Astronomy & Science

Long-Ago Supernovae Showered Earth with Debris

Sky&Telescope -

Roughly 2 million years ago, as the human ancestor homo erectus was descending from the trees, two supernovae exploded nearby and showered Earth with debris.

Supernovae Simulations Carve Local Bubble

The mass density distribution of iron-60 associated with the Local Bubble (as well as a neighboring bubble) 2.2 million years ago.
Michael Schulreich

It’s a classic doomsday scenario. A nearby star explodes in a brilliant supernova, pumping out more energy in a split second than the Sun will emit in a billion years. The blast showers Earth with radioactive elements that destroy the ozone layer and genetically mutate life. But even though astronomers think a nearby star (that is, within 100 light-years of Earth) explodes every million years or so — although not every one has such devastating results — definitive proof has been hard to pin down.

For more than half a century, scientists have recognized two tantalizing clues that nearby supernovae might have showered the Earth roughly 2 million years ago. The first hint lies deep under the Pacific Ocean, where an isotope of iron, iron-60, is embedded within the crust. Although the Earth’s formation likely produced some iron-60, its half-life is only 2.6 million years and it would have long since disappeared. So the layers must be the result of something more recent, and nearby supernovae are the likely culprits.

The other hint is the Local Bubble — a vast peanut-shaped and plasma-filled cavity surrounding the Sun. Nearby supernova probably carved out this bubble as well.

Now Dieter Breitschwerdt (Berlin Institute of Technology) and colleagues have put these pieces together to pinpoint the likely locations of ancient supernovae. The results published April 7th in Nature, show that two supernovae, both roughly 300 light-years away, exploded 1.5 million and 2.3 million years ago.

Pinpointing Vanished Stars

To find stars that likely died millions of years ago, Breitschwerdt’s team started with their surviving family. All stars are born within clusters of hundreds to thousands of stars across a wide range of masses. A cluster’s highest-mass stars explode first, while lower-mass siblings live longer lives. So when astronomers spot a cluster made of only low-mass stars, they assume the missing high-mass stars have already gone supernovae.

After digging through archived Hipparcos data, Breitschwerdt and his colleagues found just the family they were looking for: a group of some 70 low-mass stars. The team then estimated the masses of heavyweight stars presumed missing from that cluster — which told them how long those stars would have lived — in order to pin down exactly when those stars would have exploded.

Breitschwerdt and his colleagues calculated that 16 supernovae went off like popcorn during the past 13 million years. They ran computer simulations to show how those supernovae might have carved a bubble in space — and the results perfectly matched maps of the Local Bubble.

This result alone was exciting enough. But Breitschwerdt wanted to see if he could also make the leap between this result and the iron-60 deposited on the ocean floor.

The presence of iron-60 — an isotope that’s almost exclusively created in supernova explosions — in Earth’s deep-sea crusts allowed the team to nail down a specific time-period to look for the supernovae explosions. “It's a layer that grows very slowly, which means that each successive explosion deposits new iron-60 atoms,” says Breitschwerdt. “It's like the rings in trees. If you just count the rings then you know the age of the tree.” But rather than counting tree rings, the astronomers counted iron-60 layers, and found that one layer was deposited roughly 2.2 million years ago.

It was immediately evident that two of their supernovae had occurred around that time. But in order to verify that these supernovae were the true culprits, the team first had to calculate how the iron-60 fused in the stellar core gets mixed into the blast wave that eventually hits Earth.

“It’s like if you put milk in your coffee, the milk starts to diffuse out, and then you take the spoon and stir it in order to distribute it,” says Breitschwerdt. “The supernova explosions are like spoons: they stir the iron-60 into the surrounding medium. And that has to be calculated in detail to find how long it takes to travel to Earth.”

Breitschwerdt’s calculations show that two supernovae — one that occurred 2.3 million years ago and one that occurred 1.5 million years ago — contributed roughly half of all the iron-60. The rest comes from all the other supernovae combined.

Fortuitously, another paper released in Nature by Anton Wallner (Australian National University) reports iron-60 in crust samples from four different locations in the Pacific, Atlantic, and Indian Oceans. Evidence from multiple locations is exactly what scientists expect to see, given that supernovae would have rained the isotope down across the entire globe.

Did Homo Erectus Feel the Blast?

