* Astronomy

In a new study of Gamma ray bursts (GRBs) , which researchers have submitted to the Astrophysical Journal, it was found that GRBs tend to occur in small, misshapen galaxies that lack heavy chemical elements (astronomers often refer to all elements other than the very lightest ones -- hydrogen, helium, and lithium -- as metals). Even among metal-poor galaxies, the events are rare -- astronomers only detect a GRB once every few years.
But the Milky Way is different from these GRB galaxies on all counts -- it's a large spiral galaxy with lots of heavy elements.

Gamma ray bursts (GRBs) are high-energy beams of radiation that shoot out from the north and south magnetic poles of a particular kind of star during a supernova explosion. Scientists suspect that if a GRB were to occur near our solar system, and one of the beams were to hit Earth, it could cause mass extinctions all over the planet.

The astronomers did a statistical analysis of four GRBs that happened in nearby galaxies. They compared the mass of the four host galaxies, the rate at which new stars were forming in them, and their metal content to other galaxies catalogued in the Sloan Digital Sky Survey.
Though four may sound like a small sample compared to the number of galaxies in the universe, these four were the best choice for the study because astronomers had data on their composition. All four were small galaxies with high rates of star formation and low metal content.
Of the four galaxies, the one with the most metals -- the one most similar to ours -- hosted the weakest GRB. The astronomers determined that the odds of a GRB occurring in a galaxy like that one to be approximately 0.15 percent.
And the Milky Way's metal content is twice as high as that galaxy, so our odds of ever having a GRB would be even lower than 0.15 percent.

"We didn't bother to compute the odds for our galaxy, because 0.15 percent seemed low enough" - Krzysztof Stanek, associate professor of astronomy at Ohio State.

He figures that most people weren't losing sleep over the possibility of an Earth-annihilating GRB.

"I wouldn't expect the stock market to go up as a result of this news, either. But there are a lot of people who have wondered whether GRBs could be blamed for mass extinctions early in Earth's history, and our work suggests that this is not the case" - Krzysztof Stanek

Astronomers have studied GRBs for more than 40 years, and only recently determined where they come from. In fact, Stanek led the team that tied GRBs to supernovae in 2003.
When a very massive, rapidly rotating star explodes in a supernova, its magnetic field directs gamma radiation to flow only out of the star's north and south magnetic poles, forming high-intensity jets.
Scientists have measured the energies of these events and assumed that such high-intensity radiation could destroy life on a planet. That's why some scientists have proposed that a GRB could have been responsible for a mass extinction that occurred on Earth 450 million years ago.

Now it seems that gamma ray bursts may not pose as much a danger to Earth or any other potential life in the universe, either, since they are unlikely to occur where life would develop.
Planets need metals to form, so a low-metal galaxy would probably have fewer planets, and fewer chances for life.

"My initial reaction was that it's not a coincidence, and everyone just knows that GRBs happen in metal-poor galaxies. But then people asked, 'Is it really that well known? Has anybody actually proven it to be true?' And we realised that nobody had"- Krzysztof Stanek.

The GRB would have to be less than 3,000 light years away to pose a danger. One light year is approximately 6 trillion miles, and our galaxy measures 100,000 light years across. So the event would not only have to occur in our galaxy, but relatively close by, as well.

Source

-- Edited by Blobrana at 07:15, 2006-04-19

Mass extinctions seem to occur on Earth roughly every 26 million years, leading some scientists to propose that they may be caused by rare collisions with comets or asteroids.
A researcher in Poland thinks it may be possible that extraordinary predators are at fault instead.
Adam Lipowski (Adam Mickiewicz University) constructed a numerical model of many species competing for both food and living space. The model also included a term that controls mutation rates, allowing new species to develop over time.
The model shows that, much of time, the system is populated with "medium efficiency" predators whose numbers fluctuate only slightly as the prey population waxes and wanes.

Inevitably, their stable community is disrupted when mutations lead to a super predator that quickly decimates the prey population, which in turn leads to its own demise.
The few creatures that survive the predatory apocalypse gradually mutate to fill the existing ecological niches - and the cycle begins again.

The period of the cycle depends on mutation rates in the model.

The lower the mutation rate, the longer the periods between super predators. For a sufficiently low mutation rate, the model can lead to cycles that correspond to our 26 million year mass extinctions.

Previous models that do not show these sorts of cycles could be faulty, according to Lipowski, because they failed to account for the effects of limited living spaces shared by a large number of different species.