Stretching across the evening sky of September is the hazy band of the Milky Way. It is imbedded firmly on the folklore of many cultures. Richard Hinkley Allen in his book, reprinted by Dover as Star Names Their Lore and Meaning notes many titles for the Milky Way. From the German Milch Strasse (or Milk Street) close to our meaning. To the Swedish Winter Street, and many more. According to him the Ottawa Indians saw the Milky way as the stirred up muddy water of a turtle swimming along the bottom of the sky, which fits its dim appearance.
The term Milky Way galaxy, is a bit redundant because galaxy means milky in Greek. The milky appearance mostly breaks down when viewed with telescopes as small as binoculars. It was one of Galileo's discoveries, that the telescopes showed instead of a haze, a myriad of dim stars.
In the late 18th century English astronomer William Hershel made the first map of the Milky Way by making star counts in various directions. To him the greater the number of stars the greater the depth of the Milky Way, a logical assumption, not knowing anything about the structure of the Milky Way. Herschel's map showed the Milky Way to be wider than it was thick and the solar system was located near the center.
Before 1920 the Milky Way was generally thought to comprise the entire universe. The proof that this wasn't so was that large telescopes were able to photograph stars in some of the larger, thus nearer spiral nebulae. Up to that time spiral nebulae were thought to be proto-stellar nebulae, that is stars being formed. Thus these nebulae shed that appellation and became spiral galaxies.
It turns out that the Milky Way galaxy is a spiral galaxy too. We have lots of clues. The big clue is the band of the Milky Way; it's just what one would see of a disk from the inside. Along the band of the Milky Way are dark clouds, the largest of which is the great rift that splits the Milky Way from Cygnus to Aquila. Edge on spiral galaxies show the same kinds of clouds in the plane of there disks.
Since we're within the structure, how can we hope to find out the true structure of the Milky Way or its size, or where we are within it?
Our distance to the center is the easiest to find. Among the members of the Milky Way are globular star clusters. An easy way to see how helpful they are, take the globulars listed in Messier's catalog, and plot them on a grid representing the sky. Despite the lack of objects to the far south, a function of Messier's northerly latitude, it becomes apparent that most of the clusters are gathered in the area centered in western Sagittarius. It is globular star clusters that point the way to the center of the galaxy. By measuring the distances to the globulars by means of their variable stars (a subject for another time) astronomers place the solar system about 30,000 light years from the center.
Globular star clusters are very old, in the neighborhood of 10 billion years. Astronomers can measure a cluster's age by analyzing the light of the stars within it. The brightest stars in a globular cluster are red giant stars. Stars past their prime, no longer using hydrogen for fuel, but helium. There are no spendthrift bright blue starsleft. It is thought that these clusters were formed when the gasses of the Milky Way were in roughly a spherical shape before or during the collapse into a disk. In forming early the globular star clusters took themselves out of the galaxy's evolution, and follow the same paths they did when they formed, swarming, like bees, around the center of the galaxy. Stars are very massive for their size, and feel only the gravitational force of the rest of the galaxy. Gas molecules feel gravity too but also feel forces of collisions of other gas molecules being very much larger for their mass than stars. Ionized gasses also feel the forces of electromagnetic fields many times stronger than gravitation.
The hydrogen and helium gas of which the galaxy was formed, continued to collapse into a plane, forcing a common angular motion around the center of the galaxy. Later stars formed closer and closer to the rotational plane with time. We know this because stars found far from the galactic plane, in the galactic halo, appear to be older than stars close to that plane. This also is true of galactic star clusters. Also known as open clusters, these are formed even today from the nebulae or clouds of gas and dust in the. many of then have hot bright blue-white stars indicating youth. Here too the farther the star cluster is from the galactic plane the greater the age of the cluster.
Where do the spirals come from in a spiral galaxy? Stars are pretty evenly spread out in the disk of the galactic plane. Radio maps can see clouds of gas through the gas and dust that hides them in the visual part of the electromagnetic spectrum, and confirm that our galaxy has indeed a spiral structure. Perhaps the gas and the dust ended up forming density waves, while orbiting the center of the galaxy in the shape of spiral arms.
It's in the spiral arms that the bright and massive stars live out their short lives. These stars can explode as supernova's creating pressure waves, of expanding bubbles of gas, and heavy elements to mix with the hydrogen and helium already in place to enrich these clouds. Perhaps the effect is two fold, to create and keep the spiral arms intact, and not wind up with time. And to create solar systems around newly formed stars of heavy elements to make planets with.
The mystery of the spiral arms is but one of many mysteries of the galaxy. Does a black hole lurk in the galaxy's center? Where is the hidden mass of the galaxy that effects the orbital velocity of the stars about it's center? Do later generation stars with heavier elements evolve differently than first generation stars we see in the galactic halo and globular star clusters?
The galaxy is wondrous to behold on a September evening, and full of mysteries waiting to be solved.
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