DCStone wrote:Ann wrote:
Note that the blue light from the brilliant stars dominate the color of the nebula right next to the small group of hot central stars. Further away from these hot stars, the pink color of ionized hydrogen dominates.
Does the interstellar gas consist of atomic or molecular hydrogen? I would imagine that there's enough UV around to make it the former that is responsible for the observed emission, but I'd like to know for sure!
Interstellar gas consists of hydrogen plasma H
+ , neutral atomic H, and neutral molecular H
2.
(Note: the photosphere of the sun also consists of small amounts of opaque
H- anion .)
However, the amount of molecular hydrogen H
2 is very difficult to determine since all diatomic molecules
(e.g., N
2, O
2, etc.) are extremely transparent due to the high symmetry of their molecular shape.
-------------------------------------------
http://en.wikipedia.org/wiki/H_II_region wrote:
<<An H II region is a large cloud of gas and ionized (H
+) gas of glowing low density in which star formation has recently taken place. Young, hot, blue stars—which have formed from the gas—emit copious amounts of ultraviolet light, ionizing and heating the gas surrounding them. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds in which star formation takes place, and from which the stars that produce the H II region were born. The first H II known region is Orion Nebula discovered in 1610 by Nicolas-Claude Fabri de Peiresc.>>
-------------------------------------------
http://en.wikipedia.org/wiki/H_I_region wrote:
<<An H I region is an interstellar cloud composed of neutral atomic hydrogen (H I), in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. This line has a very low transition probability, so requires large amounts of hydrogen gas for it to be seen. At ionization fronts, where H I regions collide with expanding ionized gas (such as an H II region), the latter glows brighter than it otherwise would. The degree of ionization in an H I region is very small at around 10-4 (i.e. one particle in 10,000). The temperature of an H I region is about 100 K, and it is usually considered as isothermal, except near an expanding H II region. Near an expanding H II region is a dense H I region, separated from the undisturbed H I region by a shock front and from the H II region by an ionization front.
Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies. It is also used to map gravitational disruptions between galaxies. When two galaxies collide, the material is pulled out in strands, allowing astronomers to determine which way the galaxies are moving.>>
-------------------------------------------
http://home.pacbell.net/skeptica/H2.html wrote:
Is Dark Matter Just Plain Hydrogen?
Article from Sky &Telescope January 2000 page 20.
<<SEVERAL KINDS OF UNSEEN “Dark Matter” are either known or suspected to exist throughout the universe. One kind of invisible matter adds unseen mass to individual galaxies, above and beyond a galaxy’s visible stars and interstellar matter swarms of brown or white dwarfs, yet-to-be-discovered atomic particles called WIMPs or axions, and hypothetical “quark nuggets” have been proposed to account for it.
But the dark matter in galaxies may not be so exotic or even very dark. According to two Dutch astronomers, most or all of it may be ordinary molecular hydrogen (H
2), which, unlike atomic hydrogen (H), is invisible except at certain infrared wavelengths.
Using the European Space Agency’s Infrared Space Observatory (ISO), Edwin A.Valentijn (Kapteyn Institute, Groningen) and Paul P. van der Werf (Leiden Observatory) detected huge amounts of relatively warm molecular hydrogen in NGC 891, an edge-on galaxy 30 million light-years away in Andromeda. In the September 1, 1999, Astrophysical Journal Letters they claim that their result “matches well the mass required to resolve the problem of the missing matter of spiral galaxies.”
Molecular hydrogen is notoriously difficult to observe. However, the two lowest rotational energy states of this molecule produce weak spectral lines at the far-infrared wavelengths of 28.2188 and 17.0348 microns, a spectral region covered by ISO’s Short Wavelength Spectrometer. A few years ago Valentijn reported the first extragalactic detection of these lines in the center of NGC 6946. Now, the study of NGC 891 reveals that molecular hydrogen is all over the place. Valentijn and van der Werf conclude that the galaxy contains 5 to 15 times more molecular than atomic hydrogen (which is easily observed using radio telescopes). They write, “It is well established that if there is about 10 times as much molecular hydrogen as atomic hydrogen in the disks of spiral galaxies, then the missing mass problem [in galaxies] is solved.”
Since NGC 891 is a run-of-the-mill spiral, it is reasonable to assume that other galaxies may harbor similar amounts of molecular hydrogen. But this may be hard to confirm. The current observations are right at the sensitivity limit of ISO’s spectrometer. Moreover, the gas in NGC 891 is relatively warm (80º to 90º K) with still warmer patches (150º to 230º K), which makes it easier to spot. A thin background of molecular hydrogen would be much harder if not impossible to detect in our own Milky Way because the faint signal would be smeared across the whole sky.
The gas that Valentijn and van der Werf have detected resides in the galaxy’s flat disk. What about the dark matter supposedly in galaxy halos? Surprisingly, the authors claim that none may be necessary. “Our results give a much stronger footing for the ‘ordinary matter’ simple solution of the dark matter problem, in the form of massive clouds in the disks of galaxies:” they say. According to Valentijn, the “halo culture” that has grown up around the dark-matter problem might never have arisen if the ISO results had been known earlier. Nevertheless, “the problem is complex enough to avoid drawing quick conclusions:,” he says. For instance, little is known about the warming mechanism for such huge amounts of gas.>>
[quote="DCStone"][quote="Ann"]
Note that the blue light from the brilliant stars dominate the color of the nebula right next to the small group of hot central stars. Further away from these hot stars, the pink color of ionized hydrogen dominates. [/quote]
Does the interstellar gas consist of atomic or molecular hydrogen? I would imagine that there's enough UV around to make it the former that is responsible for the observed emission, but I'd like to know for sure![/quote]
Interstellar gas consists of hydrogen plasma H[sup]+[/sup] , neutral atomic H, and neutral molecular H[sub]2[/sub].
