New stars are born in the Universe around the clock – on the Milky Way, currently about ten per year. From the birth rate in the past, we can generally calculate how populated space should actually be. But the problem is that the results of such calculations do not match our actual observations. "There should actually be a lot more stars that we can see," says Dr. Jan Pflamm-Altenburg, astrophysicist at the Argelander-Institut für Astronomie of the University of Bonn.
So, where are those stars?
For years, astronomers worldwide have been looking for a plausible explanation for this discrepancy. In cooperation with Dr. Carsten Weidner from St. Andrews University, Dr. Pflamm-Altenburg and Professor Dr. Pavel Kroupa, Professor of Astrophysics at the University of Bonn, may now have found the solution. It seems that so far, the birth rate has simply been overestimated. But this answer is not quite as simple as it sounds. Apparently, the error of estimation only occurs during periods of particularly high star production.
The reason for this lies in the manner in which astronomers calculate the birth rate. "For the local Universe – i.e., the Milky Way as our home and the adjacent galaxies – it is relatively simple," explains Professor Kroupa. "Here we are able to count the young stars one by one, using huge telescopes."
The problem with this method is that it only works for our immediate vicinity. But many galaxies are so distant that even the best telescope simply overlooks their small stars. As luck would have it, however, occasionally there is an especially large whopper among the newbie's in the sky. Such a star will, even if it cannot be directly discovered as an individual star, leave its traces in the light of even the farthest galaxies. The number of large whoppers then determines the strength of this trace.
In our immediate vicinity, these large whoppers occur with a fixed probability. There are always about 300 lightweights to one "big star baby." This numerical ratio seemed to be universal. So it was sufficient for astronomers to know the number of the large whoppers, for this allowed them to determine the number of new-born stars by simply multiplying the former number by a factor of 300.
Recently, however, some Bonn astronomers around Professor Kroupa began doubting the fixed ratio. Their hypothesis is that at times when the galactic nurseries are booming, they generate a considerably higher number of stellar heavies than normal. The reason for this, according to this theory, is so-called stellar crowding. For stars are not single children; they are born in groups, as so-called star clusters. At birth, these clusters are always of a similar size – no matter whether they contain 100 star embryos - or 100,000.
Consequently, at times of a high birth rate, space can be at a premium in star clusters. Astronomers call such galaxies that are particularly rich in mass "ultra-compact dwarf galaxies," or UCD's for short. In these, things are so tight that some of the young stars fuse during formation. Thus, more stars rich in mass than normal emerge. The "small to large" ratio is then only about 50 to 1. "In other words, we used to estimate the number of newly formed small stars by far too high," explains Dr. Carsten Weidner.
The researchers from Bonn and St. Andrews have now corrected the birth rates according to the projections of the stellar crowding theory. With an encouraging result – they actually arrived at the number of stars that can be seen today.
University of Bonn
Saturday, December 4, 2010
12/04/2010 01:55:00 PM Publicado por Alquimia
12/04/2010 01:55:00 PM Publicado por Alquimia
Airplanes do not look much like birds -- unless you were to imagine a really weird bird or a very strange plane -- but should they? This question is exactly what a pair of engineers in California and South Africa inadvertently answered recently when they set about re-thinking the ubiquitous tube-and-wings aircraft architecture from scratch in order to make airplanes more fuel efficient.
The modern airplane design works well, but from a fuel efficiency standpoint, could planes be designed more aerodynamically -- to lower drag and increase lift? Geoffrey Spedding, an engineer at the University of Southern California, and Joachim Huyssen at Northwest University in South Africa, felt they could in theory, but they lacked experimental evidence. Now they have it.
Spedding and Huyssen have made a simple modular aircraft in three configurations: a flying wing alone, then wings plus body, and then wings plus body and a tail. It turns out that they had independently re-designed a bird shape, but without specific reference to anything bird-like. They presented their experimental data with these three designs, at the American Physical Society Division of Fluid Dynamics meeting in Long Beach, CA.
They started with a configuration where the entire plane is one big wing. Then they added a body designed to minimize drag and, most critically, a small tail, which essentially serves to undo aerodynamic disturbances created by the body. Spedding and Huyssen analyzed the airflows and at various relative angles for the wings, body and tail, searching for ways to achieve greater lift (the better for carrying cargo) and lower drag (for higher fuel efficiency). They made the stipulation that for any given mission, the best plane is the one that generates the least drag.
The flying wings alone provide an ideal (but impractical) baseline, since it's hard to carry people or cargo in such a shape. The presence of a body, unfortunately, immediately lowers the lift and increases the drag. The addition of just the right kind of tail, however, can restore the lift, and reduce the drag, occasionally to nearly wing-only levels.
A few years ago a glider with the modest tail design was successfully test flown, but larger and commercial test prototypes have not yet been tried. Spedding recognizes that the design of real planes is necessarily a compromise of many engineering, economic and psychological constraints. Nevertheless, he believes much can be done to make planes more energy efficient in the future.
"The most important point is that we may be wasting large amounts of fossil fuel by flying in fundamentally sub-optimal aircraft designs," says Spedding. "At the very least, we can show that there exists an alternative design that is aerodynamically superior. One may argue that there is now an imperative to further explore this (and perhaps other) designs that could make a significant difference to our global energy consumption patterns."
(Photo: RJ Huyssen/NU,RSA)
American Institute of Physics
12/04/2010 01:52:00 PM Publicado por Alquimia
Hummingbirds rank among the world's largest and most accomplished hovering animals, but how do they manage it in gusty winds?
A team of researchers at New Mexico State University, Los Alamos National Laboratory, Technische Universiteit Eindhoven, and Continuum Dynamics Inc. has built a robotic hummingbird wing to discover the answer, which they have described at the American Physical Society Division of Fluid Dynamics (DFD) meeting in Long Beach, CA.
Hummingbirds do not fly like other birds, whose wings flap up and down, explained B.J. Balakumar of the Extreme Fluids Lab at Los Alamos National Laboratory. Instead, their wings oscillate in a figure eight pattern to produce lift on both the downstroke and upstroke. They achieve the extra lift they need to hover by creating a vortex on the leading edges of their wings.
Such vortices are inherently unstable. "The birds, though, are very clever," Balakumar said. "Their wings create the vortex with a high angle of attack on the downstroke. Then they flip their wings around on the upstroke, so as they shed one vortex, they create another on the other side of the wing, thereby managing to maintain high lift forces."
A gust of wind could pull those vortices off the wing. Instead, hummingbirds continually readjust their wing angles to maintain high lift forces.
The researchers' robotic wing will attempt to replicate that feat in gusty conditions. They hope to identify robust algorithms that will allow the creation of stable ornithopters that can operate reliably under real-life conditions for surveillance and other applications.
(Photo: New Mexico State University)
American Institute of Physics