Cosmic Cartographer

Bob Kirshner wants to give the map of the universe some scale. His yardstick: the bright beacon of a supernova.

By Sam Flamsteed|Wednesday, July 01, 1992
RELATED TAGS: STARS, COSMOLOGY
Bob Kirshner is in his cramped office at the Harvard-Smithsonian Center for Astrophysics, supposedly discussing some of the weightiest issues of theoretical cosmology. In the middle of a thought, however, he suddenly remembers something he’d rather talk about.

Look at this, he says, pulling out a folder labeled JUNQUE and riffling through it. Look at this. This is great stuff!

He holds up a brochure left by a group of salesmen who came to call recently, trying to convince him that their company should get the contract for the big telescope Harvard wants to build in the Southern Hemisphere. The glossy booklet includes a bizarre picture of one of the company’s products: it’s an Army tank simulator, a full-size turret, complete with gun barrel, that sits atop a framework of piston-activated legs.

How can you say no to people who can think up stuff like this? The cold war is over, so these guys who never had time for low-budget operations like, for example, universities, are suddenly eager to talk to us.

Kirshner has a well-known taste for the absurd and a tendency to interrupt even the starchiest scientific discussion with a reference to the ridiculous. So he doesn’t really mind the extra time he’s had to spend entertaining tank-simulator salesmen and other visitors over the past two years, ever since he took over the chairmanship of Harvard’s astronomy department. Some of the rest of his schedule is taken up teaching his undergraduate course, Matter in the Universe,which regularly fills one of the largest lecture halls at Harvard. The real demands on Kirshner’s time, however, come not from salesmen or students but from stars. Kirshner made his mark in astronomy as an observer, and he still spends as many hours as he can at major observatories around the world. He has just returned from a week at the Smithsonian’s Multiple Mirror Telescope in Arizona; last October he spent a week at Las Campanas Observatory in northern Chile.

From his research posts around the world, Kirshner is attempting to answer one of astronomy’s most stubborn questions: How big--and by extension, how old--is the universe? For years astronomers have used a number of surveying methods, the details of which are somewhat complicated, to attempt to resolve the mystery. Essentially, astronomers measure the distance to nearby stars, use those figures to gauge the distance to similar stars farther away, use those to get the distance to nearby galaxies, and use those in turn to gauge the distance to remote galaxies. Each step is a little inaccurate, and the farther out you go, the worse it gets; add everything up and you know the size of the visible universe only to within a factor of 2. This imprecision--a bit like saying a friend of yours is between 6 and 12 feet tall and between 30 and 60 years old--has been more than a little frustrating.

Kirshner thinks he may now have a reliable, more direct way to measure the distance to--and infer the age of--various points in the cosmos. Those points are exploding stars, or supernovas, and they’re among the reasons he would like to spend some of his observing time with the Hubble Space Telescope.

I have a big program to study supernovas with the HST, he says. But since it developed its little problem, everybody has had to submit more paperwork explaining why their proposal can still be done, and if so, why, and if not, why not, and should they get more telescope time or less. Let no one be misled: the ability to put down arguments forcefully in a short amount of time is one of the most important skills an astronomer can have. I’m filling out these incredible forms that make your taxes look like child’s play.

Kirshner has always been smitten with supernovas. As a junior at Harvard in the late 1960s he wrote a research paper on the Crab nebula, a glowing cloud of gas left over from a gigantic stellar explosion first observed in 1054. He did his senior thesis on ultraviolet observations of the sun, but when he got to Caltech for graduate school, he returned to his original love. One thing that helped woo him back was the dramatic explosion in 1972 of a star in a reasonably nearby galaxy. The erupting body, dubbed Supernova 1972E, represented the brightest such stellar event in 35 years. At that time Caltech and the Carnegie Institution of Washington had just finished building the 60-inch telescope on Mount Palomar; it was so new that observing time hadn’t even been scheduled on it yet. Kirshner leaped to book himself a reservation.

I said to them, ‘Let me go up and do a month’s worth of observations on the supernova.’ I did go, and I got great data. I also did some work on the Crab nebula. When you start taking data that are better than anything that existed when you were an undergraduate, you start getting the feeling that you can really do something in your field.

