Reality is the sprouting of Beauty
Below are some excerpts from a submission made on August 20, 2014, to the Cornel Library University, in the subject area of "Solar and Stellar Astrophysics."
Reconstructing the cosmic evolution of the chemical elements
The chemical elements are created in nuclear fusion processes in the hot and dense cores of stars. The energy generated through nucleosynthesis allows stars to shine for billions of years. When these stars explode as massive supernovae, the newly made elements are expelled, chemically enriching the surrounding regions. Subsequent generations of stars are formed from gas that is slightly more element enriched than that from which previous stars formed. This chemical evolution can be traced back to its beginning soon after the Big Bang by studying the oldest and most metal-poor stars still observable in the Milky Way today. Through chemical analysis, they provide the only available tool for gaining information about the nature of the short-lived first stars and their supernova explosions more than thirteen billion years ago. These events set in motion the transformation of the pristine universe into a rich cosmos of chemically diverse planets, stars, and galaxies.
One beautiful afternoon I went for a run along the river. I was breathing plenty of fresh air, my face was all flushed, and I felt my heart pounding and blood flowing through my body. As air was filling my lungs, I was reminded of Carl Sagan’s saying: “We are all made from star stuff.” Indeed we are. When quenching my thirst with water, I was consuming hydrogen and oxygen in the form of H2O. When breathing, I had been taking in air made from nitrogen, oxygen, and tiny traces of other elements such as argon and neon. The red liquid of life owes its color to iron, which is embedded in our hemoglobin. But these elements do not just circulate within our carbon-based bodies: before1they became part of humans, each of these atoms was created in a grand cosmic cycle called chemical evolution that took place long before biological evolution led to life on Earth.
Most of the universe’s iron, for example, is the end result of a binary star system in which one star acquires enough material from its companion that it reaches a critical mass and erupts in a huge thermonuclear explosion, forging new elements in the process. On the other hand, the hydrogen atoms that make up water are probably nearly fourteen billion years old and were created as part of the Big Bang. And all the carbon upon which life as we know it is based was synthesized in evolved stars near the end of their lives.
Immediately after the Big Bang 13.8 billion years ago, there was a time without stars and galaxies. The hot gas left over from the Big Bang had to cool enough before the first cosmic objects were able to form. This process took a few hundred million years, but eventually the very first stars lit up the universe. The universe at that time was made from just hydrogen and2helium: heavier elements did not exist yet. As a consequence of a variety of gas chemistry and cooling processes that govern star formation, the first stars are thought to have been rather massive. Recent computations suggest these behemoths may have had up to one hundred times the mass of thesun2. In comparison, most stars today are low-mass stars with less than one solar mass.
During the explosion of a star, all the newly created elements are released into the surrounding gas. The death of the first stars marked an important milestone in the evolution of the universe: it was not pristine anymore, but3“polluted” with carbon, oxygen, nitrogen, iron, and other elements. Thus, over time, the universe became more and more enriched in the elements heavier than hydrogen and helium, which are collectively called “metals” by astronomers. In contrast, the very first stars were the only ones that formed from completely metal-free gas. All stars in subsequent generations would then form from gas clouds that contained some metals provided by at least one previous generation of stars exploding as supernovae. The sudden existence of metals in the early universe following the death of the first stars changed the conditions for subsequent star formation. Gas clouds can cool down more efficiently when metals or dust made from metals are present, leading to the collapse of smaller clouds, and thus the formation of smaller stars. Lower-mass stars like the sun could therefore form for the first time. The first low-mass stars (those with 60 to 80 percent of the mass of the sun) have long lifetimes of fifteen to twenty billion years due to their sparse consumption of the nuclear fuel in their cores. Born soon after the Big Bang as second or third-generation stars, they are still shining today. Many of these ancient survivors are suspected to be hiding in our Milky Way galaxy and, indeed, astronomers have discovered dozens of them over the past three decades. What makes these extremely rare objects so valuable is that they preserve in their atmospheres information about the chemical composition of their birth cloud, which existed soon after the Big Bang. Hence, studying their chemical composition allows astronomers to reconstruct the early era of their births.
In the earliest stages of the universe’s development, massive stars exploding as supernovae dominated the production of iron in the universe. However, this changed after about a billion years. Through the existence of the first lower-mass stars with longer lifetimes, a different pathway for iron production emerged. At the end of their long lives, low-mass stars turn into compact white dwarf remnants. If a star and a white dwarf are in a binary system and enough mass is transferred from the star to the white dwarf, the latter will undergo a thermonuclear explosion. Given the dominance of low-mass stars in the universe today, iron is thus mainly produced by this process rather than by exploding massive stars, as was exclusively the case in the early universe. After about nine billion years of this chemical evolution, driven by different types of stars at different times, our sun, together with its planets, finally formed. Its birth gas had been enriched by perhaps a thousand generations of stars and supernova explosions. That evolution pro vided the gas4with enough metals to enable the formation of planets something that may not have been possible much earlier on in the universe. Consequently, when astronomers look for extra solar planets, they focus their search on stars that are close in age to or younger than the sun.
In fact, there are recent indications that these systems are nearly as old as the universe itself: some of them may be among the first galaxies that11formed after the Big Bang. Studying these stars thus offers another chance to reconstruct the initial events of element creation within the first stars and their violent explosions, and the subsequent incorporation of this material into next-generation stars. Moreover, the existence of such old satellites may shed light on the existence of metal-poor stars in the halo of the Milky Way. Predating our own galaxy, these halo stars must have come from somewhere; perhaps they originated from dwarf galaxies when analogous systems were gobbled up by the Milky Way during its assembly process.
