Wasteful Galaxies Inhabit The Cosmos

The death of a star is a spectacular, often beautiful, farewell performance that launches newly formed atomic elements screeching into Space. Small stars, like our Sun, die in relative peace, tossing their batches of newly forged atomic elements into interstellar Space with comparative gentleness–and loveliness. On the other hand, much more massive stars perish in the brilliant blazes that herald the most powerful stellar blasts known–Type II (core collapse) supernovae–and these explosions are so brilliant that they can be observed all the way out to the very edge of the observable Universe. Only hydrogen, helium, and small amounts of beryllium were formed in the Big Bang birth of our Universe almost 14 billion years ago–all of the atomic elements heavier than helium, that astronomers call “metals”, were created by the stars in their searing-hot, roiling, nuclear-fusing fiery furnaces. In June 2016, a team of astronomers announced their discovery that galaxies “waste” great quantities of heavy metal atomic elements, generated by star formation, by hurling them up to a million light-years away into their surrounding halos and interstellar Space.

The study, led by University of Colorado at Boulder astronomers, was first published online in the Monthly Notices of the Royal Astronomical Society (MNRAS) of the UK. The new research demonstrates that more oxygen, carbon and iron atoms dwell in the surrounding, sprawling gaseous halos, on the outskirts of galaxies, than exist within the galaxies themselves–thus, leaving the galaxies with fewer raw materials so necessary in order to create stars, planets, and life itself.

“Previously, we thought that these heavier elements would be recycled in to future generations of stars and contribute to forming planetary systems and providing the building blocks of life. As it turns out, galaxies aren’t very good at recycling,” commented Dr. Benjamin Oppenheimer in a June 6, 2016 University of Colorado Press Release. Dr. Oppenheimer is a research associate in the Center for Astrophysics & Space Astronomy (CASA) at the University of Colorado at Boulder and lead author of the study.

The almost invisible surrounding reservoir of gas swirling around a galaxy, termed the circumgalactic medium (CGM), is thought to play a starring role in cycling elements in and out of a galaxy. However, the precise mechanisms that cause this relationship remain unknown. An ordinary galaxy can range in size from about 30,000 to 100,000 light years while the CGM can extend up to a million light years.

Stellar Equivalent Of Spinning Straw Into Gold

Stellar nucleosynthesis is the term used for a process by which the natural abundances of atomic elements, contained within stars, undergo a sea change as a result of nuclear fusion reactions within the stellar cores and overlying mantles. Stars grow old, evolving as they age, and experience alterations in the abundances of the atomic elements that they contain in their fiery, seething-hot stellar hearts. Core fusion increases the atomic weight of a star’s gaseous elements. This results in the loss of pressure and contraction accompanied by increasingly hotter and hotter temperatures. Stars are doomed to lose most of their mass when it is hurled out into interstellar space as they grow elderly–and this increases the abundance of atomic elements heavier than helium (metals) in the space between stars.

The term supernova nucleosynthesis is used by astronomers to describe the formation of atomic elements during the evolution, aging, and ultimate fatal explosion of a star that has come to the catastrophic end of that long stellar road. Supernovae produce the heaviest atomic elements of all, such as gold and uranium. Massive stars cannot fuse any atomic element heavier than iron in their cores, and when a massive star has reached this point, its nuclear fusing days are over. The formation of metals, within the dying hearts of elderly evolved stars, was proposed back in 1954 by the well-known English astronomer Fred Hoyle (1915-2001). The theory of nucleosynthesis in stars gained credibility when variations of the abundances of atomic elements were discovered by astronomers studying the Universe. When the astronomers plotted those varying abundances on a graph as a function of atomic number of the elements, they saw a sawtooth shape that varied by factors of tens of millions. This indicated a natural process was at work, other than a mere random distribution. Stellar nucleosynthesis is the main contributor to several processes that also occur under the general term nucleosynthesis.

