Wednesday, February 1, 2012

Star Light, Star Bright

The idea of wishing on a star is a very ancient superstition. Many wide-eyed children--as well as some unfortunate adults--have wished upon a shooting or falling star in an effort to make magic work for them. But stars are not supernatural dabs of light shimmering like angels in the clear night sky. Stars are really gigantic incandescent balls of hot nuclear-fusing gas. They are unable to hear us, and certainly cannot grant wishes. However, the stars have done much more for us than merely that--they gave us life.

The billions and billions of stars that dwell in our Universe are primarily composed of hydrogen--the lightest element in the Periodic Table--which they transform in their nuclear-fusing cores into heavier things, by way of a process termed "stellar nucleosynthesis". How were the first stars in our Universe born? How big were they? What were they like?

The first stars were not like the stars we know today; they formed directly from the lightest primordial gases--hydrogen and helium--which were born in the hot Big Bang birth of the Universe almost 14 billion years ago. In fact, the only atomic elements formed in the Big Bang inferno were hydrogen, helium, and trace amounts of lithium. The rest of the elements of the Periodic Table were cooked deep in the hearts of stars, their glowing hot interiors progressively fusing the nuclei of atoms to form heavier and heavier elements. Without these heavy elements produced by our Universe's stars, there would be no life. We would not be here. All of the carbon that is the basis for life on Earth, the oxygen we breathe, the elements composing the stones, dirt, and sand beneath our feet, were created deep inside the fiery hearts of ancient stars, billions and billions of years ago. We are made of star-stuff. When very massive stars die, they do not go gently into that good night, but blow themselves up in a magnificent supernova blast. When massive stars go supernova they hurl their newly formed batch of heavy elements out into space. The first stars were enormous, perhaps weighing as much as hundreds of times more than our own Sun. They lived fast and died young. The more massive the star, the shorter its life. When the first stars went supernova, they blasted out the very first newly-formed batch of heavy elements--so necessary for the emergence of life--into the Cosmos.

Hydrogen and helium were drawn together to create gravity-bound knots of gas. The cores of the very first protostars in our Universe ignited within the cold dark hearts of these dense knots of pristine primordial gas. The tight dark knots collapsed under their own gravitational weight, until nuclear-fusing fires started to burn. Many cosmologists think that the first stars grew to be enormous (compared to the stars of today's Universe), because they did not form from the same elements, and did not form in the same way, as stars do now. Members of the first generation of stars are termed "Population III" stars. Our Sun is a member of the most recent population of stars, and is a so-called "Population I" star. In between the first stars, and the most recent generation of stars like our Sun, are the appropriately-named "Population II" stars.

The most ancient generation of stars did not catch fire until about 100 million years after the Big Bang. How did the dramatic transition from darkness to light come about? After decades of observations, simulations, and calculations, researchers have recently made significant progress in their endeavors to answer this question. Using sophisticated computer simulation techniques, cosmologists have constructed ingenious models that reveal how the first generation of stars might have been born. Observations made using large ground-based and space-borne telescopes have also probed into cosmic history back to the remote time when the Universe was less than one-tenth of its present age.

The great scientific detective Albert Einstein once said that "The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe is as good as dead: His eyes are closed."

The birth of the first stars is one of the greatest mysteries haunting today's cosmologists. It is currently most widely thought that the ancient Population III stars were not only extremely massive, but also dazzlingly luminous, and their emergence is primarily responsible for changing our Universe from what it was to what it is now. The ordinary atomic matter that was born in the Big Bang and subsequently cooked up in the hearts of stars is composed of protons, neutrons, and electrons. Protons and neutrons are bound together into atomic nuclei surrounded by a cloud of electrons. Hydrogen is made up of only one proton and one electron. Helium is made up of two protons, two neutrons, and two electrons. Carbon is made up of six protons, six neutrons, and six electrons. Heavier elements, such as iron, lead, and uranium, contain even larger numbers of protons, neutrons, and electrons.

The "metallicity" of a star refers to the percentage of its matter that is composed of chemical elements heavier than hydrogen and helium. Because stars, which account for most of the visible matter in the Universe, are made up primarily of hydrogen and helium, astronomers and cosmologists use (for convenience) the all-encompassing term "metal" when indicating all of the elements in the Periodic Table that are heavier than hydrogen and helium. Both hydrogen and helium were born in the Big Bang--the heavier elements were formed in the nuclear-fusing hearts of our Universe's stars, or in their explosive demise. Therefore, the term "metal", in astronomical jargon, has a different meaning than the same term has for a chemist. A nebula (cloud) heavily laden with nitrogen, neon, carbon, and oxygen would be termed "metal-rich" by an astronomer, even though those elements are not metals to a chemist. Hence, this term should not be confused with the chemist's definition of "metal"; metallic bonds are impossible in the searing-hot hearts of stars, and the very strongest chemical bonds are only possible in the outer layers of cool "stars", such as brown dwarfs, which are not even stars in the strictest sense of the word because, even though they probably form as true stars form, they are too puny for their nuclear-fusing fires to ignite. The metallicity of a star may tell an astronomer its age. When the Universe first emerged, it's atomic matter was almost entirely hydrogen which, through primordial nucleosynthesis, manufactured a large quantity of helium and trace amounts of lithium and beryllium--and no heavier elements. Therefore, older stars (such as Population II and III stars) have lower metallicities than younger stars (Population I) like our own Sun.

