Even the dinosaurs, which roamed the Earth only a scant sixty-five million years ago—a blink of a cosmic eye, so to speak—knew a slightly smaller, slightly weaker sun. But this change is only relatively slight, and through the bulk of a star's lifetime, it lies on the central strip of the H-R diagram known as the “main sequence.” This longevity is provided by hydrostatic equilibrium, the same kind of force-balancing exercise practiced with gusto by galaxy clusters. In the case of stars, the outward explosion from the nuclear inferno is equalized by the inward gravitational pressure.
But eventually (where “eventually” can mean anywhere from ten million years for the biggest stars to ten billion years for the sun to trillions of years for the smallest stars), the helium detritus grows too massive and forms an unmotivated core in the center of the star, forcing the hydrogen fusion party out of the house and into the front yard. The expansion of the fusion shell pushes the outer atmosphere of the star well beyond its normal limits. Separated from the nuclear core by a larger distance, the surface now cools, and the star branches off the main sequence, becoming a red giant. (Because it's red and giant. For once, astronomers named something appropriately.)
From here, different stars have different endgames, though they're all miserable in their own unique, special ways. For stars like our sun, the interior pressures and temperatures eventually reach such incredible heights (let's say one hundred million Kelvin for good measure) that helium burning begins. But helium fusion, leading to carbon and oxygen, isn't as efficient as hydrogen burning, so to stabilize against the inevitable collapse of gravity, the reactions must take place at a feverish rate, and the helium-themed party doesn't last nearly as long. In only a few minutes, a good chunk of the helium is burned off in a flash, which inflates the core and immediately cools it, temporarily shutting off fusion and leading to, over the course of ten thousand years, the collapse of the red giant.
But once the star settles down again, the fusion game picks back up, this time with a solid core of carbon and oxygen, surrounded by a shell of helium fusion, with that surrounded by good old-fashioned hydrogen burning. The party is now so intense it's in the streets, and it can only keep it up for a hundred million years or so before it begins climbing back up to red giant status. What follows is a series of violent seizures as different layers of the star abort and restart fusion in jagged procession. The expansion of the star and the intermittent violence in the core vomit the outer layers of its own atmosphere out into the system.
Once every hundred thousand years, a new spasm hurls more of the star's mass into surrounding space. The death throes are long, slow, and painful as the star tears itself apart, one vicious episode after another. Before long, the brilliantly hot core of unburnt carbon and oxygen, about the size of a respectable rocky planet, is exposed to space for the first time, searing the surrounding system with powerful X-rays.
But that period of violence leads to a moment of ephemeral and effervescent beauty. The remnants of the star are, by now, long since blown out in complicated patterns into the depths of space, and for a brief time, barely ten thousand years, the X-rays from the leftover core illuminate the decrepit remains, energizing the gas like a neon sign, with any unique elements giving off their distinctive spectral glows. While the core (now encumbered with the name white dwarf) remains hot enough to emit X-rays, the newly spun planetary nebula glitters like an ornament, a masterpiece of interstellar art, unique to that star, never seen before and never to be seen again. But when the core shuts down, the curtain finally closes, and stars like our sun leave the cosmic stage.
For stars larger than our sun, their end comes too, but much more quickly and with efficient mercy compared to the slow, drawn-out suffering of their lower-mass siblings. For them, their increased bulk can keep a gravitational lid on things, preventing runaway expansion and expulsion as a planetary nebula. Instead, the core grows hotter and hotter, reaching new levels of fusion intensity, with carbon and oxygen fusion kicking in at the billion-Kelvin mark, followed by silicon fusion at three billion Kelvin. Each stage is shorter and shorter as the fusion processes grow ever less efficient, the ending stages leaving the star looking like a nuclear seven-layer bean dip, layers going from hydrogen in the outermost layers to iron and nickel in the center.
It's a runaway process driven by continued, pounding gravitational collapse. The hydrogen burning lasts a few million years, followed by a million years of helium fusion. The carbon can power the star for only six hundred years (notice the lack of any other word after the number), with neon lasting barely a year. Oxygen takes a turn for a mere six minutes, ending with silicon forming a solid iron ball in the center of this massive beast in less than a day.
