{ Practical astronomy | Astronomy | The interstellar medium }

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The interstellar medium

We think of the Galaxy or Milky Way primarily as huge system of stars revolving around their centre of gravity. Indeed, 90% of the non-dark matter is in stars, but there are also 10% in gas and dust in the space between the stars, the interstellar medium (ISM). Originally, before the first stars formed, there was only the "interstellar" gas of hydrogen and helium. Dynamically, the disc shape of the Galaxy is due to friction in the gas. If there were merely stars, they could merrily orbit as a spherical or ellipsoidal shape, like a huge globular cluster, or like an elliptical galaxy. The stars are also in the disc, because most of the stars we see today were made from the gas after it settled into the disc.

Observations of the rotation of the Galaxy, and of other galaxies, have shown that the mass of ordinary matter, stars and ISM, cannot account for the gravity that keeps the mass in orbit. Rather, there seems to be another five times as much dark matter, massive particles subject to gravity that are not protons, neutrons, electrons, quarks, etc., and that are not subject to electromagnetic forces.

The study of the ISM took off really in the 1950s after the advent of radio astronomy. The presence of an ISM had been apparent much earlier, in the 18th and 19th century, through the observation of bright nebulae, and to some degree through an absence of stars. Herschel (Herschel 1785a) carried out star counts to measure the extent of the Galaxy. In some directions the column of stars in the count ends because the viewing direction leaves the Milky Way disc upward or downward. In other directions the column of stars ends because interstellar dust obscures the stars beyond. Olbers, inspired by his interest in comets, formulated his famous paradox (Olbers 1823a) to argue that the darkness of the night sky pointed at the presence of an obscuring interstellar medium. The bright nebulae also pointed at an ISM, although a large number of those nebulae are now understood to be external galaxies made up of unresolved stars.

The ISM far away from stars

One might be tempted to label this the quiet ISM, but it is not quiet, not uniform, not without a wealth of physical, and even chemical processes. There is, however, a relative absence of stellar photons, which makes the majority of the interstellar space different from the places nearer to stars. This part of the ISM is indeed quiet in the sense that is is not forming stars and is suffering only moderately under the effects of dying stars.

The figure combines a model of 21-cm radio emission that I coded back in the 1980s with a current artist's impression of the Galaxy in visible light (ESO 2013a). My 21 cm model is too crude to show the spiral structure or the central bar, but it illustrates a few interesting features of the Galaxy. A more detailed model has more recently been published by Kalberla et al. (2007a).

1. The gas disc is larger than the stellar disc (25 kpc diameter).
2. The gas disc is thinner (200 pc), more concentrated than the stellar disc.
3. The gas disc is warped, in the outskirts deviating upward on one side and downward on the opposite side.
4. The gas disc is flaring: In the outskirts the disc is wider and less concentrated to its main plane.
5. The gas disc has a hole at the centre of the Galaxy.
6. The stellar disc has a central bar and two spiral arms.

The overall density of the ISM is about 10⁶ m⁻³. The 21 cm radio emission line is due to atomic hydrogen gas (HI in spectroscopic notation). However, the medium is by no means quiet and not homogeneous. Streaming motions, stellar irradiation and shock waves keep stirring the interstellar gas and dust. We distinguish a number of "phases" of the gas (Wikipedia 2026a):

• 10% to 20% of the volume is filled with (atomic) warm neutral medium at a temperature of 6000–10000 K and up to 0.5 · 10⁶ m⁻³ density.
• Up to 5% of the volume is filled with (atomic) cold neutral medium of 50–100 K. The density is correspondingly higher at up to 50 · 10⁶ m⁻³.
• Up to 50% of the volume is filled with warm ionised medium with similar temperature and density as the warm neutral medium.
• At the opposite, cold and dense end are the molecular clouds that occupy less than 1% of the volume. The temperature here is 10–20 K and the density correspondingly up to 10¹² m⁻³.

Originally, there were no stars and all the mass was in "interstellar" gas, which consisted of only hydrogen and helium (with minute traces of lithium and beryllium). In time, molecular clouds formed stars, stars fused hydrogen and helium into heavier elements, novae, supernovae etc. returned such enriched material to the ISM. Now, 70% of the gas (by mass) is still hydrogen, about 30% helium. There is an overall 1% or 2% content of heavier elements, much of which is not in the gas phase, but in dust grains. Gas and dust are very well mixed in the ISM, the mass ratio being about 100.