A better understanding of stellar nucleosynthesis was obtained during the 20th century, when scientists realized that the energy emitted from nuclear fusion reactions explained the longevity of our Sun as the source of heat and light. The fusion of atomic nuclei deep within the searing-hot heart of a star, starting from its original supply of hydrogen and helium, accounts for that energy, and synthesizes new and heavier atomic nuclei as a byproduct of the fusion process that keeps a star brilliantly glaring and hot. This phenomenon became especially clear during the decade before World War II. The fusion-formed atomic nuclei are restricted to those that are only a bit heavier than the fusing atomic nuclei–indicating that they do not contribute heavily to the natural abundances of the elements. Nevertheless, this then newly acquired understanding presented a plausible explanation for all of the natural abundances of atomic elements in the Universe. The starring energy producer in our Solar System is, of course, the Sun. Our Star fuses its necessary supply of hydrogen to create helium, which happens at a solar-core temperature of a toasty 14 million kelvin.

In 1920, the English astronomer Sir Arthur Eddington (1882-1944) proposed that the stars obtained their energy as a result of the nuclear fusion reaction that creates helium from the fusion of hydrogen atoms–thus raising the possibility that the heavier atomic elements are produced in the nuclear-fusing, seething-hot hearts of stars. Hydrogen is the most abundant, as well as the lightest atomic element in the Universe–and helium is the second-lightest. Eddington based his theory on exact measurements of atomic masses performed by the English chemist F.W. Aston (1877-1945) and a preliminary proposal by the French physicist Jean Perrin (1870-1942). In 1928, the Russian physicist and cosmologist George Gamow (1904-1968) derived what is now termed the Gamow factor, which is a quantum mechanical formula that provides the probability of bringing two atomic nuclei sufficiently close together for the strong nuclear force to fuse them. The strong nuclear force is one of the four known forces of nature, and it is responsible for holding the nuclei of atoms together. The other three known fundamental forces of nature are the weak nuclear force, the electromagnetic force, and gravity.

The Gamow factor was used during the following decade by the British astronomer and physicist Robert Atkinson (1898-1982) and the Dutch-Austrian-German nuclear physicist Fritz Houtermans (1903-1966)–and later by Gamow himself and the Hungarian-American Physicist Edward Teller(1908-2003)–to derive the rate at which nuclear reactions would proceed on and on at the extremely high temperatures thought to exist within stellar furnaces.

In 1939, in a paper entitled Energy Production in Stars, the German-American nuclear physicist Hans Bethe (1906-2005) studied the varying possibilities for reactions by which hydrogen can be fused into helium. He defined two processes that he theorized could be the origin of energy within stars. The first, termed the proton-proton chain reaction, is thought to be the main energy source in stars possessing masses of up to approximately one solar mass. The second process, the carbon-nitrogen-oxygen cycle, which was also proposed by Carl Friedrich von Weiszsacker in 1938, is the most important source of energy in stars that are more massive than our Sun.

In 1946, Hoyle proposed that a collection of extremely hot atomic nuclei would assemble into iron. Hoyle continued on with this idea, and in 1954 described in a lengthy paper how advanced nuclear fusion reaction stages within stars would synthesize elements between carbon and iron in mass. This became the primary work describing stellar nucleosynthesis, and it provided the outline of how the most abundant atomic elements on our planet had been synthesized from the original hydrogen and helium of the Big Bang. Hoyle’s work made clear how those abundant elements increased their abundances within a galaxy as the galaxy continued to evolve and age.

Stellar Heavy Metals

Astronomers think that the first stars to light up in our Universe were not like the stars we wish upon today. This is because they were born directly from the pristine gases formed in the Big Bang itself. The primeval gases, hydrogen and helium, are thought to have pulled themselves together to create increasingly tighter knots. The cores of the first protostars to dazzle the Cosmos first ignited within the mysterious dark, frigid hearts of these very dense knots of pristine hydrogen and helium–which then collapsed under the pull of their own gravity. The first stars were probably enormous because they were not born in quite the same way, or from the same ingredients, that stars do now. The first generation of stars, termed Population III stars, were likely gigantic “megastars”. Our own Star, the Sun, is a fiery member of the most youthful generation of stellar inhabitants of the Cosmos–the Population I stars. Sandwiched between the stellar populations I and III are the appropriately designated Population II stars.

