They give birth astride a grave, the light gleams an instant, then it's night once more.
The last person to see and chronicle a supernova outburst in our galaxy was Johannes Kepler. That was in 1604, when the star now named after him rivaled Venus in brightness. By some measures we're overdue for another brilliant supernova, yet the next star to explode in our galaxy is more likely to be a visual pipsqueak compared to Kepler's Star. Yet even a dim supernova is unlikely to be overlooked; its birth will be trumpeted by physicists' subterranean particle detectors rather than by astronomers' telescopes.
Supernovae are stars that brighten by a dozen magnitudes or so and at their peak are some 10,000 times more luminous than ordinary novae. (The physical processes operating during these two explosions are completely different: supernovae blow themselves to smithereens; ordinary novae don't.) The enormous luminosity of supernovae at their brightest makes it possible to readily spot them in distant galaxies — for weeks they can match the light output from all the other stars in a hefty system like our own Milky Way. Indeed, the identification of supernovae as a unique phenomenon had to wait until the 1920s, when galaxies themselves were recognized as independent star systems.
"Behold, directly overhead, a certain strange star was suddenly seen, flashing its light with a radiant gleam.... Astonished and stupefied, I stood still.... When I had satisfied myself that no star of that kind had ever shone forth before...I began to doubt the faith of my own eyes.... Having confirmed that my vision was not deceiving me...and marveling that the sky had brought forth a certain new phenomenon to be compared with the other stars, I immediately got ready my instrument."
No supernova outburst has been studied in our Milky Way for nearly 400 years. Therefore, everything we know about the workings of these stars has come from observing them in other galaxies. At such immense distances even these celestial powerhouses are dim, and their secrets have to be teased from the paltry number of photons that strike our detectors. Furthermore, catching these critters in the act has been a matter of chance, even after systematic searches began in the 1930s. The idea is simple: look at enough galaxies and, sooner or later, you'll find a supernova.
Even so, these distant supernovae are detected days or weeks after their outbursts begin. What hasn't been observed are the earliest stages of a supernova's development. Particularly interesting, for example, will be observations to assess the chemical composition and physical state of the fastest-moving pieces of the star's blown-off atmosphere. Such information should provide insight about the end point in the evolution of a very massive star as well as clues about how heavy elements are injected into the interstellar medium.
Wonderfully detailed mathematical models of supernova explosions have been built on theorists' computers (see the diagram above, right). They tell us what to expect, but wouldn't it be nice to have a sanity check? This may now be possible thanks to a group of neutrino (symbol ν, the lowercase Greek letter "nu") observatories that can warn us when a star blows up in our cosmic backyard — the Milky Way and its nearby attendants in the Local Group — even before its light begins to turn on! Although this new tool is wielded by high-energy physicists, its impact will likely rest with amateur astronomers and other small-telescope users.
ν and You
When a really heavy star (eight or more times the mass of the Sun) runs out of gas, literally, its core collapses and a so-called Type II supernova is born. Within a second, its Earth-size core crumples into a ball — a neutron star or black hole — whose density is akin to that of an atomic nucleus (a million billion kilograms per cubic centimeter). As gravity forces electrons and protons to coalesce and form neutrons, a "gazillion" ghostlike neutrinos are instantly set free to roam the universe, perhaps for eternity. (Neutrinos are chargeless, possibly massless elementary particles.)
Just after the neutrinos begin to zing merrily through space at (or very near) the speed of light, the star's core stops collapsing. Then it rebounds, causing a shock wave to travel out toward the star's surface, which doesn't have a clue about the oncoming disaster (S&T: August 1995, page 30). If the overweight star is a red supergiant (like Betelgeuse) with a hydrogen-rich envelope, nearly a day will elapse between the collapse-induced neutrino emission and the beginning of the supernova light show.
Except for SN 1987A in the Large Magellanic Cloud, no star has been observed before it blew up. As bad luck would have it, SN 1987A's progenitor (called Sanduleak –69°202) was an oddball for a Type II supernova; it was a blue (not red) supergiant and relatively lightweight (six solar masses instead of eight or more). We will probably not see another one like it "for centuries," says Stanford Woosley (University of California, Santa Cruz).
Yet SN 1987A will be remembered for producing the first supernova-spawned neutrino burst detected on Earth, though the event was recognized only after the supernova was seen shining in the sky. Now, more than a decade later, we are armed with hindsight as well as with better and more abundant neutrino detectors. Some even have cute names like Super-K, SNO, MACRO, and AMANDA.
Let's Go Get 'Em!
Before neutrinos arrive and sign the physicists' guest book, no one can predict where the next supernova will occur — except that it will likely be within the Milky Way's glowing band or enfolded by one of our neighboring galaxies. We also don't know when the first glimmer of light will appear; if there's lots of interstellar smog in the way, days or even weeks could pass before the star brightens enough to punch through.
The delay between the neutrino emission and the rebounding shock's breakout through the star's photosphere should provide ample time to mobilize the world community of amateur astronomers and other users of small telescopes. And here's a new, exciting twist: if enough neutrinos are collected by enough observatories, we should know not only that a new star will arise but roughly where in the sky we should look for it!
To find the supernova as quickly as possible, a dedicated corps of searchers is needed around the world. To guarantee complete sky coverage, given the vagaries of season and weather, it's vital that hundreds, even thousands, of observers participate. There are no qualifications — except patience! This effort is truly universal. If the dice roll just right, some naked-eye observer in Mongolia might spot the supernova first, while high-tech amateurs in Europe sip Cinzano and wait for darkness.
