It’s all Relative
I spent the first half of January traveling. The first trip was to the American Astronomical Society meeting, where the fire hose of astronomy information was at once refreshing and overwhelming! The biggest story I took away from that meeting was the amazing plethora of planets out there! It seems that there are likely millions of them out there in the galaxy. That’s based on actual observations of planets around other stars and also of the circumstellar disks of material around many stars that will eventually result in new worlds.
The second trip was a private one to celebrate a relative’s significant birthday. On the second day of the partying, I sat in the backyard of another relative’s home, watching my niece play with a new puppy. The Sun was setting, and I got to thinking about how everything in the universe grows and changes. My niece is a toddler, but soon she’ll start school. She’ll think she’s “grown up”. In due time, she’ll be 15 or so, and she’ll start dating. She’ll think she’s “grown up”. Eventually she’ll do college, get a job. At each of those points, she’ll think she’s grown up. I have done those things, and yet, I still don’t think I’m “grown up” yet. Meaning, I’m not finished growing. And, I suspect if I asked the person whose birthday we were celebrating, “Are you grown up?” the answer might be a surprising “No.”
So, I was also thinking about the Sun and its evolutionary path. It began as a small clump of gas and dust, got bigger and bigger, and eventually got hot enough in its core to start fusing hydrogen to helium. Was it grown up then? Was it mature? What does “mature” mean for a star? The Sun is still doing nuclear fusion in its core, and will for at least a billion or so years more. When it stops, does THAT mean it’s mature? Or past mature?
What about galaxies? If you look far enough across space and back in time, you can see shreds of galaxies that are really the seeds of the galaxies we know today. They combined (through collision and interaction) to form these larger galaxies. Our own galaxy formed that way, and is still cannibalizing smaller galaxies today. The Milky Way is a spiral galaxy, but does that mean it’s mature? It turns out that when spirals interact, as the Milky Way and Andromeda will do in a few billion years, they end up as ellipticals. Does that mean ellipticals are mature? In some sense, we don’t have enough data and the universe hasn’t been around long enough for us to find the “end points” of galaxies. We can see the end points of stars — planetary nebulae and supernova remnants, for example — but what does a mature galaxy look like?
Humans are short-lived compared to stars and galaxies, so anything billions of years old (or even thousands of years old) looks mighty old to us. But, the universe itself is still young. It will continue expanding for quite some time to come. The future universe — like the future of an individual human being — is an unfolding story.