Astronomy 2e · Between the Stars: Gas and Dust in Space
Interstellar Gas
On this page 9 sections
In 30 seconds
Gas is the heavyweight of the interstellar medium, making up roughly 99% of the ISM's mass — but it is not one substance in one state. It comes in three forms: cold neutral atomic hydrogen (H I), hot ionized hydrogen (H II), and cold molecular clouds (H₂), each with its own temperature, density, and — crucially — its own way of announcing its presence.
The gas is overwhelmingly hydrogen and helium, yet even a trace of heavier atoms matters enormously: molecules like carbon monoxide (CO) are the main tool for finding the clouds where stars are born. The three forms also map onto the life cycle of matter — atomic gas collects into clouds, clouds collapse into stars, and hot young stars ionize the gas around them.
Why this matters
Interstellar gas is the fuel supply of the galaxy. Every star that has ever formed condensed out of interstellar gas, and how fast a galaxy forms stars depends on how much gas it holds. Mapping the 21-cm emission of neutral hydrogen has revealed the spiral structure of our own Milky Way — structure hard to see in visible light because of dust. Molecular clouds, detected through CO, are where new stars and planets will be born.
The gas is also a diagnostic tool: emission lines reveal the temperatures, densities, and compositions of nebulae; absorption lines reveal what lies along a line of sight.
The college version
Core Concepts
Composition of the interstellar gas
By mass, interstellar gas is about 70–75% hydrogen, 25–28% helium, and ~2% heavier elements (commonly taught reference values). Hydrogen dominates for a deep reason: the universe began almost entirely as hydrogen and helium, and stars have only slowly built up heavier elements since.
Neutral hydrogen and the 21-cm line
Cold atomic hydrogen — designated H I ("H one") — is the most abundant form of interstellar matter. It is hard to see in visible light, but it emits the 21-cm line Radio emission at ~21 cm wavelength (1420 MHz) from hydrogen's spin-flip transition Full entry → at about 1420 MHz, produced by the Spin-flip transition A change in the relative orientation of an atom's electron and proton spins Full entry →: the proton and electron in a hydrogen atom behave like tiny magnets, and when the electron's spin flips relative to the proton's, the atom emits a photon of exactly this wavelength.
Any single atom waits, on average, millions of years to make this transition, but there are so many hydrogen atoms along a line of sight that the 21-cm line is easily detected. Because radio waves pass through dust unimpeded, 21-cm observations produced the first complete maps of the Milky Way's spiral arms.
Ionized gas and H II regions
Where very hot stars (spectral types O and B) exist, their intense ultraviolet light can strip electrons from hydrogen atoms, ionizing the gas into an H II region A region of hydrogen ionized by hot young stars Full entry → ("H two"): a cloud of ionized hydrogen and free electrons glowing at ~10,000 K. When electrons recombine with protons, the atoms emit characteristic lines — most famously red H-alpha — while ions like doubly ionized oxygen emit green forbidden lines (e.g., [O III]) that color many nebulae.
Because only very hot stars can create large H II regions, these glowing clouds are signposts of recent star formation. The size of the ionized zone depends on the star's ultraviolet output and the density of the surrounding gas.
Molecular clouds: the cold, dark nurseries
The coldest and densest interstellar gas exists as molecular clouds, where hydrogen has combined into molecules (H₂). Temperatures are a frigid 10–30 K and densities range from a few hundred to millions of particles per cubic centimeter. These are the only places where gravity can overcome gas pressure and collapse: star formation happens here.
Molecular hydrogen itself is nearly impossible to observe directly because it emits almost no radiation at these temperatures. Astronomers instead use carbon monoxide (CO), which is rarer but emits strong millimeter-wavelength radiation; since CO and H₂ are mixed in roughly constant proportion, measuring CO gives a reliable estimate of the total molecular gas.
