Saturday, January 23, 2016

How Old is the Milky Way?

This is a good question, because it doesn’t have a good answer. When you ask how old something is, you are asking about when was it formed. Things like trees and pigs and houses and cars and iPhones all were made over a short time, short in comparison with their age, after they accumulated some. A tree might be said to be started when the seed that formed it sprouted, or maybe the first root was put down, or the first leaf formed. These three events could be the time zero for the tree. They were inevitably the source of the whole tree. The same thing can be said of discrete plants and animals.

If you look at a blob of moss growing on the side of a tree, and ask how old it is, you might want to know when the first cell of moss landed on the tree, or when it had adhered to the bark. If you ask about how old a house is, you might want to know when the permit for construction was first pulled, or when the final inspection was done, or when someone first moved in, or when utilities were first connected. They are often about the same time, so knowing one gives you a good idea of the other.

If you see a rock on the ground, and ask how old it is, you have a slightly different problem. There are more than one thing that happened to put the rock there. That’s why people don’t often ask how old individual rocks are. It’s obvious the question has no good answer. One answer might be when did some magma cool down to solid temperatures. Another might be when did the magma block thrust itself above the surface. Another might be when the particular rock you see before you was exposed through weathering. Another might be when did a piece of rock containing the one you see fracture completely from the original block. Another might be when did the rock fragment down to the size you see, and that is fuzzy as it might have lost small pieces in the course of its rock life.

So, ages are appropriate for things which have some events which are all about the same time, with the spread of times being much less than the possible duration of the object. The rock you see doesn’t have an age, nor does the breath of air in your lungs, nor do the elements which make up your body. These things have either continuous processes forming them, or multiple events leading to their current state. What about the Milky Way?

Nowadays, when an astronomer talks about the age of the Milky Way, they are answering a slightly different question: “What is the age of the oldest stars in the galaxy?” Suppose you happen to be on an imaginary planet orbiting the oldest star in the galaxy, right after it first lights up with some fusion, and you just happen to have an equivalent of the Keck telescope in front of you. What do you see? Nothing much. Nothing else formed around you yet. So, this is the birth of the galaxy, and your star starts to pull in more gas and make the rest of the galaxy.

Or maybe you wait a while, like a few hundred million years, watching all that time even though it is as boring as can be, and you see another star light up, maybe some fifty thousand light years away. Well, neighbors have moved in! So you keep watching that star, and you notice it is doing a better job than your star in attracting gas, as it is in a denser clump of gas. More stars light up around it, and more and more and more and more. After a while, your star begins to respond to the call of gravity, and starts to move toward the group of stars already formed. You will join them in a billion years, should you care to wait for it.

Oops… Do you remember noting in your dairy that the galaxy was born on the day fusion started in your star. That may not have been correct. You were in the boondocks, and all the action was going to take place fifty thousand light years away. Your star didn’t start anything. It did the planet you were on, but not much else. So, how old is the Milky Way really?

Should the Milky Way’s age be determined by when some other event happened? Perhaps the formation of a black hole in Sagittarius, or the first spiral wave, or the bulge, or the millionth star, or something else that more knowledgeable people would list should be the trigger event for denominating the age of the Milky Way?

Even if your star was right in the middle of what is now the Milky Way, and by the way that is hard to figure as its always moving and shifting around, is the start of fusion in that first star the right thing to use for the age of the galaxy? Age in some exact sense depends on what you want to do with the number. If you are trying to qualify for buying some liquor, it doesn’t make much difference when you were conceived, or whether that was seven months before your birthdate or nine months. You might actually be one or two months organically older or younger than someone else with the same birthdate on their passport, but it doesn’t help you get the booze. So, what good is the age of the Milky Way?

One thing it is good for is bragging rights. Since we have a star in our galaxy which has an age, as we measure it, almost as old as the age of the Universe Itself, as we measure that, we can say that we are in one of the oldest galaxies in that universe. Other civilizations in other galaxies, who also can search and search and find a really old star can say the same thing.

Is there a better rule to use for defining the age of a galaxy? For uses in this blog, it is only of incidental concern what the oldest star is that either was created in the area of the galaxy or fell into it. If we want to calculate how many alien civilizations there are, who might be visiting us, we want to know how old the stars are as a whole. When did serious numbers of stars come into existence and light up? Maybe we should consider the millionth star starting to fuze as the time zero of a galaxy. Then we can get some more accuracy with our wild guesses as to alien populations.

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