On planet Earth, we sit at the edge of the Milky Way Galaxy, nestled thousands of light years from the black hole at its center, in the Orion–Cygnus Arm. In our textbooks we see beautiful images of our own celestial neighborhood, a spiral of stars emanating from a galactic core.
But if we've never sent a camera outside the solar system, how do we have pictures of the Milky Way Galaxy?
Viewing the spiral arms
We can see part of the Milky Way every time we look up at the stars – every star visible with the naked eye is within the Milky Way Galaxy. However, under the right conditions, the galactic center of the Milky Way can be seen as a diffuse band streaking across the sky. This faint band has been observed for centuries, and the streaky nature lent the adjective "milky" to the galaxy's name.
Earth lies about 27,000 light-years from the galactic center in the midst of a galaxy 1,000 light-years thick. From this position, buried within the Orion–Cygnus Arm, a top-down view of the Milky Way – the beautiful image of the galactic core with arms spiraling out – is impossible to take from the Earth. Voyager 1, launched in 1977, is only now at the edge of the solar system after thirty-five years of travel. Even if we did have a camera positioned outside of the Milky Way Galaxy, it would take thousands of years to receive image data.
Here is the fantasy crushing fact: every top-down image of the Milky Way you have seen is an artist's rendering.
These renditions, however, are not without merit. By studying the density of neutral and ionized hydrogen as well as the rotational motion of stars, we know that the Milky Way is a barrel spiral galaxy. We can also take top-down images of nearby galaxies, like the Andromeda Galaxy (M31), NGC 3344 , and the Whirlpool Galaxy (M51). These images help inform the mesmerizing top-down reconstructions of the Milky Way Galaxy we often see. The renditions are likely a very good approximation of how our galaxy looks from the outside, but they are not photographs.
What parts of the Milky Way can be photographed?
What we can photograph and reconstruct are images of the Milky Way Galaxy from the side. From Earth, we can see the the galactic center of the Milky Way from the side. Think of this side view of the galaxy like plate held flat in front of your eye – you can make out the bulge at the center and the edges, but it's impossible to see any design that is on the eating surface.
By taking a number of images, a panorama of the edge of the Milk Way Galaxy can be constructed. Since Earth is a significant distance away from the center, each band of the Milky Way seen in the night sky represents roughly 30° of the entire galaxy. One of the best is the one created by the GigaGalaxy Zoom project and the European Southern Observatory, with the final edge-on image giving the viewer an impression of the Milky Way as a disc.
Take your own picture of a streaking Milky Way
You can actually photograph the streak of Milky Way visible in the night sky with a digital SLR camera. The portion of the Milky Way visible is often very faint, with the presence of the moon's light enough to completely obscure it. If you are in the United States, the band is difficult to see east of the Mississippi River thanks to the number of dense cities that light up the sky. The galactic center rises and sets just like the sun and moon, so there is a limited viewing time. However, if you find yourself in the middle of a desert on an overnight trip with the right equipment, you have the opportunity for the shot of a lifetime.