Upon graduation Kirshner wanted to stick with supernovas. The problem, he says, is that you can’t predict when they’ll go off. So you have to be at the telescope full-time. But no one will give you telescope time just to hang out and wait for the next supernova. Kirshner’s solution was to join his friend Augustus Oemler as a postdoctoral fellow at Kitt Peak National Observatory, near Tucson. Oemler, a year ahead of Kirshner at Caltech, was working with astronomer Paul Schechter on a project designed ultimately to measure the average density of matter in the universe. To that end they were attempting to find the luminosity function, a kind of galactic census that tells both the number and distribution of galaxies in a given chunk of space. One of the chunks they were looking at--a perfectly typical chunk, they assumed, populated with a typical horde of galaxies-- was in the direction of the constellation Boötes.

It was a massive job, and it gave Kirshner plenty of time to pack in supernova observations along the way. The team kept at it even after they had left Kitt Peak for other institutions--Kirshner for the University of Michigan--and their persistence netted them a surprise. Out of the corner of our eyes, Kirshner says, we noticed something that was strange: the density of galaxies was different in different parts of our sample. So we said, ‘Let’s go fainter and see what’s happening.’ When we did look deeper, and when we plotted up the redshifts-- he starts shuffling through piles of paper. Can I find the plots? he wonders. I’ll bet I could, given a month. Well, anyway, when we plotted up the redshifts, we noticed lots of galaxies in the foreground, lots of galaxies way in the background, but nothing in the middle. And it was a big middle. There was a kind of desert there. We realized something was very strange.

The problem was not that they were finding large conglomerations of galaxies bordering a starless no-man’s-land. For decades astronomers had been finding that galaxies were not sprinkled evenly through space but were instead huddled in groups, clusters of groups, and even superclusters of clusters. What was wrong was the immensity of the hole--a huge gulf of 30 degrees on the 180-degree bowl of sky, a gigantic void 300 million light- years across. It was the largest cosmic feature ever observed, and it was further evidence for a disturbingly complex distribution of galaxies throughout the universe.

Before we dared publish the results, says Kirshner, we went on to do the next obvious thing, so that when a lot of our friends said, ‘Are you sure about this?’ we could say we were. What we did was look more closely between the fields. We kept sticking needles into this pumpkin, and it kept looking hollow, like, well, kind of like a pumpkin. We didn’t quite believe it either, but we published it.

That was in 1981, and Kirshner and his colleagues got a lot of attention. This is my personal favorite, he says. He pulls a yellowing National Enquirer from a shelf and opens to the middle. The headline reads: DID A REAL-LIFE STAR WARS CREATE HUGE HOLE IN SPACE? Then he pulls a transparency from a stack he’s been using for his course. This is the other mark of lasting fame. It’s a reproduction of a New Yorker cartoon in which one astronomer is talking to another. The caption reads, Yes, a hole in space three hundred million light-years across does make me pause and feel tiny and insignificant, but a glance around at my peers usually restores my equanimity.

Although there was some initial skepticism about Kirshner’s findings, other studies began turning up similar results before long. In 1986 a group at Harvard, led by Margaret Geller and John Huchra, conducted its own survey of the northern sky and announced that galaxies appeared to be organized into shapes that looked almost like bubbles, with the galaxies flecking the bubbles’ skin and little or nothing in the great spaces within. The Boötes Void, it appeared, was just one such empty area of the cosmos.

As Geller and Huchra continued their survey, they found the largest structure yet, a wall of thousands of galaxies, extending for at least 500 million light-years, that they named, appropriately enough, the Great Wall. Meanwhile, another group of astronomers discovered evidence in the southern sky of an enormous concentration of mass that appeared to be pulling the Milky Way and thousands of other galaxies inexorably toward it. They dubbed it the Great Attractor.

For many researchers, giant structures suddenly became the hot topic in astronomy, and Kirshner was as mesmerized as anyone--but not to the exclusion of his supernova research. Throughout the 1980s, he says, we continued trying to understand what kinds of stars become supernovas, what kind of debris they produce, how often such explosions occur, how we can use them to probe the interstellar gas. We were looking at that stuff mostly from the ground, but also with the IUE telescope.

The IUE is the International Ultraviolet Explorer satellite, a robot observatory launched into space in 1978. Most of the data Kirshner gathered with the telescope were on supernovas in distant galaxies--chiefly because that’s where most supernovas have been found. For years, however, Kirshner has submitted an annual request to the committee that governs the IUE. Essentially that request is, If a bright one goes off close to home, give me some observing time instantly. The committee has always agreed.

