The Milky Way Galaxy

    We have not always known that we live in a spiral galaxy. For the last two centuries, we have been discovering more and more details of our place in the universe. In the late eighteenth century, William Herschel had a fairly simple, and incorrect, view of our place in the universe. Although his effort to map our home galaxy was the first of its kind, Herschel did not not know of the importance of interstellar dust and its effects on the brightness of stars. It was about eighty years ago when Harlow Shapley and his contemporaries first understood the true nature of the galaxy in which we live. Using the hundred or so globular clusters of our galaxy, he plotted the location of the galaxy's center. Although the new ideas certainly were not accepted without debate.

    Our Milky Way galaxy has tens of billions of stars and Sun is only one among those billions. We now know that our home Galaxy is a spiral galaxy and that we live about halfway out from the center. We find our way around the galaxy by means of galactic longitude and galactic latitude. Here is a nice page to convert from the equatorial coordinate system to the galactic coordinate system.

    We do not yet have a very detailed map of our home galaxy. Artists have used data to render fairly detailed illustrations, showing the appearance of the spiral arms. There are a number of different interpretations of the basic data. Yet there are some overall features which we recognize:

Parts of the Milky Way

We identify three parts of the galaxy:

The Disk

The disk of the Galaxy contains the spiral arms. The origin of the spiral structure is somewhat in debate, but the prevailing explanation right now is something called spiral density waves. Which spiral arm do we live in? There is not general agreement on that point, simply because it is so hard to map the arms. Most researchers would say that the Sun is in the Orion Arm. In the middle of the last century, Walter Baade discovered two distinct populations. Population I type stars are found in the disk of the galaxy.

The Nucleus

The nucleus of our galaxy contains Population II type stars. The position of the nucleus was determined by Harlow Shapley. He found the center by plotting the X, Y, and Z coordinates of globular clusters. It lies in the direction of the constellation of Sagittarius. Here is what the sky looks like in the direction toward the center. The very center of the nucleus is thought to harbour a supermassive black hole. Here is an animation that suggests the appearence of such an object.

The Halo

The halo contains some of the oldest stars known. These are primarily Population II type stars. Here is an image of the Sombrero Galaxy, another galaxy that shows a halo much like ours. The Halo of the Milky Way is thought by some to contain WIMPs and MACHOs.

Here are the SmartBoard notes on the Milky Way.

Here is one power point presentation

And another.

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Updated April 26, 2011