To quickly tell amateurs and other small-telescope users about the nearby supernova that will surely happen someday, efficient communication is essential. Thus, when simultaneous detections at the neutrino observatories reach a predetermined level, the SuperNova Neutrino Early Warning System (SNEWS) will send its best-guess position of the supernova to AstroAlert, a network established by Sky & Telescope and several partner organizations. AstroAlert will echo that message to all who have registered with the service. The small-telescope community will then swing into action and send observations of any supernova candidate back via a standardized form.
So What Are We Looking For?
It's impossible to predict how bright the next nearby supernova will be or how long we will have to wait for it to pop off. But we can get a feel for the answers by looking at the questions in several different ways.
As a starting point, we can create a list, largely from Oriental and Arabic records, of supernovae that have been seen during the last two millenniums.
|Visual Milky Way Supernovae (A.D. 1–1999)|
|This table was compiled from a variety of sources but mainly David H. Clark and F. Richard Stephenson's book The Historical Supernovae (1977) and an article by Richard G. Strom in Astronomy and Astrophysics (Vol. 288, pages L1-4, 1994). Experts still argue over whether some of the entries represent true supernovae; the five that are boldfaced seem "gold plated." The "b" and "l" quantities are the stars' galactic latitudes and longitudes; b = 0° indicates a star exactly in the plane of the Milky Way. A kiloparsec (kpc) equals 3,260 light-years. Color was in the eye of the beholder.|
Two nearby supernovae that should have shone brightly but apparently didn't are omitted from the preceding list. SN 1680?, also called Cassiopeia A, is one of the strongest radio sources in the sky and was probably glimpsed by John Flamsteed at 6th magnitude. As shown in the images below, extensive dimming of its light by interstellar gas seems very unlikely. However, according to Thomas Dame (Harvard-Smithsonian Center for Astrophysics), one "can't rule out the possibility that the supernova went off behind a very small, very dense clump of gas."
The recently discovered X-ray remnant of SN 1320± lies at a distance of only about 650 light-years, making it the closest known supernova to Earth; its light could have equaled that of the full Moon! So why wasn't it seen? Again, extinction by interstellar gas and dust is "extremely unlikely," says Dame. Perhaps both SN 1320± and SN 1680? mark a hitherto unknown class of supernovae that are optical duds (S&T: April 1999, page 22).
All the entries in the list predate the invention of the telescope: seven conspicuous naked-eye supernovae in 1,400 years, or one every couple of centuries, on average. So why haven't we had another in 400 years? Whether because of bad statistics or bad luck, it seems we're overdue by a factor of two.
Determining how often supernovae explode in our Milky Way is fraught with uncertainties, the estimate being confounded particularly by the gas and dust that pervade the galactic plane. The rate can be judged in many ways, but all involve surrogate evidence or initial assumptions that are subject to observational bias. These methods include our galaxy's inventory of heavyweight stars (which blow up 10 million years or so after being born); the number of pulsars (spinning neutron stars, the progeny of supernovae); counts of expanding, wreathlike supernova remnants; and the determination of supernova rates in galaxies kindred to our own.
The Milky Way's supernova rate was estimated in 1994 by Richard G. Strom (Netherlands Foundation for Research in Astronomy). By comparing supernovae observed over the past two millenniums with supernova remnants of comparable age, he concludes that a star blows up near the Sun (within 5 kiloparsecs [kpc] or 16,000 light-years) every 175 years, on average. By extrapolating this rate to the whole galaxy, Strom predicts a supernova every 20 years or so.
On the other hand, a team from the University of Western Australia published a paper in 1999 that joins evidence from extragalactic sightings, stars in our galaxy, and the historical record of supernova explosions within 4 to 5 kpc (13,000 to 16,000 light-years) of the Sun. According to coinvestigator Ronald Burman, "One cannot reliably extrapolate from the rate of historical supernovae to obtain a rate for the galaxy as a whole, since we appear to live in a region of the galaxy with an enhanced event rate." Such would be the case if we were located adjacent to active star-forming regions, where supernova progenitors are most likely to be born. The bad news is that this team finds the most likely rate for Milky Way supernovae to be only about two per century. The good news is that the vast bulk of these dying stars will spit out neutrinos.
So how bright might the next Milky Way supernova be? In 1975 Sidney van den Bergh (now at Dominion Astrophysical Observatory) made a careful estimate. In preparing this article, I did my own calculation, using somewhat different rules, and got similar answers. So I combined both results in the following table.
Apparent Brightnesses of
Milky Way Supernovae
|• 10% will peak brighter than magnitude –3|
|• 20% will peak between magnitudes –3 and +2|
|• 20% will peak between magnitudes +2 and +6|
|• 20% will peak between magnitudes +6 and +11|
|• 30% will peak fainter than magnitude +11|
If this distribution is accurate, it implies that pretelescopic observers logged only a third of the supernovae that exploded in our galaxy. Historians have pointed out that a new star had to be really bright, perhaps exceeding magnitude +1.5, to stand a good chance of being noticed by ancient astronomers.
The fact that no galactic supernova has been recognized on 20th-century sky-patrol photographs seems to confirm the robustness of the table. Combined with the best-guess frequency of supernovae, it suggests that only three or four supernovae would have brightened enough to be recorded on patrol plates. And even if a maverick image had been spotted, it would likely have been dismissed as a "Kodak comet" or other defect.
Maybe those plates are worth checking again by someone armed with modern radio, X-ray, and other ledgers of supernova suspects. As van den Bergh wrote: "Very red 'novae' that exhibit a relatively slow rate of brightness decline are prime supernova suspects."
What's the chance that a supernova will jolt the neutrino detectors in the coming year? Odds of about 1 in 30 would probably satisfy Las Vegas bookmakers. The chance that the stellar fireworks will actually be seen drops to about 1 in 70, in my opinion. So, if you want instant gratification, you had better look elsewhere. But what intrigues me is that a champagne cork could pop tomorrow!