APOD: Sivan 2 to M31 (2017 Mar 03)

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APOD: Sivan 2 to M31 (2017 Mar 03)

Postby APOD Robot » Fri Mar 03, 2017 5:06 am

Image Sivan 2 to M31

Explanation: From within the boundaries of the constellation Cassiopeia (left) to Andromeda (right), this telescopic mosaic spans over 10 degrees in planet Earth's skies. The celestial scene is constructed of panels that are part of a high-resolution astronomical survey of the Milky Way in hydrogen-alpha light. Processing the monochromatic image data has brought out the region's faintest structures, relatively unexplored filaments of hydrogen gas near the plane of our Milky Way Galaxy. Large but faint and also relatively unknown nebula Sivan 2 is at the upper left in the field. The nearby Andromeda Galaxy, M31, is at center right, while the faint, pervasive hydrogen nebulosities stretch towards M31 across the foreground in the wide field of view. The broad survey image demonstrates the intriguing faint hydrogen clouds recently imaged by astronomer Rogelio Bernal Andreo really are within the Milky Way, along the line-of-sight to the Andromeda Galaxy.

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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby starsurfer » Fri Mar 03, 2017 11:30 am

This is truly remarkable! I guess it could be interpreted as an epic sequel to Rogelio Bernal Andreo's Andromeda Galaxy image! I've wanted to see Sivan 2 imaged in colour for many years now but this makes up for it! The Sivan catalogue was published in 1974 by Jean-Pierre Sivan but Sivan 2 was discovered earlier in 1972. Other interesting ones include Sivan 3, which seems to surround the Alpha Camelopardalis bowshock and Sivan 10, which envelops the whole Rho Ophiuchi Nebula complex along with IC 4592, Sh2-1 and Sh2-7!! Unfortunately not only are they faint but they are larger than 5 degrees.

There has been a previous image of Sivan 2 by Hisayoshi Kato as well as this one.

I didn't know about the MDW Hydrogen Alpha Sky Survey and it is an impressive feat so far! I hope they extend their survey to OIII in the future! :D

They also have a mosaic of Cassiopeia that includes Sivan 2 and also a colour image.

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Smoke and Mirrors?

Postby neufer » Fri Mar 03, 2017 1:19 pm

https://en.wikipedia.org/wiki/Smoke_and_mirrors wrote:
<<In mathematics, 'Name is Smoke and Mirrors' ('Name ist Schall und Rauch', literally: 'name is sound and smoke' in German) from Goethe's poem Faust was Henri Poincare's response to Felix Klein's vexation of Poincare's creation of the term 'the Kleinian function for all other cases of S'.>>
https://en.wikipedia.org/wiki/Tezcatlipoca wrote:
<<Tezcatlipoca was a central deity in Aztec religion, and his main festival was the Toxcatl ceremony celebrated in the month of May. His name in the Nahuatl language is often translated as "Smoking Mirror" and alludes to his connection to obsidian, the material from which mirrors were made in Mesoamerica which were used for shamanic rituals and prophecy.

Tezcatlipoca ("Smoking Mirror") was usually drawn with a black and a yellow stripe painted across his face. He is often shown with his right foot replaced with an obsidian mirror, bone, or a snake—an allusion to the creation myth in which he loses his foot battling with the Earth Monster. Sometimes the mirror was shown on his chest, and sometimes smoke would emanate from the mirror.

One of the four sons of Ometeotl, he is associated with a wide range of concepts, including the night sky, the night winds, hurricanes, the north, the earth, obsidian, enmity, discord, rulership, divination, temptation, jaguars, sorcery, beauty, war and strife. The four gods who created the world, Tezcatlipoca, Quetzalcoatl, Huitzilopochtli and Xipe Totec were referred to respectively as the Black, the White, the Blue and the Red Tezcatlipoca. The rivalry between Quetzalcoatl and Tezcatlipoca is recounted in the legends of Tollan where Tezcatlipoca deceives Quetzalcoatl who was the ruler of the legendary city and forces him into exile. But Quetzalcoatl and Tezcatlipoca both collaborated in the creation of the different creations and that both of them were seen as instrumental in the creation of life. Karl Taube and Mary Miller write that, "More than anything Tezcatlipoca appears to be the embodiment of change through conflict.">>
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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby Boomer12k » Fri Mar 03, 2017 5:27 pm

Interesting...

