Formation of the first galaxies was already well underway by no more than 700 million years after the big bang. Galaxies are interesting things. Compared to those commonly seen today, these ancient galaxies were mostly smaller and oddly shaped. Things were still crowded in those early times. Galaxies would form, then repeatedly collide and merge with other galaxies, resulting in new larger galaxies. Then as now, some really strange shapes and stranger effects could result when two galaxies happened to collide and merge. As their size and density grew they pulled in still more nearby galaxies until many grew to “ordinary large” galactic sizes. The Virgo cluster, around 60 million light years from Earth, contains more than a thousand of these large component galaxies.
Studying galaxies tells us a great deal about how the universe evolved. American astronomer Edwin Hubble specialized in observing galaxies, and he handed us a rich legacy. Among other things, the modern system for sorting and classifying galaxies is built on his work. Looking at those farther away is looking back in time, and Hubble’s work revealed that the ages of galaxies can often be distinguished by their shapes, of which two are most prevalent: older disks tend to be somewhat flat and oval, while younger disks tend to have spiral arms and a central bulge of older stars at the center. The Milky Way we call home is of the latter type. Elliptical galaxies are a third category that formed (usually) when two or more galaxies merged. These mostly contain only older stars because mergers occurred more frequently in the early history of the universe than they do today. A fourth type are so diverse and irregular as to defy classification.
Displaying the effects of gravitational tug, most galaxies appear to form from the inside out, starting with the spheroidal bulge at their centers. In most galaxies with a central bulge, a massive black hole is centered in the bulge. In certain galaxies, including our own Milky Way, the bulge is elongated into a “bar.” Some galaxies contain quasi-stellar objects—“quasars” —mysterious lighthouses glowing with the incredible luminosity of thousands of Milky Ways. Our universe is a strange place with strange ways…
Colors too help decipher a galaxy’s age. Galaxies whose light is on the red end of the spectrum consist of surviving low mass stars that are long lived because they have become “cooler,” relatively speaking. The older red stars typically are located predominantly in the galaxy’s central bulge. “Blue galaxies” by contrast contain stars that are often massive, super hot, and burn out quicker, hence cannot be as old as the reds. Modern telescopes show us a mixture—red galaxies where star forming has grown old and mostly ended, and beautiful blue spiral disks with many new stars still forming.
Some interesting facts attend the differences between stars and the star groupings called galaxies. The distance between galaxies averages about fifty times the average galaxy’s diameter, and it is still commonplace for galaxies to collide and merge as noted above. In contrast, the distance between stars averages millions of times the average star’s diameter, and stars virtually never collide. Galaxies throughout the universe are tenuously interconnected by enormous wispy filaments of gas and dark matter. These intergalactic filaments began forming when the universe was new and persist in the same pattern as Physarum polycephalum, a slimy yellowish lace of one-celled organisms that grow as spreading filaments on rotting tree trunks. This repetition of similar patterns in different contexts appears in many natural contexts— for example, the branching of streams in a river delta is seen also in the veins of a leaf and in the human blood and nerve systems.
Named by the early Greeks, our Milky Way came into being about 8.8 billion years ago – only five billion years or so after the big bang. A bit larger than average among galaxies in the modern sky, the Milky Way contains at least a hundred billion stars—maybe up to 500 billion. One of all those billions is our sun. We are at home in the Milky Way.
Like many other galaxies, our spiral-shaped Milky Way is turning, so that the spiral arms appear to be “trailing,” like that starfish on the potter’s wheel. “Our” overall diameter is more than a hundred thousand light years—the almighty distance light can travel at its fixed speed of 186,000 miles per second. A galactic “halo” of gas and stray stars reaches vastly farther out beyond our galaxy’s “edge.” From our home location near the outer edge of one of the spiral arms, our galactic center is around 27,000 light years distant. After you get there you’ll find the core itself is about 20,000 light years in diameter, and with a bulged shape, rather like—as one wag put it—two fried eggs back to back.
This central region contains vast swirling gas clouds, clusters of large powerful stars, and a massive black hole about two and a half million times more dense than our big sun. A black hole, remember, cannot be directly seen for the very good reason that not even light cannot escape its immense gravity and so there is nothing to see. It looks like a black circle surrounded by a background of stars. Its existence must be deduced. Astronomers have deduced this Milky Way’s monster because the stars near it, which can be seen, are observed to be orbiting the galactic center at thousands of miles per second—a speed that could only result from the presence of an enormous central mass. Cause and effect.
Such dramatic observed facts, plus reasonable inferences and deductions, help illustrate why so many scientists hold a mindset that their findings are more reliable than an ancient tribal tale which—literal interpretation or metaphor—locates heaven “between two large layers of water,” one being the earth’s oceans, the other somewhere in distant reaches of the sky. Many are the believers in science. Yet many more are believers in the literal truth of the Bible’s every word—heaven lies somewhere up there “between the waters.” Given the facts we know of the human brain and the incredible capacity of the mind that operates within it, how is such inane disagreement even possible? As global warming looms, we really, really need to answer this question, just as we desperately need to address the matter of what constitutes an adequate non-STEM education.
