Instructor
ZAHID UR REHMAN
M.Phil Space Science, M.Sc Physics, B.Ed Science Education
Topic:
Interstellar Medium
Interstellar/bodies & Interstellar
Medium
“Something occurring or situated between stars”. “The region
between the stars that contains vast, diffuse clouds of gases
and minute solid particles. Such tenuous (very small) matter in
the interstellar medium of the Milky Way system, in which the
Earth is located, accounts for about 5 percent of the Galaxy's
total mass.
Interstellar/bodies & Interstellar Medium
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
3
Examples of IS bodies
An interstellar object is an astronomical object such as an
steroid
a comet
a rogue planet (A rogue planet also termed a free-floating planet (FFP),
interstellar, nomad, orphan, starless, unbound or wandering planet)
But it is never a star.
Instead an interstellar body is not gravitationally bound to a star.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
4
Composition of interstellar bodies
The interstellar medium is filled primarily with hydrogen gas. A
relatively significant amount of helium has also been detected, along
with smaller percentages of substances such as calcium, sodium, water,
ammonia, and formaldehyde. Sizable quantities of dust particles of
uncertain composition are present as well.
(Formaldehyde is a chemical used in the production of adhesives,
bonding agents and solvents. For this reason, it is commonly found in a
variety of consumer products including: Pressed-wood products
(plywood, particle board, paneling) Foam insulation).
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
5
Properties
About 99% of this medium is gas.
1% is dust that is mostly made of
 iron
 Silicates
 carbon and
 dirty ice.
The gas component is about 90% hydrogen and 10% helium, with traces of
other elements.
The ISM is very cold and much less dense than anything created in a
laboratory.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
6
The interstellar medium is held in place by the gravitational force of
the stellar component of galaxies.
The interstellar medium contains the raw materials out of which new
stars are formed, and
As stars die, material is returned to the interstellar medium, often
violently, enriched by stellar nuclear processing.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
7
Presence & Formation
In most cases, interstellar matter occurs in cloudlike concentrations,
which sometimes condense enough to form stars.
These stars, in turn, continually lose mass, in some instances through
small eruptions and in others in catastrophic explosions known as
supernovae.
As stars die, material is returned to the interstellar medium, often
violently, enriched by stellar nuclear processing.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
8
Is there light in interstellar space?
Interstellar space is the area between the stars, but it is far from
empty.
It contains vast quantities of neutrinos, charged particles, atoms,
molecules, dark matter and photons ranging from the highest-energy
radiation to the sluggish light of the cosmic microwave background
(CMB) radiations. Due to continuous scattering and absorptions their
luster/shine ends up soon.
 That is why if you get away from city lights and look up, the sky
between the stars, it appears very dark indeed.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
9
Cooling of interstellar matter
Cooling is provided by
electronic excitation of atomic H, He and He+,
rotational and vibrational emission from H2,
metastable line emission from C, C+, O, Si and Si+ and
thermal emission from dust.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
10
Coldest form of IS medium
The molecular medium (MM) is the coldest and densest phase of the
interstellar medium and, by definition, represents regions where the
hydrogen is almost entirely in molecular form.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
11
Heating of IS medium
IS medium is usually heated by following ways;
heating by absorption of radiation
Ionization
Scattering of electrons
i.e., by photons
by collisions of energetic particles called cosmic rays
by supernova remnant shocks.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
12
Thermal equilibrium in IS medium/gas
An equilibrium may exist between the continuous heating
by energetic particles and radiation on one side and
radiative cooling on the other side.
Once out of equilibrium, gas has two ways of returning to equilibrium,
(i) by an imbalance of heating versus cooling and
(ii) by expansion or compression.
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
13
6/18/2023
ZAHID UR REHMAN, M.Phil Space Sciencs, M.Sc Physics, B.Ed
Science Education
14