The turning of the seasons usually brings a few surprises. The weather out here in California has been unsettled this winter, and uncharacteristically dry. The usual moist air from the Pacific has been held off by a persistent high pressure zone hanging over the west coast. Only last week was that high finally pushed back by a cold front that swept through bearing much-needed clouds and rain. We earthlings are so water-dependent, with our ski vacations and citrus crops alike contingent on the fickle meteorological vagaries of turbulent hydrodynamics. Terrestrial weather is a deeply complex subject, which despite countless hours of study and the application of truly humongous quantities of computer time still defies us to do better than "Thursday: 50% chance of rain."
But weather is not just a terrestrial phenomenon. Far grander pressure fronts swirl across the galaxy, and their crashing clouds light up the sky. No, I'm not talking about the hundred-kilometer lightning bolts that rip through the Jovian atmosphere, nor those which carve spokes into Saturn's rings. Those are big storms, but not the biggest. Vastly larger, but far more rarified, weather patterns move through interstellar space itself.
Space is empty, right? Well, not quite. In a typical part of the galaxy, the average density of gas in interstellar space is about one atom per cubic centimeter. Mostly those atoms are hydrogen, but about ten percent are helium, with a sprinkling of all the remaining elements, here and there bound into isolated molecules or the occasional rare dust grain. All this scattered material makes up what we call the interstellar medium, which astronomers have learned to probe and measure with ever-increasing precision. The densest parts of the interstellar medium remain far emptier than the best vacuums yet created in Earthly laboratories, and the gigantic scales over which the interstellar medium extends boggle the mind.
But despite this vast different in size and density, the same physical laws govern the interstellar medium as rule the atmosphere here on Earth, with surprisingly similar results. Terrestrial weather is an interplay between different air masses which can vary in their properties: hot or cold, high or low pressure, dry or wet. Sunlight heats the air, driving large-scale motions which give way to complex turbulence. Water vapor is present throughout the atmosphere, but only in some places are the conditions right for it to condense into clouds. The interstellar medium likewise varies in its properties from place to place, with local temperatures ranging from a frigid few dozen degrees above absolute zero to a toasty ten million kelvin (K), with densities ranging from a few atoms per cubic meter to tens of thousands. Starlight again plays the role of heat source, aided by explosive supernovae and powerful stellar jets. And only where conditions are right can atoms bond together into molecules, forming vast molecular clouds.
Meet the ISM
The interstellar medium (frequently called the ISM) pervades the entire galaxy, almost by definition. Add up all those scattered atoms, and it makes up perhaps 5 to 10 percent of all the matter in the galaxy (all the normal, baryonic matter, that is—which itself is only a small fraction of the unseen dark matter). The interstellar medium steals the show in so many spectacular images of the night sky: All those gorgeous nebulae? Part of the ISM. The famous Hubble "Pillars of Creation" photo? More ISM. You can even see the ISM with your naked eye, in the form of the dark dust lanes that block portions of the Milky Way.
The ISM isn't just a fixed, passive background to the galaxy's stars, either, but instead an active participant in their ongoing "life cycle," always changing and on the move. No matter how tenuous, all that interstellar gas and dust still feels the tug of gravity—both its own self-gravity, and the summed attraction of the rest of the galaxy. Left to its own devices, gravity would collapse all that interstellar material down into a thin, flat sheet in the galaxy's midplane, yet instead we see puffy dark clouds and expansive nebulae spread across the sky. The solution to this puzzle is that gravity is countered by pressure. Interstellar gas is so tenuous that any individual atom may go weeks or months before colliding with a neighboring atom, but these collisions are still enough to create sufficient outward gas pressure to balance gravity, just as air pressure on Earth supports the atmosphere against the downward pull of the planet. (Additional support against gravity is provided by magnetic fields and dynamic motions; the relative importance of these various pressure sources is a subject of current debate.)
This analogy only goes so far, however: the local conditions which give rise to pressure in the ISM vary far more wildly than do atmospheric conditions on Earth. Astronomers have roughly classified the variety of conditions found in the interstellar medium into a few main phases. The Cold Neutral Medium has densities of around 10 atoms per cubic centimeter and temperatures of 50-100 K, the Warm Medium (which comes in both Neutral and Ionized forms) is about ten times less dense and has temperatures of several thousand K, and the Hot Ionized Medium is a blistering million K hot, but is the most tenuous phase at no more than 0.001-0.01 atoms per cubic centimeter. It turns out that these three phases are fairly distinct, with little or no gas found at intermediate temperatures or densities, for somewhat complicated reasons having to do with the balance between stellar heating and the gas's own ability to cool. The surprising thing, given this huge diversity of properties, is that the total gas pressure (roughly, the product of temperature and density) has approximately the same value everywhere, in all three phases. This means that all three can coexist in the galaxy, without any one expanding outwards and pushing the other gas aside by virtue of greater pressure.
