Stardust and Eternity – 3.4.6

The Interstellar Medium

In our galaxy, the space between stars is not a perfect vacuum, as it was assumed before the 20th century: it is actually filled with a very tenuous gas, the “interstellar medium”. It accounts for about 5% of the total stellar mass of the Milky Way. This matter is mainly found in the spiral arms of the galactic disc, often surrounding young stars. The interstellar medium is extremely dilute, with an average density of about 1 atom per cubic centimetre – much more matter-free than any vacuum artificially created in laboratories on Earth. Since stars form from this same medium, its chemical composition is similar to that of the stars, with the most abundant element being hydrogen (about 70% by mass) and a significant amount of helium (about 28%). The remaining material is composed of heavier elements and very small solid grains, or “dust” (about a fraction of a micron in size). This dust – similar in size to the wavelength of visible light – is very effective in blocking or “switching off” the light we receive from distant stars; thus interstellar clouds often appear as dark spots in the sky, effectively absorbing light from the celestial objects behind them. The Milky Way itself, at visible wavelengths, is mostly hidden by this intervening dust, although it can be seen at higher infrared and radio wavelengths – which are not effectively absorbed by the dust.

The interstellar medium is not uniform, but turbulent and full of structures. Most of the gas and dust composing the interstellar medium is contained in denser cloud-like concentrations – the “interstellar clouds” or “nebulae”. Hydrogen gas in these celestial objects can be mostly neutral or ionized (i.e. stripped of an electron from the atom, leaving a proton). The coldest clouds, composed of neutral and molecular hydrogen, are generally “dark nebulae”. When they become gravitationally unstable and collapse under their own gravity, star formation can occur within them. “Molecular clouds” are relatively denser and very cold – about 10° above absolute zero (−273.15 °C). They typically extend a few hundred light years and have masses of a few hundred thousand solar masses, yet fill only about 0.1% of interstellar space. They are so cold that molecules can easily form and survive. Their most common component is molecular hydrogen (H₂), but traces of other molecules have also been detected – e.g. carbon monoxide (CO) and complex organic compounds, such as methanol and acetone.

The hottest clouds are generally found around recently formed hot massive stars, such as O or B types, emitting large amounts of ultraviolet radiation that ionise the gas contained in the nebula. In this case, when the electrons recombine with protons to reform neutral hydrogen atoms, they emit red light – thus producing the typical red colour of “emission nebulae”. Since ionising stars are short-lived, these emission nebulae – also called H II regions – only last for about tens of millions of years, due to the lack of ionising sources. One of the closest H II regions to the Sun is the Orion Nebula, located 1,340 light years from the Sun in the constellation Orion, where several hundred stars are forming.

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Map of the Interstellar Dust in the Milky Way – *Credit: ESA/Gaia/DPAC/CU6, N. Leclerc, P. Sartoretti and the CU6 team*

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All-sky map in infrared light with constellations and star forming regions