These nearby supernovae would have occurred shortly after human ancestors descended from the trees as Homo erectus. But would these chimp-like humans have been affected by the celestial explosions?

Adrian Melott (University of Kansas) is working on that answer now. The Nature papers refer to supernovae within several hundred light-years, but supernovae have to be much closer to do any real damage, he says. “What we call the kill zone — where you get a really big mass extinction — is like 8 or 10 parsecs [26 to 33 light-years],” Melott notes.

So the effects of supernovae at several hundred light-years from Earth won’t be large. But will they be noticeable? Save for a brilliant flash in the sky, it could be that these celestial events went over our ancestors’ heads.

References:

D. Breitschwerdt et al. “The Locations of Recent Supernovae Near the Sun from Modelling 60Fe Transport.”Nature. April 7, 2016.

A. Wallner et al. “ Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60Fe.”Nature. April 7, 2016.

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Nuevo estado de la materia detectado en un material bidimensional

Ciencia Kanija -

Artículo publicado por Sarah Collins el 4 de abril de 2016 en la Universidad de Cambridge

Los investigadores han observado la “huella” de un misterioso y nuevo estado cuántico de la materia, en el cual los electrones se rompen, en un material bidimensional.

Un equipo internacional de investigadores ha encontrado pruebas de un misterioso y nuevo estado de la materia, predicho por primera vez hace 40 años, en un material real. Este estado, conocido como líquido cuántico de espín, provoca que los electrones – que se piensa que son bloques básicos indivisibles de la naturaleza – se rompan en pedazos.

majorana-fermion-device-jpg_152002

Concepto de dispositivo que usa fermiones de Majorana Crédito: Harlan Evans

Los investigadores, que incluyen a físicos de la Universidad de Cambridge, midieron las primeras señales de estas partículas fraccionadas, conocidas como fermiones de Majorana, en un material bidimensional con una estructura similar a la del grafeno. Sus resultados experimentales encajan con uno de los principales modelos teóricos para un líquido cuántico de espín, conocido como modelo de Kitaev. Se informa de los resultados en la revista Nature Materials.

Los líquidos cuánticos de espín son misteriosos estados de la materia que se cree que se ocultan en ciertos materiales magnéticos, pero que no se habían observado de modo concluyente en la naturaleza.

La observación de una de las propiedades más intrigantes — el fraccionamiento, o división, de los electrones — en materiales reales es un avance. Los fermiones de Majorana resultantes pueden usarse como bloques básicos para los computadores cuánticos, que podrían ser más rápidos que los computadores convencionales, y podrían realizar cálculos que de otro modo serían imposibles de llevar a cabo.

“Éste es un nuevo estado cuántico de la materia, que ha sido predicho, pero no observado, anteriormente”, señala el Dr. Johannes Knolle del Laboratorio Cavendish de Cambridge, uno de los coautores del artículo.

En un material magnético común, los electrones se comportan como minúsculos imanes. Y cuando un material se enfría hasta una temperatura lo suficientemente baja, los ‘imanes’ se ordenarán, de modo que los polos norte magnéticos apunten en la misma dirección, por ejemplo.

Pero en un material que contiene un estado de líquido de espín, incluso si el material se enfría hasta el cero absoluto, los imanes no se alinearían, sino que formarían una sopa entrelazada que provocaría fluctuaciones cuánticas.

“Hasta hace poco ni siquiera sabíamos qué aspecto tendrían las huellas experimentales de un líquido cuántico de espín”, comenta el coautor del artículo, el Dr. Dmitry Kovrizhin, también del grupo de Teoría de Materia Condensada del Laboratorio Cavendish. “Algo que habíamos hecho en anteriores trabajos es preguntarnos: si realizamos experimentos sobre un posible líquido cuántico de espín, ¿qué observaríamos?”.

Los coautores Knolle Kovrizhin, dirigidos por el Dr. Arnab Banerjee y el Dr. Stephen Nagler del Laboratorio Nacional Oak Ridge, en los Estados Unidos, usaron técnicas de dispersión de neutrones para buscar pruebas experimentales de fraccionalización en el cloruro de alfa-rutenio (α-RuCl3). Los investigadores pusieron a prueba las propiedades magnéticas del polvo α-RuCl3 iluminándolo con neutrones, y observando el patrón de ondas que los neutrones producían en una pantalla cuando se dispersaban a partir de la muestra.