The enormous, massive Population III stars were brilliant, roiling, and searing-hot, and their ancient existence is considered to be responsible for causing the sea-change of our Cosmos from what it was very long ago to what it now is. These gigantic roiling, glaring stars altered the dynamics of our Universe by heating–and, therefore ionizing–the existing gases.

The metallicity of a star refers to the percentage of atomic elements it harbors that are heavier than hydrogen and helium. Metallicity also provides an important tool for astronomers to use. This is because its determination can reveal a star’s true age. When our Universe was born, its supply of atomic matter was almost entirely composed of hydrogen. It was through the process of stellar nucleosynthesis that the heavier metals were manufactured, destined to be incorporated into younger generations of stars from their cold, dark cradles within molecular clouds composed of gas and dust. Hydrogen fusion–the nuclear fusion of four protons to create a helium-4 nucleus–is the primary process that churns out energy in the seething-hot cores of active, young stars on the main-sequence of the Hertzsprung-Russell Diagram of stellar evolution. Our Sun is a main-sequence star, still burning hydrogen in active mid-life. It has lived out about half of its stellar life expectancy of 10 billion years, and at about 4.6 billion years of age, is expected to live for another 5 billion years before it dies the relatively peaceful death of similar small, solitary stars.

“Wasteful” Galaxies Inhabit The Cosmos

The team of astronomers, led by Dr. Oppenheimer, used data collected from the Cosmic Origin Spectrograph (COS), which is a $70 million instrument constructed at the University of Colorado at Boulder by Ball Aerospace Technology Corporation, for the purpose of studying the composition of the CGM. COS is installed on NASA’s HST and uses ultraviolet spectroscopy to examine the evolution of the Universe.

Our Milky Way Galaxy is a large spiral–an enormous, ancient, starlit pin-wheel whirling majestically in Space. Our Galaxy, like others of its type, vigorously forms baby stars, and displays a hot blue hue. This is in marked contrast to football-shaped elliptical galaxies that experience little in the way of star-birth, and shine with a light that is red. Both types of galaxies host tens to hundreds of billions of fiery stars that create heavy metals in their nuclear-fusing hearts.

The astronomers, after running a series of supercomputer simulations, discovered that the CGMs, in both types of galaxies, harbored more than 50% of a galaxy’s heavier atomic elements. This indicates that galaxies are not as efficient at keeping a grip on their raw materials as previously thought.

“The remarkable similarity of the galaxies in our simulations to those targeted by the COS team enables us to interpret the observations with greater confidence,” Dr. Robert Crain explained in the June 6, 2016 University of Colorado Press Release. Dr. Crain is a Royal Society University Research Fellow at Liverpool John Moores University in the UK, and a co-author of the study.

The new supercomputer simulations also explain the mysterious COS observation that there appears to be less oxygen surrounding elliptical galaxies than spirals.

“The CGM of the elliptical galaxies is hotter. The high temperatures topping over one million degrees Kelvin, reduce the fraction of the oxygen that is five times ionized, which is the ion observed by COS,” explained Dr. Joop Schaye in the June 6, 2016 University of Colorado Press Release. Dr. Schaye is a professor at Leiden University in the Netherlands and a co-author of the study.

Conversely, the temperature of the CGM gas in spiral galaxies is 300,000 degrees Kelvin, or about fifty times hotter than the surface of our Sun.

Dr. Oppenheimer told the press on June 6, 2016 that “It takes massive amounts of energy from exploding supernovae and supermassive black holes to launch all these heavy elements into the CGM. This is a violent and long-lasting process that can take over 10 billion years, which means that in a galaxy like the Milky Way, this highly ionized oxygen we’re observing has been there since before the Sun was born.”

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