Terms Developed in Context
interstellar medium (ISM)
The gas, dust, magnetic fields, radiation, and energetic particles occupying the space between stars in a galaxy. In this lecture, the term is not an isolated label: This concept is needed to reason about the multiphase interstellar medium: phases and pressure balance. Treating each ISM phase as an ideal gas of particle number density n and temperature T, the thermal pressure is P = n k T (k = Boltzmann’s constant); two phases in contact along a stable boundary must have equal pressure (ignoring, for this first approximation, magnetic and cosmic-ray pressure contributions developed later in Lecture 14), giving the pressure-balance condition n1 T1 = n2 T2.
It is tied to the lecture’s observables or evidence in this way: Optical absorption-line studies of the diffuse ISM (Na I, Ca II) reveal cold, dense clouds (T~80 K, n~30 cm^-3) embedded in a much more tenuous warm intercloud medium (T~8000 K, n~0.3 cm^-3) -- the cold neutral medium (CNM) and warm neutral medium (WNM) respectively. A second connection is important for using the term in physical reasoning: This is not merely a hypothesis: the McKee & Ostriker (1977) three-phase ISM model was constructed specifically to explain why the CNM and WNM, despite differing in density by roughly a factor of 100, are observed to coexist at nearly the same pressure P/k -- evidence that a thermal-equilibrium mechanism (developed in Lecture 02) actively regulates each phase’s temperature at fixed pressure, rather than the phases having arbitrary, unrelated temperatures. The observational interpretation is constrained by Soft X-ray all-sky background surveys (e.g., ROSAT) reveal a pervasive, million-degree, extremely tenuous hot ionized medium (HIM), thought to be maintained by the collective heating of supernova blast waves (McKee & Ostriker 1977).
Boundary and common confusion. Assuming "hot" means "energetically dominant" or "dense." The hot ionized medium (HIM) is the hottest phase by far (T~5x10^5 K) yet has the lowest density of any phase (n~3.5x10^-3 cm^-3) and a very modest thermal pressure; conversely, molecular clouds are the coldest phase yet the highest-pressure and highest-density phase. Temperature, density, and pressure must be tracked as three independent quantities, not inferred from one another by intuition. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
cold/warm neutral medium (CNM/WNM)
Two predominantly atomic-hydrogen phases: a dense, cool CNM and a diffuse, warm WNM that can coexist over a common pressure range. In this lecture, the term is not an isolated label: This concept is needed to reason about the multiphase interstellar medium: phases and pressure balance. This is not merely a hypothesis: the McKee & Ostriker (1977) three-phase ISM model was constructed specifically to explain why the CNM and WNM, despite differing in density by roughly a factor of 100, are observed to coexist at nearly the same pressure P/k -- evidence that a thermal-equilibrium mechanism (developed in Lecture 02) actively regulates each phase’s temperature at fixed pressure, rather than the phases having arbitrary, unrelated temperatures.
It is tied to the lecture’s observables or evidence in this way: Soft X-ray all-sky background surveys (e.g., ROSAT) reveal a pervasive, million-degree, extremely tenuous hot ionized medium (HIM), thought to be maintained by the collective heating of supernova blast waves (McKee & Ostriker 1977). A second connection is important for using the term in physical reasoning: The volume filling factor of each phase (the fraction of the disk’s volume it occupies) is very different from its mass fraction: the HIM fills roughly half the disk’s volume but contains a tiny fraction of its mass, while molecular clouds fill a tiny fraction of the volume but contain a large fraction of the mass available for future star formation -- exactly the volume-vs-mass distinction this course will return to throughout. The observational interpretation is constrained by Radio recombination-line and Halpha surveys reveal a warm ionized medium (WIM) filling a substantial fraction of the disk volume even far from any single ionizing star, alongside dense, compact H II regions immediately surrounding hot O/B stars.
Boundary and common confusion. Assuming "hot" means "energetically dominant" or "dense." The hot ionized medium (HIM) is the hottest phase by far (T~5x10^5 K) yet has the lowest density of any phase (n~3.5x10^-3 cm^-3) and a very modest thermal pressure; conversely, molecular clouds are the coldest phase yet the highest-pressure and highest-density phase. Temperature, density, and pressure must be tracked as three independent quantities, not inferred from one another by intuition. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
warm/hot ionized medium (WIM/HIM)
Low-density plasma phases distinguished mainly by temperature and origin: photoionized gas near 10,000 K and collisionally ionized, shock-heated gas near or above 10^6 K. In this lecture, the term is not an isolated label: This concept is needed to reason about the multiphase interstellar medium: phases and pressure balance. The volume filling factor of each phase (the fraction of the disk’s volume it occupies) is very different from its mass fraction: the HIM fills roughly half the disk’s volume but contains a tiny fraction of its mass, while molecular clouds fill a tiny fraction of the volume but contain a large fraction of the mass available for future star formation -- exactly the volume-vs-mass distinction this course will return to throughout.
