Over four millennia ago, in the final days of the Akkadian Empire in Mesopotamia, a drought swept over the region, afflicting lands as far away as Greece and what’s now Pakistan. Probably driven by the eruption of a distant volcano, the drying climate devastated local agriculture. A contemporary text, The Curse of Akkad, noted that “the large arable tracts yielded no grain … the irrigated orchards yielded no syrup or wine, thick clouds did not rain.” As once-prosperous farmlands collapsed in the northern part of the empire, people fled to the south. The southern Akkadians’ response? Build a more than 150-kilometer-long wall between the Tigris and Euphrates rivers, barring entry to any migrants. Soon after, history’s first empire crumbled, dying of thirst in the cradle of civilization.
Climate systems and civilizations are stable only up to a point. In Our Fragile Moment, climate scientist Michael Mann reminds us that today we are pushing the limits of both.
In the book, Mann looks back at episodes of global climate change over the last 4.5 billion years, from eras of deadly heat to wastelands of widespread ice. With each instance, he draws out lessons about what happens to Earth in periods of changing climate. Sometimes, the result is dramatic mass extinctions or geologic upheavals (SN: 8/28/15). Other times, as with the Akkadians, it’s societal collapse.
Earth’s climate system includes regulating forces that tend to buffer against small shifts in climate; ice caps and low clouds reflect sunlight and help cool the planet, for instance. But pushed too far, regulating forces can be overwhelmed, causing the climate to spiral out of control.
This was the case 55 million years ago. As a steady set of volcanic eruptions spewed carbon dioxide into the air, Earth warmed. The heat may have contributed to thinner and less reflective clouds. This in turn would have made the planet even hotter. Eventually, the low-lying clouds disappeared, and average global temperatures soared to 32° Celsius (90° Fahrenheit) in what is referred to as a Hothouse Earth (SN: 11/3/15).
Today, with unchecked greenhouse gas emissions, we may face a similar, though less sweltering, spiral with the disappearance of our reflective ice caps (SN: 11/9/22).
But what makes current climate change different is its source — humankind — and our ability to stop it. This is a benefit that is unique to our changing climate. It comes with blame, but it also comes with agency.
That agency is an important source of hope for Mann. Melting ice caps could raise sea levels and displace some 40 percent of the global population. Rising heat could make swaths of the planet uninhabitable (SN: 5/8/20). But if we act, we can preserve a world that looks much like ours. The limit is not geologic or even technological, Mann argues; it is political.
Despite the far-reaching themes Mann weaves throughout the book, it may not be for everyone. There is a strong academic bend to the writing, which leans heavily on jargon. The book also features a dizzying parade of researchers, and Mann often emphasizes his connection to climate researchers and events, at one point reminiscing about how he “was known as a bit of a statistics guru.” The technical terms, acronyms, initialisms and self-referential tangents can distract from the book’s broader arguments and message.
Even though Mann’s dedication to precise academic language comes at the expense of some clarity, climate buffs will appreciate the deep dives into the scientific process. Many of the dense sections reward the reader with a satisfying tidbit of fascinating information or an illuminating insight. On occasion, I laughed out loud at Mann’s puns, jokes and barbs. (A reference to The Princess Bride’s ROUSs — Rodents of Unusual Size — landed particularly well.) After journeying through the past, Mann brings us to the present and looks toward the future. Though past climates may offer lessons, those lessons only go so far. We are unlikely to bring about another Hothouse Earth, but the climate is warming faster than it has in millennia, thanks to human actions. If current climate policy holds, the best scientific predictions show things will be painful, but civilization won’t end. But climate scientists are not oracles. They can’t be sure.
That uncertainty, rather than being a cause for complacency, should spur us to action, Mann argues. “The impacts of climate change, no doubt, constitute an existential threat if we fail to act,” Mann concludes. “But we can act. Our fragile moment can still be preserved.”
Sometimes it’s surprising to discover how little we know about common plants or animals. Consider the ruby-throated hummingbird. If you live in the eastern half of Canada or the United States and have spotted a hummingbird hovering around a feeder in the backyard in summer, this is the bird you saw. But while scientists have documented many of the feeding and mating behaviors of the birds and that the birds migrate south to Central America and Cuba, there are still plenty of mysteries, such as whether the birds go the long way through Mexico when they migrate or whether they take a shortcut across the Gulf of Mexico.
