Astronomical Distances (One of the most difficult things to understand, so here's a little primer)
Fundamental to even the most basic understanding of the universe is a grasp of the vast scales of distance and size beyond the boundaries of our own small world. The transformation of the two dimensional sky to the three dimensions we know today is a great tribute to human ingenuity. The birth of this understanding began in 200BC when Eratosthenes, using basic geometric principles, calculated the circumference of the earth to within 1% accuracy. It wasn't until the 18th and 19th centuries that accumulated knowledge and observation unveiled the distance scales of our solar system. Again using the known diameter of the earth as a baseline, basic triangulation provided the means of calculating distances to the planets and closest stars. This technique is called parallax and is still used today for calculating distances out to about 150 light years.
Distances of deep sky objects (objects beyond our solar system) are measured in light years and cannot be calculated using parallax technique. A light year is the distance light travels in one year or about 5 trillion miles, or 63,241 times the distance between the earth and sun. Knowing that light can circle the earth 7 times in one second is indeed a humbling fact that begins to give the reader an idea of how truly far a light year is. These simple facts should allow for a greater appreciation of the enormous distances to even nearby deep sky objects in our own galaxy.
Many of the stars we see in the sky at night are within 100 light years of earth. The closest star to earth is alpha centuri at 4 light years. Common nebulae and star clusters within our galaxy reside several hundred to several thousand light years away. The furthest objects in our galaxy within our telescopic reach are a few tens of thousands of light years distant. The Milky Way, our parent galaxy, stretches 100,000 light years across and contains some 200 billion suns. Traveling at conventional spacecraft speeds would require over 1 billion years to traverse the galaxy!
The next step up in distance is the intergalactic scale. The closest galaxy similar to our own is the Andromeda Galaxy at 2.5 million light years. When we observe the Andromeda Galaxy through the eyepiece of a telescope we are presented a scene from the distant past. Andromeda's light began its journey toward earth approximately 2.5 million years ago, at an epoch corresponding to the dawn of human existence. Observing or photographing deep sky objects gives us an amazing opportunity to gaze back in time through a cosmic time machine. Viewed as they existed many thousands or even millions of years ago the objects remain frozen in time for us. Perhaps there is no other concept in astronomy as humbling or awe inspiring as this notion of "look-back" time. With telescope and camera we can now explore objects at far greater distances than ever before which in turn provides us with views of our universe at increasingly earlier epochs. These humbling concepts drive home the bewildering scales of intergalactic distances.
Distances to relatively nearby objects can be calculated using the principle of parallax in which an objects movement in the sky over a course of time can be used to determine its exact distance from earth. Distances to more remote objects require a "standard candle" by which astronomers can use to estimate the vast distances to objects outside our galaxy. Certain types of variable stars with fixed luminosity-brightness relationships (Cepheid Variable stars) are of paramount importance for estimating distances to galaxies within 100 million light years. Beyond 100 million light years individual stars cannot be resolved so other means of estimating distance such as bright supernovae or recessional velocities (since our universe is expanding, a galaxies recessional velocity will increase in proportion to its distance from us) are the current methods for determining distances to very remote galaxies. One of the goals of the Hubble Space Telescope and other ongoing astronomical projects is the refinement of methods to determine the distance scales of the universe.
The universe as we know it today is composed of many billions of galaxies, each one possessing countless stars and nebulae. Many of the objects in the pages ahead, especially the galaxies, are at vast distances, often exceeding our ability to easily comprehend. It may help to visualize earthly events that were occurring when these objects released their ancient photons. For example, the Virgo Galaxy Cluster released the light we see today at an epoch of time coinciding with the extinction of the dinosaurs and the rise of the mammals and primates, our ancient ancestors. (Courtesy, Robert Gendler)
© 2006 Graphically Speaking
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