It is tied to the lecture’s observables or evidence in this way: Radio recombination-line and Halpha surveys reveal a warm ionized medium (WIM) filling a substantial fraction of the disk volume even far from any single ionizing star, alongside dense, compact H II regions immediately surrounding hot O/B stars. A second connection is important for using the term in physical reasoning: Treating each ISM phase as an ideal gas of particle number density n and temperature T, the thermal pressure is P = n k T (k = Boltzmann’s constant); two phases in contact along a stable boundary must have equal pressure (ignoring, for this first approximation, magnetic and cosmic-ray pressure contributions developed later in Lecture 14), giving the pressure-balance condition n1 T1 = n2 T2. The observational interpretation is constrained by Millimeter-wave molecular-line surveys (CO and its isotopologues) reveal cold (T~10-25 K), dense (n>10^2 cm^-3) molecular clouds -- the densest, coldest phase, and the exclusive sites of star formation (Units 4-5 of this course).
Boundary and common confusion. Assuming "hot" means "energetically dominant" or "dense." The hot ionized medium (HIM) is the hottest phase by far (T~5x10^5 K) yet has the lowest density of any phase (n~3.5x10^-3 cm^-3) and a very modest thermal pressure; conversely, molecular clouds are the coldest phase yet the highest-pressure and highest-density phase. Temperature, density, and pressure must be tracked as three independent quantities, not inferred from one another by intuition. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
H II region
A volume of hydrogen kept ionized by Lyman-continuum photons from hot stars, with its size set by competition between photoionizations and recombinations. In this lecture, the term is not an isolated label: This concept is needed to reason about the multiphase interstellar medium: phases and pressure balance. Treating each ISM phase as an ideal gas of particle number density n and temperature T, the thermal pressure is P = n k T (k = Boltzmann’s constant); two phases in contact along a stable boundary must have equal pressure (ignoring, for this first approximation, magnetic and cosmic-ray pressure contributions developed later in Lecture 14), giving the pressure-balance condition n1 T1 = n2 T2.
It is tied to the lecture’s observables or evidence in this way: Millimeter-wave molecular-line surveys (CO and its isotopologues) reveal cold (T~10-25 K), dense (n>10^2 cm^-3) molecular clouds -- the densest, coldest phase, and the exclusive sites of star formation (Units 4-5 of this course). A second connection is important for using the term in physical reasoning: This is not merely a hypothesis: the McKee & Ostriker (1977) three-phase ISM model was constructed specifically to explain why the CNM and WNM, despite differing in density by roughly a factor of 100, are observed to coexist at nearly the same pressure P/k -- evidence that a thermal-equilibrium mechanism (developed in Lecture 02) actively regulates each phase’s temperature at fixed pressure, rather than the phases having arbitrary, unrelated temperatures. The observational interpretation is constrained by Optical absorption-line studies of the diffuse ISM (Na I, Ca II) reveal cold, dense clouds (T~80 K, n~30 cm^-3) embedded in a much more tenuous warm intercloud medium (T~8000 K, n~0.3 cm^-3) -- the cold neutral medium (CNM) and warm neutral medium (WNM) respectively.
Boundary and common confusion. Assuming "hot" means "energetically dominant" or "dense." The hot ionized medium (HIM) is the hottest phase by far (T~5x10^5 K) yet has the lowest density of any phase (n~3.5x10^-3 cm^-3) and a very modest thermal pressure; conversely, molecular clouds are the coldest phase yet the highest-pressure and highest-density phase. Temperature, density, and pressure must be tracked as three independent quantities, not inferred from one another by intuition. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
molecular cloud
A cold, shielded, comparatively dense region in which hydrogen is mainly H2 and where self-gravity can organize gas into star-forming structures. In this lecture, the term is not an isolated label: This concept is needed to reason about the multiphase interstellar medium: phases and pressure balance. This is not merely a hypothesis: the McKee & Ostriker (1977) three-phase ISM model was constructed specifically to explain why the CNM and WNM, despite differing in density by roughly a factor of 100, are observed to coexist at nearly the same pressure P/k -- evidence that a thermal-equilibrium mechanism (developed in Lecture 02) actively regulates each phase’s temperature at fixed pressure, rather than the phases having arbitrary, unrelated temperatures.
It is tied to the lecture’s observables or evidence in this way: Optical absorption-line studies of the diffuse ISM (Na I, Ca II) reveal cold, dense clouds (T~80 K, n~30 cm^-3) embedded in a much more tenuous warm intercloud medium (T~8000 K, n~0.3 cm^-3) -- the cold neutral medium (CNM) and warm neutral medium (WNM) respectively. A second connection is important for using the term in physical reasoning: The volume filling factor of each phase (the fraction of the disk’s volume it occupies) is very different from its mass fraction: the HIM fills roughly half the disk’s volume but contains a tiny fraction of its mass, while molecular clouds fill a tiny fraction of the volume but contain a large fraction of the mass available for future star formation -- exactly the volume-vs-mass distinction this course will return to throughout. The observational interpretation is constrained by Soft X-ray all-sky background surveys (e.g., ROSAT) reveal a pervasive, million-degree, extremely tenuous hot ionized medium (HIM), thought to be maintained by the collective heating of supernova blast waves (McKee & Ostriker 1977).
