Talk:Interstellar medium

Contents

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.

Magnetic fields doesn't seem to fit in this sentence. By following this logic, one could just as well include gravitational fields in this mixture, or neutrinos or photons? or am i getting the wrong end of the stick here? I appreciate that cosmic rays are included because they are not really rays, is there a special meaning to "(galactic) magnetic fields" 213.249.149.102 (talk) 14:29, 4 November 2009 (UTC)

A certain sentence

Using logical deduction, I'm confused about a certain sentence in the article.

Therefore, it is nearly impossible to see light emitted at that wavelength from a star, because most of it is absorbed during the trip to Earth by Lyman-alpha absorption.

The area between the sun and earth is not Interstellar Medium. While hydrogen does exist in this space, and even more light is refracted/absrobed by out atmosphere (giving us a blue sky), I'm going to remove the sentence, as it gives the impression that the space between the earth and sun is interstellar medium. --ORBIT 23:21, 26 May 2005 (UTC)

I just reread the paragraph, and while a bit confusing on my first past, does make sense, and doesn't need to be removed... Not sure how to clarify the sentence though. --ORBIT 23:23, 26 May 2005 (UTC)

I've cleaned up the paragraph and added visible extinction Mordecai-Mark Mac Low 02:05, 13 January 2006 (UTC)

Birekland's prediction

I've restored the historical paragraph, omitting the unsubstantiated claim that astronomers thought that space was a vacuum. I'll restore it, if and when I find a reference. I have no idea why you took out Birkeland's prediction, though I have qualified it as predicting both dark matter and plasma. --Iantresman 15:08, 3 February 2006 (UTC)

Gas/plasma

Why is it that the plasma article says the intersellar medium is composed moslty of a sparse plasma, and this article says it's a sparse gas? Is it a case of using "gas" & "plasma" interchangeably at this level of sparseness, or considering plasma to be a type of gas and therefore using the more general term? Just curious.

The latter. Astrophysicists routinely refer to ionized, neutral, and molecular gas, the first of which can also be called plasma. If the discussion refers specifically to plasma behaviors (eg particle orbits in tangled magnetic fields), then plasma rather than ionized gas will be used. Mordecai-Mark Mac Low 21:47, 22 February 2007 (UTC)

Picture

The caption on the picture on the main page, and the caption on the picture shown after clicking the thumbnail disagree as to the origin of the picture and what it shows. The main page caption on the H II medium image labels it as "the parts of the Galactic interstellar medium visible from the Earth's northern hemisphere (from the Wisconsin H-Alpha Mapper Survey)," while the caption on the larger picture you view after clicking on the thumbnail labels it as "a sky map of the southern sky derived from a robotic telescope in Chile," from the SHASSA team at NSF. Since the larger picture caption provides a valid linked reference, I assume it is the one labelled correctly, but I left this for someone more knowledgeable in the subject area of the article to change.

Can anyone tell which is right? Kauczuk 15:47, 9 September 2006 (UTC)

Having looked at the WHAM homepage they seem to be a lower resolution study than the SHASSA team. That image, or a higher resolution copy is from the SHASSA site so they must be the originators. --Jason Kirk 23:03, 22 September 2006 (UTC)

I don't see the image anymore on this page and my memory is foggy about what used to be here. But yes, WHAM has a lower spatial resolution than SHASSA, although it gathers spectra as a tradeoff. --mh 21:02, 25 January 2007 (UTC)

Section Edits

I've just gone through the article moving stuff around and adding new section headings. I've also reworded and expanded one or two paragraphs to make things more obvious to the non-astronomer. However, there is still a lot of stuff that I think should be included, but that something I haven't got the free time to do at the moment. Therefore I'll added a todo box to this discussion page should anybody else want to carry on. --Jason Kirk 23:03, 22 September 2006 (UTC)

As an aside: The Historical section was the biggest bugbear as its really just a collection of random quotes without any sort of context. I've tried to add bits and pieces in where the quotes seem to be taking about certain events, but its still unfinished. I also moved history to the top as the general wikipedia style seems to be for the "History" section to come first. The references in the text should also be moved to the reference section and correctly formatted. --Jason Kirk 23:03, 22 September 2006 (UTC)

I said it's not ISM. 71.237.21.218 (talk) 22:39, 7 December 2021 (UTC) Anonymous

Interstellar medium=Interstellar space?

