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I
...but not through too many difficulties. It was not that way for the R.A.F., and its motto sounds tragic: "Through difficulties to the stars."
The key to the door that leads to the stars is a drawing, and a rather pretty one (more in the colorful version than in dismal black and white). They call it now the "H-R diagram".
Astronomy is unique in that in no other science does a single graph play such a dominant role. This makes the graph a good starting point.
The only other case that bears comparison to this one is the periodic table of chemistry, also a two-dimensional arrangement of elements ("elements" in both senses here). The analogy will not be taken any further since a table is not a mathematical device, even without the chairs around it. It doesn't even have to be a picture, with colors setting off the different groups --metals, nonmetals, rare earths-- and other frills, but a mere reticulate array of rows and columns. You can even put it down on paper with a typewriter and a little patience.
A time might come when everybody will know who Hertzsprung and Russell were, just as everybody is now taught who Mendeleiev was. (The present e-book is meant to be a contribution to that end.)
Hertzsprung was first to plot the graph, but he displayed it in a journal few people were in the habit of reading. That was very early in the 20th century. A few years later Russell came up with nearly the same graph on his own. This time there were many more people listening, so he got all the praise. It was even named the "Russell diagram". A quarter of a century went by and then someone realized what Hertzsprung had been up to. Ever since then it has been the "H-R diagram".
This is almost like Leibniz and Newton, and Gregor Mendel, and even Roger Bacon and the Chinese, if you care to go that far back. It is all about doing it for the very first time and then maybe having to argue about it, or people noticing long after you're dead and a-mouldering in your grave.
The usual thing is to start out by describing another analogy, and it has to do with demography, the quantitative study of human populations.
True, this other science is not even an in-law of astronomy, yet for years now we have been hearing about "populations " of stars: Population I and Population II stars. Now they are looking around for Population III stars. It seems like a hopeless quest. That is why these ancient stars, which might not be around anymore, but dead and stone cold, or blown up and recycled, are called the Holy Grail of astronomy, just as the Holy Grail of physics is the Unified Field Theory and the Holy Grail of space exploration is alien life. Cosmology also has a Holy Grail of its own, but not many people want to know about fine-scale anisotropies in the CMBR.
On the other hand, mentioning all the Holy Grails is not the usual thing, but it can prove to be useful as the story moves along ("unfolds" if you like).
So who shall find a good Pop III star? For its price is far above rubies (Biblical quote, somewhat out of context).
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