Boundary and common confusion. Assuming "hot" means "energetically dominant" or "dense." The hot ionized medium (HIM) is the hottest phase by far (T~5x10^5 K) yet has the lowest density of any phase (n~3.5x10^-3 cm^-3) and a very modest thermal pressure; conversely, molecular clouds are the coldest phase yet the highest-pressure and highest-density phase. Temperature, density, and pressure must be tracked as three independent quantities, not inferred from one another by intuition. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
pressure equilibrium
A state in which adjacent phases have comparable total pressure, so no persistent net force drives their interface rapidly in one direction. In this lecture, the term is not an isolated label: This concept is needed to reason about the multiphase interstellar medium: phases and pressure balance. The volume filling factor of each phase (the fraction of the disk’s volume it occupies) is very different from its mass fraction: the HIM fills roughly half the disk’s volume but contains a tiny fraction of its mass, while molecular clouds fill a tiny fraction of the volume but contain a large fraction of the mass available for future star formation -- exactly the volume-vs-mass distinction this course will return to throughout.
It is tied to the lecture’s observables or evidence in this way: Soft X-ray all-sky background surveys (e.g., ROSAT) reveal a pervasive, million-degree, extremely tenuous hot ionized medium (HIM), thought to be maintained by the collective heating of supernova blast waves (McKee & Ostriker 1977). A second connection is important for using the term in physical reasoning: Treating each ISM phase as an ideal gas of particle number density n and temperature T, the thermal pressure is P = n k T (k = Boltzmann’s constant); two phases in contact along a stable boundary must have equal pressure (ignoring, for this first approximation, magnetic and cosmic-ray pressure contributions developed later in Lecture 14), giving the pressure-balance condition n1 T1 = n2 T2. The observational interpretation is constrained by Radio recombination-line and Halpha surveys reveal a warm ionized medium (WIM) filling a substantial fraction of the disk volume even far from any single ionizing star, alongside dense, compact H II regions immediately surrounding hot O/B stars.
Boundary and common confusion. Assuming "hot" means "energetically dominant" or "dense." The hot ionized medium (HIM) is the hottest phase by far (T~5x10^5 K) yet has the lowest density of any phase (n~3.5x10^-3 cm^-3) and a very modest thermal pressure; conversely, molecular clouds are the coldest phase yet the highest-pressure and highest-density phase. Temperature, density, and pressure must be tracked as three independent quantities, not inferred from one another by intuition. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
filling factor
The fraction of a specified volume occupied by a phase; it is a geometric quantity and need not track that phase’s mass fraction. In this lecture, the term is not an isolated label: This concept is needed to reason about the multiphase interstellar medium: phases and pressure balance. Treating each ISM phase as an ideal gas of particle number density n and temperature T, the thermal pressure is P = n k T (k = Boltzmann’s constant); two phases in contact along a stable boundary must have equal pressure (ignoring, for this first approximation, magnetic and cosmic-ray pressure contributions developed later in Lecture 14), giving the pressure-balance condition n1 T1 = n2 T2.
It is tied to the lecture’s observables or evidence in this way: Radio recombination-line and Halpha surveys reveal a warm ionized medium (WIM) filling a substantial fraction of the disk volume even far from any single ionizing star, alongside dense, compact H II regions immediately surrounding hot O/B stars. A second connection is important for using the term in physical reasoning: This is not merely a hypothesis: the McKee & Ostriker (1977) three-phase ISM model was constructed specifically to explain why the CNM and WNM, despite differing in density by roughly a factor of 100, are observed to coexist at nearly the same pressure P/k -- evidence that a thermal-equilibrium mechanism (developed in Lecture 02) actively regulates each phase’s temperature at fixed pressure, rather than the phases having arbitrary, unrelated temperatures. The observational interpretation is constrained by Millimeter-wave molecular-line surveys (CO and its isotopologues) reveal cold (T~10-25 K), dense (n>10^2 cm^-3) molecular clouds -- the densest, coldest phase, and the exclusive sites of star formation (Units 4-5 of this course).
Boundary and common confusion. Assuming "hot" means "energetically dominant" or "dense." The hot ionized medium (HIM) is the hottest phase by far (T~5x10^5 K) yet has the lowest density of any phase (n~3.5x10^-3 cm^-3) and a very modest thermal pressure; conversely, molecular clouds are the coldest phase yet the highest-pressure and highest-density phase. Temperature, density, and pressure must be tracked as three independent quantities, not inferred from one another by intuition. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.