If you’re hoping to read about Britney and Nicole, or Burt and Loni, or That Woman Who’s Famous But No One’s Sure Why, turn back now!

OK, if you’re still here, this is about Cosmos Episode Nine. This episode has to have my very favorite first line ever. “If you wish to make an apple pie from scratch, you must first create the universe.” Sagan then shows how the universe has evolved to its present state through the lives and deaths of the stars.

Science is all about making connections, finding simplifications. The periodic table, featured in this episode, is an elegant example of this idea. The fundamentals of the periodic table were understood before the structure was explained. There was obviously an elegant pattern hidden there, but what was the source of the pattern? The answer came when JJ Thomson, Ernest Rutherford, and James Chadwick, each a student of the one before, discovered, in turn, the electron, the proton, and the neutron. Those three particles, put together in various logical combinations, explained everything. “Chemistry,” Sagan says, “is just numbers.” A beautiful thought.

I love how Sagan links the evolution of the stars to our own existence here on Earth, and I turned to this topic many times in  my own book (shameless plug alert).

Sometimes, though, science teachers try to make connections that hide, rather than bring out, connections. I remember arguing with a college education professor that the evolution of stars (the main topic of this episode of Cosmos) is fundamentally different from the evolution of life. My argument, poorly formulated at the time, was that there was some fundamental difference between the chemical and physical changes happening in a star and the complex interactions of striving and struggling living things. I remember her saying, “Well, then, you don’t understand evolution,” and of course that got my ire up. I didn’t have the words then to explain what I meant, but I think I do now.

The fundamental difference is this: in the evolution of stars (like the evolution of planets, the evolution of the elements, etc.) why questions have exactly one answer. In the evolution of life, why questions have two answers.

For instance, if we ask why Betelgeuse is a red giant, the answer is that Betelgeuse has used up nearly all the hydrogen in its core. When the hydrogen ran low, the star started to collapse. This increased the temperature in the outer shell, causing hydrogen there to fuse into helium. This fusing hydrogen pushed the outer layers of the star out, bloating it hugely larger than before, and cooling off the now faraway surface so that the color of the star shifted toward the red. That’s the whole answer. It might be a complex answer, but it is the entire answer.

If we ask why an apple is red, on the other hand, we have two answers. One might be useful and important for one purpose, while the other is useful and important for another. An apple is red because it absorbs all colors but red – or it reflects a set of colors that when added together give red. If we wanted, we could isolate the pigments in the apple, discover the chemistry of those pigments, even reproduce them.


But if that’s the only answer we go after, we’re missing a big part of the story, perhaps the more interesting part (depending on just what we want to know.) Apples are red because trees evolved in a world in which animals can distinguish color. It is better for trees if animals eat their apples and take the seeds away to be “deposited” later somewhere else. Red apples tell us a lot about trees. Trees compete with nearby trees for light and water. Trees that can spread their seeds over large areas survive better than trees that deposit their seeds right near their trunks. Trees devote energy to apples, not to be nice to apple-hungry animals, but to spread their seeds, and so they want to advertise these juicy morsels using a color very different from the greens and browns that come from mere utility.

Red apples also tell us a lot about animals. If we were alien visitors to the Earth, and the first thing we saw was an apple tree, we could immediately deduce that there must be color-detecting, mobile organisms here that digest food and dispose of the leftover bits. We could know that not all living things on this planet harvest sunlight to make their food, but instead there is some level of parasitism on this planet. We could know that at least some plants have evolved to make use of those plant parasites (what we call animals) to help in their own reproduction. That’s a lot to know from just seeing a red apple hanging from a tree.

And that’s what I wish I’d said to my education professor. Yes, analogies are powerful, but sometimes differences are even more important and informative than similarities.