Monday, October 25, 2010


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Persistent exposure to light at night may lead to weight gain, even without changing physical activity or eating more food, according to new research in mice.

Researchers found that mice exposed to a relatively dim light at night over eight weeks had a body mass gain that was about 50 percent more than other mice that lived in a standard light-dark cycle.

“Although there were no differences in activity levels or daily consumption of food, the mice that lived with light at night were getting fatter than the others,” said Laura Fonken, lead author of the study and a doctoral student in neuroscience at Ohio State University.

The study appears this week in the online early edition of the Proceedings of the National Academy of Sciences.

If the mice are not less active or eating more, what’s causing the bigger weight gain? Results suggest that mice living with light at night eat at times they normally wouldn’t.

In one study, mice exposed to light at night – but that had food availability restricted to normal eating times – gained no more weight than did mice in a normal light-dark cycle.

“Something about light at night was making the mice in our study want to eat at the wrong times to properly metabolize their food,” said Randy Nelson, co-author of the study and professor of neuroscience and psychology at Ohio State.

If these results are confirmed in humans, it would suggest that late-night eating might be a particular risk factor for obesity, Nelson said.

In one study, mice were housed in one of three conditions: 24 hours of constant light, a standard light-dark cycle (16 hours of light at 150 lux, 8 hours of dark), or 16 hours of daylight and 8 hours of dim light (about 5 lux of light).

The researchers measured how much food the mice ate each day. They also measured how much they moved around their cages each day through an infrared beam crossing system. Body mass was calculated each week.

Results showed that, compared to mice in the standard light-dark cycle, those in dim light at night showed significantly higher increases in body mass, beginning in the first week of the study and continuing throughout.

By the end of the experiment, light-at-night mice had gained about 12 grams of body mass, compared to 8 grams for those in the standard light-dark cycle. (Mice in constant bright light also gained more than those in the standard light-dark cycle, but Nelson said the dim light-at-night mice were better comparisons to the light exposure that humans generally get.)

The dim light-at-night mice also showed higher levels of epididymal fat, and impaired glucose tolerance – a marker of pre-diabetes.

Although the dim light-at-night mice didn’t eat more than others, they did change when they ate, results showed. These mice are nocturnal, so they would normally eat substantially more food at night. However, the dim light-at-night mice ate 55 percent of their food during the daylight hours, compared to only 36 percent in the mice living in a standard light-dark cycle.

Since the timing of eating seemed significant, the researchers did a second study, similar to the first, with one important difference: instead of having food freely available at all times, food availability was restricted to either the times when mice would normally be active or when they would normally be at rest.

In this experiment, mice exposed to the dim light at night did not have a greater gain in body mass than did the others when their food was restricted to times when they normally would be active.

“When we restricted their food intake to times when they would normally eat, we didn’t see the weight gain,” Fonken said. “This further adds to the evidence that the timing of eating is critical to weight gain.”

The findings showed that levels of corticosterone, a stress hormone, were not significantly different in dim light-at-night mice compared to those living in a standard light-dark cycle.

That’s important because corticosterone has been linked to changes in metabolism, Fonken said. This shows there doesn’t have to be changes in corticosterone levels to have changes in metabolism in the mice.

So how does light at night lead to changes in metabolism? The researchers believe the light could disrupt levels of the hormone melatonin, which is involved in metabolism. In addition, it may disrupt the expression of clock genes, which help control when animals feed and when they are active.

Overall, the findings show another possible reason for the obesity epidemic in Western countries.

“Light at night is an environmental factor that may be contributing to the obesity epidemic in ways that people don’t expect,” Nelson said. “Societal obesity is correlated with a number of factors including the extent of light exposure at night.”

For example, researchers have identified prolonged computer use and television viewing as obesity risk factors, but have focused on how they are associated with a lack of physical activity.

“It may be that people who use the computer and watch the TV a lot at night may be eating at the wrong times, disrupting their metabolism,” Nelson said. “Clearly, maintaining body weight requires keeping caloric intake low and physical activity high, but this environmental factor may explain why some people who maintain good energy balance still gain weight.”

(Photo: OSU)

Ohio State University


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A giant star in a faraway galaxy recently ended its life with a dust-shrouded whimper instead of the more typical bang.

Ohio State University researchers suspect that this odd event -- the first one of its kind ever viewed by astronomers – was more common early in the universe.

It also hints at what we would see if the brightest star system in our galaxy became a supernova.

In a paper published online in the Astrophysical Journal, Christopher Kochanek, a professor of astronomy at Ohio State, and his colleagues describe how the supernova appeared in late August 2007, as part of the Spitzer Space Telescope Deep Wide Field Survey.

The astronomers were searching the survey data for active galactic nuclei (AGN), super-massive black holes at the centers of galaxies. AGN radiate enormous amounts of heat as material is sucked into the black hole. In particular, the astronomers were searching for hot spots that varied in temperature, since these could provide evidence of changes in how the material was falling into the black hole.

Normally, astronomers wouldn’t expect to find a supernova this way, explained then-Ohio State postdoctoral researcher Szymon Kozlowski. Supernovae release most of their energy as light, not heat.

But one very hot spot, which appeared in a galaxy some 3 billion light years from Earth, didn’t match the typical heat signal of an AGN. The visible spectrum of light emanating from the galaxy didn’t show the presence of an AGN, either – the researchers confirmed that fact using the 10-meter Keck Telescope in Hawaii.

Enormous heat flared from the object for a little over six months, then faded away in early March 2008 – another clue that the object was a supernova.

“Over six months, it released more energy that our sun could produce in its entire lifetime,” Kozlowski said.