One February morning in 1987--two years after Kirshner came to Harvard--he got a call from his friend Craig Wheeler at the University of Texas. He said, ‘Have you heard about the supernova in the Large Magellanic Cloud?’ Kirshner recalls. I said, ‘Oh, yeah, sure.’ That’s because we had been at a meeting together in Erice, Sicily--this really remote place--and a group of people decided it would be fun to play a trick on me. They sent this telegram saying there was a new bright supernova in a nearby galaxy. I went rushing to the travel desk, desperately trying to get a flight back to the United States. They finally let me in on the joke.

But Wheeler insisted this one was real. So Kirshner walked down the hall to Brian Marsden’s office. Marsden runs the International Astronomical Union’s Central Bureau for Astronomical Telegrams, the clearinghouse where newly discovered objects--comets, asteroids, and supernovas, in particular--are reported and announced. Brian’s teletype was going like mad, and he was taking frantic phone calls, says Kirshner. It was clear that something big was going on. I went back to my office to call the IUE people, and the phone rang. It was them, calling me.

For the astronomical community, Supernova 1987A was an extraordinary gift. There had been no major stellar explosion anywhere near Earth since the early 1600s--since just a few years, in fact, before Galileo began training the first telescopes on the heavens. What the twentieth-century astronomers were able to discover with their state-of- the-art equipment could not even have been imagined by their seventeenth- century predecessors.

We found, says Kirshner, that with 87A the heavier elements-- which are normally layered inside the star--were mixed together in the explosion. It’s important because, well, for one thing, we’re made of this stuff--oxygen, carbon, iron. Also, it’s interesting to realize that while the nuclear processes that make stars shine can’t create anything heavier than iron, the heavier elements--like gold, silver, and uranium--do exist. So there must be places where iron nuclei are bombarded with neutrons and make heavier elements. Supernovas are the places.

Gradually Kirshner began to think that his supernovas’ brilliant beacons might be able to shed some light on other mysteries of the universe--in particular, on the grandest mystery of all, the old size-and- age question. Supernovas, he realized, might provide a way of measuring cosmic distances that didn’t depend on the shaky step-upon-step methods of the traditional approach.

How could supernovas function as measuring devices? Since 1929, thanks to Edwin Hubble, we have known that the universe is expanding uniformly. This means that a galaxy 10 million light-years away will take the same time to double its distance as a galaxy 20 million light-years away; to do this, of course, the more remote galaxy must be moving twice as fast as the nearer one. This direct relationship between distance and speed of recession is known as Hubble’s law. Throughout the cosmos, an object’s speed of recession will always be directly proportional to its distance, right out to the edge of the universe, where objects are flying away at close to the speed of light.

If you can establish a distant object’s recessional velocity, therefore, you can determine its distance, and you’ll be well on your way toward drawing a three-dimensional map of the heavens. And the way you go about establishing that velocity is through redshift--the degree to which a receding body’s light waves are stretched toward the lower frequency, or red, end of the spectrum. The faster something is moving away from us, the greater its redshift.

The problem with this technique is that it gives us only a relative measurement, not an absolute one: no one knows exactly what velocity, and thus what redshift, corresponds to a given distance. In other words, we know that an object moving away from us at 20,000 kilometers a second is twice as distant as an object moving away from us at 10,000 kilometers a second, but we don’t know precisely, in absolute terms, what that distance is.

Kirshner’s supernovas, though, may offer a way out. The idea of using supernovas as cosmic measuring sticks is ultimately based on old- fashioned trigonometry: if you know how big an object appears to be and you also know how big it actually is, you can easily calculate how far away it is. Supernovas are easy enough to see; all you have to do is measure both their apparent and their actual size, and you’ve got a fine way of establishing just how far it is to the distant galaxies these cosmic beacons tend to inhabit.

Unfortunately, those measurements are not so easy to come by. At the distance of any but the nearest galaxy, a supernova’s expanding gas cloud looks like an infinitesimal speck of light, no matter what its actual size is--and an infinitesimal speck distorted by Earth’s atmosphere, at that. However, Kirshner can get at the supernova’s apparent size by taking its temperature--and this he can get by analyzing its spectrum.

If the cloud were in perfect equilibrium, he explains, then it would be what physicists call a black body, an object whose spectrum is uniquely determined by its temperature. In practice it isn’t, but the difference can be corrected for. So you model the atmosphere to get the actual temperature--you just have to solve a few simple equations. He holds up a sheet covered with abstruse mathematical symbols.

Once he has the temperature, he has a measure of precisely how much energy is coming from each square centimeter of the supernova, regardless of its distance. If he now knows the total amount of energy reaching Earth--determined by a charge-coupled device that counts the number of photons striking the telescope mirror--he can calculate the tiny area of sky the supernova covers. In effect, he has determined the supernova’s apparent size, its size as seen from Earth.