(Note: the photosphere of the sun also consists of small amounts of opaque [url=http://en.wikipedia.org/wiki/Hydrogen_anion][b]H[sup]-[/sup] anion[/b][/url] .)
However, the amount of molecular hydrogen H[sub]2[/sub] is very difficult to determine since all diatomic molecules
(e.g., N[sub]2[/sub], O[sub]2[/sub], etc.) are extremely transparent due to the high symmetry of their molecular shape.
-------------------------------------------
[quote=" http://en.wikipedia.org/wiki/H_II_region"]
<<An H II region is a large cloud of gas and ionized (H[sup]+[/sup]) gas of glowing low density in which star formation has recently taken place. Young, hot, blue stars—which have formed from the gas—emit copious amounts of ultraviolet light, ionizing and heating the gas surrounding them. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds in which star formation takes place, and from which the stars that produce the H II region were born. The first H II known region is Orion Nebula discovered in 1610 by Nicolas-Claude Fabri de Peiresc.>>[/quote]-------------------------------------------
[quote=" http://en.wikipedia.org/wiki/H_I_region"]
<<An H I region is an interstellar cloud composed of neutral atomic hydrogen (H I), in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. This line has a very low transition probability, so requires large amounts of hydrogen gas for it to be seen. At ionization fronts, where H I regions collide with expanding ionized gas (such as an H II region), the latter glows brighter than it otherwise would. The degree of ionization in an H I region is very small at around 10-4 (i.e. one particle in 10,000). The temperature of an H I region is about 100 K, and it is usually considered as isothermal, except near an expanding H II region. Near an expanding H II region is a dense H I region, separated from the undisturbed H I region by a shock front and from the H II region by an ionization front.
Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies. It is also used to map gravitational disruptions between galaxies. When two galaxies collide, the material is pulled out in strands, allowing astronomers to determine which way the galaxies are moving.>>[/quote]-------------------------------------------
[quote=" http://home.pacbell.net/skeptica/H2.html"]
Is Dark Matter Just Plain Hydrogen?
Article from Sky &Telescope January 2000 page 20.
<<SEVERAL KINDS OF UNSEEN “Dark Matter” are either known or suspected to exist throughout the universe. One kind of invisible matter adds unseen mass to individual galaxies, above and beyond a galaxy’s visible stars and interstellar matter swarms of brown or white dwarfs, yet-to-be-discovered atomic particles called WIMPs or axions, and hypothetical “quark nuggets” have been proposed to account for it.
But the dark matter in galaxies may not be so exotic or even very dark. According to two Dutch astronomers, most or all of it may be ordinary molecular hydrogen (H[sub]2[/sub]), which, unlike atomic hydrogen (H), is invisible except at certain infrared wavelengths.
Using the European Space Agency’s Infrared Space Observatory (ISO), Edwin A.Valentijn (Kapteyn Institute, Groningen) and Paul P. van der Werf (Leiden Observatory) detected huge amounts of relatively warm molecular hydrogen in NGC 891, an edge-on galaxy 30 million light-years away in Andromeda. In the September 1, 1999, Astrophysical Journal Letters they claim that their result “matches well the mass required to resolve the problem of the missing matter of spiral galaxies.”
[size=120][color=#FF0000]Molecular hydrogen is notoriously difficult to observe. However, the two lowest rotational energy states of this molecule produce weak spectral lines at the far-infrared wavelengths of 28.2188 and 17.0348 microns, a spectral region covered by ISO’s Short Wavelength Spectrometer. A few years ago Valentijn reported the first extragalactic detection of these lines in the center of [url=http://antwrp.gsfc.nasa.gov/apod/ap050125.html][b]NGC 6946[/b][/url]. Now, the study of [url=http://antwrp.gsfc.nasa.gov/apod/ap020703.html][b]NGC 891[/b][/url] reveals that molecular hydrogen is all over the place. Valentijn and van der Werf conclude that the galaxy contains 5 to 15 times more molecular than atomic hydrogen (which is easily observed using radio telescopes). They write, “It is well established that if there is about 10 times as much molecular hydrogen as atomic hydrogen in the disks of spiral galaxies, then the missing mass problem [in galaxies] is solved.”[/color][/size]
Since NGC 891 is a run-of-the-mill spiral, it is reasonable to assume that other galaxies may harbor similar amounts of molecular hydrogen. But this may be hard to confirm. The current observations are right at the sensitivity limit of ISO’s spectrometer. Moreover, the gas in NGC 891 is relatively warm (80º to 90º K) with still warmer patches (150º to 230º K), which makes it easier to spot. A thin background of molecular hydrogen would be much harder if not impossible to detect in our own Milky Way because the faint signal would be smeared across the whole sky.
The gas that Valentijn and van der Werf have detected resides in the galaxy’s flat disk. What about the dark matter supposedly in galaxy halos? Surprisingly, the authors claim that none may be necessary. “Our results give a much stronger footing for the ‘ordinary matter’ simple solution of the dark matter problem, in the form of massive clouds in the disks of galaxies:” they say. According to Valentijn, the “halo culture” that has grown up around the dark-matter problem might never have arisen if the ISO results had been known earlier. Nevertheless, “the problem is complex enough to avoid drawing quick conclusions:,” he says. For instance, little is known about the warming mechanism for such huge amounts of gas.>>[/quote]