If you live in a rural area, as I do, outside of the influence of city lights, you can often get a clear view of the night sky. I don’t normally make a special effort to look at the night sky, but on occasion I am drawn to it. I usually notice the stars when I return home on a clear evening. As I get out of my car, and before I enter the house, the night sky often grabs my attention. I pause for a moment, and try to absorb the enormity of it all. There are no words that come to mind, no thoughts, or even a sense of time. I find it difficult to focus on any particular star or any region of the sky. It’s as if I am staring into infinity—it really is an awesome sight.
The feeling of wonder that one gets when looking at the night sky is as much about the sheer amount of space, as it is about the stars that occupy that space. However, the experience doesn’t even begin to encapsulate the actual size of the universe. The size of the universe is difficult to grasp, as there is no experience in everyday life that can relate to the numbers that are required to measure the universe. The measurements of time and distance, along with the number of stars and galaxies are hard to get your head around. Nevertheless, I will try to put it in some kind of perspective.
We can all relate to a thousand, so let’s begin there. Imagine having one thousand dollars. We can do that without too much trouble, but as the numbers get larger and larger, it may not be quite as intuitive. Millions, billions and even trillions can begin to sound alike, as if there isn’t much difference between them, but there is a huge difference. One thousand, a thousand times is a million. One million, a thousand times is a billion. And one billion a thousand times is a trillion. That’s a lot of money. But it is stars and galaxies that concern us at this time. So keep these numbers in mind as we move forward.
How big is the universe? The fact is that scientists don’t know, and here is why. Light travels at 300,000 km per second, which is the fastest speed in the universe. We can never hope to see a galaxy that is farther away in light travel time than the universe is old—the light emitted hasn’t had the time to reach us yet. This cosmic speed limit prevents us from seeing anything that is farther away from us than 13.7 billion light years (the age of the universe is 13.7 billion years).
Now here is where it gets a little tricky. The most distant galaxies we can actually see are about 10 to 12 billion light years away, however, we are seeing the light that was emitted 10 to 12 billion years ago. Keep in mind a light year is a measure of distance—the distance that light travels in one year. We know that the universe is expanding. Galaxies are moving away from each other on average. Those galaxies are presently much farther away than 10 to 12 billion light years. We know at least that much. That being said, scientists can still estimate the actual size of the universe by factoring in the expansion rate since the birth of the universe.
Estimates for the rate of expansion can vary widely, and are debatable. If some of the larger estimates are taken into account, much of the light emitted from the universe will not reach us until the sun and earth have died out. To put these distances into perspective, it takes only 8.3 minutes for the sun’s light to reach the earth. If the size of the earth is used to represent the entire cosmos, the part we could see, even with the best telescopes available, would be less than a grain of sand. Wow! Although it is possible that these larger estimates are wrong, even some much more conservative estimates would still reveal a cosmos that is unimaginably large. As vast as our universe might be, we can’t rule out the possibility that there could be other universes—perhaps an infinite number of universes. The possibilities are mind boggling, but before we get carried away, let’s get back to what we know.
The speed of light and the expansion rate of the universe give us an idea of distances. Now let’s take a different perspective and look at content: the number of planets, stars and galaxies. The earth and our solar system are a small part of the Milky Way galaxy, which could be described as a stellar disk about 100 thousand light years in diameter. Our sun is located about 1/2 to 2/3 away from the center of the Milky Way. Galaxies are plentiful, as there are well over 100 billion galaxies in the observable universe alone. In an image known as the Hubble Deep Field, the Hubble Space Telescope was focused on a dark spot in the sky for a period of ten days. The spot was about the size of the opening of a drinking straw, and it covered only two parts in a million of the whole sky. In this very tiny spot 10 thousand galaxies were observed.
When numbers get significantly large they start to run together and become difficult to digest. That’s where analogies can be helpful, and when it comes to the total number of galaxies in the universe we almost need something we can visualize. How much is 100 billion galaxies? If galaxies were scaled down to the size of frozen peas, they would fill the old Boston Garden (this has actually been computed). For those of you who are not sports fans, the old Boston Garden is where the Celtics and Bruins previously played professional basketball and hockey respectively. If you don’t like peas, let’s try hamburgers. If we used hamburgers to represent galaxies, and lined them up end to end, there would be enough burgers to circle the earth fifty-two times. That’s not all. You would still have enough burgers left over to stack them to the moon and back. You may think that’s a lot of peas, burgers or galaxies. But hold on to your hats, we’re just getting started.
Galaxies are not individual objects, but vast groupings of stars. The amount of stars contained in galaxies varies by a large extent. The Milky Way contains at least 200 billion stars. The nearby Andromeda Galaxy—relatively speaking, about 2.5 million light years away from earth—is much larger than the Milky Way, and contains 1 trillion stars. From there, the numbers can get even bigger; the largest galaxy ever discovered consists of 100 trillion stars. Once again, only analogies can put these kinds of numbers into perspective; however, the sheer number of stars is so staggering that even an analogy is somewhat limiting. According to the 2010 NOVA (PBS) documentary Hunting the Edge of Space, there are more stars in the observable universe than grains of sand on all the beaches and all the deserts on earth. Yes, that’s not a misprint—all the beaches and all the deserts on earth. As difficult as that is to grasp, there is more. Imagine if you can, how many planets could be orbiting these stars—and of course you probably can’t. Out of the unimaginable number of possible planets (hundreds have already been discovered), how many of them may be able to support life? The potential is truly enormous.
I have omitted one important fact in all of this, and that is the vast amount of space that separates galaxies. Typical galaxies are usually separated by millions of light years of space, and due to the expansion of the universe the space between galaxies is increasing. Everything we can see, stars, galaxies and clusters of galaxies, make up only a tiny fraction of the entire universe. Although scientists are discovering that space may not be empty after all, in the conventional sense we could say that the universe is dominated by empty space.
When I look up at the night sky, in a way, it is the emptiness that is striking, emptiness sprinkled with twinkling yellow dots. And speaking of dots, one is suddenly reminded of just how insignificant the earth seems to be. In the immense scale of the cosmos, we make our home on a pale blue dot in an ocean of tranquility. Everything we treasure, everyone we love, our hopes and dreams, and all of human history has transpired on what is essentially a dot. And most people spend their entire lives on only a fraction of a dot. With the number of stars out there, I wonder if somewhere in a far away galaxy, someone else is also contemplating a similar situation. Due to the distances that are involved, we may never know for sure. But I think it is likely that there is intelligent life somewhere else in the universe. After all, the basic chemistry and physics is believed to be essentially the same throughout the universe. And given the number of planets that likely exist, the opportunities for life to evolve seem plentiful. Nevertheless, in the grand scheme of things, the earth appears to be a small and lonely place, but it is all that we have—our only home.
References: 2010 NOVA (PBS) Hunting the Edge of Space