I'm a little unclear: is the medium the same thing as the space? The way redirects are structured, one thinks that it is (i.e. Interstellar space redirects here). Erudy 20:53, 10 October 2007 (UTC)

Space is not the same as medium. Interstellar space is merely the (mathematical) space in which the interstellar medium (gas, dust, radiation) resides. Space can exist without a medium (it would then be empty space), but a medium can never exist without space to contain it. Maybe it would be a good idea if somebody explained on the interstellar space page what the difference is. Hope this clarifies matters a bit MHD 14:44, 24 October 2007 (UTC)

-->"Space is not the same as medium" means we have two different "things" and therefor two different articles encyclopedicaly. Interstellar space should not redirect here so I have changed that redirect to Outer space. Fountains of Bryn Mawr (talk) 19:53, 2 December 2009 (UTC)

• MHD, IS is not a mathematical but rather physical space. I.e. with all the conceptions of the nature of the continuum implied. IMHO.
• Erudy, such links should be weighed in their context and, if necessary, corrected. Lincoln Josh (talk)

NB: Hm. The IS link leads to a section in "Outer space" and that section says HERE (the "medium") is its main article... Lincoln Josh (talk) 22:21, 19 February 2013 (UTC)

• Well, I've changed the presumably main redirection on "Interstellar" (see the Talk page) and modified the "redirection" template here. Lincoln Josh (talk) 23:13, 19 February 2013 (UTC)

Hα image

I added an Hα image from the Wisconsin Hα Mapper. (A similar image with a non-commercial use only license was deleted two years ago.) Disclaimer: I am on the WHAM team. Ashill (talk) 17:03, 1 February 2008 (UTC)

Table 1

What does "Scale Height" mean? I'd like the explanation to be either before or straight after (just beneath) the table. Or it may be realised with the help of wikilinking the heading. That's it. Lincoln Josh (talk) 22:03, 19 February 2013 (UTC)

I wikified it to scale height. ★NealMcB★ (talk) 01:14, 16 July 2022 (UTC)

ISM is gas and dust

Generally the ISM is said to consist of gas and dust. I'd remove "cosmic rays" from the defining sentence. Cosmic rays are sourced and they travel through the ISM as does star light. But the gas and dust is the ISM, from which stars are formed. One also doesn't mention dark matter which is also present. ISM is the baryonic matter that makes the ISM a medium or "stuff," which will contract under the force of gravity leading to star formation. Jason from nyc (talk) 02:58, 1 December 2014 (UTC)

Cosmic rays are baryonic matter in the ISM. Not all of the ISM will ever participate in star formation -- only the dense phases (CNM and molecular gas) do, at least in the vaguely near term. And the Reviews of Modern Physics reference by Ferriere (2001) defines the interstellar medium explicitly as having "three basic constituents -- ordinary matter, comic rays, and magnetic fields". —Alex (ASHill | talk | contribs) 16:48, 2 December 2014 (UTC)

Thanks, Alex, for the reference. At first I couldn’t access a copy but finally found a pdf here: [1]. Pages 27 & 28 go into detail on the role played by cosmic rays and interstellar magnetic fields. The author says:"… interstellar magnetic field helps to support the ordinary matter against its own weight in the Galactic gravitational potential, and it confines cosmic rays to the Galactic disk. In this manner, both magnetic fields and cosmic rays partake in the overall hydrostatic balance of the ISM and influence its stability.” She cites research in 1990 that first “fully appreciated the importance of magnetic fields and cosmic rays in the hydrostatic balance.”

Research is clearly ongoing concerning the influence of magnetic fields on instability, turbulence, star formation, etc. The origin of the magnetic field is speculative. The author favors a hydromagnetic dynamo, motion of a conducting fluid through a magnetic field, which amplifies the original magnetic field. The original field, a seed field, could be extragalactic in origin, early interaction with the CMB, or 1st generation SN & solar winds.

She uses the phrases “interstellar material” and “interstellar matter” to refer to just the gas and dust while the more general term “interstellar medium” includes that plus cosmic rays and magnetic fields. In their graduate text, “Galactic Dynamics,” Binney and Tremaine, write “the gas and dust are called the interstellar medium (ISM)” thus restricting the term to what Ferriere calls interstellar material. B&T seem to have little use of the magnetic field in terms of galactic dynamics and mention it mostly to account for synchrotron radiation (p465) although Mo, den Bosch, and White discuss the role magnetic fields play in the inhibition of star formation in Chapter 9 of their “Galaxy Formation and Evolution.”

