A plasma cocoon lets growing stars keep their X-rays to themselves. Laboratory experiments that mimic maturing stars show that streams of plasma splash off a star’s surface, forming a varnish that keeps certain kinds of radiation inside.

That coating could explain a puzzling mismatch between X-ray and ultraviolet observations of growing stars, report physicist Julien Fuchs of École Polytechnique in Paris and colleagues November 1 in Science Advances.

Physicists think stars that are less than 10 million years old grow up by drawing matter onto their surfaces from an orbiting disk of dust and gas. Magnetic fields shape the incoming matter into columns of hot, charged plasma. The same disk will eventually form planets (SN Online: 11/6/14), so knowing how quickly stars gobble up the disk can help tell what kinds of planets can grow.
When disk matter hits a stellar surface, the matter heats to about 1,700° Celsius and should emit a lot of light in ultraviolet and X-ray wavelengths. Measuring that light can help scientists infer how fast the star is growing. But previous observations found that such stars emit between four and 100 times fewer X-rays than they should.

One theory why is that something about how a star eats absorbs the X-rays. So Fuchs and his colleagues re-created the feeding process in a lab. First, the team zapped a piece of PVC representing the edge of the disk with a laser to create plasma, similar to the columns that feed stars. In space, a star’s gravity draws the plasma onto its surface at speeds of about 500 kilometers per second. The star’s strong magnetic field guides the charged plasma into organized columns millions of kilometers long.
There’s not enough room or gravity in the lab to reproduce that exactly, but the plasma physics is the same on smaller scales, Fuchs says. His team applied magnetic fields up to 100,000 times stronger than Earth’s to the plasma to shape it into columns and accelerate it to the same speed it would have in space. The researchers placed a target made of Teflon representing the star’s surface just 11.7 millimeters away from the PVC, a distance equivalent to about 10 million kilometers in space.

When the plasma hits the Teflon surface, the plasma begins to ooze sideways. But the magnetic field that holds the plasma in a column stops the plasma’s spreading. Plasma and magnetic field push against each other until the buildup of pressure between them forces the plasma to curve away from the surface and back up the column, coating incoming plasma with outgoing plasma.

“This cocoon is building up,” Fuchs says. It absorbs enough X-rays to explain the surprisingly wimpy X-ray emission of growing stars, the experiment found. The team also compared the experiment setup with computer simulations of feeding stars to show that the lab configuration was a good representation of real stars.

The comparison with computer simulations makes the experiment more reliable, says experimental physicist Gianluca Gregori of the University of Oxford. “There is this reality check,” he says. “In the astrophysical community, there’s a tendency to think that there are observations, and there are simulations. But what this paper tells is that there are other ways you can understand what happens in the universe.”

People have evolved to sleep much less than chimps, baboons or any other primate studied so far.

A large comparison of primate sleep patterns finds that most species get somewhere between nine and 15 hours of shut-eye daily, while humans average just seven. An analysis of several lifestyle and biological factors, however, predicts people should get 9.55 hours, researchers report online February 14 in the American Journal of Physical Anthropology. Most other primates in the study typically sleep as much as the scientists’ statistical models predict they should.
Two long-standing features of human life have contributed to unusually short sleep times, argue evolutionary anthropologists Charles Nunn of Duke University and David Samson of the University of Toronto Mississauga. First, when humans’ ancestors descended from the trees to sleep on the ground, individuals probably had to spend more time awake to guard against predator attacks. Second, humans have faced intense pressure to learn and teach new skills and to make social connections at the expense of sleep.

As sleep declined, rapid-eye movement, or REM — sleep linked to learning and memory (SN: 6/11/16, p. 15) — came to play an outsize role in human slumber, the researchers propose. Non-REM sleep accounts for an unexpectedly small share of human sleep, although it may also aid memory (SN: 7/12/14, p. 8), the scientists contend.

“It’s pretty surprising that non-REM sleep time is so low in humans, but something had to give as we slept less,” Nunn says.

Humans may sleep for a surprisingly short time, but Nunn and Samson’s sample of 30 species is too small to reach any firm conclusions, says evolutionary biologist Isabella Capellini of the University of Hull in England. Estimated numbers of primate species often reach 300 or more.
If the findings hold up, Capellini suspects that sleeping for the most part in one major bout per day, rather than in several episodes of varying durations as some primates do, substantially lessened human sleep time.

Nunn and Samson used two statistical models to calculate expected daily amounts of sleep for each species. For 20 of those species, enough data existed to estimate expected amounts of REM and non-REM sleep.

Estimates of all sleep times relied on databases of previous primate sleep findings, largely involving captive animals wearing electrodes that measure brain activity during slumber. To generate predicted sleep values for each primate, the researchers consulted earlier studies of links between sleep patterns and various aspects of primate biology, behavior and environments. For instance, nocturnal animals tend to sleep more than those awake during the day. Species traveling in small groups or inhabiting open habitats along with predators tend to sleep less.

Based on such factors, the researchers predicted humans should sleep an average of 9.55 hours each day. People today sleep an average of seven hours daily, and even less in some small-scale groups (SN: 2/18/17, p. 13). The 36 percent shortfall between predicted and actual sleep is far greater than for any other primate in the study.

Nunn and Samson estimated that people now spend an average of 1.56 hours of snooze time in REM, about as much as the models predict should be spent in that sleep phase. An apparent rise in the proportion of human sleep devoted to REM resulted mainly from a hefty decline in non-REM sleep, the scientists say. By their calculations, people should spend an average of 8.42 hours in non-REM sleep daily, whereas the actual figure reaches only 5.41 hours.

One other primate, South America’s common marmoset (Callithrix jacchus), sleeps less than predicted. Common marmosets sleep an average of 9.5 hours and also exhibit less non-REM sleep than expected. One species sleeps more than predicted: South America’s nocturnal three-striped night monkey (Aotus trivirgatus) catches nearly 17 hours of shut-eye every day. Why these species’ sleep patterns don’t match up with expectations is unclear, Nunn says. Neither monkey departs from predicted sleep patterns to the extent that humans do.

This baby bird had barely hatched before it died 127 million years ago — and its lack of fully developed bony breastbone, or sternum, suggests it couldn’t yet fly. The tiny fossil, just a few centimeters long, is giving paleontologists a rare window into the early development of a group of extinct birds called Enantiornithes, researchers report March 5 in Nature Communications.

Previous studies of juvenile Enantiornithes have shown that the sternums of these birds ossified in a pattern different from modern and other ancient birds. The sternum’s ossification — a process in which the cartilage is replaced by bone — is a prerequisite to withstand the stresses of flight. But which parts of the sternum fuse first varies widely among modern birds. Those patterns are reflected in modern birds’ life histories, such as how soon birds can fly and how long they rely on their parents after hatching.
Similar diversity existed in how Enantiornithes developed too, the new study suggests. The baby bird’s sternum was still mostly cartilage at death, but some parts were beginning to turn to bone, which fossilized. That ossification pattern differed markedly from patterns in other known juvenile Enantiornithes, the researchers found.
It’s harder to say how these developmental features might have related to behavior. Although the baby bird couldn’t yet fly, it still might have been able to leave the nest. That’s also true of certain modern birds: Some plover chicks can walk and feed themselves shortly after hatching, but take a little longer to fly.