by Mary Hrovat
When I was 17, I took an introductory course in physical geology at a community college. I was enchanted by the descriptions of the physical processes that created land forms, and also by the vocabulary: eskers and drumlins, barchan dunes, columnar basalt. I like to know how things form and what they’re called. My strongest memory of this class, though, centers on the final lecture. The professor put Earth and its landforms and minerals in a larger context. He told us about the life cycles of stars, which have produced most of the elements on Earth.
The central fact of the lecture was that the mass of a star is a key characteristic determining how long it exists and what happens as it ages. Stars are formed when gravity causes a portion of a gas cloud to collapse until its internal pressure, and thus its temperature, are high enough for nuclear fusion to begin. The energy released when, for example, two hydrogen nuclei fuse to form helium supports the mass of the star against the pull of gravity. A star’s life unfolds as a story of the equilibrium (or loss of equilibrium) between these two forces pulling inward and pushing outward. As one fuel source is depleted (for example, as hydrogen is converted to helium) other types of fusion occur in the core of the star using the new fuel source (and creating increasingly heavier elements). At the same time, hydrogen continues to fuse in a shell surrounding the core. Mature stars may have shells dominated by various elements undergoing different fusion reactions, although the available reactions depend on the mass of the star.
The professor probably described stars according to their type. I don’t remember if he mentioned the Hertzsprung–Russell diagram, although it seems likely that he would have. The HR diagram plots the luminosity of stars (the amount of light they emit) versus their temperature. When stars are plotted this way, most of them fall on a curve called the Main Sequence, which runs from hot blue stars to cool red stars along the sequence O-B-A-F-G-K-M. (In some HR diagrams, the stellar type or color is plotted on the horizontal axis as a proxy for temperature.) Stars remain on the Main Sequence as long as the gravitational and thermal forces are in equilibrium. The larger and hotter a star is, the shorter its time on the Main Sequence, because hotter stars consume their fuel more rapidly.
As they age and leave the Main Sequence, stars undergo different processes depending on their size. The universe is still too young for the very smallest stars to have exhausted their fuel, so they’re still on the Main Sequence. Stars with masses ranging from slightly less than that of the sun to 10 times the mass of the sun go through a red giant phase, ultimately undergoing core collapse and forming dense white dwarfs. Larger stars have more complicated end-of-life scenarios, typically exploding in supernovae and leaving behind superdense neutron stars or black holes. Some elements are created only in supernova explosions.
It was a thrill to learn, even in broad terms, how black holes form. I’m slightly embarrassed that I didn’t understand this until I was 17; these days, neutron stars and black holes feature in books for children in grade school. It’s worth noting, though, that Jocelyn Bell’s discovery of pulsars, and the subsequent realization that pulsars are neutron stars that are rotating rapidly, occurred only about 10 years before I took this class. Carl Sagan’s Cosmos: A Personal Voyage wasn’t released until 1980.
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It’s hard to guess how much I knew about astronomy going into that lecture, except to say that it wasn’t a lot, despite an interest in the subject. My mother had a book called Seeing Stars (the second edition, published in 1942), which I’d read as a child. It’s a lovely little book that begins with an illustration labeled “The Clock in the Sky,” which grabbed my attention immediately. The book describes the constellations visible in the Northern Hemisphere, giving their mythological backgrounds and observing notes about stars, star clusters, nebulae. It also has brief sections on the moon, the planets, meteors, and comets.
I had also read books from the elementary school library about astronomy, but they focused mainly on observing the moon, planets, and stars rather than on the physics of stars. I probably knew that there are red giants, and that some stars expand greatly as they age; I knew about Betelgeuse and Antares. But if I’d learned any of the physics of how stars work, I had forgotten it. I had almost no sense of deep time or the age of the universe.
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I was dazzled by what I learned in class that day, by the scale and intricacy of even that very basic description of stellar evolution. The story the instructor told proceeded so logically, from elemental things (matter, gravity, the simplest atoms) to a complex sequence of events that produced the context for everything that is. I think it was the first scientific explanation that completely captivated me. However, the new knowledge didn’t fit into my life in any outwardly meaningful way.
The only person I knew at the time who might have been interested in that lecture was the person I was dating, who had shown me Jupiter and its moons in his telescope on one of our early dates. However, I don’t remember whether we talked about it. It was a busy time, with finals week coming up and Christmas after that. The world crowded in on me.
I don’t think it even crossed my mind that I might want to learn more about stars by studying science; for one thing, I thought I wasn’t good at math. In fact, it was hard for me to picture a career for myself at all other than being a wife and mother. When I took that course, I hoped to get a bachelor’s degree in history. I was thinking of being a librarian with expertise in history, with the semi-conscious assumption that this would be my backup plan if I didn’t marry.
