Page 3 of 3 FirstFirst 123
Results 61 to 72 of 72

Thread: Interstellar Medium; current leaders, current challanges?

  1. #61
    Join Date
    Jan 2011
    Posts
    122
    Quote Originally Posted by ngc3314 View Post
    Yes. Now add one more important factor strengthening that trend - the rate of energy loss to the cloud rises very rapidly toward higher density. Since the internal energy is carried mainly in the motions of particles, what general relation do you expect between the temperature and density in ISM clouds?
    The temperature would be inversly proportional to the density.
    How it happens that the cloud loses energy faster when it is more dense?
    Stars the hotter they are the faster they lose energy, right? What is the difference?
    Even assuming the logic for the clouds that more dense they are the shorter the average path between molecules and therefore molecules hit each other more often, emitting photones, the photones would be captured again by surrounding molecules, thus staying within the cloud, right?

  2. 2011-May-18, 01:22 PM
    Reason
    duplicate

  3. #62
    Join Date
    Mar 2004
    Posts
    3,162
    Quote Originally Posted by tu144 View Post
    Even assuming the logic for the clouds that more dense they are the shorter the average path between molecules and therefore molecules hit each other more often, emitting photones, the photones would be captured again by surrounding molecules, thus staying within the cloud, right?
    Not for processes that cool the clouds. The important feature is that at most temperature ranges, there are processes which give off photons at wavelengths that escape the clouds (far-infrared "fine structure" lines from C+ and O, optical forbidden lines such as [O III]). Photons will be trapped in the clouds only if they are in a resonance line - the most important is Lyman alpha of hydrogen which is easily trapped in a a cloud - or if there is enough dust to absorb all the optical and UV radiation. For H-alpha, for example, it can be absorbed only by a hydrogen atom already in the first excited state, which is very rare in the ISM because such a state has a very short decay time.

  4. #63
    Join Date
    Jan 2011
    Posts
    122
    Quote Originally Posted by ngc3314 View Post
    Not for processes that cool the clouds. The important feature is that at most temperature ranges, there are processes which give off photons at wavelengths that escape the clouds (far-infrared "fine structure" lines from C+ and O, optical forbidden lines such as [O III]). Photons will be trapped in the clouds only if they are in a resonance line - the most important is Lyman alpha of hydrogen which is easily trapped in a a cloud - or if there is enough dust to absorb all the optical and UV radiation. For H-alpha, for example, it can be absorbed only by a hydrogen atom already in the first excited state, which is very rare in the ISM because such a state has a very short decay time.
    It makes sense to me now.
    Thanks for the explanation.

  5. #64
    Join Date
    Jan 2011
    Posts
    122
    Thread checkpoint.

    1. Prohibited lines of emission in ionized nebula spectrum (IM) [PARTLY UNDERSTOOD]
    Why prohibited lines of emission show up in ionized nebula spectrum?
    Why are they so bright in ionized nebulas?
    Why they don't appear in normal conditions on Earth?
    Quote Originally Posted by EigenState View Post
    (...)Such lines would be strong in the spectra of certain astronomical objects because the physical conditions within those objects selectively populate the pertinent energy levels--that is they appear strong because the transitions you are more used to do not come into play.
    What are the physical conditions that selectively populate the pertinent energy levels, and what are the conditions that don't do that? Can someone give examples?
    What are these transitions I'm more used to?

    2. HII regions movement (IM)
    Why gas of a nebulae is set in motion when it gets ionized?
    Why most of the HII regions we see have an expansion velocity of about 10 km / s?

    3. Spectral lines of CO molecule, molecular clouds in interstellar medium (IM)
    Why are these lines so useful for studying molecular clouds?
    [UNDERSTOOD]


    4. Interstellar dust (IM)
    What effects produce interstellar dust in the spectrum of nebulae?
    [UNDERSTOOD]
    How can I quantify interstellar extinction from the nebulae spectrum? [UNDERSTOOD]


    5. Critical density of an atom level (IM)
    What is the critical density of an atom level?
    What happens to the intensity of the collisional excitation lines when it exceeds this density?

    6. Empirical diagrams in context of Interstellar Medium (IM)
    What are they?
    What could be the examples of such diagram?

    7. Temperature in ionized nebulae from collisional excitation lines(IM)
    How to calculate the electron temperature in ionized nebulae from the ratio of intensity of collisional excitation lines?

    8. Planetary nebulae vs HII region (IM)
    What are the differences between these two?
    What are they characteristic spectrums?

    9. Formation of molecules in interstellar medium (IM)
    What are the different processes of formation of molecules in interstellar medium?
    How H2 is formed?

  6. #65
    Join Date
    Sep 2006
    Posts
    1,827
    Quote Originally Posted by tu144 View Post
    Thread checkpoint.

    1. Prohibited lines of emission in ionized nebula spectrum (IM) [PARTLY UNDERSTOOD]
    Why prohibited lines of emission show up in ionized nebula spectrum?
    Why are they so bright in ionized nebulas?
    Why they don't appear in normal conditions on Earth?

    What are the physical conditions that selectively populate the pertinent energy levels, and what are the conditions that don't do that? Can someone give examples?
    What are these transitions I'm more used to?
    The pertinent difference in this case is density. In the atmosphere of the Earth, an excited molecule or atom has only a very brief time to de-excite by radiating away its energy; if it doesn't radiate the energy in a short time, a collision with another atom/molecule can excite or de-excite the molecule/atom instead.

    In an interstellar cloud, the density is so low that collisions don't occur very often. Atoms/molecules have much more time to radiate energy when they have been excited; therefore, some specific energy level changes, which take a long time to occur, may happen in space -- but not on Earth.

