Everything about Interstellar Medium totally explained
In
astronomy, the
interstellar medium (or
ISM) is the
gas and
dust that pervade interstellar space: the
matter that exists between the
stars within a
galaxy. It fills interstellar space and blends smoothly into the surrounding
intergalactic medium. The
energy, in the form of
electromagnetic radiation, that occupies the same volume is the
interstellar radiation field.
The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of
ions,
atoms,
molecules, larger
dust grains,
cosmic rays, and (galactic) magnetic fields (Spitzer 1978). The matter consists of about 99% gas and 1% dust by
mass.
Densities range from a few thousand to a few hundred million particles per
cubic meter with an average value in the
Milky Way Galaxy of a million particles per cubic meter. As a result of
primordial nucleosynthesis, the gas is roughly 90%
hydrogen and 10%
helium by number of
nuclei, with additional heavier
elements ("
metals" in astronomical parlance) present in trace amounts.
The ISM plays a crucial role in
astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM,
molecular clouds, and replenish the ISM with matter and energy through
planetary nebulae,
stellar winds, and
supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.
The history of knowledge of interstellar space
The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. However, they first had to acknowledge the basic concept of "interstellar" space. The term appears to have been first used in print by : "The Interstellar Skie.. hath .. so much Affinity with the Starre, that there's a Rotation of that, as well as of the Starre." Later,
natural philosopher discussed "The inter-stellar part of heaven, which several of the modern
Epicureans would have to be empty."
Before modern electromagnetic theory early
physicists postulated that an invisible
luminiferous aether existed as a medium to carry lightwaves. It was assumed that this aether extended into interstellar space, as wrote, "this efflux occasions a thrill, or vibratory motion, in the
ether which fills the interstellar spaces."
The advent of deep photographic imaging allowed
Edward Barnard to produce the first images of
dark nebulae silhouetted against the background star field of the galaxy while the first actual detection of cold diffuse matter in interstellar space was made by
Johannes Hartmann in 1904 through the use of
absorption line spectroscopy. In this historic study of the spectrum and orbit of
Delta Orionis, Hartmann observed the
light coming from this star and realized that some of this light was being absorbed before it reached the Earth. Hartmann reported that absorption from the "K" line of
calcium appeared "extraordinarily weak, but almost perfectly sharp" and also reported the "quite surprising result that the calcium line at 393.4 nanometres doesn't share in the periodic displacements of the lines caused by the orbital motion of the
spectroscopic binary star". The stationary nature of the line led Hartmann to conclude that the gas responsible for the absorption wasn't present in the atmosphere of Delta Orionis, but was instead located within an isolated cloud of matter residing somewhere along the line-of-sight to this star. This discovery launched the study of the ISM.
Following the identification of interstellar calcium absorption by Hartmann, interstellar
sodium was detected by through the observation of stationary absorption from the atom's "D" lines at 589.0 and 589.6 nanometres towards Delta Orionis and
Beta Sco.
Subsequent observations of the "H" and "K" lines of calcium by revealed double and asymmetric profiles in the spectra of
Epsilon and
Zeta Orionis. These were the first steps in the study of the very complex interstellar sightline towards
Orion. Asymmetric absorption line profiles are the result of the superposition of multiple absorption lines, each corresponding to the same atomic transition (for example the "K" line of calcium), but occurring in interstellar clouds with different
radial velocities. Because each cloud has a different velocity (either towards or away from the observer/Earth) the absorption lines occurring within each cloud is either
Blue-shifted or
Red-shifted (respectively) from the lines rest wavelength through the
Doppler Effect. These observations highlight that matter isn't distributed homogeneously and were the first evidence for the presence of multiple discrete clouds within the ISM.
The growing evidence for interstellar material led to comment that "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by
Kapteyn, is characteristic of a gas, and free gaseous molecules are certainly there, since they're probably constantly being expelled by the Sun and stars."
The same year
Victor Hess's discovery of
cosmic rays, highly energetic charged particles that rain down on the
Earth from space, led others to speculate whether they also pervaded interstellar space. The following year the Norwegian explorer and physicist
Kristian Birkeland wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It doesn't seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar [
sic] systems or nebulae, but in 'empty' space" .
noted that "it could scarcely have been believed that the enormous gaps between the stars are completely void. Terrestrial aurorae are not improbably excited by charged particles from the Sun emitted by the Sun. If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy."
Interstellar matter
The three-phase model
put forward the static two
phase equilibrium model to explain the observed properties of the ISM. Their ISM consisted of a cold dense phase (T < 300
K), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T ~ 10
4 K), consisting of rarefied neutral and ionized gas. added a dynamic third phase that represented the very hot (T ~ 10
6 K) gas which had been shock heated by supernovae and constituted most of the volume of the ISM. Their paper formed the basis for further study over the past three decades. However, the relative proportions of the phases and their subdivisions are still not well known .
Table 1 shows a breakdown of the properties and origin of the components of the three phases.
