Location! Location! Location! If you are buying or selling a piece of real estate, that is probably a major consideration. What about the biggest and most important real estate in our neighborhood, the earth itself? It turns out that location is very important on large scales too.
Imagine you are camping out on a cold night with just a fire to keep you warm. Too far away from the fire and you freeze, too close and you catch on fire. Guess where our earth is? We are just the right distance from our sun, that big campfire in the sky, so that water can remain in a liquid state on the surface.
We call this rather narrow area around the sun the habitable zone. Earth is known as a Goldilocks planet named after that little girl with blonde hair who had a habit of wandering around in the woods by herself and liked her porridge not too hot or too cold but just right. We are here on this planet because the temperature is just right.
It turns out our position in space going around the sun is not the only important location for our planet. There is also a habitable zone in our own Milky Way galaxy. We live in a vast pinwheel-shaped conglomeration of stars containing about 300 billion stars of which our sun is just one. Just like a pinwheel you would hold in your hand, the Milky Way has a central hub and spiral arms radiating out of that center. The scale, as you can imagine, is bigger.
Common sense might tell you that any place in that pinwheel would be fine for a star like our sun to make its home, but actually most of the galaxy is lethal. Those spiral arms are like giant traffic jams where stars get stuck and crowed together. There is a lot of scary stuff in there like exploding stars that could blast us right off our planet. We might find ourselves in close proximity to a strange star called a magnetar which have magnetic fields so intense they can hurl lethal radiation for vast distances that could cook us. I don’t want to get carried away here, but it is possible a star could come too close and jerk the earth right out of our habitable zone or hurl a barrage of comets at us like cosmic artillery. Actually that last one is going to happen to us in about a million years from now, but that is another story.
In the sweet spot
We are located in a substructures of stars known as the Orion Spur right between two of those spiral arms called the Perseus arm and the Sagittarius arm. At halfway between the center of the Milky Way and its outside edge our sun is going the same speed as the rotating arms of the galaxy.
Closer to the center we would be going faster and would fall into one of them. Farther out our speed would be slower and one of those arms would sweep us up. We find ourselves in a little sweet spot called the corotation circle where a star like our sun can hang out for billions of years allowing an incredibly rare planet like the earth to nurture that delicate commodity we call life. We are protected because of the scarcity of stars around us.
On a clear moonless night this summer be sure to spend some time out under the Milky Way. It is one of the grandest sights the human eye can see. It will look like a pale band of light stretching from north to south across the sky. High up overhead it passes through the constellation Cygnus the Swan also known as the Northern Cross.
That is the Perseus arm, the closest one to us. It is 6,000 light years away which means that glow has taken 6,000 years to reach us from the stars in that spiral arm. Be sure to thank your “lucky stars” that we are not up there in that lethal zone of our galaxy where we would probably not have had the chance to evolve much more than amoebas.