It turns out that the tiny birds, some of which are small enough to fit in your hand, could easily take the shortcut, even though they’d get no break on the journey. Based on analyses of wing shape, body size and fat reserves, some of these tiny birds could fly more than 2,000 kilometers in the right winds. That’s more than enough to get them the 1,000 kilometers across the Gulf, researchers report March 9 in The Auk. Theodore Zenzal Jr. and Frank Moore of the University of Southern Mississippi in Hattiesburg studied ruby-throated hummingbirds at the Bon Secour National Wildlife Refuge in Alabama, one of the birds’ stopovers on their journey south. From 2010 to 2014, they captured birds in the refuge during late summer and early fall. Birds were weighed, measured, banded and released.
Zenzal and Moore found that older birds tended to arrive at the refuge earlier and stayed for shorter times than younger birds. They also had more fat that could fuel a long voyage, and older males had the most. Based on these fuel loads, the birds could fly for another 2,260 kilometers on average without stopping for food, the team calculates.
That was just the average, though. Some very skinny birds arrived at the refuge, and had enough fat for just a short trip of less than 20 kilometers. This may explain why some hummingbirds stuck around in the refuge for a couple of weeks — they may have needed to bulk up before taking off again. Other birds had plenty of fat, though, enough to go more than 4,000 kilometers.
Hummingbirds’ small size may actually be an advantage when it comes to long-distance flight, the researchers note. These birds are really good at taking in a lot of fuel, and being small means that they can carry a larger percentage of their body weight as fat than can larger birds.
But just because the hummingbirds may be capable of taking the shortcut across the water doesn’t mean they actually do. Weather patterns aren’t favorable for such a flight until late fall, Zenzal and Moore say. So it may make more sense, especially for juveniles, to take the long way around since there are opportunities for pit stops should they be needed.
With the help of a neural prosthesis, a quadriplegic man used his paralyzed right hand to grab a bottle, swipe a credit card and play a guitar video game. Bypassing his damaged spinal cord, the system restored his ability to use his thoughts to command his hand to move.
Other neural prosthetic systems have allowed paralyzed people to use their brain activity to move computer cursors, robotic limbs and wheelchairs (SN: 11/16/13, p. 22). But the new approach, described online April 13 in Nature, is the first to use brain activity to control a person’s own limb. “We literally are reconnecting the brain to the body,” study coauthor Chad Bouton of the Feinstein Institute for Medical Research in Manhasset, N.Y., said April 12 in a news briefing. Decoding brain signals and correctly stimulating muscles are “really hard things to do individually,” says biomedical engineer Levi Hargrove of the Rehabilitation Institute of Chicago. Putting those together in a human subject is “very impressive,” he says. “There’s more work to be done, of course, but this is very positive and should excite people.”
In 2010, college student Ian Burkhart dived into a shallow wave and struck sand. The accident severed his spinal cord, leaving him paralyzed from the shoulders down. Burkhart volunteered to undergo brain surgery in which doctors implanted a patch of electrodes directly into his brain. These electrodes eavesdropped on the activity of nerve cells that control hand movements.
Scientists listened to these cells’ behavior as Burkhart watched a range of hand and finger movements on a screen and attempted to copy the motions. A computer system then learned to recognize the neural signals that accompanied each type of movement, and an algorithm translated those signals into movement commands. A flexible sleeve of electrodes strapped to Burkhart’s forearm delivered those instructions directly by stimulating hand muscles. In 2014, Bouton and colleagues announced that Burkhart could open and close his hand using the system. Since then, Burkhart has been able to command more complex hand movements, such as wiggling his thumb in and out and flexing his wrist. The Nature paper describes how this bypass system now allows him to pick up a cup, pour and even pinch his thumb and forefinger together to pluck a skinny stir stick. “The first time when I was able to open and close my hand, it really kind of gave me that sense of hope again for the future,” Burkhart said in the briefing.
The technology isn’t ready for life outside of the lab. In its current form, the system must be calibrated each time Burkhart uses it, and the electrodes in the brain may not perform as well with time. And bulky cables connect the brain electrodes to the computer system and forearm sleeve. Scientists are working on making the technology smaller, wireless and easier to use, study coauthor Nick Annetta of Battelle Memorial Institute in Columbus, Ohio, said in the briefing.
Neural engineer José Contreras-Vidal of the University of Houston points out that technology that can translate neural activity into electrical impulses may ultimately restore other types of muscle activity, such as walking. “What we need to do is provide solutions and options,” he says.
