Chaos has reigned chez Piquant this week, as a crew of workmen descended on our quiet DC retreat and proceeded to cover everything in plastic, strip, sand, paint, install new bathroom vanities, and replace the old drafty windows that may very well date back to 1918. So we very nearly missed the chance to wish a happy birthday to the scientists who, 20 years ago today, first detected the explosion of a supernova dubbed 1987A. It's not the catchiest moniker, granted, but at least it's easy to remember. Along with other known supernovae in the universe, 1987A is still being studied by various teams of scientists, still offering a few surprises to prompt us to re-think our working models for how these amazing celestial objects evolve.
Supernovae are essentially exploding stars. On February 23, 1987, a Canadian astronomer named Ian Shelton was manning the telescope at Las Campanas Observatory in Chile, taking images of a small galaxy some 167,000 light-years from called the Large Magellanic Cloud. He noticed an extremely bright star on one of the photographic plates that hadn't been present in prior images of the same region.
Shelton joins a long line of distinguished astronomers who've observed supernovae. For instance, more than 400 years ago, Johannes Kepler spotted a bright new object in the night sky, brighter than the planet Jupiter, so bright it was visible even without a telescope (which had not yet been invented). It stayed that bright for several weeks, gradually dimming. Today we know it as the Kepler remnant. (Jen-Luc wonders if perhaps 1987A might one day bear Shelton's name. It seems only fair.) But until the light from 1987A's explosion finally reached us, our understanding of the actual mechanisms at work were a bit on the vague side.
The new Conservapedia alternative to Wikipedia (that radical liberal bastion!) has been roundly mocked by science bloggers this past week, and rightly so. I mean, this is a site whose entry on Einstein basically says his work had nothing to do with the development of nuclear weapons. The degree of ignorance required to make that sort of statement simply boggles the mind. Does E=mc2 ring a bell? (If it doesn't, Mark Trodden of Cosmic Variance just wrote a spiffy primer post.) How about the sun? That whole energy/mass conversion thing that Einstein cobbled together in 1905 in his spare time — when he wasn't doing Nobel-Prize-winning work on Brownian motion, and coming up with special relativity — is pretty critical to human life, considering it's at the heart of nuclear reactions, and nuclear reactions are what make stars like our sun shine.
Take it from They Might Be Giants: "Oh the sun is a mass of incandescent gas/A nuclear furnace/Where hydrogen is fused into helium/At temperatures of billions of degrees." That energy warms the earth, and also keeps the sun's layers from collapsing down into its core. Eventually the fuel runs out, however, and that's when the fireworks can start. All stars die — even our sun will die one day — but really massive ones die in particularly spectacularly ways. Here's the conventional scientific explanation: As the hydrogen runs out, fusion slows down, and gravity causes the core to contract, raising temperatures even further, sufficient to give rise to a brief, shorter phase of helium fusion. What happens next depends on a star's mass. Smaller starts gradually cool to become white dwarfs, and those white dwarfs may turn into Type IA supernovae (like 1987A) if there is a near companion star.
Much larger stars (greater than 8 solar masses) will create temperatures and pressures so high that the carbon in the star's core begins to fuse once the star contracts at the end of the helium-burning stage. This halts the core's collapse, at least temporarily, and this process continues, over and over, with progressively heavier atomic nuclei. So the cores of those supernovae begin to resemble an onion, with layers upon layers of elements — the outermost layer is hydrogen, which surrounds a layer of hydrogen fusing into helium, surrounding a layer of helium fusing into carbon, and so on. In fact, most of the heavy elements in the periodic table were born in the intense furnaces of exploding supernovae that were once massive stars.
Supernovae also make great "standard candles" to help astronomers determine distances in space, because they burn so steadily and brightly — and predictably. In fact, the use of supernovae as standard candles is what enabled two separate teams of physicists in 1998 to determine that the expansion of the universe is actually accelerating… pretty much revolutionizing the field of cosmology in the process, and starting the whole "dark energy" craze.
All kinds of fascinating new results related to supernovae were reported at the 2007 meeting of the American Astronomical Society (AAS) meeting last month in Seattle. For instance, apparently stars can blow bubbles. The powerful stellar winds produced by extra-massive stars push away nearby gas and dust and form low-density cavities inside expanding bubbles. When such a star dies in a big explosion, the energy from its death throes enlarge the bubbles into huge supernova remnants (yes, just like the Kepler remnant.) If enough cavities overlap, they can form a superbubble. Scientists at the University of Illinois at Champaign-Urbana announced their observation of a superbubble in the midst of forming, which could yield new information about the formation of planetary systems among other insights.
Another hot topic at the AAS meeting: a team of researchers from the University of Notre Dame reported detection of a light echo from a Type Ia supernova called 1995E. Light echos can occur when light from a supernova is redirected towards the earth by grains of dust in the host galaxy. The explosive event is the direct rays we see, but delayed light echos from a different location because they took a more circuitous route — much like the delay between a direct flight from LA to New York versus a changeover in Chicago.
As for 1987A, University of California, Berkeley astronomer Nathan Smith announced his alternative theory for the origin of the strange, triple-ring nebula surrounding the object, produced by the star a few thousand years before it exploded. The working theory was that the original red supergiant merged with a companion and started spinning rapidly (ejecting the material that produced the strange shaped nebula), then turned into a blue supergiant and finally exploded. The sticking point for this neat little theory is that red supergiants are just too darn big to spin that fast. Smith argues that the progenitor star of 1987A might have been an unstable blue supergiant (called luminous blue variables). Such stars eject material from their surfaces in recurring eruptions, like volcanoes, before dying in a supernova explosion. His evidence: he recently discovered two such stars that have similar clouds of gas and dust.
Oh, and remember the Kepler remnant I mentioned earlier? Kepler might have lacked even a rudimentary telescope, but thanks to advanced instruments like Hubble and the Chandra X-Ray Observatory, astrophysicists are still studying it and learning new things. Chandra in particular announced a stunning new image at the AAS meeting in January, achieving unprecedented details by combining nearly nine days of observations, and determining once and for all that the Kepler remnant is the result of a Type Ia supernova. (There had been some debate about that, since prior conflicting data from radio, optical and X-ray telescopes made it difficult to determine whether it was a Type Ia, or a Type II (which results form the collapse of a single massive star that sheds material before exploding). It could also help astronomers further improve the reliability of using such objects as standard candles — and hopefully tell us more about that mysterious dark energy.
There's yet another area in which supernovae can help solve some of astrophysics' most puzzling mysteries: neutrino physics. All stars emit these so-called "ghost particles" — our sun bathes us in billions of neutrinos every day, which interact with matter so weakly that they pass right through us without being noticed. Physicists have built deep underground laboratories that have managed to detect the odd solar neutrino or two, but such events are rare; only a tiny fraction of the sun's neutrinos are detected. Supernovae like 1987A emit 1000 times more neutrinos than the sun will produce in its entire 10-billion-year lifetime. When it was first observed 20 years ago, the Super-Kamiokande detector in Japan picked up a few extra neutrinos: 19, to be exact. That's not much compared to how many neutrinos the supernova actually emitted, so it merely provide a few initial clues to the mystery of how supernovae really work.
And 1987A continues to reveal new secrets. NASA issued a press release this morning reporting that the XMN-Newton X-ray telescope has taken images showing that the supernova keeps brightening — at least since the ROSAT telescope first detected X-rays emitted by 1987A in 1992. Our plucky little Death Star Supernova now outshines all other X-ray sources in its immediate vicinity. Maybe even the curmudgeonly Simon Cowell of American Idol would be impressed.