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One of the fun things about having written thousands (!!) of articles about astronomy over the past decade or two is going through old posts looking for relevant info. If I’m writing about a black hole, say, then it helps to link to older articles that have background info, saving me the trouble of writing it again.
Every now and again I’ll be writing about some particular object and then hit the archives to see what I’ve said about it before. It doesn’t happen often, but sometimes I’ll find… nothing. Meaning, I haven’t written about this particular object before. That’s fine; I can’t write about everything, but what’s weird is when it’s about some famous or particularly iconic astronomical object. A few years ago, for example, I was looking for stuff I had written about Proxima Centauri, the closest star to the Sun, and discovered I had never written an article devoted to it! That was weird, and now happily fixed (many times over).
So. A little while back I was writing about a bright quasar found in the distant Universe, and decided to drop a line in about the very first one ever identified, called 3C273 — the story of how it was discovered is really fun, with lots of weird coincidences combined with both human failings and incredible cleverness. I looked for an old article about it to link to, but I was shocked to see I had never written about it in detail. But I could have sworn I wrote about it…
Then I remembered something funny: I did write an extensive piece about 3C273 for my book Death from the Skies!, in the chapter about how galaxies can be a danger to life inside them. But, due to space requirements, I had to leave most of the story out!
I almost never give tips on writing — it’s too individual a practice, with some advice that’s great and some being anathema to others — but here’s one that I give without hesitation: Never throw anything out.
To wit: An early draft of that chapter sitting on my disk still had all the 3C273 backstory in it. It was never published anywhere. But now I can remedy that! It took a lot of editing to make it a standalone article, but here you go: How we found out that not every galaxy is as clement as the Milky Way. Not by a long shot.
[Update (Dec. 24 2020): When researching this story, I read a bunch of papers, books, and articles, and talked to some friends about it as well. To the best of my knowledge at the time, what I wrote was correct. But not long after posting this article I got a note from an astronomer saying Milton Humason and his team had actually found objects with higher redshifts (and are therefore further away from us) years before this in 1956! I never found this paper in my research, obviously. While I'm delighted to learn something new, this does cast a different light on some of the aspects of the story, like Schmidt's confusion on the spectrum. Clearly I have some more digging to do. One of the important aspects of the story is that the object looks like a star and nothing like it had ever been seen before, and what that meant to astronomy. So that's still cool.]
In 1962, astronomers had an enigma on their hands. Radio astronomy was coming into its own, and huge dishes were scanning the heavens looking for cosmic objects that emitted radio waves. Cambridge University sponsored several such surveys, numbering them 1C through 5C. In the third catalog – called, surprise, 3C – was an object in the constellation of Virgo. It was the 273rd object listed, so it became known as 3C273. It was fairly bright in radio, and variable, too — its brightness fluctuated on a scale of days. But while the radio telescope used to make the surveys was sensitive, its eyesight was somewhat fuzzy, and an exact location for 3C273 was impossible to determine (this same situation was faced just a few years later by astronomers observing gamma-ray bursts). Without an exact location, it wasn’t possible to look for the object using optical telescopes and find out if it were a star, a galaxy, or some more exotic object. The sky is full of stars, and thousands of objects were within the uncertainty of 3C273’s location.
But astronomer Cyril Hazard got an idea. Virgo is a constellation on the zodiac, which means that the Sun and planets appear to move through it… and so does the Moon. Hazard discovered that in 1962 the Moon would pass directly over the most likely position for 3C273. What Hazard realized is that he could point a radio telescope at the radio source, then wait for the Moon to cover it (what astronomers call an occultation). At that moment the radio emission would cease, and he could measure that exact time. Since the position of the Moon is very well known for any given time, that meant he could nail down the location of 3C273.
This was a brilliant idea, but ironically he missed the actual occultation because he took the wrong train! However, his team, well trained in the telescope’s use, was able to make the observation. It went well, and they found an optical object at 3C273’s position … but it was a bit of a shock. Sitting at that location was an unassuming blue star, about 1/600th as bright as the faintest star visible to the unaided eye. This was really weird— how could something so faint in visible light be so luminous in radio?
It is said that in astronomy, a picture is worth a thousand words, but a spectrum is worth a million. OK, I’m the only one who says that, but it’s still true: By taking the light of an object and splitting it up into thousands of individual colors, you can determine lots of physical characteristics of the object. Its temperature, velocity, chemical composition, magnetic field strength, whether it’s spinning or not and even how rapidly – all are revealed by a good spectrum.
