By David Ward
They say that on a clear day you can see forever, but on a clear night you can see even farther. Have you ever wondered how far you can see when you look up at the stars at night? It is almost impossible for us to comprehend the vast distances that stretch between us and the stars. Take a quick tour with me and we will try to grasp the immensity over our heads.
For starters, the Voyager I space probe is the most-distant man-made object from earth. It was launched in 1977 and is currently about 12 billion miles out. It is traveling at a speed of 38,000 miles per hour, which sounds like a pretty good clip, but it has yet to reach the nearest star. If it were headed in that direction, it would take it another 75,000 years.
We cannot see the nearest star from here. It lies in the southern skies and it is too dim to see without a telescope anyway.
Let’s take a look at what we can see. Find the three bright stars of the Summer Triangle high up in the western sky. The bright one on the lower right is Vega, a close neighbor of ours. Not right next door, but just down the block a bit, you might say. It is about 25 light years away, which means it has taken 25 years for its light to reach us. That may not mean a whole lot, but it would take the Voyager probe 450,000 years to get there. Maybe you are thinking that little probe would be too cramped to sit in for hundreds of thousands of years. How about something bigger, like an airliner? Only problem is that planes travel slower, and the journey to Vega would take almost 30 million years.
Wait a minute — we are talking about a close neighbor of ours. How about something really far out there? Look at another star in the Summer Triangle right above Vega. That one is called Deneb, and it is right at the head of the Northern Cross. That star is about the most-distant star that we can easily see and it is so far away that no one is really sure how far away it is. Some estimates put its distance at 3,000 light years. Do not even think about taking an airplane there. The trip would take billions of years.
Can you see the Milky Way going through the Northern Cross? That faint glow is actually billions of stars in one of the arms of our galaxy. The closest part of it to us is twice as far away as Deneb. Looking up at the Milky Way is in a sense traveling back in time to before the pyramids in Egypt were built. That’s how long that light has taken to get here.
Let’s take a step farther, a big step. Do you think you can see right out of our galaxy? If you are in Seattle, do not even bother. The bright lights will wash out everything faint up there. Get away from any distracting lights and make sure there is no moon out. Give your eyes a few minutes to adjust to the darkness and get ready to take the Andromeda challenge.
Look up in the northeast for the constellation that resembles a W. That is Cassiopeia, the Queen, and you can use it as a stepping stone to Andromeda. The top half of the W points roughly south, so let it guide you about the length of the entire W to a little fuzzy spot in the night sky. Do not expect it to be bright — it is a long ways away. If you see it, that is the most-distant thing you have ever seen. I saw it a couple of nights ago, but I had the advantage of being up at 6,000 feet in the Utah desert on a very clear night.
If you are wondering what you are looking at, that little patch of light is the Andromeda galaxy, comprised of hundreds of billions of stars, perhaps twice as many as our own Milky Way contains. The light from Andromeda has been traveling across a mind-numbing void of space for 2 1/2 million years to reach your eyes. That long ago on earth, our ancestors had not even figured out fire yet.
That is as far as I am going to take you on our naked eye tour. If you have a telescope, you can go much farther. How big is the whole shebang out there? What we can see with our powerful telescopes, which is often referred to as the visible universe, is far, far larger than our little trip into the universe.
Beyond that? Most of the people thinking about questions like this believe the stars and galaxies go on forever and ever without end. Now that makes for an intriguing mystery to ponder on a chilly autumn night.