I follow quite a few scientists (mostly astronomers, of course) on the social media app Bluesky, and I wind up learning a lot of interesting stuff from them. Physgal, for example, is a university physics lecturer, and recently reposted a link to the New Humanist site, which had an article about astronomer Betty Louise Webster.

I read it with interest, because I had never heard of her, despite her being one of the two astronomers who discovered the first ever black hole known.

I mean, what. How did I not know this?

The story itself is pretty cool. In the 1960s, scientists launched sounding rockets — suborbital flights that essentially went up and then back down again — equipped with X-ray detectors, essentially fancy Geiger counters to see if astronomical sources made these high-energy X-rays. It found many! Then, in 1970 NASA launched a much more refined X-ray satellite called Uhuru (Swahili for “freedom”). It was able to nail down the direction to these cosmic sources far better. 

Two astronomers, Paul Murdin and Betty Webster, looked at the Uhuru data for one source, called Cygnus X-1 (the first X-ray source found in the constellation Cygnus), and found it was essentially coincident on the sky with a single star, a blue supergiant called HD 226868. That’s a very massive star, 20 – 40 times the mass of the Sun, making it fiercely hot and luminous. However, it doesn’t have anywhere near the wherewithal to make X-rays like the ones seen.

In 1972 they made follow-up observations on the star. They took spectra, breaking up the light of the star into many individual wavelengths (or colors). This can tell you a lot about a star, including its temperature, chemical composition, and in this particular case, its velocity. If the star is moving toward or away from Earth there will be a Doppler shift in the spectrum, a small but measurable shift in the wavelengths of its light.

They found a periodic shift for HD 226868, meaning sometimes it was blueshifted and sometimes red, in a recognizable and repeatable cycle, with a 5.6-day period. What could cause that? They argued against the star being a pulsating variable, which literally expands and contracts, because the period didn’t fit that explanation. Instead, they assumed the star is in orbit around another object. In general it’s possible to measure the actual Doppler shift and determine the orbital shape of the star, as well as the mass of the other object.

What they found though was baffling. The second object had about half the mass of the supergiant star, so at least ten times the Sun’s mass (using other arguments they found the absolute minimum mass was twice solar). A star like that is bright and should be easily detectable, but there was no sign of its contribution to the system’s light output.

How could this be? They were able to figure out the X-rays seen by the early sounding rocket and Uhuru were coming from this second object, but it was otherwise invisible. Given its mass, and the fact that they couldn’t see it, they made a bold assertion: “it is inevitable that we should also speculate that it might be a black hole.”

They were right. We now know that Cygnus X-1 is indeed a black hole, the first one ever found. Obviously, that’s a very big deal. It emits X-rays as material from the supergiant star is pulled off by the fierce gravity, and falls into an accretion disk that swirls around the black hole. This disk is the source of the X-rays, as well as other flavors of light including radio waves.

Now, I’ve heard of Paul Murdin; he’s a prolific researcher and has written several popular science books (which is how I’m familiar with him). Webster, however, is a different story. As the article in New Humanist tells, she had health problems, and apparently was modest about her achievements. Also, not so shockingly, she faced an uphill battle against the sexism of the time. She died young, at age 49, of liver cancer. Today it seems not many heave heard of her.

Astronomy is still rife with sexism even now, but things have improved since then. Many female astronomers are getting their due, including historic ones. Modern astronomy was created on the shoulders of many women, whose brilliance pushed the science forward, and today many of their names are far better known (I’m happy to say that several on that linked list are friends of mine, and are amazing scientists).

But that’s still not true for all. Betty Louise Webster deserves to have her name known as well, and I’m glad that article was written, and that Physgal pointed it out on Bluesky. I’m happy to keep that momentum going here.

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