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The Stars, Like Leaves

Week of June 21, 1999

NOTE: (June 23, 1999) I found out that the estimate I made in this Snack is off, which changes the numbers but not the idea behind the topic. I've added a bit more about it at the end of the page below.


I said in last week's Snack that astronomy follows me everywhere. It's really true.

A week before the wedding mentioned last week, my wife and I were driving back home from Kansas. We were in West Virginia, a beautiful area with relatively tall mountains covered by green trees. I had been driving a long time (it's about 2000 kilometers from Kansas to my home) and was in the passenger seat slowly drifting off to sleep. From my vantage point, slumped in the chair, I saw a green blur of unidentifiable trees, passing me at about 100 kilometers an hour.

image of sycamore leaf We slowed for a turn and one magnificent tree came into view. It was covered with large lush leaves (I think it may have been a sycamore tree). Anyway, as my mind was slowly falling asleep, it made a leap of logic that can sometimes happen in the edge between consciousness and sleep. I saw the tree covered with leaves, and thought: if those leaves were stars, how many trees will we pass before I see as many leaves as there are stars in the Milky Way?

Although I may have been sliding down the path to Morpheus' arms, the question was not an idle one. We don't really know how many stars there are in our Galaxy, though we estimate there are perhaps as many as 400 billion (that's a 4 followed by 11 zeroes). I have a personal research project looking into finding dim red stars, stars that are extraordinarily difficult to detect because they are so faint. These stars may give us a handle on the way the Milky Way behaves; its structure, density and perhaps even how it formed. There may be billions of stars like the ones for which I am searching, stars that have never been seen before. If each one were a leaf, well, that sounds like a lot of leaves!

My brain leapt ahead. Guess that there are 1000 leaves per tree, and I can see about ten trees straight back as I look out the window and up the mountainside. That's 10,000 leaves at once. When I pass 100 rows, that's a million leaves. If they're, say, five meters apart I'll pass a million leaves in, um, let's see, about 20 seconds or so (I was tired and being very rough with my numbers). So in one minute I'll pass 3 million leaves.

That's quite a few! But hold on. Three million leaves a minute is 180 million an hour. But there are 400 billion stars in the Galaxy. I'd have to wait over 2000 hours to pass that many leaves. That's about three months! Yegads, three months at 100 kilometers an hour! It takes three days of nonstop driving to cross the United States, but it takes thirty times as long to see as many leaves as stars in the Galaxy. The Milky Way is huge!

I woke up pretty fast at that point, heart racing. Literally, I was shaken awake by that little bit of personal epiphany. Space is big, and we really aren't. It's pretty daunting sometimes to think about how much more we have to learn.

Most people are lucky to have such an epiphany once in their lives; I've had two. The first occurred many years ago, again while falling asleep. I had a sudden visceral realization of just how much energy is produced by the Sun, as if I were floating over the surface, soaked by the countless photons produced by that furnace, and was jolted awake as solidly and effectively as if someone had slapped me across the face.

I love astronomy. It provides a sense of perspective, and tells us a lot about our real place in the cosmos. It has a far better and more realistic affect on us than any manmade contrivance.

So the next time you're out driving, watching the scenery go by, imagine that you're actually traveling through space. You'll never complain about how long it takes to get where you're going again!


Oops! (June 23, 1999) After I received an email from my old friend Dan Durda I've had to rethink these numbers. He claimed 1000 leaves per tree is way too low. He even called a plant biology researcher at Cornell University to confirm it! Dan is a dedicated if somewhat wacky friend. Anyway, I too called Dr. Karl Niklas at Cornell, and he said that some trees can have upwards of a million leaves on them, but it's not uncommon for larger leafed trees to have much fewer. Sycamore leaves are indeed quite big (the tree where I grew up commonly had leaves 20 or more centimeters across), and Dr. Niklas agreed that an estimate of 1000 is probably too low, and 10000 is probably better (I'll say here that Dr. Niklas is not responsible for any errors I make here, including the estimate I'm using!). So instead of 90 days to pass as many leaves as stars in the Milky Way, it may only take 9. And it would be fewer if you look at, say, mature oak trees. Then you'd only need to pass a few hundred thousand trees. Still, that's a long time. The numbers are impressive: there are more stars in our Galaxy than we can name, let alone even see.
Yet another note: (July 2, 1999) I had the opportunity today to get a good long look at a sycamore in the peak of health. 10,000 leaves is certainly too few, and I bet 50,000 is more like it. Need I say it? There are still lots of stars in the Galaxy!



©2008 Phil Plait. All Rights Reserved.

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