[BRIEF NOTE] Not this sort of apocalypse, at least

[rfmcdonald]

Gamma ray bursts were first detected by the American Vela satellites, deployed in Earth orbit in the 1960s to detect nuclear weapons explosions on Earth. Bursts astonished astronomers of the time by their sheer power: the most powerful bursts manage to convert nearly a Solar mass into gamma rays in the space of a few seconds. Current astronomical theories suggest that gamma-ray bursts are product either of the collapse of supermassive Wolf-Rayet stars into black holes or of the collisions of two neutron stars orbiting in a binary.

Gamma-ray bursts are relevant to people on Earth because of their exceptional power. The brightest gamma-ray bursts, GRB 990123, was briefly as bright as Neptune despite being roughly twenty trillion times as far away from Earth. That gamma-ray burst took place 9.6 billion light years away. A less powerful burst has been proposed several years ago as the cause of the Ordovician mass exintction 443 million years ago. If, for whatever reason, that gamma ray burster had been stronger, Earth almost certainly would have been sterilized. Might bursts, some wonder, explain the apparent and mysterious lack of extraterrestrial civilizations? (And what about us in the future?)

Fortunately it seems that we don't have to worry about death coming from the stars in that way. Via alexpgp, Space Daily reports that we in the Milky Way Galaxy seem to be safe. Maturity counts for much.

Reporting in the May 10 online issue of Nature, a team of astronomers said they analyzed 42 long-duration GRBs - those lasting more than two seconds – using the Hubble Space Telescope. They discovered that the galaxies in which the bursts originated tended to be small, faint and misshapen. Only one burst occurred in a large spiral galaxy similar to the Milky Way.

In contrast, supernovae - also the result of collapsing massive stars - were found in Milky Way-sized spiral galaxies roughly half of the time.

The results indicate GRBs form only in very specific environments, which tend to be different from those found in the Milky Way.

"Their occurrence in small irregulars implies that only stars that lack heavy chemical elements (elements heavier than hydrogen and helium) tend to produce long-duration GRBs," said lead author Andrew Fruchter of the Space Telescope Science Institute.

This means long bursts occurred more often in the past when galaxies did not have a large supply of heavy elements.

Galaxies build up a stockpile of heavier chemical elements through the ongoing evolution of successive generations of stars. Early generation stars formed before heavier elements were abundant in the universe.

The astronomers also found the locations of GRBs differed from the locations of supernovae - which occur much more frequently. The GRBs were far more concentrated in the brightest regions of their host galaxies, where the most massive stars reside. Supernovae, on the other hand, seem to occur throughout galaxies.

"The discovery that long-duration GRBs lie in the brightest regions of their host galaxies suggests that they come from the most massive stars - perhaps 20 or more times as massive as our Sun," said co-author Andrew Levan of the University of Hertfordshire, in the United Kingdom.

Massive stars abundant in heavy elements are unlikely to trigger GRBs, because they may lose too much material through stellar winds emanating from their surfaces before they collapse and explode. When this happens, the stars retain too little mass to produce a black hole, a necessary condition to trigger a GRB.

I'd much prefer for there not to be any anomalous Wolf-Rayet stars or neutron stars on collision course hidden in the centre of our Galaxy. There aren't any, of course. Right?