Galactic Archaeologists Build A Stellar Family Tree
By Judith E Braffman-Miller
Traditional archaeologists uncover the many mysteries of our ancient human ancestors by digging for the lingering remains of long-lost civilizations that can be excavated and observed today. Astronomers are now borrowing the same principles applied in archaeology and biology in order to piece together the mysterious family tree of the stars that dwell within our Milky Way Galaxy. Galactic archaeologists do this by carefully tracing the history and formation of our Milky Way from detailed observations of the stars, gas and other structures that can be observed from our own planet today. In February 2017, a team of astronomers announced that by studying chemical signatures found in the stars, they have pieced together stellar evolutionary family trees by observing how the stars were born and how they are all connected to each other. The stellar signatures act as a proxy for DNA sequences in genetics.
In 1859, Charles Darwin published his revolutionary theory of evolution proposing that all life forms on Earth are descended from one common ancestor. This theory has been the basis of evolutionary biology ever since. However, Darwin’s theory is now being adopted by some scientists who are in an entirely different field. One such “adoption” happened as the result of a recent chance encounter between an astronomer and a biologist over dinner at King’s College in Cambridge, UK. The conversation inspired the astronomer to entertain the possibility of applying what she had learned from the biologist about biological evolution to the host of dazzling stars that dance around within our large barred-spiral Galaxy.
Writing in Monthly Notices of the Royal Astronomical Society in London, England, Dr. Paula Jofre, of the University of Cambridge’s Institute of Astronomy, describes how she went about creating a phylogenetic “family tree” that connects a large number of stars that inhabit our Galaxy.
“The use of algorithms to identify families of stars is a science that is constantly under development. Phylogenetic trees add an extra dimension to our endeavors which is why this approach is so special. The branches of the tree serve to inform us about the stars’ shared history,” Dr. Jofre noted in a February 20, 2017 University of Cambridge Press Release.
All stars are enormous, roiling spheres of glaring, searing-hot mostly hydrogen gas. Hydrogen is both the lightest and most abundant atomic element in the Universe, and the stars transform their supply of hydrogen fuel into heavier atomic elements in their nuclear-fusing cores by way of a process termed stellar nucleosynthesis.
The first stars were not like the stars that we are familiar with today. The first stars were born directly from the lightest primordial gases–hydrogen and helium–which formed in the Big Bang birth of the Universe almost 14 billion years ago (Big Bang nucleosynthesis). In fact, the only atomic elements that were manufactured in the wild, exponential inflation of the Big Bang are hydrogen, helium, and trace amounts of lithium and beryllium. All of the rest of the atomic elements, listed in the familiar Periodic Table, were created deep within the secretive, nuclear-fusing hot hearts of the stars, their stellar furnaces progressively fusing the nuclei of atoms into heavier and heavier elements–or, in the case of the heaviest atomic elements of all (such as gold and uranium), in supernovae blasts that mark the violent demise of massive stars.
Without these heavy atomic elements, created within our Universe’s vast multitude of stars, there would be no life. The oxygen we breathe, the carbon that is the basis for life on Earth, the elements that compose the stones and sand that we walk upon, were all manufactured in the hidden hearts of hot ancient stars–billions and billions of years ago. We are the dust of stars. When massive stars perish, they meet their inevitable doom brilliantly and explosively. It is thought that the first stars to dance around in our Cosmos were huge, possibly weighing-in at hundreds of times solar-mass. When massive stars go supernova they violently hurl their batch of freshly formed heavy elements out into interstellar space. Because the first stars were so huge, they lived fast and paid for it by dying young. The more massive the star, the shorter its “life”. When the first generation of stars died in horrific supernova blasts, they tossed out into the space between stars the very first batch of heavy elements–so necessary for the emergence and evolution of life in the Universe.
Pristine primordial hydrogen and helium were pulled together to form knots of gas that were bound by gravity. The cores of the first generation of newborn baby stars (protostars) caught fabulous fire within the frigid dark hearts of these very ancient dense knots of pristine gas. The dark tightly bound knots collapsed under the relentless pull of their own gravity, until their raging nuclear-fusing stellar fires at last ignited.