There’s nothing like a guy doing all the child care to win female favor, even among giant water bugs.
Thumbnail-sized Appasus water bugs have become an exemplar species for studying paternal care. After mating, females lay eggs on a male’s back and leave him to swim around for weeks tending his glued-on load.
For an A. major water bug, lab tests show an egg burden can have the sweet side of attracting more females, researchers in Japan report May 4 in Royal Society Open Science. Given a choice of two males, females strongly favored, and laid more eggs on, the one already hauling around 10 eggs rather than the male that researchers had scraped eggless.
Females still favored a well-egged male even when researchers offered two males that a female had already considered, but with their egg-carrying roles switched from the previous encounter. That formerly spurned suitor this time triumphed.
A similar preference, though not as clear-cut, showed up in the slightly smaller and lighter A. japonicus giant water bug. “We conclude that sexual selection plays an important role in the maintenance of elaborate paternal care,” says study coauthor Shin-ya Ohba of Nagasaki University.
Brain waves during REM sleep solidify memories in mice, scientists report in the May 13 Science.
Scientists suspected that the eye-twitchy, dream-packed slumber known as rapid eye movement sleep was important for memory. But REM sleep’s influence on memory has been hard to study, in part because scientists often resorted to waking people or animals up — a stressful experience that might influence memory in different ways.
Richard Boyce of McGill University in Montreal and colleagues interrupted REM sleep in mice in a more delicate way. Using a technique called optogenetics, the researchers blocked a brain oscillation called theta waves in the hippocampus, a brain structure involved in memory, during REM sleep. This light touch meant that the mice stayed asleep but had fewer REM-related theta waves in their hippocampi. Usually, post-learning sleep helps strengthen memories. But mice with disturbed REM sleep had memory trouble, the researchers found. Curious mice will spend more time checking out an object that’s been moved to a new spot than an unmoved object. But after the sleep treatment, the mice seemed to not remember objects’ earlier positions, spending equal time exploring an unmoved object as one in a new place. These mice also showed fewer signs of fear in a place where they had previously suffered shocks.
Interfering with theta waves during other stages of sleep didn’t seem to cause memory trouble, suggesting that something special happens during REM sleep.
Sea turtles do not have an easy start to life. After hatching, they have to break out of their shell, dig their way out from beneath the sand, then make a mad dash across the beach to the water where they may or may not find food and safety — hopefully without getting snapped up by a predator. All of this requires a bit of luck and a lot of energy. And the energy a hatchling expends on breaking out of the nest is energy that can’t be used on surviving the rest of the journey.
Now, a new study has quantified the amount of energy a baby sea turtle uses to dig itself to the surface. Having lots of siblings — and, thus, lots of help — can really be a time and energy saver, researchers report May 18 in the Journal of Experimental Biology. That also implies that the conservation technique of dividing clutches may instead make hatchlings worse off.
Figuring out the energy expenditure of baby sea turtles took some trial and error. Mohd Uzair Rusli of the University of Malaysia Terengganu and colleagues started by burying newly hatched green turtles beneath 40 centimeters of beach sand, but the hatchlings never started digging and the researchers abandoned the experiment after 48 hours. They suspected that the turtles might need a pocket of air, something that would naturally be found in between eggs.
The team then tried eggs that were just starting to hatch, orienting them so that the top of the egg — where a turtle had started to emerge — would be toward the sand surface. But instead of digging upwards, many of the turtles dug toward the side of the big sand-filled chamber. The researchers thought that the babies may have been drawn to light entering through the transparent chamber walls. “It appears that they can be attracted to light even when buried underground,” they note. This is perhaps not all that surprising given that researchers knew that baby turtles use cues from the sun to emerge most often at night or on cloudy days.
For the final experiment, the scientists buried clutches of eggs just about to hatch beneath 40 centimeters of beach sand in a chamber with opaque walls. Just above the eggs sat a strip of aluminum foil that, when broken, signaled the start of the digging-out process. A 24-hour webcam monitored the top of the sand so researchers could see when digging ended. The whole setup was then enclosed so that the scientists could measure oxygen consumption — a stand-in for energy expenditure. And the team was careful to stay quiet near the experiment, because they learned that talking near the buried turtles prompted the tiny hatchlings to dig.