With iron, the cops finally come in to break up the scene. Fusing elements lighter than iron leads to energy gains as the binding glue increases. But above iron, it takes energy to form heavier elements. So the fusion process still happens, but there's no leftover energy to stop the crush of 1031 kilograms of material on top of it. The result: any errant electrons buzzing around get shoved into nearby protons, converting them into neutrons via the weak nuclear force. In a dozen minutes, the entire core of iron is transformed into a dense neutron sphere the mass of the sun and the size of your neighborhood.
Without overwhelming pressure, that's as compact as the neutron sphere can get. The superlative tons of atmosphere crush inward, meet resistance, and rebound outward.
As they say, boom.
These supernova explosions signal the deaths of the most massive stars in our universe, flinging newly fused material out into the surrounding medium. The energetics of the detonation itself, an instant that releases more energy than the entire lifetime output of a hundred suns, fuse a slew of elements heavier than iron, enriching the interstellar expanses with the rest of the periodic table.
These titanic outbursts, known as type II or core-collapse supernovae, aren't the only flare-ups in the cosmos. This is the common fate of massive, but isolated and lonely, stars. But most stars are found in double or triple (even septuple!) systems, and that leads to some other interesting situations worthy of note—mainly because they're hard to ignore.
A star's fate is largely set the day it's born. Its mass determines its future track on the H-R diagram. Low-mass stars lead long, but uneventful and not very bright, lives. Medium-mass stars like our sun burn for billions of years before turning inside out. Massive stars burn the nuclear candle at both ends, snuffing themselves out in millions of years. When stars are born in the same system, they will inevitably have different masses, because why should they be the same?
Thus the more massive sibling will go through its life faster, leading to the inevitable sad-sack white dwarf or neutron star. Its lower-mass relative, bound by gravitational chains, is forced to watch the whole ugly process play out. But it too eventually succumbs to a similar fate. As the star expands into its red giant phase and draws close, its outer envelope funnels onto its long-dead companion. When enough material accumulates on the surface, the increased pressures can ignite a nuclear burp—an intense but brief flash of energies, a nova.
In some rare cases, enough material can spill onto the surface of a companion white dwarf without small eruptions, saving its energies for a single insane outburst that triggers a chain reaction in the carbon-oxygen soup of the dead star itself, ripping it apart in an instant, ecstatic whoosh. Since white dwarfs always have about the same mass, the explosions have roughly the same brightness every time they go off. A standard candle, if you will; a powerful supernova of another kind—the type Ia—first recorded in detail by Tycho Brahe himself, and used hundreds of years later to map the expansion history of the universe and discover dark energy.
And you thought I would just leave you hanging about type Ia supernovae, didn't you?
Baryons may make up a small fraction of the contents of the universe, but they can pack quite a punch. But it's not all fusion factories and energetic explosions out there in the v
astness of space. Stars may get all the attention, constantly strutting their radiative stuff and occasionally blowing themselves up, but there's another side to the baryonic coin in our universe—the nebulae.
Thin, wispy tendrils containing up to thousands of suns’ worth of raw material. Mostly hydrogen and helium—no surprises there—but always sprinkled with traces of metals (in the joys of astronomical terminology, anything heavier than helium is called simply “a metal,” because don't ask). With the naked eye, or telescope-enhanced eye, they generally come in two different colors: bluish and reddish. The red ones are typically near very bright stars, absorbing their high-energy output and spitting it back in more familiar colors. The blue ones hang out near less intense stars and simply reflect and scatter any incoming light, which, as in our own atmosphere, produces a warm blue tint.
While a cloud of gas and dust can hang out in the middle of nowhere for as long as it wants, occasionally it will receive a kick—say, from a nearby supernova or passing cloud—and when that happens, troubles set in. As the cloud pulls in on itself, pieces pinch off and catastrophically collapse, their internal densities and pressures climbing higher and higher. The more they collapse, the more they heat up, and the more they heat up, the more they radiate away that energy, cooling themselves and collapsing even further. It's the result when hydrostatic equilibrium is neither static nor in equilibrium.
