Gold from the Heavens

An astronomy story has made big news headlines this week.  Usually most people are not unduly interested in the happenings in the heavens (either because such events are difficult to comprehend, or because they are regarded as remote to human interests), however this story does directly involve matters which humans take great interest in. Scientists and theorists working for the Harvard-Smithsonian Center for Astrophysicists have announced a spectacular new theory concerning the origin of gold (and other heavy elements like platinum and uranium): the cosmologists believe that the heaviest natural elements are created when two neutron stars collide or when a neutron star collides with a black hole (here is an easy summary of neutron stars, extremely tiny supernova remnants with a mass greater than the sun).  Elements as complicated as iron are manufactured by normal stars in the course of their lifetime, however the creation of heavier elements is more mysterious.  Until now, chemists and physicists had imagined that gold, platinum, uranium, and what have you, come from supernovae—however computer models of various types of supernova events did not supporting that conjecture. The scientists at the Harvard-Smithsonian Center for Astrophysicists based their hypothesis partly on the massive gamma radiation burst detected on June 3rd, 2013 from 3.9 billion light years away in a galaxy located in the constellation Leo.  Gamma ray bursts tend to be associated with hypernovae/supernovae caused by the collapse of super-giant stars, but the June 3rd burst was different.  In certain rare circumstances, two neutron stars are in a binary system together.  Over time, the orbits decay and the stars come together in a cataclysmic event which releases energy tantamount to that of a supernova.    Based on the unusual exotic energy signatures of the June 3rd gamma ray burst,  it seems that scientists caught a rare peek at such an event.

Neutron Star Collision

I will confess that I am having trouble imagining two objects the size of small cities (yet each with a mass greater than the sun) slamming into each other at astronomical speeds.  Apparently such events only happen every 100,000 years or so in a galaxy the size of the Milky Way.  When the neutron stars come together, a black hole is ripped in the fabric of spacetime.  Huge parts of the neutron stars fall into the black hole and vanish from this universe, but other portions of the neutron stars (which, as the name hints, are made up largely of neutrons) are jettisoned into space.  Edo Berger, one of the astrophysicists who authored the new theory described the process with an earthy metaphor, saying, “several exciting things happen very quickly…. Most of the material collapses to form a black hole. Some of it is spewed into space. That material is rich in neutrons, which drives the formation of heavier and heavier elements, the way mud piles up on an off-road vehicle.” The gold, platinum, and heavy elements are created in astonishing mass (like many earths made entirely of gold).  The elements are diffused through the cosmos and become part of newly forming star systems. Gold is strange stuff anyway.  The gold present on Earth during its nebular formation is believed to have sunk deep into the center of planet’s molten core where it is inaccessible.  All the gold that rappers and kings wear (and that Ron Paul and draugers hoard) first began falling to Earth 200 million years after the planet’s final formation on asteroids.  The great gold strikes are well named: gold on the surface of Earth is there because of meteor strikes (although billions of years of geology have buried and twisted and hidden these cosmic remnants).

Yeehaw! There’s asteroid fragments!

Advertisement

Neutron Stars

Mind-blowing diagram comparing Vancouver to a Neutron Star (by Christian Joore)

Most items in the heavens are inconceivably large.  The sun, a fairly ordinary star has a diameter of 1,391,000 kilometers (864,327 miles).  Even a tiny planetoid like the moon has a diameter of 3,474 km (2,159 miles).  However a few noteworthy items in the heavens are so small that we can think of them in human terms—like neutron stars, which are the size of a town or small city with a diameter of only 20 or 30 kilometers (about ten to 15 miles  miles).   But even though they are the size of a small asteroid or Manhattan Island, neutron stars are hardly inconsequential.  These dinky stars can have more mass than the entire glorious sun (which itself is 332,946 times more massive than the Earth and everything on it).  A 1.27 cubic centimeter block of such material (approximately the size of a half an inch sugar cube) would weigh approximately the same as all of the human inhabitants of Earth (give or take).

Neutron stars are left-over fragments of supernovae explosions.  When a star 4 to 8 times more massive than our sun burns through all available fuel, its outer layers blow apart in a supernova which spreads glittering matter across great swaths of space.  The dense remaining portion of the stellar core undergoes a titanic battle between electron degeneracy pressure and gravity.  If the fragment has more than 1.44 stellar masses, gravity wins and the electrons and protons of its constituent matter are crushed into super dense neutrons.  Such explosions are tremendously dynamic and bright.  In 1054 AD, Sung dynasty astronomers recorded such an explosion which outshone the moon.  Contemporary astronomers have determined that the 1054 AD supernova created the Crab Nebula, an oval shaped mass of hydrogen, carbon, oxygen, nitrogen, neon, sulfur, and iron.

The Crab Nebula (which measures 11 light years across and lies 6,500 light-years from Earth) NASA/CXC/SAO/F. Seward

In the center of the Crab Nebula is a spinning neutron star which is emitting jets of particles at a tremendous velocity from its magnetic poles. These jets produce very powerful beams of electromagnetic radiation (which varies in intensity and wavelength according to elaborate nuclear & stellar physics, much of which is not yet understood).  The forces which create neutron stars often leave the stars spinning and pulsing with energy in such a way that they become pulsars.  These pulsars are useful for studying gravity, general relativity, and the behavior of matter at nuclear densities (albeit indirectly).  They also make accurate time measurement devices and useful beacons.  It is strange to think that stars so prominent for vast distances and so useful to astronomers actually have such minimal volume.

A Detailed x-ray image of the pulsar at the center of the Crab Nebula (Chandra)