Un imán normal crearía unas lineas definidas muy distintivas, pero era un misterio qué tipo de patrón crearían los fermiones de Majorana en un líquido cuántico de espín. La predicción teórica de las firmas realizada por Knolle y sus colaboradores en 2014 encajan bien con los grandes montículos en lugar de las líneas definidas que los experimentadores observaron en la pantalla, proporcionando por primera vez una prueba directa de un líquido cuántico de espín y la fraccionalización de electrones en un material bidimensional.

“Ésta es una nueva adición a una corta lista de estados cuánticos conocidos de la materia”, apunta Knolle.

“Es un avance importante en nuestra comprensión de la materia cuántica”, explica Kovrizhin. “Es divertido tener otro estado cuántico nuevo que nunca antes habíamos observado – nos muestra nuevas posibilidades para probar nuevas cosas”.

Referencias

A. Banerjee et al. ‘Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet.’ Nature Materials (2016). DOI: 10.1038/nmat4604

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Up, Up, and Away with Comet 252P

Sky&Telescope -

With the Moon finally put to bed and Comet 252P still bright, there's no better time than now to see it. Nearby Mars and Saturn only sweeten the deal. 

252P Drags Its Tail

Comet 252P/LINEAR displays a faint, 30′ long ion tail  pointing west in this photo taken through a 10-inch reflector on April 3, 2016.
Michael Jaeger

A wonderful thing happened this week. The morning Moon exited the sky, and Comet 252P/LINEAR finally came into its own. Naked-eye sightings began to trickle in as early as April 1–2, while the moon was still a thick crescent. Most magnitude estimates at the time put the comet around +5.5.

On April 1st, I suspected the 252P with the naked eye using averted vision but saw it plainly in 10×50 binoculars from a moderately dark site, even in moonlight. The coma was impressive — a soft, smoky patch almost 1° across with a slightly brighter center. No tail was visible.

Comet observers rate how the brightness of a comet increases towards its center — its Degree of Condensation (DC) — on a scale from 0 to 9, where 0 indicates a diffuse coma of uniform brightness and 9 a stellar coma. DC increases as the coma shrinks and become more sharply defined.  Unlike many comets that possess a bright nuclear region, 252P coma has been consistently rating a DC of 2: diffuse with a slight but definite brightening toward the center.

Streak and Smudge

This wide-angle photo of 252P/LINEAR was made at the start of dawn on April 1st and gives a good impression of the comet through binoculars. The comet is the fuzzy spot to upper left of the streak made by a tumbling satellite that happened to pass through the field during the 15-second time exposure.
Bob King

Australian amateur Chris Wyatt and others report that a Swan Band comet filter strongly enhances the comet's contrast and brightness. The filter isolates the green light of diatomic carbon emissions in gaseous comets like 252P, making them stand out, the same way a nebular filter enhances an emission nebula. I've often used it at the telescope, but Wyatt held it up to his eye to get a better naked-eye view.

As of April 5th, the comet hovers at the naked-eye limit of magnitude +6. Yes, it's fading, but not so fast that you can't still get a wonderful view in either binoculars or telescope. As seen in a 10-inch or larger scope, 252P fills most of the field of view and reveals a very faint, not-quite-stellar nucleus. Since it's still moving along at a fairly good clip — still more than 1° a day or about 3′ per hour — you can detect its motion in a half hour or less through a scope.

Roller Coaster Ramble

Use this map to help you track Comet 252P/LINEAR through June. The comet's position is marked every three days at 5 a.m. local DST starting April 5th. Stars are labeled with their Greek (Bayer) letters and Flamsteed numbers and plotted to magnitude +8.0. Saturn's and Mars’s positions are for early April. All the Greek-lettered stars are bright enough to see with the naked eye.
Chris Marriott’s SkyMap

Plus it's all gravy from here on out: no Moon to madden, and the comet climbs steadily northward through the constellation Ophiuchus. Only recently has 252P begun to show a tail. Photos taken in the past few days show a stub of an ion tail issuing from the nuclear region. Why no tail until now? It's probably a matter of spatial perspective. Earlier, we were looking down the tail. Now that the comet has passed Earth, we're seeing it more from the side. Try playing around with this interactive 3D orbital diagram of 252P/LINEAR created by Dominic Ford and see what you think.