Absorption lines from interstellar gas
Interstellar gas also reveals itself in absorption: as starlight passes through cold foreground gas, atoms such as calcium and sodium absorb specific wavelengths, imprinting narrow dark lines on the star's spectrum. These lines differ from the star's own because they are extremely narrow (the cold gas's atoms move slowly) and sit at the interstellar radial velocity — a line of sight may cross several clouds at different speeds, giving several distinct sets of lines.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| H I region | H II region | H I = neutral hydrogen; H II = ionized hydrogen. H I is cold and mapped by 21-cm radio; H II is hot (~10,000 K) and glows in visible lines |
| Molecular hydrogen (H₂) | Atomic hydrogen (H) | H₂ is a bonded pair of atoms in cold dense clouds; H is a single free atom in warmer, thinner gas |
| Forbidden lines | "Impossible" lines | Forbidden lines are just unlikely at Earth-like densities; in space they are often the brightest lines (e.g., [O III]) |
| The 21-cm line | Any hydrogen line | The 21-cm line comes only from neutral hydrogen's spin flip; ionized gas does not emit it |
| CO as a "tracer" | CO being the main gas | CO is rare; it is used to measure H₂, which is abundant but nearly invisible |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Interstellar gas comes in three "flavors": plain cold hydrogen atoms, hot hydrogen that has lost its electrons, and super-cold hydrogen molecules. Each flavor announces itself differently — cold atoms hum on the radio, hot gas glows red like a neon sign, and cold molecular gas is found using carbon monoxide as a "tracer."
Worked example
Imagine a cold molecular cloud drifting near a region where massive stars have just formed. A millimeter telescope detects carbon monoxide: the cloud is molecular, full of H₂, and destined to collapse into new stars — the cloud as fuel. When a hot young star forms inside it, its ultraviolet light carves out an H II region: an optical telescope shows red H-alpha glow — the cloud as signpost of star birth. Finally, a radio telescope tuned to 21 cm catches the neutral hydrogen the star has not yet ionized. One cloud, three states of matter, three wavelengths, three different stories — this is how astronomers build a complete picture of interstellar gas.
Key takeaways
- Interstellar gas is ~99% of the ISM's mass: ~70–75% hydrogen, ~25–28% helium, ~2% heavier elements (reference values).
- H I (neutral hydrogen) is traced by the 21-cm spin-flip radio line (1420 MHz) — the key tool for mapping the Milky Way's spiral structure.
- H II regions are produced by ultraviolet light from hot O and B stars; they glow in H-alpha and forbidden lines like [O III] and mark recent star formation.
- Molecular clouds (H₂) are the coldest (~10–30 K), densest gas and the only sites of star formation; they are traced with CO.
- Interstellar gas also shows up as narrow absorption lines (calcium, sodium) at the gas's own velocity.
- H I = neutral hydrogen; H II = ionized hydrogen — a classic exam trap.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What are the three main forms of interstellar gas, and what are their approximate temperatures?
Show answer
Cold neutral atomic hydrogen (H I, ~100 K), ionized hydrogen (H II, ~10,000 K), and molecular clouds (H₂, ~10–30 K).
Why is the 21-cm line such a powerful tool for mapping the Milky Way?
Show answer
It comes from neutral hydrogen, the most abundant interstellar gas, and radio waves pass through the dust that blocks visible light — so it maps hydrogen across the whole galaxy.
Why do H II regions mark places where stars have recently formed?
Show answer
Only very hot, massive (O and B) stars produce enough ultraviolet light to ionize large volumes of hydrogen, and such stars are young — so H II regions pinpoint recent star birth.
Why do astronomers use carbon monoxide to study molecular clouds instead of observing H₂ directly?
Show answer
H₂ has no strong emission at cold-cloud temperatures, so it is nearly invisible; CO emits strong millimeter radiation and is mixed with H₂ in a known ratio, making it a reliable tracer.
How can you tell an interstellar absorption line from a line produced in the star's own atmosphere?
Show answer
Interstellar lines are extremely narrow (cold gas), sit at the interstellar cloud's velocity rather than the star's, and often appear as multiple components from several clouds along the line of sight.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- H I region
- A cloud of cold, neutral (unionized) atomic hydrogen
- 21-cm line
- Radio emission at ~21 cm wavelength (1420 MHz) from hydrogen's spin-flip transition
- H II region
- A region of hydrogen ionized by hot young stars
- Forbidden line
- An emission line from a transition that is extremely unlikely at normal densities (e.g., [O III])
- Molecular cloud
- A cold, dense cloud where hydrogen exists as H₂ molecules
- Giant molecular cloud (GMC)
- A very large molecular cloud, up to ~10⁶ solar masses
- Spin-flip transition
- A change in the relative orientation of an atom's electron and proton spins
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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