Welcome to the page for my July 19 365 Days of Astronomy podcast.  In this episode I wanted to do something a little different. I wanted to take listeners back through their lineage as citizens of the cosmos. To do that, I began with the continents of our planet Earth.

As most folks who have studied geology and Earth’s history know,  the continents we know today aren’t the same ones that Earth had a few hundred million years ago. At several points in the past, our planet had only one giant continent, called Pangaea. But it wasn’t even the first continent — there were probably others before that.

But, Pangaea is usually considered the first big one.  Over time, that continent broke up and the pieces drifted apart as Earth’s tectonic plates moved them around. This is called “continental drift” and is a pretty well-accepted mechanism to explain how we get the continents we see today.

Plate tectonics is active on Earth, and scientists are still working to understand why we see it here but don’t see it at Mars, for example.  Something happened to Mars, or maybe it never had extensive plates as our planet does.  But, for Earth, plate motions and continental drift are accepted facts, and there is no question that they affect life (and have done so throughout Earth’s history).  As part of the life on this planet, our ancestral species (going back to the earliest life forms) no doubt were affected by the changing positions of the continents over time.

Our next step out is to the stars (since as citizens of Earth we are also citizens of the solar system).  The stuff of our bodies comes from the stars — and the study of astrobiology tries to trace the path between the deaths of stars and the rise of life on Earth.  But, the chemical elements of stars DO course through our veins and in our lungs, and so we can say we are citizens of the stars.  The stars themselves are part of the Milky Way Galaxy, which is the stellar city in which we live.  But, the Milky Way does not travel the universe alone — it’s part of the Local Group of galaxies, as well as the Virgo Cluster (a larger collection of galaxies), which is itself part of the Virgo Supercluster.  And, there are superclusters as far as we can see — back to nearly the beginning of time.  The first galaxies began being born not long after the Big Bang, which was the birth point of the universe we live in. As I mentioned in the podcast, the Big Bang created all the hydrogen in the universe. There were also traces of helium and lithium, but that was about it. The rest of the elements that make up the second and later generations of stars, the planets, the nebulae, our bodies… all came from stars, through a process called nucleosynthesis.

So, we are citizens of Earth, the solar system, the Milky Way Galaxy, the Local Group, the Virgo cluster and supercluster, and ultimately, the cosmos — through a lineage that stretches back some 13.7 billion years!  We aren’t necessarily the ultimate endpoint of that lineage — it could be there’ll be further evolution and development of life. So, we aren’t at the top of the tree of life — but we ARE a self-aware collection of cells and neurons.  Welcome to the cosmos, fellow citizens!