There seems to be a broader definition and a more narrow definition. Still, Ferriere is making the “interstellar environment” the focus of her study and that is hard to dispute. Jason from nyc (talk) 17:30, 3 December 2014 (UTC)

Extremely low frequency

I understand that these won't travel in the interstellar medium? Maybe worth mentioning and explaining. --Trickstar (talk) 09:14, 2 May 2017 (UTC)

How do photodissociation regions (PDRs) fit in?

From photodissociation region I guess that PDRs include the neutral atomic H components (in the table), but it is unclear to me if that includes just Cold neutral medium (CNM), just Warm neutral medium (WNM), or both. Could the article be expanded in the right places to show where PDRs fit in? And then could the PDR article be fleshed out a bit more? Thanks! ★NealMcB★ (talk) 01:10, 16 July 2022 (UTC)

Strange section on "The atomic hydrogen model"

The following section was added on 1st July 2017 by User:Jamtby and since then has been subject to only minor copy-editing. However, it was at best grossly out of place and at least partially cranky (the ISM does not produce "flares"!). I have copied it below in case anyone can make sense of parts of it, and replaced it with a brief summary of the physics behind the 3-phase ISM model. PaddyLeahy (talk) 18:56, 12 June 2023 (UTC)

The atomic hydrogen model

This model takes into account only atomic hydrogen: A temperature higher than 3000 K breaks molecules, while that lower than 50000 K leaves atoms in their ground state. It is assumed that the influence of other atoms (He ...) is negligible. The pressure is assumed to be very low, so the durations of the free paths of atoms are longer than the ~ 1 nanosecond duration of the light pulses that constitute ordinary, temporally incoherent light.

In this collisionless gas, Einstein's theory of coherent light-matter interactions applies: all the gas-light interactions are spatially coherent. Suppose that a monochromatic light is pulsed, then scattered by molecules with a quadrupole (Raman) resonance frequency. If the “length of light pulses is shorter than all involved time constants” (Lamb (1971)), an “impulsive stimulated Raman scattering (ISRS)” (Yan, Gamble & Nelson (1985)) applies: the light generated by incoherent Raman scattering at a shifted frequency has a phase independent of the phase of the exciting light, thus generating a new spectral line, and coherence between the incident and scattered light facilitates their interference into a single frequency, thus shifting the incident frequency. Assume that a star radiates a continuous light spectrum up to X-rays. Lyman frequencies are absorbed in this light and pump atoms mainly to the first excited state. In this state, the hyperfine periods are longer than 1 ns, so an ISRS “may” redshift the light frequency, populating high hyperfine levels. Another ISRS “may” transfer energy from hyperfine levels to thermal electromagnetic waves, so the redshift is permanent. The temperature of a light beam is defined by its frequency and spectral radiance with Planck's formula. As entropy must increase, “may” becomes “does”. However, where a previously absorbed line (first Lyman beta, ...) reaches the Lyman alpha frequency, the redshifting process stops, and all hydrogen lines are strongly absorbed. But this stop is not perfect if there is energy at the frequency shifted to Lyman beta frequency, which produces a slow redshift. Successive redshifts separated by Lyman absorptions generate many absorption lines, frequencies of which, deduced from absorption process, obey a law more dependable than Karlsson's formula.

The previous process excites more and more atoms because a de-excitation obeys Einstein's law of coherent interactions: Variation dI of radiance I of a light beam along a path dx is dI=BIdx, where B is Einstein amplification coefficient which depends on medium. I is the modulus of Poynting vector of field, absorption occurs for an opposed vector, which corresponds to a change of sign of B. Factor I in this formula shows that intense rays are more amplified than weak ones (competition of modes). Emission of a flare requires a sufficient radiance I provided by random zero point field. After emission of a flare, weak B increases by pumping while I remains close to zero: De-excitation by a coherent emission involves stochastic parameters of zero point field, as observed close to quasars (and in polar auroras). PaddyLeahy (talk) 18:56, 12 June 2023 (UTC)