How many ways can a star go “kaboom!”? It might depend on what kind of galaxy the star lives in, astronomers said last week.
For the last 20 years, astronomers seeking to measure the cosmos have used a special type of exploding star, known as Type 1a supernovas, as distance markers. They are thought to result when stars known as white dwarfs grow beyond a certain weight limit, setting off a thermonuclear cataclysm that is not only bright enough to be seen across the universe but is also remarkably uniform from one supernova to the next. Using them, two teams of astronomers a little more than a decade ago reached the startling and now widely held conclusion that some “dark energy” was speeding up the expansion of the universe.

But astronomers, to their embarrassment, have not been able to agree on how the white dwarf gains its fatal weight and explodes, whether by slowly grabbing material from a neighboring star or by crashing into another white dwarf.

In a telephone news conference on Wednesday and a paper published Thursday in the journal Nature, Marat Gilfanov and his colleague, Akos Bogdan, both of the Max Planck Institute for Astrophysics in Garching, Germany, said that for at least one class of galaxies in the universe, the roundish conglomerations of older, redder stars known as ellipticals, these supernovas are mostly produced by collisions.

“We have revealed the source of the most important explosions in cosmology,” Dr. Gilfanov said, adding that until now “we didn’t know exactly what they were.”

Reasoning that white dwarfs slowly gobbling gas from neighbors would emit X-rays as the captured gas fell and was heated, Dr. Gilfanov and Dr. Bogdan used NASA’s Chandra X-Ray Observatory to look at five elliptical galaxies and the central bulge of the nearby Andromeda galaxy — all of which are composed of older stars. The satellite recorded only about one-thirtieth to one-fiftieth of the X-rays that would be expected from such white dwarfs, leading the astronomers to conclude that no more than 5 percent of the supernovas in those types of stellar systems could be produced by accreting white dwarfs.

The observations leave open the possibility that accreting dwarfs might be responsible for more of the supernovas in spiral galaxies like our own, which tend to have younger, more massive stars.

That leaves open the possibility of two different kinds of Type 1a supernovas at loose in the universe and could add extra uncertainty into efforts to use exploding stars as standard candles to make precise measurements of the universe. Accreting white dwarfs go off at a precisely determined mass known as the Chandrasekhar limit, but a pair of colliding dwarfs could have a range of masses.

Mario Livio, a theorist at the Space Telescope Science Institute, said that while the new results and the idea of two classes of supernovas “muddies the water,” they would not affect the measurements of dark energy. Most of the supernovas in those studies, he said, came from spiral galaxies, and the astronomers, moreover, were very careful to calibrate their data.

“The main results so far will remain unchanged,” Dr. Livio said.

Adam Riess, of Johns Hopkins University and the Space Telescope Science Institute and a first-rate dark energy hunter, called the new paper “an interesting study.” He compared the two theories of supernovas to lighting a stick of dynamite with a fuse versus banging them together to see if they would go off. “If we find a connection to where nature does it one way versus the other, we could use that information to improve the use of these candles,” Dr. Riess wrote in an e-mail message. “I think we are getting close to that point now.”

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Suppose, as some experts advise, that the new national dietary guidelines due this spring will lower the recommended level of salt. Suppose further that public health officials in New York and Washington succeed in forcing food
companies to use less salt. What would be the effect?
A) More than 44,000 deaths would be prevented annually (as estimated recently in The New England Journal of Medicine).

B) About 150,000 deaths per year would be prevented annually (as estimated by the New York City Department of Health and Mental Hygiene).

C) Hundreds of millions of people would be subjected to an experiment with unpredictable and possibly adverse effects (as argued recently in The Journal of the American Medical Association).

D) Not much one way or the other.

E) Americans would get even fatter than they are today.

Don’t worry, there’s no wrong answer, at least not yet. That’s the beauty of the salt debate: there’s so little reliable evidence that you can imagine just about any outcome. For all the talk about the growing menace of sodium in packaged foods, experts aren’t even sure that Americans today are eating more salt than they used to.

When you don’t know past trends, predicting the future is a wide-open game.

My personal favorite prediction is E, the further fattening of America, but I’m just guided by a personal rule: Never bet against the expansion of Americans’ waistlines, especially not when public health experts get involved.

The harder the experts try to save Americans, the fatter we get. We followed their admirable advice to quit smoking, and by some estimates we gained 15 pounds apiece afterward. The extra weight was certainly a worthwhile trade-off for longer life and better health, but with success came a new challenge.