The interstellar medium is constantly churned by various events. Strong UV radiation from young stars, stellar winds, expanding shells from novae and supernovae send shock waves through the medium. Some material is then expelled from the galactic disc into the halo above or below and eventually falls back to collide with the gas of the disc, giving rise to more shocks; astronomers speak of a galactic fountain (de Boer and Savage 1982a). All such shocks can sweep clean an area, but at the same time they can also compress the material that they run into.

In denser, colder regions the atomic gas can convert to molecular gas. Along with H₂ molecules CO molecules form. The former have no line emission, but CO is readily detectable as tracer or proxy. Such clouds have an outer layer where incoming UV still keeps CO dissociated, but the H₂ molecules of this layer deplete the UV flux, so that the interior is shielded and CO molecules can form. The image shows a translucent cloud, separating out into colours the atomic hydrogen outwith the cloud (blue), the diffuse molecular hydrogen subject to dissociation by starlight (green), and the dense molecular hydrogen shielded from starlight (red).

The image also shows the filamentary appearance of almost all structure of the cloud. This is a widespread phenomenon and called galactic cirrus. It is a signature of the continuous stirring and churning that can stretch clumps into linear filaments or compress them into thin sheets best seen edge-on.

Dark nebulae

The densest regions of the ISM, dense cores in large molecular clouds, can contract under their own gravity and form new stars. In these cases the gravitational collapse ends only when the centre begins to fuse hydrogen into helium.

One type of interstellar nebula is the dark nebula. Basically, this is a quiet, cold, dense, molecular cloud. Far away from stars this is observable mostly by radio astronomers by the radiation of CO molecules; the H₂ itself is virtually invisible when cold. In visible light a dark nebula can be seen mostly near young stars; they are the still dark and cold remnants of the molecular cloud from which the stars formed. Large dark nebulae can also be seen away from young stars, in silhouette against the stars behind.

The ISM near young stars

HII regions

Young blue stars dominate their surrounding interstellar medium with a strong UV flux capable of dissociating hydrogen molecules and ionising hydrogen atoms. The result is a region of (singly) ionised hydrogen (HII in spectroscopic notation). The HII region is limited by the density of the gas and the flux of UV photons. Some distance from the stars, all UV photons have been used up to keep the intervening region ionised. Beyond this no UV remains and the gas further out remains atomic or molecular.

One type of interstellar nebulae observable in visible light are these HII regions. The light is in the form of emission lines like Hα and Hβ (both from the HII species), but also "forbidden" transitions of doubly ionised oxygen ([OIII]; in spectroscopic notation the roman numeral is the degree of ionisation minus one and square brackets signify forbidden transitions).

The photon flux of a bright star pushes against the surrounding interstellar medium, as does a stellar wind. These are sources of shock waves in the interstellar medium.

Reflection nebulae

A different type of bright interstellar nebula, the reflection nebula, occurs near blue stars. The interstellar dust in front of the star scatters the starlight, giving the dust a blue hue. While the HII region is physically related to the UV-emitting stars, the reflection nebula can originate from dust that is not directly related to the star. Although the nebula is grey or blue, the light is not in emission lines, but a continuum spectrum like that of the star.

Still, both types of bright nebula in our field of view are associated with stellar nurseries, young blue stars, etc. As such open star clusters, HII emission nebulae and reflection nebulae, and even the dark nebulae, often are in close vicinity to each other.

The ISM near old stars

Planetary nebulae and supernova remnants

Middle-aged stars hardly interact with the interstellar medium, except perhaps by a stellar wind. Old stars, running out of fusion fuel, become unstable. Red giants and novae expel shells of material and supernovae explode to expel much of themselves into a supernova remnant. The visible result are planetary nebulae and supernova remnants. Although they also show HII emission, they are rather hotter regions that exhibit also more exotic emission lines from less common elements and higher ionisation levels. The forbidden [OIII] emission lines hint at hot regions of low density. "Forbidden" means that the transition of the ion is rare and that normally a collision with another gas particle will happen sooner than the emission of the photon. When the gas density is low, collisions are rare and emission has a better chance.

The expansion of the planetary nebulae and supernova remnants pushes against the surrounding material. They are sources of shock waves in the interstellar medium.