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Galactic Death Stars And Extinction

by John G. Cramer

Alternate View Column AV-178
Keywords: gamma ray bursts, hypernovas, extinctions, death stars,
Published in the July-August-2015 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 02/05/2015 and is copyrighted ©2015 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

The Universe is perhaps a more dangerous place than we had ever imagined. This column is about the death stars that produce gamma ray bursts and the implications of such events for life in the Universe and its extinction. We will start with a review of the gamma ray burst phenomenon.

In 1973, a declassified paper appeared in Astrophysics Journal Letters describing the unexpected observation of gamma ray bursts (GRB). The discovery was an accidental spinoff of the Cold War.  Los Alamos National Laboratory and the U.S. Air Force had launched the top-secret VELA series of satellites, which had been designed to detect any clandestine nuclear weapons tests by foreign powers.  Instead, at the rate of about one per day, the satellites had detected bursts of gamma radiation (high energy photons) that were ultimately time-triangulated as originating somewhere outside the Solar System and coming in from all directions.  I discussed the mysterious gamma ray burst phenomenon in my October 1995 column (AV–74), and placed them as Item 3 of my July/August 1999 end-of-millennium list of "Things We Don't Understand"(AV–96).

Five years later, NASA's Swift satellite, launched in November 2004 for the explicit purpose of investigating GRB, used gamma-ray detectors to identify and locate incoming bursts. Swift could point on-board X-ray and optical telescopes in the detected burst direction in well under a minute.  In many cases, Swift was able to observe the X-ray and optical components of such GRB events even before the gamma rays from the burst had stopped arriving. Distant galaxies were observed to "light up" in the visible and X-ray regions as they emitted these huge bursts of gamma radiation.  The conclusions were that the GRB sources were extra-galactic in origin, were billions of light years away from the Earth, and represented the emission of an incredibly huge amount of energy—up to 1047 joules—in the form of gamma rays.  To put this quantity of energy in perspective, if an antimatter star with the mass of our Sun were to completely annihilate with our Sun and 28% of the liberated energy went into gamma radiation, the event would produce about 1047 joules of gamma rays.

With new observations and with improved modeling, we now understand much more about the origins and characteristics of GRB. First, there are at least two distinct GRB production mechanisms.  Those that are relatively short in time duration (< 2 seconds) are designated as sGRB ("s" is for short) and originate in the "merger" of compact astrophysical objects like neutron stars or black holes.  These events have an average rate of occurrence of about 0.04 per year per cubic gigaparsec (a gigaparsec is 3.09 x 1025 meters or 3.26 billion light years, and a cubic gigaparsec is a volume of space that would include a large number of galaxies). An sGRB event has a total liberated energy between 5 x 1042 and 1 x 1046 joules. (For comparison, the total energy output of the Sun in one second is 3.85 x 1026 joules.)

The GRB that are relatively long in time duration (> 2 seconds) are designated as LGRB ("L" is for long), and modeling suggests that they are produced when a rapidly rotating, high-mass star has a core-collapse to a black hole in a hyper-supernova event.  These events have an average rate of occurrence of about 0.15 per year per cubic gigaparsec and a total liberated energy between 1 × 1042 and 1 × 1047 joules. In other words, LGRB events occur about four times as often as sGRB and may liberate up to an order of magnitude more energy.  The massive stars that produce these super-explosions are truly death stars, exploding with such huge energies and temperatures that the thermal photons they produce are gamma rays. The LGRB events have the largest potential impact on life (and death) in the Universe.

However, observations indicate that the probability of a LGRB event is larger when the metalicity of the galaxy from which it originates is low.  "Metalicity" is a term used by astronomers to indicate the fraction of the mass of a star that is not in the form of hydrogen or helium.  As a reference, our Sun has a rather high metalicity of about 0.02, meaning that 2% of its mass derives from heavier elements like carbon and iron.  The first generation of stars that formed soon after the Big Bang had a metalicity of essentially zero, because no elements heavier than helium (and a tiny amount of lithium) had been produced by primordial nucleosynthesis of the early Universe.  These early stars, which tended to be hot, massive, and short lived, consumed their hydrogen fuel rapidly, and then went on to fuse their helium into carbon, neon, magnesium, silicon, sulfur, etc., making the light elements up to iron. When those fusion processes ran out of fuel, the stars exploded into supernovas, producing floods of neutrons that synthesized elements heavier than iron and that blasted part of their matter out into the Universe, where it became the material supply for the formation of the next generation of stars.  Thus, the metalicity provides an indication of the time after the Big Bang at which a star was formed, with low metalicity earlier and high metalicity later.  The recent studies of LGRB indicate that they occur preferentially in those galaxies that have a low metalicity, i.e., those representing the early generation of star formation.  This is probably because the higher density of hydrogen in that era led to the formation of very massive stars.  We are fortunate to live in a high metalicity galaxy, where such massive stars are rare.


