Back in 1969, singer and songwriter Joni Mitchell composed a beautiful, haunting song immortalizing the young hippies of Woodstock Nation. “We are stardust; we are golden,” she wrote–“long time ago,” as Peter, Paul, and Mary would say.
Truer lyrics have never been spun into a melody.
Stars are enormous incandescent balls of searing-hot gas. The billions and billions of glittering stars that haunt our sky at midnight are primarily composed of hydrogen–the lightest element in the Periodic Table, as well as the most abundant in the Universe. The stars transform their hydrogen into progressively heavier and heavier elements, deep down within their fiery-hot, nuclear-fusing hearts, by way of a process termed stellar nucleosynthesis–spinning star-stuff into gold!
It all began very long ago, when our almost-14-billion-year-old Universe was young. It is believed that the first stars born in the ancient Universe bore little resemblance to the stars we observe today. The first stars formed directly out of the lightest of all primordial gases, hydrogen and helium, which came into being during the hot Big Bang birth of our Universe itself. In fact, there were only three atomic elements born in the Big Bang fireball–hydrogen, helium, and wispy traces of lithium. The rest of the elements of the Periodic Table were manufactured deep within the hearts of stars, their extremely hot interiors fusing the nuclei of atoms into increasingly heavier and heavier elements. All of the atomic elements heavier than helium, all the way up to iron and nickel, were made deep within the hearts of stars–carbon, oxygen, neon, magnesium, silicon, sulfur, argon, calcium, titanium, chromium, nickel, and iron would not exist in our Universe, if the stars had not been there to manufacture them. Elements heavier than iron are cooked up in supernova blasts themselves. Massive stars die explosive deaths (unlike smaller stars like our Sun whose demise is relatively peaceful), and when they blow themselves up, the horrific blast creates the heaviest of elements–mercury, uranium, and gold.
When the outer layers of a massive star that has gone supernova are hurled back out into space, its newly forged batch of heavier elements may eventually be reincorporated into cold dark molecular gas clouds that will later give birth to new generations of stars, their retinues of planets and moons and other objects, and living creatures, such as the people who dwell on our planet. Without these heavier atomic elements, cooked up in the searing-hot cauldrons of our Universe’s stars, there would be no life. The oxygen we breathe, our bones, our blood, the elements composing the dirt, stone, and sand beneath our feet, the carbon that is the basis for all life on Earth, were all cooked up within the fiery cores of ancient stars, billions and billions of years ago. We are stardust!
It is believed that the first stars to light up our Universe switched on almost at the beginning of Time itself. These stars were born when hydrogen and helium were drawn together by gravity to create knots of pristine gas. The fiery, searing-hot, nuclear-fusing hearts of the very first generation of protostars formed when the knots collapsed under their own gravitational weight. These first stars were probably gigantic because they did not form from the same elements, and did not form in precisely the same way, as the familiar stars that we now know and love. Members of the first generation of stars are termed Population III stars. Our Star, the Sun, is a youthful, bouncy member of the youngest generation of stars, the so-called Population I stars. In between the first stars, and the youngest generation of stars like our Sun, are the appropriately designated Population II stars.
The metallicity of a star refers to the percentage of its material that is made up of chemical elements heavier than hydrogen and helium. Astronomers use (for convenience) the all-embracing term metal when referring to all of the elements of the Periodic Table heavier than helium. Therefore, the term metal possesses a different meaning for astronomers than it does for chemists. A cloud floating around in space that is richly endowed with nitrogen, neon, carbon, and oxygen would be designated “metal-rich” by an astronomer, even though those elements are not metals to a chemist.
The stellar Populations I, II, and III, show decreasing metal content with increasing age. Therefore, the first stars to catch fire in the Universe were depleted of metals. The Population II stars, which are also very ancient, but not as old as the Population III stars, bear the metals manufactured in the hearts of the first stars. Population I stars like our Sun, the bouncing babies of the Universe, have the greatest metal content. As each subsequent generation of stars came into being, they became increasingly more heavily metal-enriched, as the gas clouds from which they were born were gifted with metal-enriched stardust, spun up and scattered into space by earlier generations of stars.