Escaping from the sand took between 3.7 and 7.8 days, with larger clutches taking less time to emerge and also using less oxygen per hatchling. Digging behavior was not consistent during the whole time; the oxygen consumption rate rose and fell in peaks as the turtles dug and dug and dug together, rested and then started again. “In nature, it is likely that hatchlings receive a significant benefit by belonging to a large clutch,” the team concludes. They use less energy in their escape, leaving more for the mad dash to the sea and finding a first meal.
The researchers note that in some regions of the world, it is a common conservation strategy to split up clutches when relocating them into hatcheries. But this practice, they warn, could leave baby turtles with reduced energy reserves when they reach the ocean.
Americans have climate change to thank for a decades-long spate of milder winters. Around 80 percent of U.S. residents live in counties where the weather has become more pleasant over the last four decades (see map). That trend won’t last, however: Researchers predict in the April 21 Nature that 88 percent of Americans will experience noticeably worse weather by 2100 than they do today.
The researchers created a weather pleasantness index to rank weather conditions. Hot, humid summers cost points, while mild winters added points. In the contiguous United States, winter warming has outpaced increases in summertime temperature and humidity. But if greenhouse gas emissions continue unabated, summer weather will become less pleasant over the coming decades, potentially sparking increased public interest in combating climate change, the researchers predict.
An experimental drug swept sticky plaques from the brains of a small number of people with Alzheimer’s disease over the course of a year. And preliminary results hint that this cleanup may have staved off mental decline.
News about the new drug, an antibody called aducanumab, led to excitement as it trickled out of recent scientific meetings. A paper published online August 31 in Nature offers a more comprehensive look at the drug’s effects.
“Overall, this is the best news that we’ve had in my 25 years doing Alzheimer’s clinical research,” study coauthor Stephen Salloway of Brown University said August 30 at a news briefing. “It brings new hope for patients and families most affected by the disease.” The results are the most convincing evidence yet that an antibody can reduce amyloid in the brain, says Alzheimer’s researcher Rachelle Doody of Baylor College of Medicine in Houston, who was not involved in the study.
Still, experts caution that the results come from 165 people, a relatively small number. The seemingly beneficial effects could disappear in larger clinical trials, which are under way. “These new data are tantalizing, but they are not yet definitive,” says neuroscientist John Hardy of University College London.
Like some other drug candidates for Alzheimer’s, aducanumab is an antibody that targets amyloid-beta, a sticky protein that accumulates in the brains of people with the disease. Delivered by intravenous injection, aducanumab appeared to get inside the brains of people with mild Alzheimer’s (average age about 73) and destroy A-beta plaques, the results suggest. After a year of exposure to the drug, A-beta levels had dropped. This reduction depended on the dose — the more drug, the bigger the decline in A-beta. In fact, people on the highest dose of the drug had almost no A-beta plaques in their brains after a year.
“I know of no other antibody that leads to this degree of amyloid removal,” study coauthor Alfred Sandrock of Biogen in Cambridge, Mass., said at the news briefing. For several decades, scientists have been trying to figure out whether A-beta is a cause, or just a symptom, of Alzheimer’s (SN: 3/12/11, p. 24). With its ability to reduce A-beta plaques in the brain, aducanumab may help settle the debate.
The bigger question is whether the drug can preserve thinking skills and memory. The new study was not designed to detect improvements in mental performance. Yet it turned up hints that aducanumab may help. Compared with participants who received a placebo, people who took aducanumab showed less decline on standard tests of memory and thinking skills over the course of a year. And like the reductions in brain amyloid, better performance seemed to come with higher doses. During the study, people who received the placebo lost just under three points on average on a 30-point cognitive test. In contrast, people on the highest dose of aducanumab lost a little over half a point.
“One needs to take the cognitive data with a grain of salt at the moment, given the small number of people who enrolled and completed the study,” says neuroscientist Eric Reiman of the Banner Alzheimer’s Institute in Phoenix. But if larger studies show a similar benefit, “it would be a game changer for the field,” says Reiman, who wrote an accompanying commentary in Nature.
Aducanumab targets several forms of A-beta — including both small, soluble bits called oligomers and larger clumps called fibrils — that make up plaques. Both forms may cause trouble. Once aducanumab sticks to A-beta, specialized brain cells called microglia may come in and remove the buildup, lab experiments suggest.
Twenty-seven people in the study had an adverse drug reaction known as ARIA, marked by changes in brain fluid detected by brain scans. The side effect is often without symptoms, but can cause headaches or more serious trouble in some people. ARIA was more common at higher doses, the researchers found. The larger studies of aducanumab that are under way may help scientists pinpoint the most effective dose with the fewest side effects.
