"In the beginning, there was an explosion. An explosion which occurred everywhere, with every particle rushing apart from every particle."
-- Steven Weinberg, The First Three Minutes
The Big Bang happened, symmetry was broken and matter began to form, creating space with great density as energy converted to matter.
0.01 s after the Big Bang, the temperature was 100 billion degrees C. No atoms existed, only constituent pieces, electrons, positrons, neutrinos, photons. The density was 4 billion gm/cm3.
1 s after the Big Bang the volume of space was growing, rapidly decreasing the density of the Universe and letting it cool to only 10 billion degrees C.
180 s after the Big Bang, at a temperature of 1 billion degrees, protons and neutrons became stable forms of matter, providing H and He nuclei. In this instant the Universe became comprised of 73% H, 27% He.
From there on, expansion, cooling, aggregation into galaxies and stars (after a few billion years) took place. From an instant of 300,000 years after the Big Bang to the present, events have radiated light propagating in the now transparent Universe (low enough density so that photons could survive without being scattered), and we can 'see' back to that time, but no earlier.
Looking at the stars is like looking back in time, as we look to more and more distant objects we are seeing events longer and longer ago. This is because it has taken longer for light from distant objects to reach us today, so that light had to have left the object longer ago. Can we see all the way to the Big Bang?
In part, the answer is yes. In 1965 Bell Lab and Princeton astrophysicists observed the Background Radiation. This is the "original flash" of light, radiation persisting from the Big Bang once the Universe became transparent. It is so far red-shifted that we see it to day as light in the radiowave (microwave) spectrum. It is the flash of the Big Bang! Well, actually from the point of decoupling, when photons became free to fly in space. The Big Bang commenced a process that persists to today: the creation of larger and larger atomic structures by Nucleosynthesis at various locations in the Universe. In the first 100 seconds of the Universe, the temperatures and pressures were such that Protons and Neutrons collided and bound together due to the strong nuclear force, producing first a Deuterium nucleus (1 Proton + 1 Neutron), and then a Tritium nucleus (1 Proton + 2 Neutrons), and finally a Helium nucleus (2 Protons + 2 Neutrons). The binding together of the Helium nucleus released energy, and is the process called fusion. This is the same energy we tap in thermonuclear bombs. By three minutes into the existence of the Universe there was 1 He atom for every 10 H atoms, and this set the 24% mass of He that exists in the Universe today. The continuing expansion, cooling and decreasing pressure of the Universe prevented much more in the way of Nucleosynthesis of larger elements during the Big Bang, although some were created.
From 10,000-300,000 years after the Bang, the expanding, cooling Universe progressively became transparent, and the free photons from the early glowing explosion are seen even today in the microwave background radiation. Over the next 1-2 billion years the debris from the Big Bang, expanding outward and defining space, underwent turbulent mixing and aggregation, with matter sorting into galactic masses, and within those masses the process of star formation began. All of the early stars consisted of H and He alone. As they grew, however, Nucleosynthesis began anew, and the expansion of the Periodic Chart of the Elements began.
So, we must understand stars, if we are to understand how the Earth System came to be what it is today. If not for the processes operating in stars, and their catastrophic terminal phases, there would be no stones, no iron, no flesh, no humans to think about these things. This is the substance of today's lecture.
"Every atom in each of our bodies was once inside a star. The iron in your blood came from the central regions of stars more massive that our Sun. Who can contemplate this notion and not feel at one with the Universe."
-- Thorne Lay
Stars are radiating bodies, massive enough to ignite their own nuclear furnaces with the onset of fusion processes. Their behavior differs widely, and is largely dependent upon the mass of material in the star. Gravity pulls the gas and dust material (mainly H and a little He in these first generation stars) inward as a star grows, and the pressure and collision rate of the matter increases the temperatures. As the internal temperatures rise to at least 10 Million degrees K, fusion begins, and once again H atoms are slowly converted to He, with a large release of energy. Eventually, meaning after some billions of years, the fuel that is available within the hottest interior of the star will be burned up (fused) into stable larger elements, and the star will cease to radiate energy, with fusion turning off. What happens then is very dependent on the mass originally swept into the star.