Officials responded by advising Americans to shun fat, which became the official villain of the national dietary guidelines during the 1980s and 1990s. The anti-fat campaign definitely made an impact on the marketing of food, but as we gobbled up all the new low-fat products, we kept getting fatter. Eventually, in 2000, the experts revised the dietary guidelines and conceded that their anti-fat advice may have contributed to diabetes and obesity by unintentionally encouraging Americans to eat more calories.

That fiasco hasn’t dampened the reformers’ enthusiasm, to judge from the growing campaign to impose salt restrictions. Pointing to evidence that a salt-restricted diet causes some people’s blood pressure to drop, the reformers extrapolate that tens of thousands of lives would be saved if there were less salt in everybody’s food.

But is it even possible to get the public to permanently reduce salt consumption? Researchers have had a hard enough time getting people to cut back during short-term supervised experiments.

The salt reformers say change is possible if the food industry cuts back on all the hidden salt in its products. They want the United States to emulate Britain, where there has been an intensive campaign to pressure industry as well as consumers to use less salt. As a result, British authorities say, from 2000 to 2008 there was about a 10 percent reduction in daily salt consumption, which was measured by surveys that analyzed the amount of salt excreted in urine collected over 24 hours.

But the British report was challenged in a recent article in The Clinical Journal of the American Society of Nephrology by researchers at the University of California, Davis, and Washington University in St. Louis. The team, led by Dr. David A. McCarron, a nephrologist at Davis, criticized the British authorities for singling out surveys in 2008 and 2000 while ignoring nearly a dozen similar surveys conducted in the past two decades.

When all the surveys in Britain are considered, there has been no consistent downward trend in salt consumption in recent years, said Dr. McCarron, who has been a longtime critic of the salt reformers. (For more on him and his foes, go to nytimes.com/tierneylab.) He said that the most notable feature of the data is how little variation there has been in salt consumption in Britain — and just about everywhere else, too.

Dr. McCarron and his colleagues analyzed surveys from 33 countries around the world and reported that, despite wide differences in diet and culture, people generally consumed about the same amount of salt. There were a few exceptions, like tribes isolated in the Amazon and Africa, but the vast majority of people ate more salt than recommended in the current American dietary guidelines.

The results were so similar in so many places that Dr. McCarron hypothesized that networks in the brain regulate sodium appetite so that people consume a set daily level of salt. If so, that might help explain one apparent paradox related to reports that Americans are consuming more daily calories than they used to. Extra food would be expected to come with additional salt, yet there has not been a clear upward trend in daily salt consumption evident over the years in urinalysis studies, which are considered the best gauge because they directly measure salt levels instead of relying on estimates based on people’s recollections of what they ate. Why no extra salt? One prominent advocate of salt reduction, Dr. Lawrence Appel of Johns Hopkins University, said that inconsistent techniques in conducting the urinalysis surveys may be masking a real upward trend in salt consumption.

But Dr. McCarron called the measurements reliable and said they could be explained by the set-point theory:

As Americans ate more calories, they could have eased up on some of the saltier choices so that their overall sodium consumption remained constant. By that same logic, he speculated, if future policies reduce the average amount of salt in food, people might compensate by seeking out saltier foods — or by simply eating still more of everything.

The salt reformers dismiss these speculations, arguing that with the right help, people can maintain low-salt diets without gaining weight or suffering other problems. But even if people could be induced to eat less salt, would they end up better off? The estimates about all the lives to be saved are just extrapolations based on the presumed benefits of lower blood pressure.

If you track how many strokes and heart attacks are suffered by people on low-salt diets, the results aren’t nearly as neat or encouraging, as noted recently in JAMA by Michael H. Alderman, a hypertension expert at Albert Einstein College of Medicine. A low-salt diet was associated with better clinical outcomes in only 5 of the 11 studies he considered; in the rest, the people on the low-salt diet fared either the same or worse.

“When you reduce salt,” Dr. Alderman said, “you reduce blood pressure, but there can also be other adverse and unintended consequences. As more data have accumulated, it’s less and less supportive of the case for salt reduction, but the advocates seem more determined than ever to change policy.”

Before changing public policy, Dr. Alderman and Dr. McCarron suggest trying something new: a rigorous test of the low-salt diet in a randomized clinical trial. That proposal is rejected by the salt reformers as too time-consuming and expensive. But when you contemplate the potential costs of another public health debacle like the anti-fat campaign, a clinical trial can start to look cheap.

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"Without the breath of the tiger there will be no wind, only clouds, and certainly no rain." —The I Ching
"I'll be quite honest with you," says Ron Tilson, director of conservation at
Minnesota Zoo, co-author of a new edition of the encyclopedic Tigers of the World and, with decades of fieldwork in Asia's tiger habitats under his belt, an authority — maybe the authority — on our most endangered big cat. "I've never seen a wild tiger."

