What is Interstellar medium: Definition and 16 Discussions

In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.
The interstellar medium is composed of multiple phases distinguished by whether matter is ionic, atomic, or molecular, and the temperature and density of the matter. The interstellar medium is composed, primarily, of hydrogen, followed by helium with trace amounts of carbon, oxygen, and nitrogen comparatively to hydrogen. The thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important, dynamically, than the thermal pressure is.
In all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules per cm3 (1 million molecules per cm3). In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions per cm3. Compare this with a number density of roughly 1019 molecules per cm3 for air at sea level, and 1010 molecules per cm3 (10 billion molecules per cm3) for a laboratory high-vacuum chamber. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. The hydrogen and helium are primarily a result of primordial nucleosynthesis, while the heavier elements in the ISM are mostly a result of enrichment in the process of stellar evolution.
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, which ultimately contributes to molecular clouds and replenishes 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.
Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025. Its twin Voyager 2 entered the ISM on November 5, 2018.

View More On Wikipedia.org
  1. C

    I How Bright Would the Milky Way Look If It Had No Dust?

    Interstellar dust prevents us from seeing most of the light of the Milky Way. Does anyone know how bright it would look if it had no dust at all? As it is, the Milky Way is impossible to see from cities and many suburbs, but how would the situation improve if there were no dust between the stars?
  2. Dr Wu

    Traveling through the Interstellar Medium

    I understand that the interstellar medium (ISM) consists mostly of neutral hydrogen gas, plus a smaller fraction of helium gas. Furthermore, I gather that between 0.1 and 1.0 atoms per cm3 correspond to the average density values pertaining in our "Local Bubble". The above, however, refers...
  3. S

    A What is stochastic heating of very small grains?

    I recently came across a paper where there was a term ''stochastic heating of very small grains". I went through some papers on this area but I am still confused about it. What is the vibrational temperature associated with very small grains? Does temperature has got a meaning if it's a highly...
  4. J

    Nebulosity of an O Star (Homework Question)

    1. Question Suppose you examined the spectrum of some nebulosity surrounding a main-sequence spectral-type O star and found that it contained no emission lines, only the continuous spectrum of the star. What conclusions could you draw about the nature of the interstellar material around that...
  5. U

    A Calculating the ionization rate in the Interstellar medium?

    I hope putting this in the high energy section is the right section (if not, please let me know which would be more appropriate!) I felt this was appropriate since the work I am doing is high energy astrophysics. So I'm doing some research this summer, and my tasks were to take some data...
  6. C

    Hubble Finds Origin of Huge Cloud Crashing into our Galaxy

    In 2008, astronomers discovered that a multi-million-solar-mass cloud named Smith's Cloud would hit the Milky Way's disk in about 27 million years. Now new Hubble observations have identified the mysterious cloud's likely origin. Link: New Scientist
  7. H

    Vibrational Continuum in Interstellar Medium: A Closer Look

    Hello everyone, this is my first poster here. I met this problem when I was attending class of interstellar medium, and I was confused by the question.
  8. S

    Physics of ISM: Pressure, Volumes, Masses

    Homework Statement Answer the following questions. Pleas show all of your work and your line of thinking and state your assumptions. There are several phases of the ISM. Consider the following three phases with different typical number densities ## n ## and temperatures ## T ##. The cold...
  9. P

    The propagation of disturbances in interstellar gases

    Homework Statement Show that the solution of the form ρ1 = ρ1(x±a0t) satisfy the equation: ∂2ρ1/∂t2 - a02∂2ρ1/∂x2 = 0 and that they correspond to waves propagating in the directions x increasing or decreasing. Homework Equations P = P0 + P1 ρ = ρ0 + ρ1 u = u1 The Attempt at a Solution P1 =...
  10. MattRob

    Interstellar Medium Around Sol / Interstellar Ramscoop

    So, I've read here http://en.wikipedia.org/wiki/Interstellar_medium that the Interstellar medium varies greatly throughout space. Of the categories listed; Molecular clouds, Warm Neutral Medium, Clound Neutral Medium, Warm Ionized Medium, H II regions, and Coronal Gas/Hot Ionized Medium...
  11. M

    Interstellar Medium, 21 cm line

    Somewhere in our Galaxy resides a cloud of neutral hydrogen gas with a radius of 10 pc. The gas density is 10^7 atoms/m^3. (a)How many 21-cm photons does the cloud emit every second? (b)If the cloud is 100 pc from the Sun, what is the energy flux of this radiation (in W/m^2) at the Sun...
  12. P

    Interstellar Medium: Est. Mean Mass of Particle

    Hey, can anybody tell me what the estimate is on the mean mass of a particle in the interstellar medium? I would guess around 1-10 proton masses?
  13. M

    (VLISM) Interstellar medium characteristics

    Hi all, I am writing you because in my research I need to characterize the space environment for an interstellar flight from the Earth to Alpha Centauri (more exactly proxima centauri). So i need densities of atoms and ions (in terms of cm^-3), energies (MeV), magnetic field strength...
  14. C

    Interstellar Medium: Herbig-Haro Objects

    Hi all. First post and all that, so I hope I don't do anything wrong, and that if I do, you'll cut me a little slack :P Anyway, onto the question I'm wanting help with: A J-shock in a Herbig-Haro object is propagating through neutral hydrogen gas at speed 100km/s. The gas has number...
  15. H

    Interstellar medium: Theory and models.

    I am trying to get a basic understanding of the physics of the interstellar medium and the current models which describe it (FGH and McKee-Ostriker). A first question arises regarding the possible assumption of a gas in ionization equilibrium, in which ionizarion and recombination occur at...
  16. Norman

    Interstellar Medium and Pulsars

    Originally posted in College Level Homework help but I got no responses there. Please help if you can. I am studying for my qualifier and doing problems out of Jackson. I am stuck on this one... any help would really be appreciated... I am unsure how to begin: Jackson 7.15 The partially...