This feature is inspired by the questions we have received over the years from interested readers. If you have a question about an astronomical topic, please forward it to us.

In this issue, Dr. Wm. Bruce Weaver answers (or discusses, perhaps) a question that is asked dozens of times at any given star party.

Q: How far away is it?

A: This is probably the most frequently asked question when we're showing folks something through the eyepiece of a telescope. It is a perfectly reasonable question. And, for several reasons, it is surprisingly hard to answer.

In the first place, with the advent of quantum mechanics in the 20th Century, astronomers turned from positional astronomy (where is it on the sky?) to trying to understand the underlying physics of celestial objects (what is its true nature?). Now where it was on the sky had become much less important than the information derived from its spectra1.

How far away it is, however, becomes very important because that information, combined with its apparent brightness, tells astronomers how intrinsically bright it is. For example, quasars appear as insignificant faint blue objects. Once you know that they are half way across the Universe from the Earth, you suddenly realize that not only are they the intrinsically brightest things in the Universe but are more powerful per cubic inch than an exploding thermonuclear device!

But the focus is on the nature of the objects. For planets, stars, nebulae, and all the galaxies that look like galaxies, how far away they are from the Earth is just a matter of where we happen to be. If we learned anything from Copernicus, it was that there is nothing astronomically special about where we live. So most professional astronomers don't really care much about how far away it is except when they are trying to calculate intrinsic brightness.

In the second place, it is very, very difficult to measure how far away it is. The distance scales change so dramatically as we move from the Earth-Sun distance to the distance to the nearest star (about 200,000 times), or from the nearest star to nearby clusters of stars (hundreds to thousands of times), or from the nearest star to features of the Milky Way (thousands of times), that different techniques must be bootstrapped one after the other to come up with these distances.

By the time the distance of a distant galaxy is published in a handbook, seven or more different techniques had to be layered on top of each other to arrive at the answer. What professional astronomers know is that each of these techniques is difficult to do accurately, so each has a substantial error. When several are bootstrapped on top of each other the errors accumulate rapidly. We know that the distance listed in the book, even though it may represent hundreds or thousands of hours of painstaking research, is not very accurate.

But we are now in the 21st Century and the focus will soon change again. Encouraged by the success of recent astrometric2 satellites, unmanned space missions are now planned that will be able to make direct measurements of the distance to objects throughout our Galaxy, skipping over several of these rather inaccurate intermediate measurements. These measurements will lift a centuries-old fog from our mapping of nearby stars, clusters, nebulae, and the structure of spiral arms of our galaxy. There are already some hints, from the first astrometry missions, that there may be some rude surprises about what it really is when we find out exactly how far away it is.

Notes

1. There are still some branches of astronomy - for example, the study of the distribution of different types of stars in our Milky Way Galaxy - where the distribution of celestial objects on the sky remains important.
2. Astrometry is the astronomical science of measuring the positions of objects on the sky.

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