The astronomers knew that if the source were a supernova, the extreme amount of energy it emitted would qualify it as a big one, or a “hypernova.” The temperature of the object was around 1,000 Kelvin (about 700 degrees Celsius) -- only a little hotter than the surface of the planet Venus. They wondered -- what could absorb that much light energy and dissipate it as heat?

The answer: dust, and a lot of it.

Using what they learned from the Spitzer survey, the astronomers worked backward to determine what kind of star could have spawned the supernova, and how the dust was able to partly muffle the explosion. They calculated that the star was probably a giant, at least 50 times more massive than our sun. Such massive stars typically belch clouds of dust as they near the end of their existence.

This particular star must have had at least two such ejections, they determined – one about 300 years before the supernova, and one only about four years before it. The dust and gas from both ejections remained around the star, each in a slowly expanding shell. The inner shell – the one from four years ago – would be very close to the star, while the outer shell from 300 years ago would be much farther away.

“We think the outer shell must be nearly opaque, so it absorbed any light energy that made it through the inner shell and converted it to heat,” said Kochanek, who is also the Ohio Eminent Scholar in Observational Cosmology.

That’s why the supernova showed up on the Spitzer survey as a hot dust cloud.

Krzysztof Stanek, professor of astronomy at Ohio State, said that stars probably choked on their own dust much more often in the distant past.

“These events are much more likely to happen in a small, low metallicity galaxy,” he said -- meaning a young galaxy that hadn’t been around long enough for its stars to fuse hydrogen and helium into the more complex chemicals that astronomers refer to as “metals.”

Still, Kozlowski added that more such supernovae will likely be found by NASA’s Wide-field Infrared Explorer (WISE), which was launched in December 2009. “I would expect WISE to see 100 of these events in two years, now that we know what to look for,” he said.

Because of the alignment of the galaxy with Earth and our sun, astronomers were not able to see what the event looked like to the naked eye while it was happening. But Kochanek believes that we might see the star brighten a decade or so from now. That’s how long it will take for the shockwave from the exploding star to reach the inner dust shell and slam it into the outer shell. Then we’ll have something to see here on Earth.

We do have at least one chance to see a similar light show closer to home, though.

“If Eta Carinae went supernova right now, this is what it would probably look like,” Kochanek said, referring to the brightest star system in our Milky Way Galaxy.

The two stars that make up Eta Carinae are 7,500 light years away, and they host a distinctive dust shell dubbed the Homunculus Nebula, among other layers of dust. Astronomers believe that the nebula was created when the larger of the two stars underwent a massive eruption around 1840, and that future eruptions are likely.

(Photo: NASA/JPL-Caltech/R. Hurt)

Ohio State University


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Physicists at Rutgers University have discovered new properties in a material that could result in efficient and inexpensive plastic solar cells for pollution-free electricity production.

The discovery, posted online and slated for publication in an upcoming issue of the journal Nature Materials, reveals that energy-carrying particles generated by packets of light can travel on the order of a thousand times farther in organic (carbon-based) semiconductors than scientists previously observed. This boosts scientists’ hopes that solar cells based on this budding technology may one day overtake silicon solar cells in cost and performance, thereby increasing the practicality of solar-generated electricity as an alternate energy source to fossil fuels.

“Organic semiconductors are promising for solar cells and other uses, such as video displays, because they can be fabricated in large plastic sheets,” said Vitaly Podzorov, assistant professor of Physics at Rutgers. “But their limited photo-voltaic conversion efficiency has held them back. We expect our discovery to stimulate further development and progress.”

Podzorov and his colleagues observed that excitons – particles that form when semiconducting materials absorb photons, or light particles – can travel a thousand times farther in an extremely pure crystal organic semiconductor called rubrene. Until now, excitons were typically observed to travel less than 20 nanometers – billionths of a meter – in organic semiconductors.

“This is the first time we observed excitons migrating a few microns,” said Podzorov, noting that they measured diffusion lengths from two to eight microns, or millionths of a meter. This is similar to exciton diffusion in inorganic solar cell materials such as silicon and gallium arsenide.

“Once the exciton diffusion distance becomes comparable to the light absorption length, you can collect most of the sunlight for energy conversion,” he said.

Excitons are particle-like entities consisting of an electron and an electron hole (a positive charge attributed to the absence of an electron). They can generate a photo-voltage when they hit a semiconductor boundary or junction, and the electrons move to one side and the holes move to the other side of the junction. If excitons diffuse only tens of nanometers, only those closest to the junctions or boundaries generate photo-voltage. This accounts for the low electrical conversion efficiencies in today’s organic solar cells.

“Now we lose 99 percent of the sunlight,” Podzorov noted.

While the extremely pure rubrene crystals fabricated by the Rutgers physicists are suitable only for laboratory research at this time, the research shows that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors. Continuing development could result in more efficient and manufacturable materials.

The scientists discovered that excitons in their rubrene crystals behaved more like the excitons observed in inorganic crystals – a delocalized form known as Wannier-Mott, or WM, excitons. Scientists previously believed that only the more localized form of excitons, called Frenkel excitons, were present in organic semiconductors. WM excitons move more rapidly through crystal lattices, resulting in better opto-electronic properties.

Podzorov noted that the research also produced a new methodology of measuring excitons based on optical spectroscopy. Since excitons are not charged, they are hard to measure using conventional methods. The researchers developed a technique called polarization resolved photocurrent spectroscopy, which dissociates excitons at the crystal’s surface and reveals a large photocurrent. The technique should be applicable to other materials, Podzorov claims.

(Photo: Rutgers University)

Rutgers University




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