APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

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APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

Post by APOD Robot » Wed Oct 28, 2020 4:06 am

Image NGC 6357: The Lobster Nebula

Explanation: Why is the Lobster Nebula forming some of the most massive stars known? No one is yet sure. Cataloged as NGC 6357, the Lobster Nebula houses the open star cluster Pismis 24 near its center -- a home to unusually bright and massive stars. The overall blue glow near the inner star forming region results from the emission of ionized hydrogen gas. The surrounding nebula, featured here, holds a complex tapestry of gas, dark dust, stars still forming, and newly born stars. The intricate patterns are caused by complex interactions between interstellar winds, radiation pressures, magnetic fields, and gravity. NGC 6357 spans about 400 light years and lies about 8,000 light years away toward the constellation of the Scorpion.

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Dobby or not Dobby?

Post by neufer » Wed Oct 28, 2020 1:56 pm

https://en.wikipedia.org/wiki/Pismis_24-1 wrote: <<Pismis 24-1, also known as HD 319718, is the brightest star of the open cluster Pismis 24 within the nebula NGC 6357 about 6,500 light-years away. It was once thought to be the most massive star known, but is composed of at least three individual objects, each still among the most luminous and most massive stars known.

Pismis 24-1 was first catalogued as HD 319718, later resolved into both Pismis 24-1 and the fainter Pismis 24-16. The surrounding HII region NGC 6357 is prominent, but the compact 10th magnitude open cluster Pismis 24 was not identified until 1959.

In 1973, Pismis 24 was resolved into 15 components of which 12 were considered member stars. The brightest was numbered first as Pismis 24-1 and tentatively considered a supergiant. It was later resolved into an O3.5 supergiant spectroscopic binary and an O4 giant star separated by approximately 500 AU.

Pismis 24-1 has been resolved visually into two components, usually labelled as NE and SW from their orientation with each other. Pismis 24-1NE is slightly more luminous and hotter than 24-1SW, but is known to be a spectroscopic binary. This is surprising given the spectral luminosity classes, because it would make the individual supergiant stars less luminous than a single cooler giant star. It could be that the interaction between the components of 24-1NE is confusing its classification, or the O4 giant may also be a close binary.

Pismis 24-1 is actually a shallow eclipsing binary with a period of 2.4 days. It is presumed to be the NE component which produces the eclipses, but the separate light curves for the components have not been resolved. The light curve is symmetrical, indicating a near circular orbit, and the two eclipsing stars have very similar masses and temperatures.

The Catalog of Components of Double and Multiple Stars lists Pismis 24-1 as having two fainter companions 5.5 and 16.4 arc-seconds distant. This is not surprising since it is a member of a rich open cluster only 1.5 arc-minutes across.

The two components of Pismis 24-1NE cannot be detected separately, but analysis of their eclipses shows that they are almost identical, with temperatures around 42,000 K. The pair combined is nearly 800,000 times as luminous as the sun, making each individual star likely to be under 400,000 L. The spectral type of the combined object is O3.5 If* indicating an expanded star with strong emission lines of highly ionised nitrogen. Separate spectral signatures cannot be detected so it is assumed that both stars have similar spectra. The mass as a single object has been calculated to be 74 M but each star would have a smaller mass. Hard x-ray radiation from the vicinity of Pismis 24-1 is assumed to be caused by the colliding winds of these two supergiants.

Pismis 24-1SW is apparently a single star with a spectral type of O4 III(f+), indicating a temperature around 40,000 K and emission lines of ionised nitrogen, silicon, and helium. The luminosity is around 650,000 L, the radius 17 R, and the mass 66 M. It is classified as a giant star on the basis of its spectrum, but the hottest O stars develop these spectral features while still burning hydrogen in their cores, as a result of vigorous convection and powerful stellar winds.

When first modelled, Pismis 24-1 was thought to be a single star with a mass of 300 M or more, higher than expected to be theoretically possible. This has been steadily reduced as the star was discovered to be double, then triple, and as newer models of stellar atmospheres have been developed. The latest mass estimates are well within theoretical expectations for star formation without exotic explanations.>>
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Re: APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

Post by Ann » Wed Oct 28, 2020 8:58 pm

APOD Robot wrote:

The overall blue glow near the inner star forming region results from the emission of ionized hydrogen gas.
Well, in mapped color images like this, blue usually means doubly ionized oxygen - particularly if the blue glow is seen near the hot bright stars, which is the case here.