There's more. "I'm actually allergic to tigers," continues Tilson, 66. "If I touch them, I break out in hives." He chuckles. "Figure that. I'm the world expert. Never seen one. And I'm allergic to them."

While the allergy is incurable, Tilson might yet see his first wild tiger, in a central Chinese wilderness he is playing an almost godlike role in creating. Thanks to a unique collaboration between Minnesota Zoo, China's State Forestry Administration (SFA), and a Bangkok-based environmental financier called International Consultancy Europe (ICE), a plan is under way to reintroduce the South China tiger, the rarest of the world's five surviving subspecies, back into its natural habitat. In this Year of the Tiger, the project has secured $3 million to restore a 250,000-acre (100,000 hectare) nature reserve straddling the borders of Hubei and Hunan provinces.

Half of this grant has been provided by the Chinese government, whose high-level interest in the project is easy to understand. Panthera tigris amoyensis is the progenitor of all modern tigers and the only subspecies unique to China. "You have a culture that reveres the tiger," says Tilson. "It's part of their fabric." By pulling a Chinese subspecies from the brink of extinction, China seeks not only to overturn an appalling record on conservation and the environment but also to gain a powerful new icon of national resurgence — not a cuddly giant panda this time but a formidable predator that eats herbivores for breakfast.

Tigers have never before been reintroduced to the wild. This is partly because scarce conservation resources are usually devoted to what Tilson calls "a failed strategy": protecting what few tiger habitats remain. "There needs to be a new paradigm," says Tilson. His answer? "Let's create wildernesses, as opposed to trying to protect the little fragments that are left." Hopes for resurrecting the South China subspecies rest largely on a captive population of 67 tigers, held in zoos across China. It will be challenging. Derived from just six animals — two male, four female — caught between 1958 and 1970, they are so inbred that they are virtually brothers and sisters. But, Tilson adds, "China is an economic juggernaut, a military powerhouse. As part of that portfolio they need to bring back the icon of Asian wilderness. And that's the tiger."

In half a century, the wild South China tiger population in China has been reduced from perhaps 4,000 to — Beijing disputes this — none. During Mao Zedong's time they were considered a pest and extermination campaigns were launched against them. Also taking a toll were loss of habitat, declining prey numbers and, as the economy took off, growing demand from traditional Chinese medicine for every part of the animal: whiskers, penis, bone, even feces.

Not one but four subspecies of tiger — Siberian, Indochinese, Bengal and South China — have been all but killed off within China's borders. In 1993, Beijing banned the nation's domestic trade in tigers and their parts and, today, China is one of 175 parties to the Convention on International Trade in Endangered Species of Wild Fauna and Flora, which outlawed tiger trafficking globally. But Chinese demand still drives a lucrative pan-Asian trade in poached tigers, which other countries blame for the accelerating decline in their own wild populations. In India, 88 tigers were killed in 2009 — double the previous year's figure.

China, where tiger-hunting was legal until 1977, is not the only country with a poor record of conservation. Tigers are also found in Bangladesh, Bhutan, Burma, Cambodia, India, Indonesia, Laos, Malaysia, Nepal, Russia, Thailand and Vietnam — but only just. A century ago, there were an estimated 100,000 tigers in the wild in Asia. Now their numbers are at an "all time low" of 3,200, estimates the WWF, which in January warned the animals could be extinct in the wild by 2022 — the next Year of the Tiger — unless new efforts are made to save them.

Today, penalties for harming tigers in China are harsh — a villager in Yunnan province was recently jailed for 12 years for killing and eating what might well have been the country's last wild Indochinese tiger. But the laws are patchily enforced. In December the SFA released a directive promising better protection of wild tigers and a sterner crackdown on the illegal trade. Many conservationists remain unconvinced. "We've heard these words before from China," says Mike Baltzer, leader of the World Bank – backed Global Tiger Initiative at the conservation group WWF. "We're waiting to see if they really have any teeth." Vivek Menon, executive director of the Wildlife Trust of India, says China's responsibilities are clear. "They've saved the panda," he says. "Now they must do the same for the tiger."

Tilson is used to skeptics. "Don't you read the newspapers?" he was asked by an outraged prospective donor in the U.S. "The goddamn Chinese eat their tigers and put them into medicine." But Tilson is convinced that China's economic and human resources make it uniquely placed to put tigers back in the wild. The South China project could help revolutionize Chinese attitudes to endangered species and kick-start other attempts to revitalize biodiversity. "China is at a tipping point in its conservation history," he says.

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