Whisper of a Tail

A deep exposure made on April 5th shows the tail more clearly. The coma measures 80′ across, the ion tail, 100′.
Michael Jaeger

252P is best viewed just before the start of dawn when it stands well up in the southern sky not far from the meridian. Use the accompanying map to help guide you to its lair. As payback for the trouble of getting up and losing sleep, two other bonbons await — Mars and Saturn. Time with Saturn is never wasted. Is there a more unique object anywhere in the celestial sphere? The rings are within a fraction of a degree of fully open, their north face tilted in our direction. What more of an invitation do you need?

Then there's Mars. I examined it right after the comet and was frankly shocked at how big it's grown. (Guess I need to get out more.) Over the coming week, the Red Planet swells in the eyepiece from 12.6″ to 13.5″  as it hurries toward its May 22nd opposition.

Bonbons Anyone?

Don't pass up the chance to view Mars and Saturn after looking at the comet. Both planets are located in the same neighborhood in the constellation Ophiuchus, not far from Antares in Scorpius.
Bob King

For western hemisphere observers, the feature-rich half of Mars has been slowly coming around. During the next few mornings, watch for the dark thumb of Mare Acidalium in the northern hemisphere and the chicken drumstick-shaped Sinus Meridiani/Sinus Sabeus closer to the equator. A week from now (around April 14–20), the planet's most prominent feature, Syrtis Major, an enormous ancient shield volcano, will dominate the center of the disk. You might notice a faux polar cap to the south of Syrtis. What you're really seeing are bright clouds over the huge impact basin Hellas.

Mars Map

These bright and dark albedo features on Mars, which bear traditional names, are what observers are most likely to see with a telescope. North is up; longitude is labeled along the bottom. Click on the image to see a larger map.

If you want to know which side on Mars faces your way at any hour, visit and bookmark Sky & Telescope's Mars Profiler. Set your alarm and enjoy a fantastic pre-dawn sky!

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Crescent Moon To Cover Aldebaran in Blue Sky

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Crescent Moon occults Aldebaran

Seconds before the waxing crescent Moon covered Aldebaran exactly 19 years earlier, on the night of April 10, 1997, Los Angeles astrophotographer Michael Stecker caught this shot using a 5-inch refractor. (The image in the homepage slideshow is a colored photo-illustration to suggest Sunday's blue-sky view.)

On Sunday afternoon, April 10th, you'll see the waxing crescent Moon hanging high in a sunny blue sky if the weather is as good as we hope it'll be. Look with a telescope from nearly anywhere in North America, and you can find something else too. Somewhere in the Moon's vicinity will be 1st-magnitude Aldebaran: a tiny yellow-orange spark shimmering in the blue. And if you're watching at the right moment, you can see Aldebaran snap out of view in the blink of an eye as the Moon's dark limb (invisible in the daylight!) covers it up.

The farther east you are, the later in the afternoon the occultation will happen, so the lower the Sun will be and the more readily you should be able to sweep up Aldebaran.

The event happens a little after sunset in much of the Canadian Maritimes. Along the East Coast from Maine to Miami, the Sun will still be about 3° to 12° above the western horizon. Farther west the Sun will be higher and the sky brighter, so the clarity of your air will matter more. Only northern Canada and Alaska miss the occultation completely.

Aldebaran will reappear from out from behind the Moon's bright limb up to an hour or more later, when the Sun will be lower and even, for the East Coast, may have recently set. But Aldebaran will be harder to see as it buds out from the sunlit moonscape, so you'd still need the telescope.

The Moon will be a thinnish, 17%-illuminated crescent. You’ll find it about 50° to the Sun's celestial east.