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NGC 6357: The Madokami Nebula

Post by neufer » Wed Oct 28, 2020 9:21 pm

https://en.wikipedia.org/wiki/NGC_6357 wrote: <<NGC 6357 contains many proto-stars shielded by dark disks of gas, and young stars wrapped in expanding "cocoons" or expanding gases surrounding these small stars. It is also known as the Lobster Nebula and the Madokami Nebula by some fans of the anime Madoka Magica due to its resemblance to the main character from the anime and a popular fan petition to rename it. Located in our galaxy about 5,500 light years from Earth, NGC 6357 is actually a “cluster of clusters,” containing at least three clusters of young stars, including many hot, massive, luminous stars. The X-rays from Chandra and ROSAT reveal hundreds of point sources, which are the young stars in NGC 6357, as well as diffuse X-ray emission from hot gas. There are bubbles, or cavities, that have been created by radiation and material blowing away from the surfaces of massive stars, plus supernova explosions. Astronomers call NGC 6357 and other objects like it “HII” (pronounced “H-two”) regions. An HII region is created when the radiation from hot, young stars strips away the electrons from neutral hydrogen atoms in the surrounding gas to form clouds of ionized hydrogen, which is denoted scientifically as “HII”. Researchers use Chandra to study NGC 6357 and similar objects because young stars are bright in X-rays. Also, X-rays can penetrate the shrouds of gas and dust surrounding these infant stars, allowing astronomers to see details of star birth that would be otherwise missed.>>
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Re: NGC 6357: The Madokami Nebula

Post by Ann » Wed Oct 28, 2020 9:31 pm

neufer wrote:
Wed Oct 28, 2020 9:21 pm
https://en.wikipedia.org/wiki/NGC_6357 wrote: <<NGC 6357 contains many proto-stars shielded by dark disks of gas, and young stars wrapped in expanding "cocoons" or expanding gases surrounding these small stars. It is also known as the Lobster Nebula and the Madokami Nebula by some fans of the anime Madoka Magica due to its resemblance to the main character from the anime and a popular fan petition to rename it.
Yeah, that's an uncanny resemblance.

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Re: APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

Post by neufer » Wed Oct 28, 2020 11:14 pm

Ann wrote:
Wed Oct 28, 2020 8:58 pm
APOD Robot wrote:
Wed Oct 28, 2020 4:06 am

The overall blue glow near the inner star forming region results from the emission of ionized hydrogen gas.
Well, in mapped color images like this, blue usually means doubly ionized oxygen - particularly if the blue glow is seen near the hot bright stars, which is the case here.
https://en.wikipedia.org/wiki/Reflection_nebula wrote:
<<Reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars. The energy from the nearby stars is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible. Thus, the frequency spectrum shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized.

Analyzing the spectrum of the nebula associated with the star Merope in the Pleiades, Vesto Slipher concluded in 1912 that the source of its light is most likely the star itself, and that the nebula reflects light from the star (and that of the star Alcyone).Calculations by Ejnar Hertzsprung in 1913 lend credence to that hypothesis. Edwin Hubble further distinguished between the emission and reflection nebulae in 1922.

Reflection nebulae are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets). Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. Some 500 reflection nebulae are known. A blue reflection nebula can also be seen in the same area of the sky as the Trifid Nebula. The giant star Antares, which is very red (spectral class M1), is surrounded by a large, red reflection nebula. Reflection nebulae may also be the site of star formation.>>
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Re: APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

Post by Ann » Thu Oct 29, 2020 5:26 am

neufer wrote:
Wed Oct 28, 2020 11:14 pm
Ann wrote:
Wed Oct 28, 2020 8:58 pm
APOD Robot wrote:
Wed Oct 28, 2020 4:06 am

The overall blue glow near the inner star forming region results from the emission of ionized hydrogen gas.
Well, in mapped color images like this, blue usually means doubly ionized oxygen - particularly if the blue glow is seen near the hot bright stars, which is the case here.
https://en.wikipedia.org/wiki/Reflection_nebula wrote:
<<Reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars. The energy from the nearby stars is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible. Thus, the frequency spectrum shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized.
Art, are you suggesting that the inner part of NGC 6357 is blue because it is a reflection nebula???? :facepalm:

I also object to the picture you have chosen to illustrate the concept of a reflection nebula. :twisted:
ESO wrote:

This image of the region surrounding the reflection nebula Messier 78, just to the north of Orion’s belt, shows clouds of cosmic dust threaded through the nebula like a string of pearls. The submillimetre-wavelength observations, made with the Atacama Pathfinder Experiment (APEX) telescope and shown here in orange, use the heat glow of interstellar dust grains to show astronomers where new stars are being formed. They are overlaid on a view of the region in visible light.
So the purpose of the image with the glowing orange dust was to call attention of the cosmic dust clouds surrounding reflection nebula M78 by imaging the dust clouds at submillimeter wavelengths. The portrait of the glowing orange clouds was then overlaid on a visible-light image, creating a weird (and to me, disturbing) effect. :evil:

To get a reasonable idea of what reflection nebula M78 would look like to our eyes if the color receptors of our eyes were many magnitudes more sensitive to faint light than they are, look at the ESO image at right.

