By Les Johnson
The Hubble Deep Field, showing about 3000 previously unknown galaxies. (Image courtesy of NASA, R. Williams and The Hubble Deep Field Team (STScI).)
For ten days in 1995, the Hubble Space Telescope pointed its mirror to a small patch of seemingly empty sky near the Big Dipper and started collecting light. (“Seemingly empty” means that no stars or galaxies were at that time known to be in that particular piece of the sky.) The part of the sky being imaged was no larger than the apparent size of a tennis ball viewed from across a football field. It was a very small portion of the sky. What they found was awe-inspiring. Within that small patch of nothingness was far more than nothing. The image revealed about three thousand previously unseen galaxies, creating one of the most famous of Hubble’s images and my personal favorite. The sky is not only full of stars but also of galaxies and they are very, very far away.
One of my favorite daydreams is also one of my scariest. When I am outside on a clear, cloudless night, I like to imagine that I am on a spaceship in the deep between the stars, looking out at the vastness of the universe. During this daydream, I often fondly recall my favorite science fictional spaceships – the Enterprise (Star Trek Classic, of course!), the Drusus (from the pulpish German language serial Perry Rhodan), or the Nostromo (Alien) – and wonder what it would be like to be truly in the middle of deep space, far from Earth and our familiar solar system. My thoughts alternating between the wonder of it all and the terrifying thought of what it would be like to be stranded there, so far from home.
I’ve told some of my friends and colleagues about this daydream and many have asked, “Why do you place yourself in interstellar space and not just somewhere in our solar system between the planets? After all, those distances are so large that you would probably experience the same thing there!” Intellectually I know they are right. Buy hey, it’s my daydream and if that’s how I can get to the stars, then so be it! The distances between planets are enormous and, given our current technological capabilities, if I were to be stuck there it would look and feel much the same as if I were stuck between the stars Alpha Centauri and Epsilon Eridani. In both cases, the stars would be my only companions within the void.
That says a lot actually. Space is big. Huge. Incomprehensibly large. It is so large that I contend none of us can really understand how big it really is. Most Earthly distances are experiential – we can go from here to there and experience traveling the miles between. Save for a very few, this is not so for space. I believe that even those who have been to space don’t really comprehend the distances they’ve traveled and they are just as clueless as the rest of us when it comes to what lies beyond.
In the 21st Century, those of modest means can purchase airline tickets for a reasonable price and travel virtually anywhere on the globe. Catching a flight from my adopted hometown of Madison, Alabama to Tokyo last year cost about $1000 with a total in-the-air flight time of sixteen hours. The distance between Madison and Tokyo is approximately nine thousand miles, which means my effective speed was about five hundred and seventy-five miles per hour. For someone used to traveling by car at seventy miles per hour, that’s pretty fast. And yet it still took sixteen hours to get from here to there. To fly around the globe would take, at that speed, over forty hours.
In 1969, Neil Armstrong, Buzz Aldrin and Michael Collins made the trip from the Earth to the Moon in three days using rockets. If they’d been traveling by the airplane I used to get to Japan, knowing full well that airplanes cannot fly through space, then it would have taken over four hundred hours – or seventeen days – to make the journey.1 The rockets used in Project Apollo reached speeds of over twenty-four thousand miles per hour. This staggering speed would allow me to travel the 9000 miles from Madison to Tokyo in a mere twenty-two minutes. By anyone’s measure, that’s fast. Unfortunately, to travel within our solar system that’s not fast enough.
At about the same time Armstrong was walking on the Moon, Dr. Werner Von Braun, the father of America’s space program, met with President Nixon and unveiled his plans for sending people to Mars within the next decade. Using rockets larger than the Saturn V, the round-trip voyage to Mars would nonetheless have required approximately three years to complete. Granted, this included some time actually at Mars, but the majority of the astronauts’ time away from home would have been spent traveling between worlds, not exploring. The situation really hasn’t changed since then; we’re still using chemical rockets and even if we made the leap to using rockets powered by nuclear fission, the trip time would remain about the same – we’d only reduce the number of rockets required to be launched from Earth to space, most carrying rocket propellant, in order to complete the voyage. And Mars, while not the closest planet to the Earth, is still not very far away when compared to its planetary siblings. We are not even close to having the technology required to send humans to visit the other planets in our solar system.
Because of their much smaller size, we have been able to send robotic explorers outward to every other planet in the solar system. Their voyages have taken years.