Table 1: Phases of the interstellar medium
Adapted from>
| Component |
ractional
Volume |
cale Height
(pc) |
emperature
(K) |
ensity
(atoms/cm³) |
tate of hydrogen |
Primary observational techniques |
| Molecular clouds |
< 1% |
70 |
10—20 |
102—106 |
molecular |
Radio and infrared molecular emission and absorption lines |
| Cold Neutral Medium (CNM) |
1—5% |
100—300 |
50—100 |
20—50 |
neutral |
H I 21 cm line absorption |
| Warm Neutral Medium (WNM) |
10—20% |
300—400 |
6000—10000 |
0.2—0.5 |
neutral |
H I 21 cm line emission |
| Warm Ionized Medium (WIM) |
20—50% |
1000 |
8000 |
0.2—0.5 |
ionized |
Hα emission and pulsar dispersion |
| H II regions |
< 1% |
70 |
8000 |
102—104 |
ionized |
Hα emission and pulsar dispersion |
Coronal gas
Hot Ionized Medium (HIM) |
30—70% |
1000—3000 |
106—107 |
10-4—10-2 |
ionized
(metals also highly ionized) |
X-ray emission and absorption lines of highly ionized metals, primarily in the ultraviolet |
Structures
The ISM is turbulent, and therefore full of structure on all spatial scales.
Stars are born deep inside large complexes of
molecular clouds, typically a few
parsecs in size. During their lives and deaths, stars interact with the ISM physically.
Stellar winds from young clusters of stars (often with giant or supergiant
HII regions surrounding them) and
shock waves created by
supernovae inject enormous amounts of energy into their surroundings, which leads to hypersonic turbulence. The resultant structures – of varying sizes – can be observed, such as
bubbles and
superbubbles of hot gas seen by X-ray satellite telescopes or turbulent flows observed in
radio telescope maps.
The Sun is currently traveling through the
Local Interstellar Cloud, a denser region in the low-density
Local Bubble.
Interaction with interplanetary medium
The interstellar medium begins where the
interplanetary medium of the
Solar System ends. The
solar wind slows to
subsonic velocities at the
termination shock, 90—100
astronomical units from the
Sun. In the region beyond the termination shock, called the
heliosheath, interstellar matter interacts with the solar wind.
Voyager 1, the furthest human-made object from the Earth, crossed the termination shock on
2004-12-16 and may eventually enter interstellar space, providing the first direct probe of conditions in the ISM .
Interstellar extinction
The ISM is also responsible for
extinction and
reddening, the decreasing
light intensity and shift in the dominant observable
wavelengths of light from a star. These effects are caused by scattering and absorption of photons and allows the ISM to be observed with the naked eye in a dark sky. The rifts that can be seen in the band of the Milky Way are caused by absorption of background starlight from the uniform disk of stars by molecular clouds within a few thousand light years.
Far ultraviolet light is absorbed effectively by the neutral components of the ISM. For example, a typical absorption wavelength of atomic
hydrogen lies at about 121.5 nanometers, the
Lyman-alpha transition. Therefore, it's nearly impossible to see light emitted at that wavelength from a star farther than a few hundred light years from Earth, because most of it's absorbed during the trip to
Earth by intervening neutral hydrogen.
Heating of the interstellar medium
The ISM is usually far out of
thermodynamic equilibrium. Collisions establish a
Maxwell-Boltzmann distribution of velocities, and the 'temperature' normally used to describe interstellar gas is the 'kinetic temperature', which describes the temperature at which the particles would have the observed Maxwell-Boltzman velocity distribution in thermodynamic equilibrium. However, the interstellar radiation field is typically much weaker than in thermodynamic equilibrium; it's most often roughly that of an
A star (surface temperature of ~10,000 K) highly diluted. Therefore,
bound levels within an atom or molecule in the ISM are rarely populated according to the Boltzmann formula (Spitzer 1978 S. 2.4).
Heating by low-energy cosmic rays
The first mechanism proposed for heating the ISM was heating by low energy
cosmic rays. Cosmic rays transfer energy to gas (through both ionization and excitation) and to free electrons through Coulomb interactions. Low energy cosmic rays (a few MeV) are more important because they're far more numerous than high-energy cosmic rays. Cosmic rays are an efficient heating source able to penetrate in the depths of molecular clouds.
Photoelectric heating in grains
The
ultraviolet radiation emitted by hot stars can remove electrons from dust grains. The photon hits the dust grain and some of its energy is used in overcoming the potential energy barrier (due to the possible positive charge of the grain) to remove the electron from the grain. The remainder of the photon's energy heats the grain and gives the ejected electron
kinetic energy. Since the size distribution of dust grains is:
where T is the gas temperature,
the dust temperature, and
the post-collision temperature of the gas atom/molecule. This coefficient was measure by Burke & Hollenbach (1983) as
.
Other heating mechanisms
A variety of macroscopic heating mechanisms are present including:
Cooling of the interstellar medium
Fine structure cooling
This process is dominant in most regions of the ISM, except regions of hot gas and regions deep in molecular clouds. This occurs most efficiently with abundant atoms having fine structure levels close to the fundamental level such as: CII and OI in the neutral medium and OII, OIII, NII, NIII, NeII and NeIII in HII regions. Collisions will excite these atoms to upper levels, which will eventually de-excite through photon emission, which will carry the energy out of the region.
Cooling by permitted lines
At higher temperature more levels than fine structure levels can be populated via collisions. For example collisional excitation of the n=2 level of hydrogen will release a Ly
photon upon de-excitation. In molecular clouds excitation of rotational lines of CO is important.
Further Information
Get more info on 'Interstellar Medium'.
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