Book Review: A Classification System for All of Astronomy

Title: Classifying the Cosmos: How We Can Make Sense of the Celestial Landscape

Authors: Steven J. Dick.

Author Institutions: (currently none), Baruch S. Blumberg NASA/Library of Congress Chair in Astrobiology (2013 – 2014), NASA Chief Historian (2003 – 2009), US Naval Observatory astronomer (1979-2003).

Status: Published by Springer [closed access.]

Kingdom, phylum, class, order, family, genus, species.  Many high school students can recite the order of biological taxonomy from memory.  Its modern origin is traced to Carl Linnaeus in the 18th century but Aristotle and the ancient Egyptians are known to have devised early taxonomies as well.  Taxonomy in biology is successful for two main reasons: it has been widely adopted by the entire field and the classification itself reveals important distinctions between organisms.  That is, assigning taxonomy to a new species tells scientists how it relates to other species in the same genus, for example.

Other fields use taxonomy as well.  Particle physics uses the Standard Model and the US Department of Agriculture even established a system of soil taxonomy

Classification in Astronomy

Astronomers are no strangers to classification at the subfield level. One notable example is the Harvard (spectral type) and Yerkes (luminosity class) classification system for stars (the Sun is G2V in the combined system).  Stellar astronomers use this framework to understand every known class of stellar object.  These classification schemes are also prominent in planetary science, most notably in 2006 when they were used to designate Pluto as a dwarf planet. This helped clarify differences between planets and dwarf planets.

These are robust classification systems but nothing yet unifies them.  Steven Dick wants to change that. In his new book Classifying the Cosmos: How We Can Make Sense of the Celestial Landscape, Dick undertakes the monumental task of designing a classification system for all of astronomy.  He even goes a step further by incorporating existing subfield classification schemes.

In each object entry, Dick expertly employs his background as NASA Chief Historian and his decades of experience as an astronomer at the US Naval Observatory to provide detailed historical context and rigorous scientific explanation.  He calls his taxonomy the 3K or Three Kingdom System

Introducing the 3K System

The first goal of any taxonomy system, Dick explains, is that it be widely accepted and used.  He took care to choose names of the taxa themselves that would align somewhat with biology but also incorporate words astronomers already use for classification.  He settled on Kingdom, Family, Class, and Type. (To align with the book’s notation, this article capitalizes the names of the taxa used in the 3K system.)

At the top of the 3K system are the Kingdom of the Planets, the Kingdom of the Stars, and the Kingdom of the Galaxies.  The author argues that gravity, as the most important of the fundamental forces to astronomy, most naturally separates objects into these three categories.  This has been standard in astronomy for some time. 

Each of the three Kingdoms is divided into six Families.  This is where the magic of Dick’s classification emerges. One of the six Families is the main object in the Kingdom and the other five are named based on their relation to it.  For example, the Kingdom of the Planets contains the Planet Family, the Protoplanetary Family, the Circumplanetary Family, the Subplanetary Family, the Interplanetary Medium Family, and the Planetary Systems Family. 

The same bold sections (Proto-, Circum-, Inter-, Sub-, Systems) are applied to stars and galaxies for the remaining Families.  It is a beautifully simple and yet fully inclusive system. Few objects in the universe do not fall within one of the eighteen Families.

By using a parallel structure for each Kingdom, Dick invites comparison between them, even directly saying so in the book’s Introduction.  What he fails to mention, however, is the potential danger of false equivalency. For example, circumgalactic satellites may be situated around galaxies just as circumplanetary moons are situated around planets, but the physics that govern their interactions are significantly different.  An experienced astronomer will appreciate the full complexity of the subjects and see the classification only as a guide, but a non-expert may not realize the nuance. Like early models of the atom showing electrons orbiting the nucleus, these sorts of false equivalencies have the potential to spawn inaccurate conclusions.

The author, however, spends little time at the Family level, reserving most of the book to discuss each of his 82 Classes.  No longer following parallel structure, the Class divisions of each Family make use of existing classifications schemes. Distinguishing between Classes requires asking the question how are these objects physically different?  For example, Dick divides the Star Family into Classes following the Yerkes luminosity class system (the Roman numerals) instead of the Harvard spectral class system (OBAFGKM).  If he had used the Harvard system primarily, there would be a class of M stars including both low-mass main sequence stars and red supergiants. Those two types of objects have significantly different physical properties, making the Yerkes system more appropriate for Classes.  

The 82 Classes are further divided into Types.  Though Dick alludes to some well-known groups of objects that would make natural Types, such as supernovae, he does not attempt to create an exhaustive list.  Should the 3K system be adopted by astronomers, it will quickly fill up with Types from every subfield. Figure 1 shows a summary of the taxa, the motivations behind them, and an example.

Taxon Total Number Groups primarily divided by… Example
Kindom 3 gravity. Kingdom of the Stars
Family 18 gravitational relationship to the central object. Star Family
Class 82 object physical characteristics. An effort is made to retain existing groups. Dwarf
Type N/A subfield specifics, many of which are yet to be determined. Lower Dwarfs (low-mass)
Figure 1. Chart outlining the taxa of the 3K system, the primary separation between the groups, and an example. Adapted from the book.

How to Use the 3K System

The author’s extensive research and reliance of relevant literature are demonstrated in the care he takes in choosing designations between the taxa.  By linking each level to distinct astronomical properties, he creates a classification scheme with potential for widespread use. 

That being said, I find it hard to imagine this book will serve a purpose other than pedagogy.  Certainly, this in and of itself is a worthwhile enterprise. Students of astronomy, including Astrobites writers and many Astrobites readers, may find Classifying the Cosmos a useful companion to course studies.  The book is unparalleled in its breadth of astronomy and casual, approachable tone.  But anyone with even a moderate level of expertise in a topic will exceed the depth of this work, and Dick can hardly be faulted for that. 

The problem, as I see it, is that Classifying the Cosmos may be relegated between a lay audience—with a genuine interest in astronomy—that has no practical use for the 3K system, and a professional audience that is too specialized to find the classification relevant for their subfield.  Convincing the field to adopt standardized nomenclature, undoubtedly a venerable goal, will remain an uphill battle for some time. 

I encourage students especially and all those interested in astronomy to explore the 3K system. Hierarchy personally intrigues me because I find the exercise of organization better enables my understanding of context.  Next time I find myself writing an Astrobite on a new solar system object, I plan on referring to my copy of Classifying the Cosmos to ensure I describe it with the most appropriate term.  Next time I attend a talk on an object outside my subfield, I will use the 3K system to help me understand where it belongs in astronomy.  Dick’s approachable writing and extensive expertise makes that possible. 

About Will Saunders

I am a Ph.D. candidate at Boston University, where I study planetary atmospheres. My dissertation research involves using new and archival stellar occultations to measure the upper atmospheres of Uranus and Neptune. My work aims to better understand how the atmospheres of the ice giants are heated to hundreds of degrees. I received my Bachelors in Physics from the University of Pennsylvania. Be sure to check out astro[sound]bites, the only podcast combining Astrobites posts with lighthearted discussion about the latest astronomy research. Find us on, Apple Podcasts, Google Play, SoundCloud, and Spotify. In my free time, I enjoy cycling, exploring New England, and trying new wines.

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  1. Thank you for this review. I am always on the lookout for good astronomy texts!

  2. Thanks for your informative review. The goal is indeed venerable, and laudable too. However, and I don’t mean to belittle the initiative, to rapidly gain real traction for its ideas, this book would need to be a small fraction of its current price. That’s not the only key to success of course, but it’s certainly one of them.



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