Original Star Mass Fate 1 Sun Red Giant/White Dwarf > 6 Suns Supernova/Neutron Star > 30 Suns Supernova/Black Hole
Let's think about this further: A star involves a balance of heat and gravity. The heat results from energy release during fusion and tends to expand the star outward. Gravity opposes this, drawing the material of the star toward its center. If you shut down the fusion reactions, in the center the core of the star begins to collapse and the outer region cools and expands. This results in a Red Giant phase, involving a collapsing inner region surrounded by a planetary nebula. The luminosity of the star grows, in this terminal phase (for modest size stars like our sun), and then the nebula cools and dissipates, leaving a white dwarf (or black dwarf) relic.
But, if there is enough mass in the central region, as the collapsing core shrinks, it heats back up, possibly reaching a critical temperature of 100 million K. At that temperature, the collisions of He atoms in the core are so severe and frequent that 3 He atoms can collide to produce a Carbon atom. This is called Helium burning, and the release of energy of this fusion renews the star. If the central regions of the star are at yet higher temperatures (and they will be at higher pressures because the star is massive), Carbon burns to give Oxygen, which burns to give Silcon, Magnesium and Sulfur. The star assumes a layered structure where each deeper layer involves the fusion of smaller elements to make larger ones. The reason this did not happen in the Big Bang is that the temperature and pressure decreased too rapidly to enable much fusion. The confined interior of a star sustains the necessary conditions to generate substantial amounts of heavier elements.
Thus: STARS ARE THE FORGES OF THE HEAVY ELEMENTS
In a massive star, at least 6 times larger tahn the Sun, the deepest layer of fusion involves the production of a central core of iron, at temperatures in excess of one billion degrees K. But at that level, there is a problem. Iron (Fe) does not burn to produce heavier elements at any attainable pressure and temperature. Thus, the process of a layered star, no matter how massive, is such that in the end it will burn up all of the fuel available to produce its stratified structure, and then it will run out of fusion energy. Gravitational collapse of the iron core will begin, and if the core has more than 1.4 solar masses the heating up of the core as the iron collapses can achieve temperatures of 50 billion degrees! At these temperatures and pressures, Protons and Electrons can collide and recombine to give the neutral charge particle called a Neutron, with tremendous amounts of energy released as neutrinos. The entire core will convert to neutrons in less than 5 seconds, with 99% of the atomic energy released as neutrinos. The core thus collapses into a neutron star, and the vast energy release blows out the surface layers of the star, expanding at more than 15,000 miles/s. This is a SUPERNOVA. It can be 1 billion times brigher than the Sun! In fact, at the time of the initial explosion, the energy released is about the same as that released in that same second by the entire universe! These are rare events (thank goodness), but they are very important in terms of nucleosynthesis, and indeed in the history of our thought about the Universie aroundus.
The transient conditions of vast temperatures and pressures in the shock wave of the Supernova allow small trace amounts of elements heavier than iron to be produced, building up the periodic chart of the atoms all the way to Uranium or Plutonium. These atoms, plus all the elements already generated in the layered star, are dispersed into space.
The dust and gas from a supernova then can reaccrete to build a second generation star which now has a distribution of elements already containing elements heavier than H and He. Our star is not big enough to produce very heavy elements, nor will it explode in a supernova. Yet its spectroscopic signature indicates the presence of a rich distribution of minor elements that are the products of many previous supernovas that yielded the materials comprising our star. As we have already seen, if the Earth consists of the same materials comprising the Sun (minus the very light gases, mostly H and He), then all the heavy elements that make up rocks, and that make up lifeforms including humans, were once forged in stars that have blown up.
So, what happens to the collapsed core of either a Red Giant or a supernova? Lacking any rekindling fusion, the core matter packs closer and closer together, with no counter force to gravity. A dense, small core will result for our star, making a white dwarf. A massive star that goes supernova will leave a neutron star or pulsar. On the surface of a neutron star a 150 pound human would way 1 million tons. All humans together would pack into the volume of a pea. Neutron stars spin very fast (any initial spin of the star is vastly exaggerated when it collapses, just like an iceskater pulling in her arms), and the 1.4 solar masses would pack into no more than a 10 mile diameter object. If the star had more than 30 solar masses to begin with, the portion of the iron core that collapses would pack matter into even greater density, producing a Black Hole. Such an object has so strong a gravitational field that photons cannot escape it (only a few), and it strongly perturbs local space.