The collapse continues unabated until, you guessed it, the pressures and temperatures reach that special critical threshold and hydrogen fusion ignites deep in the core—a star is born, surrounded by the beginnings of a planetary system. Once the star gets fully pumped up, it clears away its dusty nest, leaving behind any planets that may have formed from the leftover materials. The star lives its life, how long depending on how much gas got pumped into it during its nursing phase, and eventually dies, in one form or another, spewing its contents back to where they came.
That newly formed nebula, freshly ejected from the dying star, mingles back with the general interstellar milieu, finding new friends and new hangouts. The next generation of nebulae live their lives unmolested until a new instability crisis, repeating the pattern.
Stars. Nebulae. Stars. Nebulae. A cycle of cosmic birth, death, transformation, and rebirth, repeated since the first stellar denizens appeared on the scene all those billions of years ago. Each successive generation a little bit richer in elements, with more carbon, silicon, oxygen, iron, plutonium, potassium, and you know the rest of the periodic table.
After enough generations some of those stars could host planets composed of oxygen and silicon bound together to make a material known as “rock,” with enough oxygen leftover to bond with hydrogen to make “water.” Maybe some spare nitrogen gas to make “air.” Add a healthy dose of sulfur, calcium, and phosphorus and a sprinkling of a few others, and you get “living creatures.” Star stuff (or star poop, if you're feeling juvenile), formed in fusion factories in the hearts of stars or in cataclysmic explosions, regurgitated and recycled for eons. A process set in motion billions of years ago in a dark and lonely universe, hydrogen moved by simple gravity, reconstituted into bizarre and complex organisms capable of locomotion and cognition.
Sometimes those organisms even start asking questions.
Maybe Kepler was right after all, after a fashion. We're not communicating substantially with the celestial realm through any physical force, and the stars certainly don't tell us who to marry or the best time to wager on that horse. But the heavenly denizens did break themselves down to form the solar system, the sun and all the planets. So in a sense—a technically narrow but hauntingly beautiful sense, a sense that takes deep time and expanded horizons to realize—we are indeed connected to the stars, and they to us.
And beyond the scale of stars and nebulae tracing out their interconnected, sometimes-blowing-up lives lie the galaxies themselves. We've already taken the measure of the Milky Way, but we haven't cataloged its contents. It may not be a significant player on the cosmological scene, no more or less so than any other galaxy, but dang it, it's our home. We should be proud of it.
Galaxies got a major boost in the hearts and minds of astronomers about a hundred years ago, when Hubble firmly concluded that the strange, dusty spirals, at one time considered to be just another variety of nebula, were instead island universes—galaxies—in their own right, far removed from our own Milky Way by a vast expanse of vacuum.
But as our surveys grew broader and deeper, galaxies once again lost their importance, or at least their uniqueness. One speck of aggregated light isn't much different from another, as long as it faithfully traces out the underlying dark matter and gives us frames of reference for the occasional supernova. To a cosmologist, galaxies are a means to an end, a tool for measuring even greater things.
Still, that's not the whole story. There's more to a galaxy than meets the eye, and there's a heck of a lot that meets the eye. As we added more galaxies to our collections and mounted them in our display cases, we of course noticed all the peculiar differences. Many had the iconic spiral arms, quite a few were just general balls of gas and stars, and some were very lumpy and irregular. Of the spirals, some had dazzling pinwheels; some had just a couple of spokes; some had central bulges; and others had long, extended bars in their cores. Some galaxies were busting with star formation activity; some were long dead, dim, and red.
Besides the big ones (the biggest a few times larger than the Milky Way), the universe was littered with small dwarf galaxies. These little intergalactic rug rats averaged a bare 1 percent the size of our own galaxy. Sometimes they orbited larger galaxies, like the Magellanic Clouds, and sometimes they were isolated in the cold vastness. They too had a diversity of shapes: sporting spirals when they could, settling into elliptical monotony, or getting torn into irregular clumps.