Our galaxy is like a tremendous fractal sponge, with the various types of interstellar medium haphazardly arrayed in large-scale structures: walls of cool gas between bubbles of warm, layers of hot gas wrapped around cooler filaments, with subsheets folding back upon themselves and striated layers wrapping together like a ten-thousand-light-year-wide marbled cake. These structures are all in motion, though over galactic timescales that make glaciers look positively speedy. Driven by slight pressure differences and other energy inputs (more on those later), different portions of gas and dust occasionally come together in collisions that slowly play out over millions of years. Space isn't empty; it's a lively and sometimes violent place. Nor is there truly no sound in space: the ISM's crashing waves boom out across the galaxy in vast whispered thunder, trillions of times too faint for the human ear to hear and taking millennia to cross each light year.
The Birth of Stars
If these motions compress interstellar gas to a high enough density, a remarkable transformation can occur. Denser gas clouds can better block out sources of heat, such as starlight and cosmic rays. Without this heating, the central regions of the clouds can then cool by emitting radiation of their own, primarily due to faint atomic transitions of carbon and oxygen. If the gas cools past a certain point, atoms will begin to bond into molecules. The more complicated electron configurations of molecules offer many more possibilities for emitting radiation than solo atoms do, and so as more molecules form, the gas will cool faster and faster, in turn forming more and more molecules. Pairs of hydrogen atoms bond into molecular H2, while carbon and oxygen join to form first carbon monoxide, then complicated organic hydrocarbons, even alcohols and amino acids. Over a hundred kinds of molecules have been detected in deep space, revealed by their characteristic radio frequencies. This runaway process can convert vast amounts of interstellar atomic gas into giant, cold "molecular clouds," which can contain tens of thousands or millions of times the sun's mass packed into a volume a few hundred light years across. Just as water vapor condenses into clouds only when air falls below the dew point, only where conditions are right can interstellar matter coalesce into molecular clouds.
The most famous example of such a giant molecular cloud is the Great Nebula in Orion, the second "star" in the sword hanging from his belt. Take a look at this nebula through a telescope, or even a small set of binoculars, and you will see glowing gas and darker lanes of dust. But you will also see hundreds of young stars scattered about the nebula, for it is in such places that stars are born. Recall that it was the thermal pressure of the interstellar gas which supported it against gravity. As the gas cools, and as its atoms join together into molecules (thereby greatly reducing the effective number of particles present), the pressure in the cloud will drop precipitously. Here and there, in particularly cool and dense clumps, it may fall below the level needed to support the cloud against its own gravity. When that happens, the 'molecular clump' (yes, that really is an actual scientific term!) will collapse inwards in the blink of an eye, over a mere few thousand years or so. Falling inwards, the densities and pressures will rise tremendously. As atoms and molecules slam together, shock waves heat the gas to thousands, then millions of degrees. Eventually, conditions become so violent that two protons collide with such force that they merge together into one nucleus. Fusion begins. A star is born.
Feedback and Self-Regulation (or, how clouds blow themselves away)
If this process has been going on for billions of years, why then is there any remaining interstellar material at all? Why hasn't it all formed into stars long ago? It turns out that star formation is a remarkably inefficient process. In any given molecular cloud, only a percent or two of the gas and dust will ever form into stars before the cloud is destroyed by its own success. One of the enduring mysteries of astronomy is explaining why stars form with certain masses. The sun is a medium-large star, but many more small stars are seen than large stars. Wherever stars form, this pattern remains the same. Most of the stars that arise are small, but here and there, giants are born: stars of ten, twenty or even more solar masses. These stars burn ferociously, tens of thousands of times brighter than the sun, and inject tremendous amounts of energy into their surrounding molecular clouds. This powerful radiation vaporizes dust grains, destroys molecules, rips atoms apart into ions, and heats the gas to thousands of degrees. The pressure rises, and the once-placid molecular cloud is forced apart by powerful winds. To add insult to injury, massive stars burn their nuclear fuel so fast they only last a few million years before dying as supernovae. For a moment, an exploding star will outshine the summed total light of all hundred billion other stars in the galaxy. The poor molecular cloud hasn't got a chance. Barely a few million years after it formed, the cloud has been blown to bits, most of its cold molecular material heated back up to thousands or millions of degrees long before it had a chance to form into stars.
But like a phoenix, this particular death carries the seeds of a new beginning. As the blast wave propagates outward, it jostles surrounding portions of the ISM, providing new energy keeping the gas moving and setting the stage for further collisions, for future molecular clouds and more star birth. Supernovae and their cousins, the less-violent deaths of smaller stars, are also responsible for spreading out into the galaxy more of the heavy elements beyond helium, formed by nuclear burning deep in stellar cores. These elements enrich the interstellar medium, giving subsequent generations of stars more of the rocky and icy building blocks necessary for planetary systems. Over the last few years, observations have clearly proven that stars which formed from gas with more heavy elements have a much higher probability of having planets.
So next time you're walking down the street and you see clouds massing on the horizon, take a moment to think about the weather on the other side of the sky. The space between the stars is neither empty nor silent, and as a result the galaxy appears to be becoming a more interesting place all the time.