The interstellar medium
The space between stars is not empty. It contains gas and dust that both block starlight and provide the raw material for new stars.
What the interstellar medium is
The interstellar medium (ISM) is the gas and dust between stars, about 99% gas (mostly hydrogen and helium) and 1% dust by mass. It is extremely thin, on average about one atom per cubic centimetre, far emptier than the best laboratory vacuum, but across vast distances it adds up to about 10-15% of the Milky Way's visible mass.
Phases of the gas
Interstellar gas exists in several phases: cold, dense molecular clouds where stars form; cool and warm neutral hydrogen; warm ionised gas around hot stars; and very hot, thin gas heated by supernovae, at millions of kelvin. These phases interact as stars form, shine, and explode.
Emission nebulae
Near hot young O and B stars, ultraviolet light ionises surrounding hydrogen, which glows red as electrons recombine with protons, creating emission nebulae (H II regions) such as the Orion and Lagoon Nebulae. They mark sites of recent star formation.
Dust: reddening and extinction
Interstellar dust grains, tiny particles of carbon and silicates, absorb and scatter starlight. Dust makes distant stars appear dimmer (extinction) and redder, because blue light is scattered more than red, similar to why sunsets are red. Dark nebulae, such as the Horsehead Nebula, are dense dust clouds silhouetted against brighter regions.
Reflection nebulae
When dust clouds lie near stars that are not hot enough to ionise gas, they scatter starlight, appearing as bluish reflection nebulae, blue for the same reason the sky is blue. The wisps around the Pleiades are a famous example.
The 21-cm line
Cold neutral hydrogen emits radio waves at a wavelength of 21 centimetres. This radiation passes through dust, so radio astronomers use it to map hydrogen throughout the Milky Way, revealing the spiral arms and measuring how gas rotates around the galaxy.
Cosmic recycling
The ISM links generations of stars. Dying stars return gas enriched with heavy elements through winds, planetary nebulae, and supernovae. Supernova shock waves can compress nearby clouds, triggering new star formation. Over time, each generation of stars is slightly richer in heavy elements.
Worked example
A star appears much fainter and redder than expected for its spectral type. Astronomers compare its observed colour with the colour its spectrum predicts; the difference reveals how much dust lies along the line of sight. Correcting for this extinction gives a more accurate distance, showing why ignoring dust once led astronomers to misjudge the Milky Way's size.
Key takeaways
The ISM is about 99% gas and 1% dust, extremely thin, yet a significant fraction of the galaxy's visible mass. Gas exists in cold molecular, neutral, ionised, and hot phases. Emission nebulae glow red around hot young stars. Dust causes extinction and reddening and forms dark nebulae; reflection nebulae look blue. The 21-cm radio line maps hydrogen through dust. The ISM recycles material enriched by dying stars into new stars.
Flashcard
Gas and dust between stars
Check your understanding
Why do astronomers use the 21-cm radio line to map the Milky Way's spiral structure?