Even more galaxies awaited the patient astronomer capable of deep observations at radio wavelengths: the blazars, the quasars, the LINERs, the Seyferts, the LIRGs. Fanciful acronyms and names used in place of real understanding. Some galaxies are quiet as a mouse, and some blast so loudly they can shine across the visible universe without breaking a sweat.
What's the common thread? Hubble himself attempted a classification scheme that, while not quite right, is still used because Hubble was awesome and I guess we respect the guy or something.5 Once we figured out that the universe evolves, which itself was a major breakthrough, pieces of the galactic family puzzle began to click into place—though I should repeat my usual caveat that there's still a lot we don't understand.
There's something you should know about galaxies: they have secrets. Deep, dark secrets they hold close to their hearts. In fact, their hearts themselves are the secrets. They're infinitely black and all-consuming. As we've seen already, it's now understood that almost every galaxy hosts a massive black hole in its core. These black holes are tremendous beasts—at the low end, millions of times the mass of the sun.
Known appropriately as supermassive black holes, they are for the large part quiet. With no stars, dust, or gas nearby, they sit there, lurking in the shadows, sleeping monsters. But when material falls in, the release of gravitational energy ignites the infalling gas, causing it to glow intensely as it crushes in toward oblivion. The swirling gas is a mad rush of plasma, with charged particles whipping furiously around from the tremendous forces. Generating loops of self-reinforcing magnetic fields, some material can eject itself to safety before crossing the surface of blackness that marks the outer edge of the black hole itself. The ejected material can stretch for thousands of light-years, powered and sustained by those same twisting magnetic fields, piercing beyond the host galaxy and into the surrounding cluster itself.
Thus, an active galaxy, the most powerful engine known in the universe, among the most energetic events since the big bang itself, driven by the extreme gravities of the true monsters of the cosmos: the black holes. When galaxies are active, they scorch themselves with radiation, heating their own g
as and preventing the cooling of nebulae, slowing down star formation. Thankfully, the Milky Way is dormant now, as are most galaxies in the present-day cosmos. But in the distant, more crowded past, when larger structures like clusters first started forming, galaxy mergers were much more common, and these active galaxies were much more numerous. It was dangerous times then, and much louder in the radio—these were the bright but distant sources seen in the 1950s that started putting the screws to Hoyle's steady-state model and early credence to the big bang that we all know and love.
Every galaxy is built from the successive mergers of smaller ones, with many repeated feedings of the central black holes (which you'll be unsurprised to learn also grow as more galaxies combine). Eventually, though, for most galaxies, the mergers cease, and they can begin to live in peaceful bliss, resulting in the development of elegant spiral arms. That's right: it's thought that spiral arms are the natural state of a galaxy when left to its own devices. Density waves spread through each galaxy like ripples in a pond, but unlike a pond, the galaxy is spinning with its inner bits faster than its outer bits, causing some of the waves to pile up on each other in, well, a spirally fashion. The density differences in an arm are pretty mild—just a few percent—but can trigger the collapse of nebulae and lead to a new round of star formation wherever they occur. Hence the beautiful spiral patterns. It's not that the arms are much more populous than the galactic average, but their demographics are shifted toward recently formed young, blue—and blazing hot—stars. You know, the ones that stand out in visible light photography.
Crash two spiral galaxies together, and it makes things messier. But “crash” is really the wrong word. Merge? Meld? Combine? Galaxies are so much empty space that even the vast nebula clouds are bare flecks of dirt in the galactic landscape. When galaxies collide, it's more like two swarms of bees coming together (and yes, I'm using the same analogy as I did with the larger galaxy clusters, because it's a good analogy). The individual stars won't collide (for the most part; you just know someone's going to be obnoxious, though), but all the gravitational interactions will tear the arms apart, leading to a torn and tattered mix—an irregular galaxy. Over enough time the galaxy heals from its wounds and settles down, but the collision triggered a flash of new star creation, exhausting a galaxy's supply of usable gas. In the long term, the result of a major merger event is a dead (or at the very least, dying) galaxy, full of old red stars: an elliptical.
Your Place in the Universe Page 23