As it happened, the following summer I married the man I’d been dating. I gave birth to two children and focused on caring for them and the house we lived in; I stopped taking classes altogether for five years. What I had learned about the stars was a cherished part of my inner world, along with countless scraps of poetry and tidbits of history, and that was about it.
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Now, though, I can see how that knowledge shaped my later life. When I went back to school, after that first marriage ended, that lecture was one of the things that influenced my decision to study astrophysics. I’m not sure why I decided that I could handle the math after all, or at least that it was worth trying. (I wound up graduating with distinction, with a B.S. in astrophysics and a minor in mathematics.)
As an undergrad, I learned much more about the HR diagram and how it’s used by astronomers, as well as the underlying physics. Stellar evolution and stellar classification are much more complex than I had learned at 17, of course. I was impressed with the richness of detail that can be observed in stellar spectra and how much a spectrum can tell us about a star (for example, the density of its atmosphere and its rotation rate). The deductive process that led astronomers from these tiny details to information about a distant star fascinated me. I love compact representations of tremendous quantities of data, and the HR diagram remains one of my favorite examples.
When I was an undergrad, I took to drawing little HR diagrams—in the corner of a chalkboard, in the sand at the beach, in the snow. I drew just the two axes and a curve representing the Main Sequence, with perhaps an X, or a bit of twig, representing the sun. (I still draw the occasional HR diagram in the snow.)
I even met Margaret Russell Edmondson, the daughter of one of the astronomers who developed the HR diagram, Henry Norris Russell. She was married to Professor Frank Edmondson, who was an emeritus faculty member at Indiana University when I was a student.
As an undergrad, I spent a summer at a solar observatory as a research assistant. But I didn’t go on to grad school in astronomy, for various reasons, mainly financial. In the years that followed my graduation, I periodically attempted to bring my astronomical interests into my work life. These attempts usually began with an application to one graduate program or another and ended when I dropped out, usually for financial reasons.
One of my shorter stints in grad school was in astronomy. I considered writing a dissertation on a particular type of star. I can’t remember now what type, but I think the work would have involved the study of stellar spectra. So I might have wound up as a stellar spectroscopist, if I’d followed that path. It’s a pleasing thought, although I’m not sure that academia would have been a good home for me.
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Another attempt at grad school involved a Ph.D. program in the history and philosophy of science. Before I dropped out, I’d been planning to write a dissertation about the women who worked in astronomy around the turn of the 20th century. I was especially interested in a large classification project at Harvard College Observatory that took place in that period. Several women played key roles, not only in examining several hundred thousand stellar spectra and classifying the stars according to their spectral lines, but in understanding what underlying reality stellar types described.
Stellar types were originally assigned in alphabetical order and later rearranged into the OBAFGKM sequence on the basis of physics, but nothing I’d read had ever explained clearly how this was done, or what happened to all the missing types. I might have done the research and written a dissertation to tell that story. As a grad student in the history of science, I did write three encyclopedia articles on stellar evolution, but I never wrote a dissertation.
That regret has been more poignant than any regret I felt about not becoming a stellar spectroscopist. For Christmas 2016, a friend gave me Dava Sobel’s book The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars, which is about those women and the work they did. The title refers to the glass plates used to photograph the spectra they studied. Sobel wrote the story of how the OBAFGKM sequence was developed and provided many details about the lives and work of the women at Harvard College Observatory.
I enjoyed that book tremendously and learned so much from it. Still, my pleasure was mixed with wistfulness: Could I have written that story, either as a dissertation or as an independent research project? I wish I’d had the chance to try.
Well, as Regina Spektor wrote, “A heart can’t be helped, and it gathers regrets.” Even if financial means are available, time is always limited. My life is full of good things that I wouldn’t have had if I’d followed one of my other possible paths. I didn’t tell the story of that work at Harvard Observatory, but I have other stories to tell, and to live.
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Notes
The Regina Spektor quote is from her song “Firewood.”
At some point I found a copy of Seeing Stars, the book I read as a child. Looking at the book now, I see that Pluto is named, but very little is said about it. This makes sense, because Pluto was discovered only 12 years before the book was published. In addition, the Whirlpool Galaxy (in Ursa Major) is listed as the Whirlpool Nebula. The book was published only 17 years after Edwin Hubble had announced his discovery that some of the nebulae, notably the spiral nebulae like the Whirlpool, are not part of the Milky Way but in fact separate galaxies (“island universes,” in the parlance of the time) at great distances. It amazes me now, and thrills me a little, that the first book about astronomy that I remember reading was written that long ago. At the time, however, I had very little sense of even the most recent history of astronomy.
I wish I could name the professor who gave that wonderful lecture. However, I can’t remember his name. Yvonnie Bryant at Phoenix College kindly answered my request for help. Although there’s no record of who taught the class I was in, she told me that the only two geology instructors at the time were Gilbert Mills and Milford Benham. I’m grateful to whichever of them it was.
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You can see more of my work at MaryHrovat.com.
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