Space matter overview

 There are stars and planets and black holes and dark matter and comets and asteroids floating around in space.  There is also gas and dust, as individual particles and organized in clouds.  Tell me about these.

Absolutely. The interesting thing is that space is not actually empty. It is extraordinarily empty compared with anything we experience on Earth, but between the stars there is a thin, complicated mixture of matter—and much of the visible universe is built from it.

A useful way to think about the contents of the universe is to start with the ordinary matter and then work outward to the stranger stuff.
Gas: the invisible majority of ordinary matter

Most of the ordinary matter between stars is gas, overwhelmingly hydrogen and helium. In the space between stars, the density can be astonishingly low—sometimes only a few atoms per cubic centimeter, and in particularly empty regions far less. That's an almost perfect vacuum by terrestrial standards.

But "gas" in astronomy doesn't necessarily mean something like the air in a room. At those densities, atoms can travel enormous distances without colliding. The particles are often electrically charged, making the gas a plasma, particularly around hot stars and in energetic regions.

There are several broad environments:

The interstellar medium is the gas and dust occupying the space between stars within a galaxy. It isn't uniform. There are hot bubbles, warm diffuse gas, cold clouds, and everything in between.

Some of the gas becomes sufficiently cold and dense that gravity begins to gather it together. These are molecular clouds, where hydrogen atoms combine into H₂ molecules. They can contain thousands or millions of solar masses of material. Within their densest portions, stars are born.

So there is a rather beautiful cycle:

gas → clouds → stars → stellar evolution → gas returned to space → new stars.

The atoms in your body have therefore been through this sort of recycling. Carbon, oxygen, nitrogen, calcium, iron and so forth were manufactured in previous generations of stars and scattered into space.
Dust: tiny solid particles drifting between the stars

Astronomical dust sounds rather mundane, but it is fascinating stuff. These aren't clouds of household-sized particles; they're microscopic grains, typically containing silicates, carbon compounds, ices, and other materials.

Some are roughly comparable to smoke particles, some considerably smaller.

Dust has an enormous effect despite representing only a small fraction of the ordinary matter in interstellar space. It absorbs and scatters visible light, which is why distant stars can appear dimmer and redder than they really are.

You can see the effect dramatically in photographs of the Milky Way: those black, apparently empty lanes aren't necessarily empty at all. They are often dark clouds of dust blocking the light behind them.
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Dust also provides surfaces on which molecules can form. In the coldest clouds, dust grains acquire coatings of ice. Complex chemistry can take place there, producing molecules that eventually become incorporated into new stars and planets.

And there's an important connection to planets: the solid material from which rocky planets are made is essentially the descendants of this cosmic dust.
Clouds: enormous, but extraordinarily thin

Astronomical clouds can be almost comically large. A molecular cloud can extend for dozens or hundreds of light-years.

Yet its material is so thin that if you somehow stood inside one, you wouldn't see anything resembling a terrestrial cloud. There would be no fog bank and probably nothing visible at all. You would simply be surrounded by an extremely thin collection of molecules and dust.

The spectacular clouds seen in astronomical photographs are usually visible because they're being illuminated by nearby stars, emitting their own light, or glowing at particular wavelengths.

There are several types worth knowing:

Emission nebulae glow because energetic radiation from nearby stars ionizes their gas.

Reflection nebulae shine by scattering starlight.

Dark nebulae contain enough dust to block light from objects behind them.

And planetary nebulae, despite the misleading name, have nothing to do with planets. They're shells of gas expelled by aging stars like the Sun.
Comets and asteroids: leftover building material

Move from interstellar space to individual planetary systems and you encounter asteroids and comets.

Asteroids are mostly rocky or metallic bodies. Many are leftovers from the formation of planets—material that never got incorporated into one.

Comets are different in their traditional composition: lots of frozen volatile material mixed with dust and rock. When one approaches the Sun, its ices begin turning directly into gas. The escaping material produces the spectacular coma and tail.

The tail isn't actually streaming behind the comet like smoke from a moving automobile. Solar radiation and the solar wind push the material away from the Sun, so the tail generally points more or less away from the Sun, even when the comet is moving in another direction.

And comets are another wonderful example of astronomical recycling. Some of their water and other volatile compounds may preserve material that dates back to the formation of the Solar System—or even earlier.
Stars: where the recycling gets dramatic

Stars are enormous balls of plasma held together by gravity. They are born when portions of molecular clouds collapse.

At their cores, pressure and temperature become sufficient for nuclear fusion. A Sun-like star spends most of its life converting hydrogen into helium.

But stars are also cosmic chemical factories.

Massive stars can manufacture progressively heavier elements, and when they die spectacularly as supernovae, they eject much of that material into space. Other elements are produced in neutron-star mergers and other extreme events.