How do we know that any of this is true? Nucleosynthesis is very well understood as a result of:
Historical Observations:
About 10 supernovas have been witnessed in the last couple millenia. Chinese records show 'new stars' (in Latin, Nova Stelli) at 185, 393 and 1006AD. Some of these were bright enough to be seen even during the day! They would typically last for a couple years before fading.
1054 Crab Nebula, (which we now know is 5000 light-years away). The remains of this event are the Crab Nebula, a set of filamentous clouds. Telescopic photographs of this object taken many years apart document the fact that it is still expanding, a millenium later. Turning the clock backward, one can project where and when these gases were in one place -- at about 1100. The object in the center is what is called a pulsar, emiting energy that arrives on earth about every few seconds, in pulses. This is the very dense rapidly rotating remnant, sending out radiation like a search light that passes the earth every few seconds.
The next supernova in 1572 is called Tycho's Star. It is named for
Tycho Brahe, a Danish astronomer who had made it his business to
chart the heavens. This was important because at the time a debate
was growing about the very nature of the cosmos -- whether it was
earth-centered, or sun-centered (heliocentric). The latter cosmic
view claimed that the earth was in the middle, and was surrounded by
a set of spheres, the nearby ones occupied by the planets, and the
last one, the eighth, occupied by star. These were supposed to be
immutable, unchanging. That a new star (Nova Stelli) could be borns
within this sphere, and could be demonstrated not to be moving (as
the planets and comets do), flew in the face of this theory of the
cosmos. (By 1576 Galileo produced telescope and could look closely at
this one as it faded.)
1604 Renaissance Star, ... Shakespeare noted this one... This too was
very important for the history of ideas, coming on the heals of
Tycho's star, and, given the invention of the telescope in the
intervening few decades, could be studied more closely.
All of these supernovas must have been in our galaxy, as the naked eye, and even these early telescopes, were unable to distinguish even stars from galaxies, the nearest one to our own being too far to see individual stars. Given the number of stars in our galaxy, we would expect to see about 1 supernova/century. However, after 1604 none were seen... until 1987.
In that year, on Feb. 24, the light first arrived from a star burst in the Large Magellanic Cloud, 175,000 light years away (old). The spectral signature of the radiation from the supernova showed the burning of Cobalt to produce Fe, from which the original star (Sanduleak-69202) was determined to have been 15 solar masses. In fact, close scrutiny of records from experiments set up to capture the rare high energy neutrinos that are presumably the harbingers of these events showed that on the night of February 22nd, a brief 20 second pulse of neutrinos whizzed by (and through) the earth. Continuing study of the radiation has confirmed most basic ideas about supernovas.
In summary, then, while much of the Hydrogen was synthesized in the Big Bang, all of the elements heavier than Helium were synthesized in stars. Elements as heavy as Fe were generated by fusion reactions. Elements heavier than Fe were generated in supernova shock waves. Supernovas therefore both fill out the periodic table and fling most of the synthesized elements into space
In addition, these rare events produce a specific kind of very high energy radiation, very high speed particles (mainly Hydrogen nuclei -- Protons) that we call 'cosmic radiation'. These cosmic rays play a significant role in life, as it is the damage from cosmic radiation that causes mutations of genes, which is a fundamental process in the evolution of life.
So, the early Universe involved formation of galaxies and stars, many so massive that they synthesized much of the periodic chart in their interiors, then disgorged these elements to the surrounding gas clouds in supernovae. This process continued for about 9.5 billion years after the Big Bang, enriching second generation stars in heavy elements that did not exist for the first generation stars. In one gas and dust rich region of our galaxy gravity began to aggregate material, collapsing a spherical cloud of gas and dust into a nebular disk. And that spinning disk formed a star with some planets in orbit around it; planets and star all made of similar proportions of heavier elements, but with the star and larger planets having enough gravity to hold onto lighter gases. This became the Solar System, which formed about 4.6 billion years ago; and the Earth was born!
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