“How old would you be if you didn’t know how old you are?” -Satchel Paige
Today marks another year and another trip around the Sun for me. For you, and me, and everything on Earth that makes it through another year on this world, there’s a whole lot we get to experience.
Some things are tiny: the Earth’s rotation slows by about two millionths of a second each year, while some are large: we hurtle over 900 million kilometers in outer space as we orbit around the Sun. Our Earth spins just over 366 times on its axis, while our one revolution around the Sun leaves us with 365 (and an occasional 366) days in a year.
But looking beyond the Earth, practically everything you can imagine in the Universe has undergone a lot in the last year you’ve been on Earth.
Every satellite we’ve put up, including the vaunted International Space Station, races around the Earth as we go about our daily lives. The low-Earth-orbit ones (like the ISS) race around the Earth at speeds around 17,000 miles-per-hour (27,000 km/hr), meaning that over the course of a year, they orbit the Earth around 6,000 times.
But while our Solar System is full of interesting facts and motions, everything I’ve mentioned and much, much more hurtles around the galaxy — imperceptibly — as the years unfold.
The entire Solar System speeds through the galaxy, traveling 7 billion kilometers each year in our Sun’s journey through the galaxy’s disk. But life goes on in the galaxy over that time, as gas clouds contract, forming new stars all the time.
around a million new stars about a Solar Mass’s worth of stars (reference here) form in the galaxy, mostly in star forming regions like the Eagle Nebula, above. At the same time, stars in our galaxy burn out and die, but not quite at the same rate.
Stars die spectacularly throughout the galaxy, but the vast majority of stars we form have a lifetime that’s far greater than the present age of the Universe. As a result — even optimistically — only dozens of stars die each millenium in our Milky Way, meaning that each year that passes marks an extra new star or so in every galaxy in the Universe.
And those galaxies, themselves, are different on this day than they were even a year ago.
The nearest large cluster of galaxies — the Virgo Cluster, above — has around 1,600 large, Milky Way-sized (or larger) galaxies in it, and lives just 53 million light years away. It’s also speeding away from us at over 1,200 kilometers per second, meaning that each year that goes by sees that entire cluster wind up another 40 billion kilometers farther away from us.
In fact, if you lived to be 229, you would find this cluster an extra light year distant from us than when you were born. But that’s for the closest cluster of galaxies to us; what about the farthest galaxy we’ve ever discovered?
The light from this galaxy — found in the Hubble Ultra Deep Field — took 13.1 billion years, or 96% of the age of the Universe, to reach us. But the Universe is expanding so quickly that each year that passes sees this galaxy more than two extra light years distant from us! Even if we launched a spaceship today at 99.999999999%+ the speed of light at this galaxy, it would never reach it. As each year passes, maybe 20 or so galaxies newly suffer this fate: that they become unreachable, even in theory, from our place in the Universe.
So happy birthday to me and everyone who shares another year on the planet with me, and may the Universe continue to amaze and educate us all.