Okay, Art, I was too hard on you here, so I... I guess I apologize. 💐 Yes, but you know what happens when people lecture me on color and get it wrong!!!


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Re: APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

Post by neufer » Thu Oct 29, 2020 1:19 pm

Ann wrote:
Thu Oct 29, 2020 5:26 am

Okay, Art, I was too hard on you here, so I... I guess I apologize. 💐

Yes, but you know what happens when people lecture me on color and get it wrong!!!
I'm lecturing you on physics not on color.

O & B blue stars are hot enough to produce photons beyond the (912 Å) Lyman limit to ionize hydrogen.

But only white dwarfs are hot enough to produce photons that doubly ionize (electron loving) oxygen
.
https://en.wikipedia.org/wiki/Lyman_limit wrote: The Lyman limit is the short-wavelength end of the hydrogen Lyman series, at 91.2 nm (912 Å). It corresponds to the energy required for an electron in the hydrogen ground state to escape from the electric potential barrier that originally confined it, thus creating a hydrogen ion. This energy is equivalent to the Rydberg constant.
NGC 6357 O & B blue stars blew out holes in the hydrogen clouds in which they were embedded
...but they only partially blew out holes in the dust clouds in which they were embedded.
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Re: APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

Post by Ann » Fri Oct 30, 2020 7:01 am

neufer wrote:
Thu Oct 29, 2020 1:19 pm
O & B blue stars are hot enough to produce photons beyond the (912 Å) Lyman limit to ionize hydrogen.

But only white dwarfs are hot enough to produce photons that doubly ionize (electron loving) oxygen[/b].
https://en.wikipedia.org/wiki/Lyman_limit wrote: The Lyman limit is the short-wavelength end of the hydrogen Lyman series, at 91.2 nm (912 Å). It corresponds to the energy required for an electron in the hydrogen ground state to escape from the electric potential barrier that originally confined it, thus creating a hydrogen ion. This energy is equivalent to the Rydberg constant.
NGC 6357 O & B blue stars blew out holes in the hydrogen clouds in which they were embedded
...but they only partially blew out holes in the dust clouds in which they were embedded.
NGC6357schedler_S2HaO3_60[1].jpg
NGC 6357. Image data: Ha-O3-S2-LRGB
(Ha 8x40m, O3 8x40m, S2 13x40m, L 11x20m, R 7x20m, G 7x20m, B 7x20m),
total 26,3 h combined. Photo: Johannes Schedler.


























Above you can see two pictures of NGC 6357, one in mapped color (OIII-Hα-SII), one in RGB (and possibly Hα).

The RGB image at right is completely dominated by the red color of Hα. But the red color is a bit less compact and dominant in the inner cavity, which has been carved by hot stars. The bright region "below" the hot stars is pink instead of red. In the mapped color image at left, the cavity is dominated by blue light and the region that was pink in the RGB image is now bright turquoise. In view of the fact that OIII is given its own channel in the mapped color image, we surely have to assume that OIII emission is present in NGC 6357.

Where are you likely to find that OIII emission in NGC 6357, if not in the cavity blown by the hottest stars, or in the brightest part of the nebula? And if those parts of the nebula are blue or turquoise in the mapped color image, shouldn't we assume that the blue and turquoise colors signal the presence of OIII?

You wrote:
But only white dwarfs are hot enough to produce photons that doubly ionize (electron loving) oxygen[/b].

That is only partly true. Yes, only white dwarfs are hot enough to produce so much OIII emission that many planetary nebulas look bright green to the eye when observed through a telescope. But main sequence and giant O-type stars are hot enough to ionize some OIII emission. Not a lot, and not enough for "normal emission nebulas" to ever look green in RGB images. But the OIII emission is there, and it is detected by photographing the nebula through an OIII filter. The OIII image is then usually mapped as blue in narrowband images.

Ann
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Re: APOD: NGC 6357: The Lobster Nebula (2020 Oct 28)

Post by neufer » Fri Oct 30, 2020 1:18 pm

Wolfgang Pauli wrote:
"Das ist nicht nur nicht richtig; es ist nicht einmal falsch!"
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