The space between the stars is not a pure vacuum, as is attested by the gaps in the Milky Way; cool clouds of gas and dust within the Galactic plane block out the light of the more distant stars. Less apparent is the tenuous, warm gas that pervades the Galactic disk. Together these two components of the interstellar medium (ISM in the astronomical jargon) fill about half of the space of the Galactic disk. The remainder is fill with extremely low-density gas, much of which is extremely hot.
Even the most dense regions of the Galaxy are hard vacuums, with the number of molecules reaching only 105 per cubic centimeter. These regions are the cores of molecular clouds, and they are very cold, with a temperature of around 10° Kelvin. The more common cool regions have a density of several 10s of atoms per cubic centimeter and a temperature below 100° K. In the warm-gas regions of the Galaxy, the density is only about 1 atom per cubic centimeter, but the temperature is several thousand degrees K. The region within 50 parsecs of the Sun is an example of the lowest-density regions of the Galaxy; the hot bubble we sit in has a density of only 0.1 atoms per cubic centimeter and a temperature of about 1 million degrees K. Besides these hot low-density regions, there are cool low-density regions. All total, the average density at the Galactic plane is a little over 1 atom per cubic centimeter. But because of the great distances between the stars, the amount of mass in the interstellar medium rivals the amount of mass in stars. Recent research places the mass density at the galactic plane at 0.076 solar masses per cubic parsec, which means the fraction of mass in gas and dust is about 30% of the total mass.
The interstellar medium has two principal constituents: gas and dust. The gas is primarily hydrogen and helium left over from the big bang. This gas pool has been enriched by stellar winds with carbon, nitrogen, and oxygen and by supernova shock waves with elements heavier than iron. The stars that precipitate from the high-density clouds of the interstellar medium reflect this changing composition. The heavier elements are better at trapping radiation, particularly at the stellar photosphere, and the presence of carbon, oxygen, and nitrogen allows a star to convert its hydrogen into helium through the CNO cycle rather than through the PP cycle. Because stars born ten billion years ago contain far fewer “metals” (elements heaver than helium) than stars born today, their structure and evolution differ from that of young stars.
The gas of the interstellar medium appears in many different states. In the coldest, densest clouds, it is composed of molecules of hydrogen, carbon monoxide (CO), ammonia (NH3), and other carbon-based and nitrogen-based molecules. In the cool regions and the warm regions, it is composed of neutral atomic gas (called H I regions). In hot regions, the gas is principally ionized hydrogen (called H II regions).
The dust is made up of carbon, silicon, iron, and nickel. In very cold regions, the dust is coated with ice. The dust is responsible for most of the absorption of light within the galactic plane. The dust is also responsible for an infrared background; the dust reradiates the starlight it absorbs in the infrared band.
Besides gas and dust, the interstellar medium is threaded by magnetic fields. At about 5 micro-Gauss (the Earth's magnetic moment is 1/3 Gauss), these fields are strong enough to provide a pressure comparable to the pressure of the gas. The magnetic fields therefore affect the dynamics of the interstellar medium.
Finally, there are the cosmic rays, which are atomic nuclei and fundamental particles that have been accelerated by supernova shock waves and other mechanisms to nearly the speed of light. A proton moving with half the speed of light spirals around a 5 micro-Gauss magnetic field with a radius of 0.02 AU, while a proton moving with a kinetic energy that is 1 million times its rest mass (only 0.5×10−12 smaller than the speed of light) has a radius of 0.17 parsecs. Because of these small gyroradii, cosmic rays remain trapped within the Galaxy by the magnetic fields. The cosmic rays strike and heat the dust grains; although starlight dominates the heating of dust grains in most regions of the interstellar medium, cosmic rays dominate their heating in the densest regions.
Despite the wide variety of densities and temperatures found in the interstellar medium, the pressure is fairly uniform within the Galactic plane. This uniformity is why the temperatures within the gas are inverse to the densities: in most regions the gas pressure provides at least half of the pressure—the magnetic field provides the remainder of the pressure—and because pressure is proportional to density times temperature, a region of high density has a proportionally lower temperature than a neighboring region of low density. In the rare places where density and temperature are low, pressure is fully supplied by magnetic fields.
One can think of the interstellar medium as an atmosphere with a pressure that counteracts the gravitational force of the Galactic disk. The pressure is highest at the Galactic plane, and it drops as one moves above or below the galactic plane. The scale height of this atmosphere is several-hundred parsecs in the warm regions; the cool, high-dense regions have a smaller scale height of about 100 parsecs. The scale height of the stars in the Galactic disk is of order 100 parsecs, so the cool gas has the same scale height as the disk stars, and the warm gas extends far above and below the disk of stars.
In astronomy, the ionization stage of an atom is noted by a roman numeral after the element's chemical symbol. If an atom is neutral, a roman numeral I is appended, so neutral hydrogen is H I, neutral helium is He I, etc. When an atom is short 1 electron, a roman numeral II is appended, so ionized hydrogen is H II, singly-ionized helium is He II. The Roman numeral increases with each additional loss of an electron.