To make the distance between worlds in our solar system easier to discuss, scientists devised a new unit of measurement called the “Astronomical Unit,” or AU. One AU is ninety-three million miles – the distance between the Earth and the Sun. On this scale, Jupiter is about four AU from the Earth, Neptune is about twenty-nine and Mars (the planet it would take us three years to explore) is only one-half AU away! If we were talking about distances in our more-familiar miles, then Neptune would be two billion, six-hundred-ninety-seven million miles away. I challenge anyone to tell me they can really comprehend such a vast distance.
Beyond Neptune lies the former-planet, Pluto and a legion of its icy rock cousins in the Kuiper Belt – all are still well within the solar system, orbiting Sol, our sun. The very edge of Sol’s influence, and hence the acknowledged outer boundary of our solar system, is known as the heliopause. It is at this point that the sun’s outward radiation pressure, the light streaming forth from the sun’s nuclear processes, is balanced by the light coming in from all other stars in the galaxy combined. The heliopause is thought to be about two-hundred-fifty AU’s away. That’s two-hundred-fifty times ninety-three million miles.
On this scale, the nearest star system – Alpha Centauri – is about two-hundred fifty-thousand AU’s more distant from Earth than the heliopause! Once again, our units of measurement fail us. Fortunately, nature provided us with a pretty cool way to measure distances as large as this: the Light Year (LY). A LY is the distance light, in a vacuum, will travel in one year’s time. On this scale, Alpha Centauri’s 4.2 LY distance makes it sound like it is pretty close. Until we remember that a LY is really 63,239 AU or 5,878,000,000,000 miles. Are you, like me, starting to feel pretty small?
Before we go farther into deep space, it might be useful to find a way to at least visualize the comparative distances discussed so far. If you imagine that a one AU distance can be shrunk down to be one inch, then the Earth would be orbiting the Sun at a distance of one inch. Mars would be orbiting the Sun at one and a half inches, Jupiter at five inches, and Pluto at thirty-nine inches. The heliopause would be about two hundred fifty inches – twenty feet – away. On this scale, the distance to Alpha Centauri would be about four miles. That's a lot of inches.
Okay. Now we can discuss and visualize the distances to some pretty cool places like Barnard’s Star (5.9 LY) or Wolf 359 (7.8 LY) or Epsilon Eridani (10.5 LY) and not trip over ourselves reciting the zeros. Figure 1 shows the relative position and distance to several nearby stars.
Figure 1 shows the stars nearest our sun and within about ten Light Years. (Image courtesy of Richard Powell and atlasoftheuniverse.com.) http://www.atlasoftheuniverse.com/12lys.html
These are the stars closest to us. But what about the really interesting places that may not be so close? Our home galaxy, the Milky Way, has over two hundred billion stars and is about one hundred thousand LY in diameter. Since we are embedded within it, we cannot take a picture of our own galaxy from above. Based on what we can see, we know that it looks a lot like our neighboring galaxy, Andromeda, shown in Figure 2. For the record, the Andromeda Galaxy, a close neighbor in galactic terms, is about two and a half million LY away. The light that left Andromeda and was captured by the camera that took the photograph in the figure had been traveling through space at 186,000 miles per second for over two-and-a-half million years.
Figure 2 shows the Andromeda galaxy, which is very similar in structure to our own. The distance across our galaxy is approximately 100,000 light years. (Image courtesy of Mike Herbaut & the ESA/ESO/NASA Photoshop FITS Liberator.)
The daydream I mentioned at the beginning of this article first occurred after I read a story in the Perry Rhodan series in which our hero finds himself in contact with an ancient and highly-advanced technological civilization. In order for them to show him their capabilities, they send him and his interstellar spaceship – the pride of a growing Earth-based spacefaring civilization encompassing stars out to about one hundred LY from Earth – into the void between the Milky Way and Andromeda galaxies and strand him there for several days. That image of being so far away from home, where only the light of galaxies, not stars, can be seen in the mostly-dark sky, haunts and captivates me still today. They, of course, bring Rhodan back home so that he can have many more adventures in deep space.
I’m feeling small again... If the universe were composed of just the Milky Way, as was thought to be the case only about one hundred years ago, then our place within it would now be understood. Alas, the universe is far larger still.
The next step outward into the universe can only be accomplished using our telescopes. And as we gaze outward, shock of shocks, we discover that there are more than just a few galaxies. There are billions of galaxies out there, each containing billions of stars. They are not evenly distributed. In fact, they seem to be grouped together in distinct sets. The set in which we find ourselves is called the Local Group and it is comprised of about thirty galaxies with our Milky Way galaxy roughly at its center. The diameter of the Local Group is approximately ten million LY. That’s 10,000,000 LY.
