                    *          0*  * * *          
                     *         ^ *             *
    Crux-Centaurus --->*       |    *  *  * 
    Arm                    *   |
                               |   *
                               |
                          Location of
                          Solar System

---

Note - The diagrams display a pictorial representation of the galaxy to the best of my HTML artistic capabalities, so please view with compassion.

There seems to be quite some disagreement about the number of stars in our Milky Way, but the most commonly given figure is 100 billion stars. (That's 100x10⁹ or 10¹¹ to be exact)

Other sources put the estimate as low as 4 or as high as 400 billion.

The reason for this inaccuracy is mainly that we cannot see (and therefor cannot count) most of the stars in our galaxy, due to clouds of dark matter, lack of telescopes with high enough resolution and the fact that a far-away galaxy appears the same as a not-so-far-away star. Scientist thus extrapolate the number from what they can see and comparisons with other galaxies that are believed to be similar to our own.

The human eye, btw, can only distinguish several thousand stars on the clear night sky.