Evolution of stars, the element factories
Suppose there are “only” 200 billion galaxies (rather than that 500 billion estimate above) in the universe. That’s still quite a number, considering that a single galaxy may contain hundreds of billions of stars. And don’t forget, every bigger newer better telescope enables discovery of a lot more than we thought we knew before. Some galaxies have quite many more than the 200 to 400 billion stars in our Milky Way, so simple arithmetic thus tells us there are uncountable quintillions of stars in the wide universe. The true figure may range into the septillions or octillions, who can say? Let’s say a whole lot. How all these stars form(ed) is both interesting and important for our own human story.
Being at the smaller end of large-scale structure formation, each and every star came into being by the same gravity-driven process of self-organizing evolution that brought the galaxies and other large-scale structures into existence. At first, only large intra-galactic clumps condensed—big clumps forming within bigger clumps. The irregular clumping process that had begun soon after the big bang now continued inside millions of more localized gaseous clouds of matter and energy that formed within every protogalaxy.
In each of these large local clouds of gas and space dust, gravity continued pulling vast clouds of atoms inward until huge amounts of material, mostly hydrogen, accumulated at a center. As the centers grew in size and became ever more dense, heating up as inward pressure grew, they further condensed into mighty spheres that would become stars. The process was fairly straightforward. When the atoms of these spherical clouds were squeezed down sufficiently tight to form a star-sized ball, the increasingly intense heat (around 18 million degrees Fahrenheit) caused the ball to self ignite and start burning as an atomic fusion furnace. You could do this yourself by squeezing a golf ball in your hand if—a rather big if—you could squeeze the ball hard enough. Thus a star is born. But don’t squeeze too hard, or you might go beyond a star to form a black hole.
The first stars were massive—many over a hundred times the mass of our present-day sun. These early suns started the process that would eventually produce all our elements: oxygen for breathing, iron for trains, gold for rings, the everyday stuff we take for granted. But, at first, almost no elements heavier than hydrogen and helium yet existed.
Using hydrogen atoms as fuel, each star-furnace first fused hydrogen into helium, while releasing heat and light—sunlight—into the surrounding space. When the hydrogen was about burned up, the star started burning the helium—which produced other new elements. The process then repeated, each repetition producing a newer element to be burned in its turn, each new generation of elements slightly heavier (more protons, more electrons, in each new type of atom) than the one before. Coincidentally, radiation spreading outward from the furiously burning hot core kept knocking electrons back out of the recently-formed atoms, so that eventually each new star was surrounded by a huge bubble of hot gas thousands of light years across. You can see these “halos” tonight in certain telescopes.
With the first generation of stars populating the young galaxies, millions—eventually uncountable billions—of these new beacons poured light energy into the universe. But not yet did a single planet orbit any star. To have planets, gravity needs more than just hydrogen and helium to work with. Some heavier elements are required.
Star formation actively continues today in the parts of galaxies that are richest with gas and dust, particularly out in the spiral arms. Our sun and its planets reside at one edge of the Milky Way’s “Orion” spiral arm, which has been called a nursery for star formation.
In a pattern now familiar to us, stars are born, live vigorously, grow old, and die. In a less familiar pattern, an older generation’s death helps give phoenix-like birth to the next generation of stars. Which generation a star happens to be born into determines which elements will be available for it to use as fuel, how long it will live, and what sorts of elemental products it will leave behind when it exits—often by dramatic explosion.
Stars are element factories. Those in the first generation were necessarily composed of only the elements produced directly by the big bang: a great deal of hydrogen, much less helium, a wee dab of lithium. By orders of magnitude, hydrogen was the most plentiful element in the early stars, hence it became the primary fuel for making stars. Hydrogen burned intensely hot and relatively fast, so first-generation stars had short lifetimes of only a few million years. Their enormous mass also contributed to exhausting their fuel.
As hydrogen atoms burned in a star’s atomic furnace, the intense heat and pressure transformed them, alchemy-like, into increasing amounts of helium atoms. In turn, heat and pressure then turned some of the helium into lithium—and so the process continued, in turn transmogrifying each product element into another “new” element. The atoms produced by the first stars were mainly lighter elements, “lighter” meaning the atoms’ nuclei contained fewer protons and neutrons than do “heavier” elements. But the process could go only so far in these first stars, for when their primary hydrogen fuel became completely used up most stars collapsed in upon themselves and exploded.
When a star explodes, its newly-forged elements are broadcast far across its galaxy. From such debris clouds, fuel for newer stars’ fusion furnaces, more new elements gradually formed. As the clouds grew slowly larger, enriched with heavier elements produced by the first-generation stars, their growing gravity drew in still more gas from nearby smaller clouds. Once again, cloud centers became the compressed seeds for formation of stars—a second generation came into being. When enough light-element debris from previously exploded stars was incorporated into the new generation of protostars, they too ignited as nuclear furnaces, but with a difference.
The products of burning and churning in these newer stars were elements that were heavier (more protons and neutrons in the atom’s nucleus) than the first-generation stars had necessarily started with. Successive stellar generations gradually produced more and more ever-heavier elements. After lithium and deuterium, along came oxygen, nitrogen, silicon, sulfur, and many others including carbon—the very carbon of which our bodies are made. Our human bodies are formed of dust—immortal elements—that were forged in the hearts of stars. And you know what that’s called, as did Hoagie Carmichael when, inspired, he named his immortal melody.
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