In the December 5, 2014 issue of Physical Review Letters, astrophysicists Tsvi Piran of the Hebrew University and Raul Jimenez of U. Barcelona and Harvard have used the new information on gamma ray bursts to estimate their impact on the probability for sustaining life on Earth-like planets in our galaxy and in the Universe in general.  They argue that when a GRB occurs close enough to a life-bearing planet like the Earth, the gamma rays will form nitric oxide in the stratosphere, which will deplete or destroy the protective ozone layer for a period of months, exposing the planetary surface to bombardment by UVB sunlight. Intense UVB irradiating the surface of the ocean will destroy surface marine life including plankton, depriving all other marine life of their main nutrients.  The result will be a massive extinction event for the marine life of the planet, and probably also for plants and animals inhabiting the land masses.

Calculations indicate that an incident gamma ray intensity of about 104 joules per square meter would remove about 68% of the Earth's ozone layer for about a month, and that 105 and 106 joules per square meter would remove about 91% and 98%, respectively.  Piran and Jimenez assume that a gamma ray intensity of 104 joules per square meter will cause significant damage to planetary life and that 106 joules per square meter will cause a catastrophic life extinction event. They take 105 joules per square meter as their threshold for lethal gamma ray intensity and use this value, together with modeling of GRB probability and intensity, to estimate the impact of GRBs on the sustainability of planetary life during the evolution of the galaxy.

They have computed the probability of having more than one lethal GRB per 109 years to be about 95% if the star is two thousand parsecs from the galactic center (about 25% of the Milky Way's stars are closer than this), about 50% if the star, like our Sun, is eight thousand parsecs from the galactic center (about 90% of the Milky Way's stars are closer than this), and about 20% if the star is two thousand parsecs from the galactic center, at the outer edges of the Milky Way.


The geological record indicates that there have been at least five major extinction events during the evolution of life on Earth.  Piran and Jimenez speculate, based on these numbers, that at least one of these was probably due to a lethal gamma ray burst from within the Milky Way. For GRB events at distances greater than fifty thousand parsecs, there is essentially no danger from gamma radiation, so the other members of our Local Group, e.g., Andromeda and the Magellanic Clouds, pose no threat to life in our galaxy.  The implication is that, although most of the stars in the Milky Way are closer to the galactic center than is our Sun, there is a low probability of finding Earth-like life in that more densely populated region of our galaxy due to the higher frequency of extinction events. The Earth seems to be in a preferred location.

Piran and Jimenez also consider what constitutes a friendly neighborhood for life elsewhere in the Universe.  The best places are regions where the galaxies formed rather late, are large and diffuse, and have a high metalicity (at least 1/3 that of our Sun).  They conclude that the galaxies we observe with a red-shift factor z greater than 0.5 (i.e., galaxies about 3 × 109 years older than ours) would be unlikely to harbor life.  Such galaxies have higher stellar density and lower metalicity, and in these the LGRB events will always be sufficiently near to any Earth-like planet as to cause frequent extinctions.  Thus, life in the Universe may have become possible only relatively recently, in only the past few billion years of the 13.8 billion years that the Universe has existed.


There remains the question of how the occurrence of extinction events is connected to the evolution of life, and particularly intelligent life.  Clearly, extinctions are not good, per se, but I argued in a previous AV column ("The Pump of Evolution," AV–11 in the January 1986 Analog) that the evolutionary record, as characterized by "punctuated equilibrium," suggests that occasional extinction events may jar embedded species out of entrenched ecological niches and promote positive change and species improvement.  However, the "pump stroke" of extinction events must be at an optimum rate.  The extinctions cannot be too severe or occur too often, or there will not be sufficient time for recovery, evolutionary development, and repopulation.

The Fermi Paradox, the question of why, despite optimistic Drake equation estimates, we have been unable to find any evidence of intelligent life elsewhere in the Universe, is not completely answered by the realization that much of the galaxy and the Universe are hostile to the development of life.  There are still many suitable stars in the outer reaches of the Milky Way, some better protected from LGRB than is the Solar System.  However, the work of Piran and Jimenez adds another constraint on the evolution of life, and contributes another element to the growing realization that the Earth, the cradle of the only life we have so far found in the Universe, is a very special place. We really need to take better care of it.


References:

"Possible Role of GRBs on Life Extinction in the Universe," Tsvi Piran and Raul Jimenez, Physical Review Letters 113, 231102 (2014); arXiv preprint: 1409.2506v2 [astro-ph.HE].


John Cramer's new book:  a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a printed or eBook at:  http://www.springer.com/gp/book/9783319246406 . 

SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 180 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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