The sparkling members of the first generation of stars are termed Population III stars. Our own Sun is a member of the youngest generation of stars, and is a Population I star. Sandwiched between the oldest generation of stars (Population III) and the youngest (Population I) stars, are the Population II stars.
Population III stars did not ignite until approximately 100 million years after the Big Bang, and their ancient birth is one of the greatest mysteries in scientific cosmology. It is currently thought that the Population III stars were not only very massive, but also brilliantly bright, and their birth is primarily responsible for causing a sea-change in our Cosmos–changing it from what it was into what it is. The so-called “ordinary” atomic matter that formed first in the Big Bang fireball, and was later manufactured in the hot hearts of stars, is made up of protons, neutrons, and electrons. Protons and neutrons are glued together into atomic nuclei encircled by a cloud of electrons. Hydrogen is composed of only one lone proton and one electron. Helium is composed of two protons, two neutrons, and a surrounding cloud of electrons. The nucleus of a carbon atom is made up of six protons and six neutrons. The nuclei of heavier atomic elements, such as iron, lead, gold, and uranium, contain larger numbers of protons and neutrons.
The metallicity of a star describes the percentage of its matter that is made up of chemical elements heavier than helium. Because stars account for most of the visible matter in the Cosmos, and are mostly composed of hydrogen with smaller amounts of helium, scientific cosmologists (for convenience) use the general term metal when they are referring to all of the atomic elements listed in the Periodic Table that are heavier than helium. Therefore, the term metal in astronomical jargon is not defined the same way that it is in chemistry. A nebula (cloud) that has been abundantly gifted with nitrogen, carbon, oxygen, and neon would be termed metal-rich by an astronomer, even though those elements are not defined as metals by a chemist.
Stellar metallicity can reveal the well-kept secret of a star’s true age to the prying eyes of curious astronomers. When the Universe first came into existence, its supply of atomic matter was almost entirely hydrogen. However, through the process of primordial nucleosynthesis, an abundant amount of helium formed, as well as very small quantities of lithium and beryllium–but nothing heavier. Therefore, the older generations of stars–Populations II and III–have lower metallicities than bouncing stellar babies, like our Sun, that are designated Population I.
The stellar generations I, II, and III, exhibit decreasing metal content with increasing age. Therefore, relatively young stars, like our Sun, show the highest metal content when compared to their ancient forebears. The first generation of stars (Population III) were born completely barren of metals. Population II stars are very ancient, but they came after the first generation of stars (Population III), and before relatively youthful Population I stars. Population II stars are not barren of metals because they inherited the freshly forged metals manufactured by the primeval Population III stars.
Even though the most ancient stars contain lower metal contents than their stellar descendants, the fact that literally all of the stars so far observed by astronomers contain at least some small quantity of metals is a perplexing puzzle. The currently favored theory proposes that Population III stars must have inhabited the baby Cosmos in order for these heavier atomic elements to have been manufactured–even though not one Population III star has ever been observed. According to this explanation, in order for the ancient Population II stars–which have been detected–to contain their relatively puny quantities of metals, their metals must have been manufactured in the nuclear-fusing hot hearts of an earlier generation of stars–the Population III stars. Soon after the Big Bang, the Universe had no metal content. For this reason, theory suggests that only stars with masses hundreds of times that of our own Sun could have been born in the primeval Cosmos. As they approached the grand finale of their hydrogen-burning (main-sequence) “lives”, the Population III stars fused the first 26 atomic elements listed in the Periodic Table by way of the process of stellar nucleosynthesis.
As later generations of stars were born in the Cosmos, they became increasingly more and more metal enriched. This is because the gas-laden natal clouds–from which the later generations of stars emerged–contained metals cooked up in the stellar cauldrons of earlier generations of stars. The youngest generation of stars–like our Sun–have the largest metal content. However, all stars–of any stellar generation–are mostly composed of hydrogen. Even metal-rich stars contain only small amounts of any element heavier than helium. In fact, metals (according to the definition used by astronomers) make up only a small fraction of the total chemical composition of the Cosmos.