Kirshner’s next step is to determine the supernova’s actual size- -or rather, the change in its actual size over a period of time, since the exploding star’s atmosphere is constantly expanding in all directions. The only part we can see is the part that is coming toward Earth, but that is enough to tell us the star’s rate of expansion. Just as light receding from us is stretched toward the red end of the spectrum, light racing toward us is compressed, pushed toward the high-frequency blue end. And just like the redshift, this blueshift can be correlated with a given velocity. By measuring the blueshift of the expanding stellar atmosphere, then, Kirshner can determine how fast the supernova is growing.

All he has to do now is just wait awhile, then repeat his calculations for the supernova’s apparent size. The two measurements will give him a percentage of change in apparent size that necessarily equals the percentage of change in actual size. Since he knows the rate at which the supernova is getting bigger, he can thus easily determine its actual size. At that point, all he needs is a little simple trigonometry to determine how far away the giant body has to be to appear as the tiny speck of light in the telescope.

We believe we can get accurate distances to within twenty percent, says Kirshner, which is an improvement over existing techniques. Those calculations are what peg the overall size of the universe at 10 to 20 billion light-years and the overall age at 10 to 20 billion years. But the oldest stars, which are in globular clusters, are something like 12 to 14 billion years old. This leads to the embarrassing situation of having a universe that’s potentially younger than its oldest stars. We shouldn’t panic yet, but we’d better solve this problem.

As Kirshner gears up to help solve it--using historical data on existing supernovas and waiting for new ones to go off--he is also resuming his research into how the universe is arranged, hoping to determine whether there are even larger structures beyond the Great Wall. Many astronomers around the world are investigating the same question, getting ready to do deep, massive redshift surveys to determine the relative velocity--and thus the relative position--of hundreds of thousands of galaxies. In typical fashion, however, Kirshner and company are taking a different, relatively low-tech approach. Many of these other guys are from the beat-your-brains- out school of observation, Kirshner says. We’re from the good-enough-is- good-enough school. If you can do good, leading-edge work and it’s easy, then hey, why not?

The trick in doing high-volume surveys is taking many redshifts at once. Traditionally, astronomers get the redshifts of galaxies by positioning their telescopes so that a single galaxy’s light falls on the aperture of a spectrometer. When they have all the information they need on this lone galaxy, they move on to the next one, then the next and the next, and so on.

Kirshner, Oemler, and colleague Steve Shectman do the same thing, only a hundred galaxies at a time. Guided by surveys they’ve taken previously of the southern sky to mark the positions of appropriate galaxies, they drill tiny holes in a round aluminum plate 36 inches across, each hole corresponding to the point where a galaxy will appear in the telescope’s field of view. Then, down in Chile, the astronomers fit the plate to the telescope and attach a fiber-optic cable to the back of each hole. When the plate is perfectly aligned with its designated piece of sky, the light from 112 galaxies enters 112 holes, bounces down a cable, and enters a spectrometer. Each exposure lasts for two hours; it then takes about 20 minutes to switch the optical fibers to a new set of holes in the plate representing a new set of galaxies.

This particular spectrometer, says Kirshner, is called the two-dimensional, or 2D, photon counter, so we call it the 2D-Frutti for short. We call the whole setup the Fruit and Fiber.

The Fruit and Fiber survey has been going on since 1987, but it took a while to work out the bugs, and the results so far are only a fraction of what they eventually will be. After several thousand redshifts, however, some interesting trends are appearing. We’re already getting an idea of how galaxies are organized across perhaps a tenth of the visible universe, says Kirshner, and while we see the same sorts of structures people have found in earlier surveys, we haven’t seen anything bigger. This is good. It means that maybe after the Great Wall and the Great Attractor, we’re coming to the end of the Age of Greatness and entering the Age of Mediocrity.

For the moment, however, Kirshner must put aside these grand thoughts. On this afternoon it’s time for him to go back to his role as a Harvard chairman. A department party is about to begin, and he’s required to go mingle. Before he does, Kirshner has to get dressed for the occasion. Other department chairs at prestigious Ivy League universities are probably, even now, putting on their pinstripe suits and preparing to down some sherry. Kirshner’s drink will be V-8 juice, drunk straight from the can. And his badge of office, which he puts on with a flourish before heading to the conference room, is a vest embroidered with brightly colored felt stars, planets, and comets. For an astronomer with a taste for the absurd, it’s perfectly appropriate.
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