Some predicted times of the star's disappearance and reappearance, at cities from east to west:

Halifax: disappearance 8:00 p.m., reappearance 8:56 p.m. ADT.
Montreal,  d. 6:47,  r. 7:48 p.m. EDT.
Boston,  d. 6:51,  r. 7:56 p.m. EDT.
Toronto,  d. 6:39,  r. 7:46 p.m. EDT.
Washington, DC,  d. 6:43,  r. 7:55 p.m. EDT.
Atlanta,  d. 6:34,  r. 7:49 p.m. EDT.
Miami,  d. 6:57,  r. 7:50 p.m. EDT.
Chicago,  d. 5:25,  r. 6:39 p.m. CDT.
Kansas City,  d. 5:11 , r. 6:31 p.m. CDT.
Austin,  d. 5:07, r.  6:22 p.m. CDT.
Winnipeg,  d. 5:17,  r. 6:13 p.m. CDT.
Denver,  d. 3:51,  r. 5:11 p.m. MDT.
Edmonton,  d. 4:12, r.  4:36 p.m. MDT.
Vancouver,  d. 2:48,  r. 3:28 p.m. PDT.
Berkeley,  d. 2:21,  r. 3:37 p.m. PDT,
Los Angeles,  d. 2:21,  r. 3:42 p.m. PDT
Honolulu,  d. 10:29,  r. 11:28 a.m. HST.

Detailed timetables for 570 cities and towns, including in Mexico and the Caribbean, are on the massive website of the International Occultation Timing Association (IOTA). In the timetables are the altitudes of the Sun and Moon at the time of the event for each location.

That link brings up three tables: for Aldebaran's disappearance, reappearance, and the locations of the cities. The three are stacked without very obvious demarcations between them, so watch for the breaks as you scroll. The two capital letters designate the country; remember CA means Canada, not California.

Can you get good pix of this event? Post 'em in our Gallery!

 

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U.S. Navy Resumes Celestial Navigation Training

Sky&Telescope -

Modern security threats have brought back an old method — celestial navigation — to help U.S. Navy sailors navigate the high seas.

Night sky at sea

Brief star trails mark the sky above the Arleigh Burke-class guided-missile destroyer USS Preble as it patrols the U.S. 7th Fleet area of responsibility.
Mass Communication Specialist 3rd Class Paul Kelly / U.S. Navy

It's a scene straight out of a Tom Clancy novel. An adversary, seeking to gain the upper hand, manages to blind GPS satellites in a first strike. As alert levels rise and military leaders attempt to assess the situation, ships at sea must somehow get an accurate fix of their position . . . without the use of modern technology.

The United States Navy recently recognized modern vulnerabilities by bringing back an old method for navigating at sea: the Naval Academy in Annapolis, Maryland, has just resumed training officers in the lost art of celestial navigation. Although this training used to be standard in the U.S. Navy, the advent of GPS technology so simplified and improved the ability to find a ship's position at sea that the Navy ROTC ended celestial navigation training in 2000, and the U.S. Naval Academy phased it out as well in 2006.

But the U.S. Navy and the Department of Defense are taking cyber threats to technological infrastructure seriously. Commercial GPS jammers are now readily available on the internet. And while the U.S., China, the European Union, Russia, and India are all moving to assure they have their own exclusive GPS network in orbit, a deliberate attack may not even be necessary. A space debris chain reaction known as an ablation cascade could knock out our GPS capability, or a strong Earth-directed solar storm such as the 1859 Carrington super-flare event could do the job just as well.

"There is an effort throughout the Navy for midshipmen, officers, and sailors to become more familiar and comfortable with celestial navigation," says Lt. Daniel Stayton, an instructor of the U.S. Naval Academy CELNAV course. "We are currently in the first steps of reintroduction back into the fleet."

The U.S. Naval Academy brought back celestial navigation theory for its 2015 summer session, and the graduating class of 2017 will be the first in more than a decade with basic instruction in celestial navigation theory.

U.S. Navy officer using sextant at sea

A U.S. Navy officer uses a sextant at sea.
Mass Communication Specialist 2nd Class Scott Raegen / U.S. Navy

Losing Our Way — and Finding It Again

This sort of "back to basics" approach echoes a growing refrain: our over-reliance on GPS has made navigational skills all but vanish. My wife and I are the first to admit, it's both amazing and a little scary just how reliant we've become on Google Maps as we travel in Europe. Then again, I also remember that same GPS technology used to lead would-be visitors to our Florida home down a nearby dead-end street.

CELNAV instruction

Lt. Daniel Stayton instructs a classroom on celestial navigation in the Navy's CELNAV course.
Mass Communication Specialist 2nd Class Tyler Caswell / U.S. Navy

Although the U.S. military pioneered the development of GPS technology in the 1970s, widespread civilian use only came after May 2000, when President Clinton directed the removal of intentional inaccuracy (known as selective availability, a precaution to assure an enemy wouldn't utilize GPS) that had been built in to early commercially available systems. The U.S. Navy now typically uses the Voyage Management System developed by Northrop Grumman for navigation at sea.

