On dark, clear summer nights we see a dusty trail crossing the sky as a background to the stars. It’s called the Milky Way. The Milky Way is a galaxy, made up of hundreds of billions of stars - each one a sun. The ones in the dusty trail are so far away and there are so many of them that, instead of seeing them as individual points of light, we see their combined light as a faded band in the dark sky. Look at it with binoculars and you begin seeing the individual stars that make up the band.There are countless numbers of other galaxies in the universe. However, the Milky Way is special because it is our home galaxy. Our Sun and solar system are in it. So are all the other individual stars we see with the unaided eye. How can it be that we see it as a far-away band of light, yet we are in it? Why do we see some stars in the Milky Way as individual points of light and others as part of this faded band?
Think of the Milky Way as a round, "deep dish" pizza, that is about ten times as wide as it is thick. Think of our sun as one of the billions of atoms that makes up the pizza. In this analogy our solar system - the Sun, Earth and other planets - would be located about 2/3 of the way out from the center toward the edge of the pizza.
If we were to look out in the direction of the top or bottom of the pizza (through the thickness), or even out toward the near edges, there would be a lot less pizza to look through. The atoms we would see in these directions are relatively close, so we would see them mostly as individual atoms. However if we look toward the center of the pizza, and to the far edges left and right of center, we would be looking in a direction in which there are a lot more pizza atoms, because they extend out further away from us. From our line of sight these atoms would appear a lot denser. Some of these would be close enough to see as individual atoms, but most would be too far away to see as individuals. Most would become part of a background band that stretches across our line of sight.
Using the pizza and its atoms as an example think back to the Milky Way and its billions of stars.
In the late winter and early spring, during evening and early night hours, Earth faces predominantly in the direction of the near edges of our galaxy and the directions out its top and bottom. We see less stars. In the summer and early fall, during evening and early night hours, we face in the direction of the center of our galaxy, its nucleus, and toward the far edges beyond the center. We see more stars, visually more closely packed together, many of them further away than those of winter.
Stretching across the sky, we see a dusty trail made up of the combined light of the billions of stars that share the same galaxy we are in, but are too far away to see as individual points of light. We call this dusty trail the Milky Way, the same as the name of the galaxy it is part of.
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