  7. #66
    Join Date
    Jan 2011
    Posts
    122
    Understood. Thank you.

    As we are talking about collisions,
    How to calculate the electron temperature in ionized nebulae from the ratio of intensity of collisional excitation lines?

  8. #67
    Join Date
    Dec 2006
    Posts
    2,221
    Quote Originally Posted by tu144 View Post
    Understood. Thank you.

    As we are talking about collisions,
    How to calculate the electron temperature in ionized nebulae from the ratio of intensity of collisional excitation lines?
    You really should go check out Osterbrock's book. I think you will find that many of your questions will have detailed answers and you will be able to find them quickly.

  9. #68
    Join Date
    Jan 2011
    Posts
    122
    Quote Originally Posted by Amber Robot View Post
    You really should go check out Osterbrock's book. I think you will find that many of your questions will have detailed answers and you will be able to find them quickly.
    Don't have it. I'll try to scan thorugh The physics of the interstellar medium by Dyson J., Williams D. and come back afterwards.

    I'll be back.

  10. #69
    Join Date
    Mar 2004
    Posts
    3,162
    You could look here, which includes very brief summaries of these points in Osterbrock's notation. Examples of density- and temperature-sensitive line ratios are given.

  11. #70
    Join Date
    Jan 2011
    Posts
    122
    Quote Originally Posted by ngc3314 View Post
    You could look here, which includes very brief summaries of these points in Osterbrock's notation. Examples of density- and temperature-sensitive line ratios are given.
    I'm on it. Reading it over and over again mixing with some other stuff.
    Meanwhile
    I have spectral distribution of energy for HII Regions & Planetary Nebulas:


    Now, around 10^13.1 Hz looking in the direction of lower and lower frequencies the lines for HII Regions and Planetary Nebulas split. Emited energy rises for HII and fals down for Planetary Nebulas.
    The opposite split happens at 10^13.7 Hz.
    What is it?
    Why these spectrums are so different?
    Both HII regions and Planetary Nebulas are ionized hydrogen.
    These are the physical characteristics I have from wikipedia:
    Planetary Nebula density~100-10 000 particles/cm^3, T~10,000-25,000K
    HII density~0-1 000 000 particles/cm^3; T~10,000K
    This doesn't make sense, because both of the clouds can have the same characteristics, yet the spectrum is different.
    What I am missing here?

  12. #71
    Join Date
    Mar 2004
    Posts
    3,162
    Quote Originally Posted by tu144 View Post
    I'm on it. Reading it over and over again mixing with some other stuff.
    Meanwhile
    I have spectral distribution of energy for HII Regions & Planetary Nebulas:

    Now, around 10^13.1 Hz looking in the direction of lower and lower frequencies the lines for HII Regions and Planetary Nebulas split. Emited energy rises for HII and falls down for Planetary Nebulas.
    The opposite split happens at 10^13.7 Hz.
    What is it?
    Why these spectrums are so different?
    Both HII regions and Planetary Nebulas are ionized hydrogen.
    These are the physical characteristics I have from wikipedia:
    Planetary Nebula density~100-10 000 particles/cm^3, T~10,000-25,000K
    HII density~0-1 000 000 particles/cm^3; T~10,000K
    This doesn't make sense, because both of the clouds can have the same characteristics, yet the spectrum is different.
    What I am missing here?
    That part of the spectrum doesn't come from the gas itself, but is reradiated from heated dust grains. Cooler grains (longer wavelengths) would mostly be farther from the heating source (star or cluster). Planetary nebulae may run out of dust grains before getting to such low temperatures.

    At the short-wavelength end, I have to wonder whether the those data include the ionizing star for planetary nebulae but not for star-forming regions. Dust doesn't get hot enough to be an important contributor in the J band before it evaporates.

  13. #72
    Join Date
    Jan 2011
    Posts
    122
    Quote Originally Posted by StupendousMan View Post
    (...)
    Quote Originally Posted by Amber Robot View Post
    (...)
    Quote Originally Posted by ngc3314 View Post
    (...)
    Quote Originally Posted by EigenState View Post
    (...)
    Quote Originally Posted by astromark View Post
    (...)
    Ok, thanks to all of you, and the links you've send me, I have most of my questions answered and understood.
    With this two I still have problem:

    2. HII regions movement (IM)
    Why gas of a nebulae is set in motion when it gets ionized?
    Why most of the HII regions we see have an expansion velocity of about 10 km / s?

    I've found some documents that say the sound speed is 10km/s in HII regions, but I can't connect it in my mind with expansion.

    6. Empirical diagrams in context of Interstellar Medium (IM)
    What are they?
    What could be the examples of such diagram?

    This one is unfindable on the internet, lecture presentations, nowhere. I have no idea what is this stuff.

Similar Threads

  1. Your current weather?
    By Buttercup in forum Off-Topic Babbling
    Replies: 6901
    Last Post: Yesterday, 06:02 PM
  2. Birkeland current
    By Adamsavage in forum Astronomy
    Replies: 12
    Last Post: 2012-Jan-15, 08:17 PM
  3. Current technology interstellar propulsion - Torusail Drive
    By IsaacKuo in forum Space Exploration
    Replies: 70
    Last Post: 2005-Dec-14, 05:41 PM
  4. Current value for Omega?
    By TriangleMan in forum Astronomy
    Replies: 11
    Last Post: 2003-Jun-10, 09:09 PM
  5. current Maid 4TV .Doc
    By in forum Conspiracy Theories
    Replies: 2
    Last Post: 2003-Apr-12, 12:05 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •