by neufer » Fri Mar 03, 2017 7:06 pm
MarkBour wrote:
Every time a hydrogen atom emits an H-alpha photon, it does so because it was previously excited from the absorption of a photon. So, if we can image this, it shows that this hydrogen is also attenuating the light passing through it, right? Take the bright star imaged in the center of the frame. In this image it looks fuzzy. Doesn't this hydrogen reduce its brightness as we view it? But how large is the effect? Would it actually noticeably impact our measurement of the luminosity of Andromeda?>>
Every time a hydrogen atom emits an H-alpha photon, it does so because
it was (at some point previously) IONIZED by the absorption of a short wave (~10 nm to ~100 nm) UV photon. Eventually, some free electron will find & combine with the freed proton and cascade down to produce visible Balmer radiation.
The attenuated short wave (~10 nm to ~100 nm) UV photons are not even visible from the location of the Earth thanks to hydrogen absorption by the solar wind; however, it was announced that Voyager 1 detected the first Lyman-alpha radiation originating from the Milky Way galaxy on December 1, 2011.
https://en.wikipedia.org/wiki/Lyman_series wrote:
<<The Lyman series is a hydrogen spectral series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 the lowest energy level of the electron. The transitions are named sequentially by Greek letters: from n = 2 to n = 1 is called Lyman-alpha (121.6 nm), 3 to 1 is Lyman-beta (102.6 nm), 4 to 1 is Lyman-gamma (97.3 nm),
and so on up to ionization at 91.18 nm. On December 1, 2011, it was announced that Voyager 1 detected the first Lyman-alpha radiation originating from the Milky Way galaxy. [Red shifted] Lyman-alpha radiation had previously been detected from distant galaxies, but due to interference from the Sun, the radiation from the Milky Way was not detectable.
The Galaxy Evolution Explorer (GALEX) is an orbiting ultraviolet space telescope launched on April 28, 2003, and operated until early 2012. It can see medium wave UV
from 135 nm to 280 nm, with a field of view of 1.2 degrees wide.>>
https://en.wikipedia.org/wiki/H_II_region wrote:
<<An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. (H is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H+ (free protons) in other sciences) It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm. The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds.
In H II regions the dominant spectral line has a wavelength of 656.3 nm. This is the well-known H-alpha line emitted by atomic hydrogen. Specifically, a photon of this wavelength is emitted when the electron of a hydrogen atom changes its excitation state from n=3 to n=2. Such state-changes happen very frequently when an electron is captured by an ionised hydrogen atom (a proton), and the electron cascades down from some higher excitation state to n=1. Thus, it was concluded that H II regions consist of a mix of electrons and ionised hydrogen that are constantly recombining into hydrogen atoms.
Such regions may be of any shape, because the distribution of the stars and gas inside them is irregular. They often appear clumpy and filamentary, sometimes showing bizarre shapes. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.
Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically. Some galaxies contain huge H II regions, which may contain tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy.
The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered. William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns". In early days astronomers distinguished between "diffuse nebulae" (now known to be H II regions), which retained their fuzzy appearance under magnification through a large telescope, and nebulae that could be resolved into stars, now know to be galaxies external to our own. Confirmation of Herschel's hypothesis of star formation had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae.
During the 20th century, observations showed that H II regions often contained hot, bright stars. These stars are many times more massive than the Sun, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years). Therefore, it was surmised that H II regions must be regions in which new stars were forming. Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse. The Pleiades are an example of a cluster which has 'boiled away' the H II region from which it was formed. Only a trace of reflection nebulosity remains.>>.
[quote="MarkBour"]
Every time a hydrogen atom emits an H-alpha photon, it does so because it was previously excited from the absorption of a photon. So, if we can image this, it shows that this hydrogen is also attenuating the light passing through it, right? Take the bright star imaged in the center of the frame. In this image it looks fuzzy. Doesn't this hydrogen reduce its brightness as we view it? But how large is the effect? Would it actually noticeably impact our measurement of the luminosity of Andromeda?>>[/quote]
Every time a hydrogen atom emits an H-alpha photon, it does so because [b][u][color=#FF00FF]it was (at some point previously) IONIZED[/color][/u][/b] by the absorption of a short wave (~10 nm to ~100 nm) UV photon. Eventually, some free electron will find & combine with the freed proton and cascade down to produce visible Balmer radiation. [b][u][color=#0000FF]The attenuated short wave (~10 nm to ~100 nm) UV photons are not even visible from the location of the Earth[/color] thanks to hydrogen absorption by the solar wind[/u][/b]; however, it was announced that Voyager 1 detected the first Lyman-alpha radiation originating from the Milky Way galaxy on December 1, 2011.
[quote=" https://en.wikipedia.org/wiki/Lyman_series"]
[float=left][img3="[b][color=#0000FF]Medium wave UV (135 nm to 280 nm) GALEX view of Cygnus loop[/color][/b]"]https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/Ultraviolet_image_of_the_Cygnus_Loop_Nebula_crop.jpg/420px-Ultraviolet_image_of_the_Cygnus_Loop_Nebula_crop.jpg[/img3][/float]<<The Lyman series is a hydrogen spectral series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 the lowest energy level of the electron. The transitions are named sequentially by Greek letters: from n = 2 to n = 1 is called Lyman-alpha (121.6 nm), 3 to 1 is Lyman-beta (102.6 nm), 4 to 1 is Lyman-gamma (97.3 nm), [b][u]and so on up to ionization at 91.18 nm[/u][/b]. On December 1, 2011, it was announced that Voyager 1 detected the first Lyman-alpha radiation originating from the Milky Way galaxy. [Red shifted] Lyman-alpha radiation had previously been detected from distant galaxies, but due to interference from the Sun, the radiation from the Milky Way was not detectable.
:arrow: The Galaxy Evolution Explorer (GALEX) is an orbiting ultraviolet space telescope launched on April 28, 2003, and operated until early 2012. It can see medium wave UV [b][u]from 135 nm to 280 nm[/u][/b], with a field of view of 1.2 degrees wide.>>[/quote][quote=" https://en.wikipedia.org/wiki/H_II_region"]
<<An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. (H is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H+ (free protons) in other sciences) It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm. The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds.
In H II regions the dominant spectral line has a wavelength of 656.3 nm. This is the well-known H-alpha line emitted by atomic hydrogen. Specifically, a photon of this wavelength is emitted when the electron of a hydrogen atom changes its excitation state from n=3 to n=2. Such state-changes happen very frequently when an electron is captured by an ionised hydrogen atom (a proton), and the electron cascades down from some higher excitation state to n=1. Thus, it was concluded that H II regions consist of a mix of electrons and ionised hydrogen that are constantly recombining into hydrogen atoms.
Such regions may be of any shape, because the distribution of the stars and gas inside them is irregular. They often appear clumpy and filamentary, sometimes showing bizarre shapes. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.
Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically. Some galaxies contain huge H II regions, which may contain tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy.
The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered. William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns". In early days astronomers distinguished between "diffuse nebulae" (now known to be H II regions), which retained their fuzzy appearance under magnification through a large telescope, and nebulae that could be resolved into stars, now know to be galaxies external to our own. Confirmation of Herschel's hypothesis of star formation had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae.
During the 20th century, observations showed that H II regions often contained hot, bright stars. These stars are many times more massive than the Sun, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years). Therefore, it was surmised that H II regions must be regions in which new stars were forming. Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse. The Pleiades are an example of a cluster which has 'boiled away' the H II region from which it was formed. Only a trace of reflection nebulosity remains.>>.[/quote]