Going On A Stellar “Dig”
Galactic archaeologists work on a wide range of areas and problems within the general theme. These include:
-understanding the dynamical and chemical properties of various stellar populations dwelling within our Milky Way Galaxy.
-hunting for very metal-poor stars. Stars with low metallicity include the most ancient stars–Population III–that were born early in the history of our very ancient Galaxy.
-discovering the mysterious origins and characteristics of globular clusters, which are very common spherical stellar clusters.
-Hunting for our Milky Way’s retinue of satellite galaxies.
-mapping our Milky Way’s structure. This would include the stellar streams and substructure that developed during the accretion of neighboring galaxies.
-investigating the properties of the Large and small Magellanic Clouds. This nearby duo of dwarf galaxies are relatively close neighbors of our Milky Way.
Galactic Archaeologists Build A Stellar Family Tree
Dr. Jofre’s team chose twenty-two stars, including our own Sun, to study. The astronomers carefully measured the stars’ chemical elements, based on information derived from ground-based high-resolution spectra, collected by large telescopes situated in the north of Chile. Once the stellar families were identified, using the chemical DNA, their mysterious evolution was investigated–using what was known of their true ages and kinematical properties that had been obtained from the space mission Hipparcos.
A space observatory of the European Space Agency (ESA), Gaia is the successor to the Hipparcos mission. Designed for astrometry, Gaia’s goal is to create the most precise, as well as the largest, 3D space catalog ever constructed–totalling approximately 1 billion astronomical objects including stars, planets, asteroids, comets, and quasars among others. The spacecraft will monitor each of its target stars approximately 70 times over a five year span to study the exact motion of each star relative to our Galaxy. This will include about 1% of the Milky Way’s population with all stars brighter than magnitude 20 in a broad photometric band that spans most of the visible range. Part of ESA’s Horizon 2000+ long-term scientific program, Gaia was launched on December 19, 2013 by Arianespace using a Soyuz ST-B/Fregat-MT rocket soaring from Kourou in French Guiana. In addition, Gaia is expected to spot thousands to tens of thousands of Jupiter-sized exoplanets in orbit around stars beyond our own Sun, 500,000 quasars, and tens of thousands of new comets and asteroids within our own Solar System.
Stars are born when especially dense blobs, embedded within their natal giant, cold molecular clouds collapse under the merciless and powerful tug of their own gravity. A duo of sister stars with the same chemical compositions probably were born within the same natal molecular cloud. Some lucky stars “live” longer than the age of the Universe, and because of their longevity, they can serve the important role of fossil records revealing the composition of the gas at the time they were born. The oldest star in the sample analyzed by Dr. Jofre’s team is estimated to be almost ten billion years old, which is more than twice the age of our 4.56 billion year old Sun. The youngest star studied by the team is a “mere” bouncing baby at 700 million years old.
According to Darwin’s theory of evolution, all organisms on Earth are linked together by a pattern of descent with modification as they evolve. Because stars are not living, they are very different from living organisms on our planet. However, stars do have a history of shared descent as they are born from their natal giant, cold molecular clouds of gas and dust. The stars, in a way that evokes a comparison to living things on Earth, carry their history in their chemical structure. By applying the same phylogenetic methods that biologists use to trace descent in plants and animals on Earth it is possible to investigate the mysterious “evolution” of stars dwelling within our Galaxy.
“The differences between stars and animals is immense, but they share the property of changing over time, and so both can be analyzed by building trees of their history,” explained Dr. Robert Foley in the February 20, 2017 University of Cambridge Press Release. Dr. Foley is of the Leverhulme Centre for Human Evolutionary Studies at Cambridge.
With more and more datasets being made available from both Gaia and more advanced telescopes on Earth, as well as on-going and future large spectroscopic surveys, astronomers are progressing ever closer to being able to assemble one stellar family tree connecting all the stars in our Milky Way.
The new research is published by Monthly Notices of the Royal Astronomical Society, DOI 10.1093/minras/stx075, under the title Cosmic phylogeny:reconstructing the chemical history of the solar neighborhood with an evolutionary tree (Paula Jofre et al).
Judith E. Braffman-Miller is a writer and astronomer, whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy, because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.