But for centuries sailors had found their way using the stars.

Celestial Navigation: The Basics

In the Northern Hemisphere, it's pretty straightforward to find latitude by noting the elevation of the star Polaris, which marks the celestial north pole. Navigators can also measure the Sun's elevation at noon. Discerning longitude, however, is trickier and requires accurate timekeeping.

Celestial Sphere

Measuring stars' positions on the celestial sphere (particularly their right ascension) is crucial to celestial navigation. (Click for bigger view.)
University of Durham

From the surface of the Earth, the imaginary sphere of the sky along the celestial equator appears to rotate 15 degrees per hour. In fact, we mark off right ascension in the sky — analogous to longitude of the surface of the Earth — in hours, minutes, and seconds. A land-based observatory will therefore see a celestial object rise, transit the local meridian bisecting the sky from north to south, and set at a different time than a ship at sea.

A navigator would note stars' positions using a sextant to measure the angle between each star and the horizon. Then the navigator would compare the measurement to an almanac published for a particular location. With this painstaking method, a seasoned celestial navigator could reckon a ship's position down to about two kilometers.

Getting an accurate fix on longitude, however, had to wait for accurate timekeeping. Early efforts relied on complex lunar tables. By the mid-19th century, such methods improved and longitude measurements became increasingly accurate. Some clever methods were devised as well: Christopher Columbus and Captain Cook both made use of lunar eclipses to gain one-time measurements of their positions at sea. Astronomers of the day also proposed using the transits of Jupiter's moons in a similar fashion, though observing such events from the deck of a pitching ship at sea proved problematic.

Apollo astronauts even practiced celestial navigation on their missions to the Moon, in the event that a loss of communication meant they had to find their own way home.

"Redundancy improves readiness," notes Stayton. "If the Navy is able to add an additional layer of redundancy . . . by merely teaching a skill, then an opportunity exists to improve readiness throughout the fleet."

It's great to see an old method brought back to guide the modern tech-savvy military. Perhaps we should all take a cue from the U.S. Navy and keep those paper maps handy the next time we head out on our next GPS-guided adventure . . . just in case.

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El lado oscuro de la religión

Ciencia Kanija -

Artículo publicado el 5 de abril de 2016 en el Instituto Max Planck

Cómo los sacrificios humanos rituales ayudaron a crear unas sociedades desiguales.

Los sacrificios humanos rituales desempeñaron un papel central ayudando a aquellos en la cima de la jerarquía social a mantener el poder sobre los que estaban por debajo. Éste es el hallazgo clave de un estudio publicado hoy en la revista Nature. Investigadores de la Facultad de Psicología de la Universidad de Auckland, del Instituto Max Planck para Ciencias de la Historia Humana, en Alemania, y de la Universidad Victoria, querían poner a prueba el vínculo entre lo desigual, o jerárquica, que era una cultura – lo que se conoce como estratificación social – y los sacrificios humanos.

human sacrifice

Sacrificio humano

“La religión se ha visto tradicionalmente como una guía de moralidad y cooperación, pero nuestro estudio ha hallado que los rituales religiosos tenían también un papel más siniestro en la evolución de las sociedades modernas”, señala el autor principal del estudio, Joseph Watts, de la Universidad de Auckland.

El equipo de investigación usó métodos computacionales derivados de la biología evolutiva para analizar datos históricos procedentes de 93 culturas ‘austronesias’. La práctica de los sacrificios humanos estaba extendida por toda Austronesia: 40 de las 93 culturas incluidas en el estudio practicaron alguna forma de asesinato humano ritual. El término austronesio  se refiere a una gran familia de lenguajes cuyo país de origen es Taiwan y cuya distribución abarca gran parte del Océano Índica y partes del Pacífico. Las culturas austronesias forman una especie de laboratorio natural para los estudios interculturales, dado que tienen una enorme variedad de religiones, lenguajes, tamaños y formas sociales, y se sitúan en distintas regiones climáticas y geográficas.