The Local Group of galaxies is a small member of the Local Supercluster, which has a diameter of one-hundred-ten million LY. The best visualization I’ve seen for understanding our place in The Local Supercluster was created by Andrew Colvin and is shown in Figure 3.
Figure 3 traces our place in the solar system out to The Local Supercluster. (Image courtesy of Andrew Colvin.) http://upload.wikimedia.org/wikipedia/commons/archive/a/a7/20100606032423!Universe_Reference_Map_(Location)_001.jpeg
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And now for something completely different and very, very strange: In the 1980s, astronomers noticed that our galaxy and all of the galaxies in the Local Group are being pulled toward something we cannot see at over one million miles per hour. The something toward which Andromeda, our galactic cousins and we are being pulled is called, “The Great Attractor.” My advice is to try not to think about it...
Zooming yet further out, we can see the distribution of superclusters surrounding our home supercluster, also known as the Virgo Supercluster (Figure 4). Remember that each of the dots making up this illustration represents a galaxy, not an individual star. Don’t forget that each galaxy contains hundreds of billions of stars.
Figure 4 shows the distribution of superclusters surrounding our own, The Virgo Supercluster, centered in the middle of the sphere of observation. Please note that we are placed in the center of the sphere because we are making the image by looking out in all directions – not because we are actually at the center of anything in particular. (Image courtesy of Richard Powell and atlasoftheuniverse.com.) http://www.atlasoftheuniverse.com/superc.html
The known universe is comprised of multiple galactic superclusters separated by ... well, essentially nothing, creating immense structures. Early on, astronomers assumed that these structures were pretty much uniformly distributed in the universe. After all, what could possibly cause structure on such a large scale? This notion was shattered when the Great Wall, a group of galaxies five hundred million LY long, two hundred million LY wide and fifteen LY thick was discovered. (See Figure 5.) Within a few years, another “wall” of galaxies was found and called the Sloan Great Wall.
Figure 5 shows the distribution of galaxies as seen from Earth. The figure shows the galaxy groupings, which, when looking from the top left to corner of the image, forms what appears to be a wall -- the "Great Wall." (Image copyright by Mario Juric and J. Richard Gott, Department of Astrophysical Sciences, Princeton University.)
For those still trying to put their arms around the distances involved, the Great Wall is over one billion LY away from Earth.
We’re finally near the end and close to knowing, if not really grasping, the size of it all. Based on very recent findings from many deep space missions, including the Hubble and Chandra Space Telescopes, the Wilkinson Anisotropy Probe, and others, we have learned the universe is about ninety-three billion light years in diameter.2 Just for perspective, let me convert that to miles just so we can at least say we’ve done it: 550,000,000,000,000,000,000,000 miles.
Now, back to my daydream – where was I? Oh yes, somewhere near that Great Attractor...
End Notes
1 For the sake of simplicity, I am ignoring the fact that the distance one must travel between objects in space is actually much greater than the straight line distance between them. The path you follow is curved; and everything is moving. The Apollo spacecraft was directed toward a point in space where the Moon would be when it, the Moon, and the spacecraft, arrived. The Moon may be two hundred forty thousand miles away, but to get there you have to travel a distance greater than that due to the curved path that physics demands spacecraft follow.
2 For some, this number may not make sense. After all, haven’t we learned that the approximate age of the universe is only 13.7 billion years and, if we are limited by the speed of light, shouldn’t the universe be 13.7 billion LY in diameter? That would be true if the universe weren’t expanding. But it is expanding, rapidly, and that means that the actual distance (“space time”) through which the light travels is actually much larger than it would be if it weren’t expanding. Think of running across a bridge while someone is stretching the bridge. You may be running at a constant rate (the speed of light) but the distance you cover once you’ve crossed the bridge is larger than it was when you first began.
FOR MORE INFORMATION:
JPL has created an elegant interactive map of our galaxy that is worth checking out: http://planetquest.jpl.nasa.gov/SIMGuide2Galaxy_508.html
The Scale of the Universe is an excellent interactive tool that lets the user zoom out from our experiential frame of reference here on the surface of the Earth to the ends of the universe: http://primaxstudio.com/stuff/scale_of_universe/
There’s the classic short film, “Powers of Ten” that begins at a Chicago picnic and every ten seconds takes us an order of magnitude father away into we are well into deep space: http://www.youtube.com/watch?v=0fKBhvDjuy0
The updated version of “Powers of Ten” that takes into account what we’ve learned since the original was made in 1977: http://www.youtube.com/watch?v=aPm3QVKlBJg
The End
Copyright © 2011 by Les Johnson