This is not chemist Phil Baran’s first rodeo: It’s clear that he has done media interviews before. In his Scripps Research Institute office perched above a golf course along the Pacific Ocean in La Jolla, Calif., he is at ease, helpful and patient answering basic questions — why is it important to develop a new way to make a carbon-carbon bond? — as well as opining about how chemists would be better off going after more private research funding, why mentorship is still the best model for science training and his sense that the public (unjustly) thinks of chemistry as a bad word. People take for granted the many products that we rely on every day, he says. At 39, Baran is still young but already experienced and accomplished enough (he has been a tenured professor with his own lab for more than a decade) to be wary of the breathless attention journalists and prizes can bring.
That attention has come in many forms, notably a MacArthur Fellowship (worth $625,000) in 2013 and more than a dozen other awards, the most recent one from the New York Academy of Sciences. He’s been featured in write-ups in Forbes, Chemical & Engineering News, Chemistry World magazine and The Scientist, among others. He travels often, consulting with a long list of Big Pharma companies. His lab has a blog, Open Flask, and he has cowritten an e-textbook aimed at research chemists. He is frequently mentioned on organic chemistry insider blogs and appears to have his own devoted internet troll (a sure sign of fame). He has even been featured in a local fashion shoot for Modern Luxury. Reporters often call him for comments on other people’s work. But when I met him in his office this summer, he didn’t dwell on any of this. What Baran wants to talk about is chemistry. “Phil is extraordinarily passionate about his science,” says Andrew G. Myers, a synthetic chemist at Harvard University. “It is clear that he is driven by his love of chemistry.”
Baran’s specialty is designing sophisticated chemical routes to concoct large, complex molecules from scratch, or from commercially available starter materials. It’s like cooking, but much more precise and finicky. His field, known as total synthesis, largely targets natural molecules first isolated from plants or other organisms. Such molecules are used in everything from fragrances and flavorings to pesticides, and they are hugely important in the drug development pipeline.
Most such molecules are easier and cheaper to extract from their natural sources than to make in the lab. But in some cases the plants might be rare (as was the case with the Pacific yew tree that produces the anticancer drug Taxol) or yield small amounts insufficient even for testing. Synthetic techniques also enable scientists to create closely related derivatives of a natural molecule, which may prove more useful than the original. A few decades ago, total synthesis targeted natural molecules as a tour de force of pure chemical prowess, earning accolades and advancing techniques but concocting pathways too complicated and costly to be practical. Baran’s work represents a shift in the field.
“I’m less interested in proving that something can be made chemically,” Baran says. “We want to invent new chemistry that will be useful.”
The lab is Baran’s element. It’s a calling, he says, what he can’t wait to get back to. “I have always found playing with chemicals an escape of sorts,” he says. He doesn’t recall ever being that interested in science as a child, and was not a motivated student until he found science as a high school sophomore, taking an astronomy class at a local community college in Florida, where he grew up. While simultaneously earning his high school diploma and an associate’s degree, and working long hours at Friendly’s, he took high school chemistry, where his teacher indulged his interest in reactions by letting him drop in to the lab and do experiments. “I liked mixing things and making new forms of matter. It was really cool,” Baran says.
As an undergraduate, he studied with chemist David Schuster at New York University, where he would spend long hours in the lab, and chemistry became more important to him than breathing oxygen, he says. “There was no end goal. I was more like an artist.” He did, however, become fluent in the language of molecules. From there, he got a spot in the Ph.D. program at Scripps, where he worked in the lab of synthetic chemist K.C. Nicolaou, a giant in the field. “I was an animal,” Baran says. “I had a singular purpose.” It wasn’t until his third or fourth year of graduate school that he even went to the beach.
Earning his doctorate at 24, he did a postdoc with another star of organic synthesis, E.J. Corey, a Nobel laureate at Harvard. “As a graduate student, you get paid to play in the sandbox, essentially. You are starting to correlate reality with your imagination,” Baran says. “But as a postdoc, you have real responsibility. You have to translate ideas in your head to things in the hood.”
That responsibility jumped again when, at age 25, Baran returned to Scripps to run his own lab. “It’s exhilarating and also terrifying.” Managing and funding a team, and dealing with not just your own lab failures but other people’s, that was new and challenging to someone who had lived for the chemistry itself. Now, the focus is on “sustaining the momentum,” Baran says. “I’m constantly thinking about what’s next.”