Los métodos de asesinato ritual en estas culturas eran diversos y, a veces, extremadamente crueles. La razón para estas muertes eran, por ejemplo, el enterramiento de un jefe, la inauguración de un nuevo barco, o una casa, o el castigo por la violación de las tradiciones o  tabúes. Las víctimas normalmente pertenecían a un estatus social bajo, como esclavos, aunque los instigadores normalmente eran de una capa social superior, como sacerdotes o jefes.

El estudio dividió las 93 culturas distintas en tres grupos principales de estratificación social alta, moderada, o baja. Se encontró que las culturas de alto nivel de estratificación era más probable que practicasen sacrificios humanos (67%, o 18 de 27). De las culturas con una estratificación moderada, el 37% usó sacrificios humanos (17 de 46) y las sociedades más igualitarias eran las menos propensas a practicar el sacrificio humano (25%, o 5 de 20).

“Mediante el uso del sacrificio humano para castigar las violaciones de los tabúes, desmoralizar a las clases bajas, e infundir miedo a las élites sociales, los poderosos lograban mantener y construir un control social”, señala Joseph Watts.

Russell Gray, Director del Departamento de Lingüística y coautor del estudio, señala que “el sacrificio humano proporcionó un medio particularmente efectivo de control social, debido a que proporcionaba una justificación sobrenatural para el castigo. Los gobernantes, tales como jefes o sacerdotes, a menudo creían que eran descendientes de los dioses, y los sacrificios humanos rituales eran de demostración definitiva de su poder”.

Un rasgo único de la investigación fue que el uso de los métodos tomados de la evolución computacional permitieron al equipo reconstruir la secuencia de cambios en los sacrificios humanos y el estatus social a lo largo de la historia en el Pacífico. Esto permitió al equipo poner a prueba si el sacrificio precedió o siguió a los cambios en el estatus social.

El coautor, Quentin Atkinson, señala que: “Lo que hemos encontrado es que el sacrificio era la fuerza motriz, haciendo que las sociedades fuesen más propensas a adoptar estatus sociales altos y menos a volverse una estructura social igualitaria”.

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Láseres gigantes podrían ocultar la Tierra de los ojos alienígenas

Ciencia Kanija -

Artículo publicado por Belinda Smith el 4 de abril de 2016 en Cosmos Magazine

Aunque buscamos de forma activa señales de vida en otros planetas, ¿quién dice que los extraterrestres no están haciendo lo mismo?, y ¿queremos que nos encuentren? Belinda Smith informa de una capa de invisibilidad a nivel planetario.

Hacer que la Tierra sea invisible a los ojos extraterrestres, según unos astrónomos de la Universidad de Columbia, en Nueva York, podrías ser posible gracias a unos potentes láseres.

laser

Láser Crédito: Andrea Pacelli

En un artículo teórico publicado en la revista Monthly Notices of the Royal Astronomical Society, David Kipping y Alex Teachey esbozan cómo podrían crear una capa planetaria hecha de luz, si los alienígenas mirasen en nuestra dirección.

La idea subyacente es una técnica de búsqueda de planetas conocida como el “método de tránsito”. Cuando un planeta pasa entre nosotros y su estrella, la luz estelar disminuye ligeramente, y podemos detectar esta diferencia de luz.

Aunque esto sólo nos permite “ver” exoplanetas directamente en nuestro plano, unos tres cuartos de los exoplanetas descubiertos entre 2005 y 2015 se hallaron usando el método del tránsito, de los cuales, más del 80% se captaron usando el telescopio espacial Kepler.

Y buscar una disminución en la luz estelar es sólo la punta del iceberg. Para ampliar la visión de los exoplanetas habitables (o que potencialmente podrían contener vida), la “espectroscopia de tránsito” busca señales de vida molecular, tales como oxígeno o contaminantes procedentes de la actividad industrial, que absorben varias longitudes de onda de la luz estelar cuando se filtra a través de ellas.

Debido que los humanos han desarrollado el método del tránsito, tal como indican Kipping y Teachey, es factible que otra civilización lo suficientemente avanzada también lo haya logrado. Y si no vienen en son de paz, lo mejor será que quedemos fuera de su detección.

Podemos evitar la detección mientras orbitamos al Sol si emitimos la misma cantidad de luz que bloqueamos, cancelando de un modo efectivo la “atenuación” de la luz estelar. Una forma de lograr esto sería construir enormes espejos controlables, pero esto sería muy caro.

