Aug 6, 2008

A Milky Way Primer

It's not just a candy bar!


Marshall Space Flight Center

Back to the Science@NASA story "The Fading Milky Way"

What is that gorgeous band of light...the Milky Way?

It stands out as noticeably brighter than the rest of the nighttime sky for at least a couple of reasons. First, the stars are more numerous here, and from within its borders, a warm iridescent glow emanates even from the empty void between the stars. But exactly what is the Milky Way and why is it so stunningly different from the other parts of the summer sky?

To answer that question we must first understand that our sun is one of about 200 billion other suns that make-up a group of stars called a galaxy. Viewed from above or below, this collection of stars and gas looks something like a gigantic pinwheel, with huge arms of stars and gases spiraling out from the center. Viewed edge-on, it looks a bit like two fried eggs placed back-to-back. This is our home galaxy, and it is so huge that light takes about 80,000 years just to travel from one side to the other. Our sun and its retinue of planets reside in one of these spiral arms about two-thirds, or 30,000 light years out from the center. Because the stars are more closely packed together in the center of the galaxy, this part looks brighter. Even here, however, the stars are trillions of miles apart. (The closet star to the earth besides the sun is 4.3 light years. You may know that a light year is the distance light can go in a whole year, travelling at 186,000 miles a second).

What we call the Milky Way is in reality a nickname of sorts for the view we get of our own galaxy by looking at it from the inside out.

Consider this analogy: Imagine you could miniaturize yourself so that you could get INSIDE the glass of a half-inch thick windowpane. Suppose that as the glass was being made someone sprinkled millions of grains of sand all throughout its length, width and depth. Now looking from inside the glass, you gaze, not toward the outside, but down the length of the glass itself. You would likely see a multitude of sand grains stretching from your location to the edge of the pane. Some grains would be very close to you, some, an intermediate distance and others would be quite far away. But all the grains would be visible at the same time because they would all be in the same line of sight...the same plane.

That scenario approximates the situation we earthlings find ourselves in as we look at the Milky Way. Just as the tiny observer who gets inside the glass can see the many grains of sands at varying depths when looking down the entire length of the window, so are we able to see the many stars inside our own galaxy as we look down its length. Some stars may be only 10-15 light years away, while others may be hundreds or thousands, but they all appear to lie in the same general direction, one behind the other, stretching across the summer sky to the edge of the galaxy.

The Milky Way is truly a view of our own home galaxy from the inside out.

Back to the Science@NASA story "The Fading Milky Way"

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