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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby MarkBour » Fri Mar 03, 2017 5:28 pm

Simple smoke-and-lenses question here. 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?

A deeper question this leads to is where is this hydrogen? It seems it would be just as difficult to answer that, if not more so, than how distant a given star is. Because this diffuse hydrogen could be at lots of different distances, involving patches of gas that are unrelated to each other, except in terms of our line-of-sight. The APOD caption indicates that "The broad survey image demonstrates the intriguing faint hydrogen clouds recently imaged by astronomer Rogelio Bernal Andreo really are within the Milky Way ...", but I don't follow why that conclusion seems clear (unfortunate pun).
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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby 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.

: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 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.>>.
Art Neuendorffer

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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby neufer » Fri Mar 03, 2017 7:11 pm

MarkBour wrote:
The APOD caption indicates that "The broad survey image demonstrates the intriguing faint hydrogen clouds recently imaged by astronomer Rogelio Bernal Andreo really are within the Milky Way ...", but I don't follow why that conclusion seems clear (unfortunate pun).

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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby MarkBour » Fri Mar 10, 2017 5:47 pm

neufer wrote:... 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. ...

Thanks for clarifyng that Art, and also for the additional reference material.

So, you're telling me that if we can image a cloud full of hydrogen, registering as having Lyman-alpha radiation, it does not do so directly from absorption of Lyman-alpha radiation. (I don't know what wavelength Rogelio was using to capture these images.) And indeed the wavelength it absorbed was mostly in the UV. But it does still absorb and scatter lots of frequencies of light, does it not? (That last sentence came out sounding almost rhetorical, but really, I'm asking because I don't know.) If it does, I have no idea what wavelengths and in what proportions. The basic question, from me, is: We can tell there is a cloud there. I wonder how much effect that cloud is having on any observations that are being attempted through it. Perhaps that's a first-year astronomy question, perhaps it's a research question, I don't know.
Mark Goldfain

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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby neufer » Sat Mar 11, 2017 3:41 pm

neufer wrote:
... 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. ...

I need to correct what I stated above :!: :oops:

Once hydrogen has been preionized by short wave (~10 nm to ~100 nm) UV photons it is then transparent to both those wavelengths as well as to the strong 121.6 nm Lyman alpha line absorption. It is only clouds of neutral hydrogen (such as those that lie within our heliosphere) that limit our view of UV photons (particularly at the 121.6 nm wavelength).

:arrow: Neutral hydrogen cloud Lyman alpha absorption line at cosmological distances obscures quasar light at longer wavelengths as well.
MarkBour wrote:
So, you're telling me that if we can image a cloud full of hydrogen, registering as having Lyman-alpha radiation, it does not do so directly from absorption of Lyman-alpha radiation. (I don't know what wavelength Rogelio was using to capture these images.) And indeed the wavelength it absorbed was mostly in the UV. But it does still absorb and scatter lots of frequencies of light, does it not? (That last sentence came out sounding almost rhetorical, but really, I'm asking because I don't know.) If it does, I have no idea what wavelengths and in what proportions. The basic question, from me, is: We can tell there is a cloud there. I wonder how much effect that cloud is having on any observations that are being attempted through it. Perhaps that's a first-year astronomy question, perhaps it's a research question, I don't know.
Once hydrogen has been ionized it is no longer good at absorbing or scattering (though it can polarize) light.
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Re: APOD: Sivan 2 to M31 (2017 Mar 03)

Postby MarkBour » Tue Mar 14, 2017 6:36 pm

neufer wrote:Once hydrogen has been ionized it is no longer good at absorbing or scattering (though it can polarize) light.

Yes, but these atoms are participating in an ongoing cycle of ionization and re-capture (emission), over and over again. I suppose there's the question of what proportion are in each state, which might be what I'm missing. If 99% of them are ionized, that changes my view of what's going on in a significant way, and your statement becomes quite relevant.
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