Pero, ¿qué hay de los láseres?

Bueno, ya tenemos tecnología láser. Algunos telescopios, como el Very Large Telescope en Chile, usan brillantes láseres para tomar imágenes más claras.

Además, ni siquiera se necesitaría que el láser estuviese encendido todo el tiempo. La pareja calculó que se necesitaría un láser de 30 megawatts apuntando hacia un sistema estelar que pudiese observarnos mientras estamos en una etapa detectable del tránsito, sólo unas 10 horas cada año.

Esta “capa de banda ancha” podría protegernos en luz visible, pero nos dejaría a la vista en otros rangos del espectro de luz, como el ultravioleta o el infrarrojo.

Extender el láser a esas longitudes de onda más cortas y largas requeriría un ejército de láseres ajustables que pudiese barrer todo el espectro a alta velocidad. Esta “capa cromática” sería un enorme consumidor de energía: calcularon que consumiría unos 250 megawatts.

Y en el otro extremo del espectro de la solución láser está la “biocapa”, donde sólo se ocultan las señales atmosféricas que delatan nuestra presencia, tales como el oxígeno.

Esto, comenta Teachey, “puede lograrse con una energía de láser de apenas 160 kilowatts por tránsito. Para otra civilización parecería que la vida nunca arraigó en la Tierra”.

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It’s International Dark-Sky Week!

Sky&Telescope -

In 2002, high-school student Jennifer Barlow had a simple idea: let's take some time to appreciate the beauty of the cosmos and consider ways to reduce the spread of light pollution. Here's how you can join the celebration!

Milky Way from Arches National Park

This is how the summer Milky Way looks in a sky without light pollution. It's a spectacular backdrop for Delicate Arch in Arches National Park near Moab, Utah.
Jacob W. Frank / NPS

Have you ever stepped outside to take in a view of the starry sky overhead — only to have your celestial panorama ruined by the ugly glow of light pollution? Maybe it's coming from a neighbor's security light or the combined glow over your whole town. Either way, light pollution prevents virtually all of us from viewing a pristinely beautiful night sky.

Back in 2002, a Virginia high-school student named Jennifer Barlow decided to do something about this, at least in her hometown of Midlothian, Virginia. First, she encouraged her friends and neighbors just to enjoy the beauty of a star-filled night sky — then she urged them to take some modest, common-sense steps to reduce the light pollution that outdoor lighting creates.

Barlow is all grown up now (she's earning a Ph.D. in Spanish literature at the University of Virginia), but her simple idea has blossomed into an annual event called International Dark-Sky Week. It's always celebrated in April, which is also Global Astronomy Month. This year celebrations begin Monday, April 4th, and run through Sunday, April 10th.

Now, as then, IDSW's goals are simple:

  • Inspire people to celebrate the beauty of the night sky
  • Raise awareness about the negative effects of light pollution
  • Embolden everyday citizens to take action!
Harsh lights in residential neighborhood

Do these harsh, glaring lights look like those in your neighborhood or, worse, the ones in your front yard?
S&T: J. Kelly Beatty

These days, IDSW is coordinated by the International Dark-Sky Association, whose members campaign year-round for better outdoor lighting. The goal is not to simply turn off all lights at night. Like it or not, we've become a 24/7 society — and, like it or not, nighttime activity requires some illumination to help us find our way along streets and sidewalks.

But there's a huge difference between the harsh, glary lights pictured at right and lighting that's well designed to illuminate the ground so that glare and light pollution are minimized. Too many fixtures send light streaming up into the sky, or provide far more light than is necessary, or are simply left on when they're not needed.

You Can Make a Difference

So, you might be thinking, "How can I reduce light pollution?" Easy! The first steps can be as simple as holding a neighborhood stargazing session sometime this week, or installing a motion detector on your outdoor security lights.

If you're a regular visitor to SkyandTelescope.com, you know my views on light pollution. I'm especially eager to see more support and involvement from amateur and professional astronomers — the very individuals who stand to gain the most from reducing light pollution.

So please take a few minutes to learn more about IDSW and the many ways, big and small, that you can participate. Meanwhile, here's a list of public events planned for this week, and here's more information about Global Astronomy Month.

The post It’s International Dark-Sky Week! appeared first on Sky & Telescope.

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