With some regularity, I get asked the difference between astronomy and astrophysics. I suppose I鈥檓 a good person to ask: Of my three degrees, two of them are in 鈥渁stronomy and astrophysics鈥�. But what this merger means is that even we 鈥渁stronomers and astrophysicists鈥� have given up telling the difference between the two. How does astronomy fit into physics? Well, my position is that astronomy is now essentially an area of physics, but I know some people will be unhappy I said that!
What鈥檚 interesting here isn鈥檛 how emotional astro folks get about their sometimes tense relationship with physics, but the idea that there are neat categories in the first place. There is some logic to it: astronomy is a much older area of research than physics. People have been looking at the heavens and trying to explain them for a long time.
In Muslim communities, this had practical importance: figuring out accurate prayer times depended on careful timing predictions, including sunrises and sunsets. The Muslim astronomers tasked with these calculations learned an extraordinary amount about the heavens, and their observations set the stage for the scientific revolution for which Europeans are often solely credited.
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This is a reminder of how curiosity can take us in brand-new directions. These observations are also one reason that during the last 500 years, a consistent theory emerged connecting astronomical observations with the study of moving objects on Earth. What we now call Newtonian physics linked astronomy and physics, laying the groundwork for the transition of astronomy into the field of astrophysics.
One of the lessons that I take from this history is that categories are rarely hardwired into the universe and are a lot more about what we know at any given time. In the early 20th century, for example, Annie Jump Cannon catalogued stars based on their visible characteristics. In this system, the letters O-B-A-F-G-K-M are each assigned to a category of stars that have a certain temperature and colour. To remember these letters in order, most people my age had a mnemonic: 鈥淥h be a fine guy, kiss me.鈥� Around the time I started my master鈥檚, our instructors were inserting 鈥済al鈥� instead of guy.
There aren't fixed boundaries between one colour and another, just a continuous shifting of the colours
These days, people assign their students with coming up with their own. University of Louisville professor Benne Holwerda shared a few with me that his students had created. My favourite was, 鈥淥nly bad astronomers forget generally known mnemonics.鈥�
What鈥檚 funny about this is that these stellar categories are actually fairly incomplete. When they were created in the early 1900s, they were based on what is, by today鈥檚 standards, a fairly rudimentary understanding of stars. Back then they didn鈥檛 even know what made stars burn so brightly. Only after the advent of the nuclear era did scientists start to understand that fusion is the engine that makes stars visible to us. To understand stars required applying known physics to observations of distant objects: this is the nature of modern astrophysics.
There is a lesson in here about method in science. Although the scientific method is typically discussed like it is a fixed set of practices, how we conduct science changes with time. This includes categories, which necessarily must shift as we learn new information. It used to make sense to categorise stars solely by colour because we didn鈥檛 know what the colours meant. Today, we know the colours give us information about a star鈥檚 composition and its place in cosmic history. If scientists had known that 100 years ago, they might have constructed different categories to organise their knowledge of the stars.
Importantly, colours lie on a spectrum. There aren鈥檛 fixed boundaries between one colour and another, just a continuous shifting of the colours as we move from one part of the spectrum to the other. So any categorisation based on colour imposes boundaries that don鈥檛 physically exist, even if we find them helpful for organising information. The history of stellar categorisation is a reminder not to confuse method with fact. Our methods are there to help us establish and organise facts, but they are not synonymous with facts.
There are many examples of this across science. For instance, notions about how sexual organs develop in humans have tended to reflect social beliefs that sex is a fixed binary. Today, biology tells us that human embryos don鈥檛 experience organogenesis 鈥� organ development 鈥� until several weeks into existence. In other words, human embryos have no sex at conception. This is a scientific fact that defies traditional categorisation methods. Our norms will have to shift accordingly.
Chanda鈥檚 week
What I鈥檓 reading
I contributed an essay to I Know What the Red Clay Looks Like: The Vision & Voice of Black Women Writers, and I鈥檓 enjoying the interviews in it.
What I鈥檓 watching
While on bedrest for a week, I marathoned the show Younger. Miriam Shor is great!
What I鈥檓 working on
I鈥檓 teaching a new course this semester, so the new material and students are keeping me on my toes.
Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women鈥檚 studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, and dreams deferred