Astronomer Maarten Schmidt knew this very well, and obtained a spectrum of 3C273 not long after the optical position was determined. What he found was, to be charitable, odd. It looked nothing at all like a star, a galaxy, or anything ever seen before. Schmidt puzzled over it, and then had a flash of insight many astronomers wait a lifetime to experience. He suddenly understood why the spectrum was so odd: It was hugely redshifted.
Just like sound waves can change pitch if the source is moving toward or away from you (via the Doppler shift), light can too. In this case, pitch = color. An object moving away from you has its wavelength stretched out, and we call that a redshift. If it’s heading toward you the wavelength is compressed: a blueshift*.
In the 1960s, most astronomical objects measured had relatively low shifts. Even something moving away from you at, say, a few hundred kilometers per second has a low redshift, changing its wavelength only a few percent.
Schmidt’s insight was that the spectrum of 3C273 was enormously redshifted, moved to longer wavelengths by a stunning 16%, the highest ever seen! He knew right away that this was a special object: Its great speed meant that it must be terribly far away.
Almost half a century before, astronomers discovered the Universe itself was expanding. This meant that galaxies that were farther away were moving away from us faster than ones closer in. This in turn meant they had higher redshifts. So, by measuring the redshift, the distance to an object could be found. Back then the numbers used weren’t as accurate as today’s, but the overall idea was correct.
Using this method, Schmidt realized that 3C273 was tremendously far away, farther away than anything ever before seen. Far from being an innocuously faint and nearby blue star, 3C273 must be the most luminous known object in the Universe. It was so far away that it had to be hellishly luminous to be seen at all!
3C273 was the first object of this kind to be identified, but several more were to follow (and in fact the very similar object 3C48 was actually found first, and even had an optical counterpart found, but it was too faint to analyze well enough to get a distance). These new celestial beasties were dubbed quasars, short for quasi-stellar radio sources (or sometimes QSOs for quasi-stellar objects). 3C273 is the nearest quasar, at a distance of a staggering 2 billion light years. It is truly a monster, emitting several trillion times the Sun’s energy, hundreds of times the total output of our whole Galaxy!
Eventually, of course, more were found. Optical telescopes, taking very deep exposures of quasars, found some have “fuzz” around them, a faint extended source of light. Eventually, as technology got better, astronomers figured out this fuzz was actually an entire galaxy, the light from which was dwarfed by the quasar phenomenon itself! More observations revealed more details — quasars and other types of so-called active galaxies had extremely bright and very small cores; their light could change brightness on very short time scales, implying the source of the light was small; they emitted light across the electromagnetic spectrum, from radio waves to gamma-rays. And whatever was powering them had a lot of energy at its disposal.
Only one object in the Universe can fit all those clues: a black hole.
And no ordinary black hole. It had to be a supermassive black hole. One with millions or even billions of times the Sun’s mass.
Now we know this in fact to be the case, and we also think almost every big galaxy in the Universe has one of these monsters in its heart. While the details are complex and fierce, all the different kinds of active galaxies we see, including quasars, are variations of the same type of object: A supermassive black hole greedily gobbling down material from its host galaxy.
The black hole at the center of 3C273 probably has a mass a million or more times the Sun, which ironically makes it a lightweight as such things go. It just happens to be actively feeding, making it extremely luminous. But far more luminous ones are known, some that positively dwarf 3C273.
We’ve come a long way since those first observations in the early 1960s. We now know of tens of thousands of active galaxies, and have learned vast amounts about them. They taught us about supermassive black holes, and that the birth and evolution of galaxies depend on them. We know the Milky Way has one, and that it profoundly affects the environment around it. We’ve used them to test relativity. And now we’ve even been able to take images of one!
Our Universe has grown considerably since that time, as has our understanding of it. But remember: This happened over a human lifetime. My lifetime; I can remember when articles were written about the mysterious quasars, speculating on what they might be. And now I can look back on those and chuckle; they were far, far weirder than anyone thought at the time.
Isn’t that always the way? The Universe is a pretty weird place, and it’s always throwing curveballs at us.
But oh my, isn’t that the fun of it?
* The words redshift and blueshift are not meant to mean that an object turns red or blue! We use them as adjectives here: “Red” in this case means longer wavelength, and “blue” shorter. An object might start off emitting ultraviolet light and get “blueshifted” to the X-ray, even though the wavelength is moving away from the literal blue part of the spectrum, just as an object may start off emitting red light and get “redshifted” to infrared.