The massive Hubble portrait of the Crab Nebula, my favourite deep-sky object, has decorated my computer desktop since 2005. Recently I stumbled across some amazing historical tidbits about the Crab on the World Wide Web. The highlight was a you-are-there website describing the thrilling discovery of the neutron star at the Crab’s core. Thinking about the advance of observing technology over the years covered by the websites leads me to pose an observing challenge to those among you who track the pulsations of variable stars.
In many ways, the first item on Monsieur Charles Messier’s list of fuzzy non-comets is the most peculiar. His 1774 list includes many galaxies, globular clusters, and planetary nebulae, but M1 is none of these. The smudge in Taurus received the nickname “Crab Nebula” a century later from Lord Rosse, who saw filaments like crab’s legs extending from the central mass. It looked strange enough, but how strange it actually was became apparent only in the 20th century.
With the gradual understanding of the mechanism of supernovae came the conception of a supernova remnant, or SNR. The debris remaining after a star explodes can take many forms, and naturally it changes over time. Changes in the Crab’s shape were evident over a few decades, with the filaments appearing to be expanding from the centre. Based on the expansion rate, astronomers estimated the supernova occurred almost a millennium ago. Old Chinese records indicate that the Crab Nebula is the remnant of a star that exploded in 1054 -- or at least that’s when the light from the explosion reached Earth. Since then, the puff left behind has been expanding and evolving.
Our understanding of SNRs has also been evolving, thanks to both theoretical advances (especially general relativity) and observations at wavelengths outside the visible range. Observations made using early radio telescopes linked the Crab to a bright radio source. Both X-ray and gamma imaging confirmed there was a compact, energetic enigma there. The hot, bluish 16th-magnitude star named Baade’s Star or CM Tauri at the heart of the Crab was found to have a peculiar light spectrum, but seemed too faint to be the source of all the electromagnetic energy observed. Then, not long after the announcement in early 1968 of the accidental discovery of pulsars, the compact source was found to emit radio pulses at a rate of thirty times a second. That was convincing evidence that the core of the Crab Nebula is a rapidly rotating neutron star, a bizarre object as massive as the Sun but only a dozen kilometers in diameter. As it rotates, emissions from its magnetic poles sweep across the sky, with the radio waves flashing past the Earth like a searchlight beam.
Astronomers immediately wondered if the visible light from the neutron star at the Crab’s core was also flashing. The discovery of the optical pulsations of CM Tauri is an exciting chapter in 1960s astronomy, and one that’s extremely well documented. Thanks to the physicist Philip Morrison and the American Institute of Physics, the website http://www.aip.org/history/mod/pulsar/pulsar1/01.html
contains descriptions, interviews, images, and audio from the very moment of discovery. It is a fascinating look at how real science advances, by a combination of enthusiasm, knowledge, risk-taking, and a sense of adventure. John Cocke and Michael Disney, two young theoretical astronomers, found themselves at the University of Arizona’s Steward Observatory on Kitt Peak late in 1968. They needed experience at actually using a telescope and somehow realized that they could put together all the pieces they needed to attempt this challenging observation. Variable stars don’t get much more variable than this. How do you detect a star flashing 30 times a second? You need fast imaging instrumentation, plus electronics to compile data in special ways over an extended period, plus a decent-sized telescope, of course. (The one they used was a 36-incher.)
With electronics expert Don Taylor and night assistant Bob McCallister, the astronomers booked a few nights on the scope and set up to collect data. Even pointing the telescope was a challenge, since the target star was too faint to observe visually. Three nights of observations produced no pulsations, and then cloudy nights scrubbed the next sessions. After some head-scratching they realized they had miscalculated the expected pulse period: they had used the wrong formula to correct for the Doppler effect caused by the earth’s revolution around the sun. A last-minute cancellation on January 15, 1969 gave them access to the telescope once again. The website puts us right in the room with them as they began their observing run that night. What makes the setting so real is that they had a tape recorder running while making their observations, so they could keep track of the data they were collecting. Their voices on the tape reveal a mix of frustration, delight, incredulity, and finally the flash of success as they saw a clear signal rising out of the electronic noise. After the initial thrill, they knew they had to prove to themselves (and others) that the pulse was real. In order to do this, they demonstrated that it disappeared when they changed the target frequency, and also when they moved the telescope off the target star. The website also puts this moment of discovery in context, with transcripts and audio of several recent interviews, and a variety of images and background information. Their observation was quickly confirmed by another group using another telescope, and the papers announcing the results and confirmation appeared in Nature the following month.
And now, a challenge. We’re in 2009, almost half a century after this discovery. Telescopes, even large ones, are much more accessible to amateurs, and modern electronic imaging brings the power of 1960s professional observatories to our own back yards. Can we detect the flashing of that distant neutron star? What would it take to succeed? I picture a clever amateur using some fast pixel shuffling on a sensitive CCD chip attached to a high-end scope to do the trick. Simpler techniques involving a slotted disk spinning in front of the detector at 30 rpm might also work. Seeing the outcome, say as a pair of images with the neutron star “on” and “off,” would make for an awesome Galileo moment for 2009’s International Year of Astronomy. Let me know if you’re up to the challenge.