He claims he works less now than he once did, but he’s still at the lab 12 hours a day during the week, and half a day on Saturday. Surrounded by drawings made by his three young children, an old-fashioned candy dispenser filled with jelly beans and a sleek black desk, he prints out just a few of his team’s recent papers. Science, Nature, Science. A whiteboard covers the wall across from his desk — it’s littered with drawings of chemical structures, reaction designs as well as cartoons, including one of a Santa Claus the size of a fire hydrant. None of this hides Baran’s intensity. “His productivity is nothing short of remarkable,” Myers says. Many of his team’s more than 100 papers describe quicker routes to synthesizing a molecule. He and colleagues have cooked up ingenol, a molecule whose derivatives have antitumor potential, in 14 steps and were among the first to synthesize the technically complex palau’amine in 25.
Baran does this by a kind of high-risk, fearless approach to reactions, says his postdoc Joel Smith — instead of designing elaborate chemical routes filled with detours, added to protect one part of a molecule while changing another, for example, he looks for more direct paths. “He’s looking for something creative, what he calls ‘powerful,’” says Smith, who has been working on making thapsigargin, an intimidating-looking molecule made previously in a 42-step synthesis, in a new 11-step protocol. In another example, Baran’s team found a new way to make phorbol, a molecule chemists have known about for more than 80 years. For the last 40 years, they’ve struggled to build it efficiently. Last April in
Nature
, Baran’s group reported a route that would create the complex molecule in
just 19 steps
— a huge improvement over the 40 to 52 steps scientists had needed before. Like most products first isolated from plants or other natural sources, phorbol is gangly, a jungle gym of a molecule with a basic structure of carbon dotted with other chemical groups, and (to a synthetic chemist) a disturbing number of oxygens in inconvenient spots. But molecules related to phorbol are of great interest to pharmaceutical companies.
Drawing inspiration from biological systems, Baran’s team relied on a new strategy, based on a two-step technique used to successfully synthesize ingenol in 2013. It involves first building the carbon skeleton (in phorbol’s case, that’s 20 atoms) and then adding the oxygens — mimicking the way that natural enzymes build the molecule within plants. The result still doesn’t compete with natural product isolation in terms of yield, but the strategy will be helpful for creating related molecules.
His philosophy, outlined in 2010 in a well-known Journal of Organic Chemistry paper on what he and coauthor Tanja Gaich call “ideality,” is to hew closely to the way that nature makes molecules. Sometimes this means recombining older techniques in new ways or inventing new chemical strategies. In January in Science, for example, he reported on a new technique to add “spring-loaded” carbon-carbon and carbon-nitrogen bonds to certain kinds of ringed molecules important for drug discovery. In May, also in Science, Baran’s team described a way to use alkyl carboxylic acids, cheap starting materials that are easy to work with, to introduce new carbon-carbon bonds to complex molecules.
Baran says he wants to choose targets for synthesis that have the most potential to help people, which is why so much of his work is now done in collaboration with companies like Pfizer, Bristol-Myers Squibb and the Danish firm LEO Pharma. Like many in his field, he’s drawn to solving real-world problems. But it’s also obvious that Baran finds the creativity of synthetic chemistry the biggest draw. “Total synthesis is a venture into partly unknown territory,” he says. “It’s like building a house while trying to juggle and play chess in 15 dimensions all at the same time.”
Our local galactic neighborhood might be more expansive than previously thought. Rather than being stuck in some backwater galactic community, our solar system lives along a major spiral arm of the Milky Way, researchers report online September 28 in Science Advances.
Astronomers suspected that our arm of the Milky Way — the Orion Arm — was just a bridge connecting two bands of stars and gas, the Sagittarius and Perseus spiral arms that wrap around the galaxy. Ye Xu, an astronomer at Purple Mountain Observatory in Nanjing, China, and colleagues measured distances to about two dozen stellar nurseries and found that they — and the sun — are scattered along an arm over 20,000 light-years long that parallels the two neighboring arms. This arc of a presumably larger spiral arm is comparable in length to what we can see of the Sagittarius and Perseus arms.
Despite being nestled inside the Milky Way, we know surprisingly little about it. Like trying to map a forest while standing among the trees, our view of the galaxy is limited, blocked by walls of interstellar gas and dust. Astronomers must infer the overall structure of the Milky Way through measurements like these and by comparisons to other nearby galaxies.