The stellar Populations I, II, and III, show decreasing metal content with increasing age. Therefore, Population I stars, the youngest in the Universe, have the highest metal content. The first stars to catch fire in the universe (Population III) were depleted of metals. Population II stars are very ancient, but not as old as the Population III stars, or as youthful as our bouncy baby of a Sun. Population II stars bear the metals produced by the first generation of stars.

Even though older stars carry fewer heavy elements than younger stars, the fact that all stars thus far observed by astronomers have at least some metals, presents a delectable mystery. The currently favored explanation for this mysterious observation is that Population III stars must have existed in order for these heavier elements to have been produced--even though not one Population III star has ever been observed! According to this line of reasoning, in order for the Population II stars--which have been observed--to carry their relatively scanty amounts of metals, their metals must have been created in the nuclear-fusing hearts of a pre-existing generation of stars. Soon after the Big Bang birth of our Universe, there were no metals. Therefore, it is hypothesized that only stars with masses hundreds of times that of our Sun could have been born in the very ancient Universe. Near the end of their hydrogen-burning (main-sequence) lives, these first stars fused the first 26 elements up to iron in the Periodic Table by way of stellar nucleosynthesis.

Because of their heavy mass, current stellar models indicate that the ancient Population III stars would have rapidly burned their supply of hydrogen fuel and exploded in extremely violent supernovae. Those extremely violent blasts would have completely strewn their material all over the Universe, shooting newly-forged metals throughout the once-pristine Universe to be incorporated into subsequent generations of stars--the stars that we observe, and wish upon, today. No Population III stars have ever been observed because of their hypothesized great mass. Because these stars lived fast and furiously, they died young, and thus blew themselves to smithereens in mighty supernova explosions in the very ancient Universe. Hence, Population III stars can only be observed dwelling in the most remote galaxies inhabiting the early Universe, and finding such stars or establishing their non-existence is extremely difficult.

As subsequent generations of stars were born in the Universe, they became increasingly more heavily metal-enriched, as the gas-laden clouds from which they were born were bestowed with metal-enriched dust cooked up by previous generations of stars in their nuclear-fusing hearts. The youngest stars, like our own Sun, therefore have the greatest metal content in our Universe today. However, this must be kept in its proper perspective. Even metal-rich stars contain only small quantities of any element heavier than hydrogen or helium. In fact, metals (in the astronomical sense of the term) make up only an extremely small percentage of the overall chemical composition of the Universe.

Although it is generally believed that the ancient Population III stars were extremely massive, cosmologists are by no means in complete agreement on this issue. However, because smaller stars live much longer than more massive ones, it would explain why there has never been a metal-free low-mass star observed by astronomers. If Population III stars were small, some of them should still be floating around our Universe today.

There is a beautiful story to be told here, to anyone who wants to understand the plot. Our Universe was born almost 14 billion years ago in the Big Bang, and it grew from a small microscopic crumb into an entity where incandescent stars could be born, dangling like sparkling baubles within billions and billions of galaxies. The elements that made life possible were cooked up slowly in the hearts of very ancient stars, which then flung these newly forged elements out into space when they went supernova. Some of these newly formed heavy elements ultimately merged together and mixed on a small, watery, pale blue Planet, circling an ordinary fiery golden Star, dwelling in a typical spiral Galaxy. About 300,000,000 years ago, humanity's ancestral organisms first commenced their momentous Great Crawl out of Earth's primeaval oceans, eventually to evolve into land-dwelling creatures. The transition from fishes to limbed vertebrates (tetrapods) occurred about 370,000,000 years ago, when "Tiktaalik Roseae", better known as the "fishapod", emerged from the ancient, fertile seas of our Earth. It was a fish, but a fish with fins that flexed and extended like arms and hands. It was a flattened, superficially crocodile-like animal. Hundreds of millions of years later some of its descendants would evolve into us, land-dwelling creatures able to wonder about the Universe, able to observe planets circling distant stars beyond our Sun, where life may also have evolved out of non-living substances. That silly-looking little "fishapod" left the imprints of its momentous journey in the mud near ancient seas. Eventually, over the eons, the record of this journey was frozen in stone. Millions and millions of years later, descendants of that funny little animal would leave imprints of yet another momentous journey in the dust of the Moon.

As physicist Dr. David A. Clarke and his co-authors noted in the article "Astrophysical Jets", a publication of the Canadian Association of Physicists: "Nature has devised numerous mechanisms by which the Universe could become self-aware, and where humanity could spring forth from the ashes of ancient supernovae and gaze back upon the heavens to contemplate its origins."

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