So the periodic table isn't simply something that existed at the beginning of the universe. Much of it was built later inside stars and violent cosmic events.
Black holes: matter that has fallen too far

A black hole isn't really a "thing floating around" in quite the same sense as an asteroid. It's a region of spacetime where gravity has become so strong that beyond the event horizon, nothing can escape—not even light.

Black holes can nevertheless have enormous amounts of ordinary matter around them. Gas falling toward a black hole can form a rapidly rotating accretion disk, becoming extremely hot and luminous before it disappears across the horizon.

Interestingly, the black hole itself doesn't necessarily suck up everything nearby. If the Sun were magically replaced by a black hole of exactly the Sun's mass, the Earth would continue orbiting almost exactly as it does now. We would have other problems—rather severe ones—but we wouldn't suddenly spiral inward.
And then there's dark matter

This is where things get genuinely weird.

Dark matter doesn't appear to interact with light in the ordinary way. We don't see it, photograph it, or detect it directly in the way we detect stars and gas.

We infer that it exists because of gravity.

Galaxies rotate as though they contain substantially more mass than we can see. Clusters of galaxies have the same problem. And gravitational lensing—the bending of light by gravity—reveals mass that isn't accounted for by visible matter.

The simplest explanation is that galaxies are embedded in enormous halos of dark matter.

And there's an important distinction: dark matter isn't simply ordinary dust that happens to be dark. Dust blocks light; dark matter appears to be fundamentally different from ordinary atoms.

We still don't know what the stuff actually is.

That's one of the great unsolved questions in physics.
There is also something even stranger: dark energy

If we're making the inventory, we can't leave this one out.

The universe's expansion is accelerating. The simplest explanation is something called dark energy, which seems to be associated with space itself.

Unlike dark matter, which attracts gravitationally, dark energy has the apparent effect of driving accelerated cosmic expansion.

But "dark energy" is partly a name for our ignorance: we know quite a lot about what effect it has, but considerably less about what it actually is.
So what is floating around?

If you could somehow turn off the light and look through the universe, you would find a remarkably varied place:

Stars — enormous fusion reactors.

Planets — rocky, icy and gaseous worlds orbiting stars.

Moons — satellites of planets and other bodies.

Asteroids — rocky/metallic leftovers.

Comets — icy, dusty leftovers.

Gas — mostly hydrogen and helium, ranging from nearly empty space to dense star-forming clouds.

Dust — microscopic solid grains that obscure, cool and chemically enrich interstellar clouds.

Black holes — extraordinarily compact concentrations of gravity.

Dark matter — apparently invisible mass shaping galaxies through gravity.

Dark energy — whatever is causing the expansion of the universe to accelerate.

And there's one final thing I rather like about this picture: the boundaries between these categories aren't as clean as they sound. A star is born from a cloud of gas and dust; it returns material to space; that material becomes part of another cloud; eventually some of it becomes a new star and planets. Asteroids and comets are remnants of that planetary formation. And the heavy elements in those rocks—and in us—were manufactured by earlier stars.

In other words, the universe isn't a collection of isolated objects sitting in an otherwise empty box. It's a gigantic recycling system, with gravity doing much of the organizing.

We say "floating" but that is not quite right; they are in orbit.

Exactly. "Floating" is a useful visual metaphor, but physically it is misleading. Almost everything we casually imagine as "floating around in space" is actually participating in gravitational motion.

The interesting part is that "orbit" has a much broader meaning than just "a planet going around a star."

The Earth orbits the Sun. The Moon orbits Earth. The Sun orbits the center of the Milky Way. The Milky Way itself is moving within the Local Group, which is gravitationally bound to some extent. And the Local Group is moving relative to the larger-scale structure of the universe.

Even an asteroid that appears simply to be traveling through empty space is generally following a curved trajectory determined by gravity. It might be orbiting the Sun on a very elongated orbit, for example.

Gas and dust are a little more complicated because individual particles can be moving in many different directions. In a molecular cloud, they aren't necessarily neatly orbiting some central object. They're jostling and drifting under the combined influence of gravity, pressure, magnetic fields, radiation, and the motions of the surrounding material. But when gravity dominates on a large enough scale, the cloud itself can collapse and develop organized rotation and orbital motion.

And there is a delightful hierarchy to it:

Moon → planet → star → galaxy → galaxy group → larger cosmic structure.

Each is moving within a gravitational environment while simultaneously participating in larger-scale motion.

There's also a subtle point: nothing is really "at rest" in the universe. There isn't a universal cosmic coordinate system in which we can say, "Ah, that object is stationary and everything else is moving around it." Motion is always relative to something else.

So when we say a star is "floating through space," what we really mean is something closer to:

    It is moving through spacetime along a trajectory determined by gravity and its initial motion.

Which is considerably less poetic, but much more accurate.