APOD: SH2-308: The Dolphin-Head Nebula (2021 Oct 21)

[APOD Robot]

SH2-308: The Dolphin-Head Nebula

Explanation: Blown by fast winds from a hot, massive star, this cosmic bubble is huge. Cataloged as Sharpless 2-308 it lies some 5,000 light-years away toward the constellation of the Big Dog (Canis Major) and covers slightly more of the sky than a Full Moon. That corresponds to a diameter of 60 light-years at its estimated distance. The massive star that created the bubble, a Wolf-Rayet star, is the bright one near the center of the nebula. Wolf-Rayet stars have over 20 times the mass of the Sun and are thought to be in a brief, pre-supernova phase of massive star evolution. Fast winds from this Wolf-Rayet star create the bubble-shaped nebula as they sweep up slower moving material from an earlier phase of evolution. The windblown nebula has an age of about 70,000 years. Relatively faint emission captured by narrowband filters in the deep image is dominated by the glow of ionized oxygen atoms mapped to a blue hue. Presenting a mostly harmless outline, SH2-308 is also known as The Dolphin-head Nebula.

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[Ann]

SH2-308: The Dolphin-Head Nebula. Image Credit & Copyright: Nik Szymanek

The star of the show is the star of the show!

And the star of the show is EZ Canis Majoris, a Wolf-Rayet star of spectral class WN4. According to Wikipedia, the temperature of a star of spectral class WN4 is typically around 70,000 K, more than eleven times hotter than the Sun, and hotter than any "normal" O-type star like, for example, Theta 1 Orionis C in the Trapezium.

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So this star is not red hot, and it is not even blue hot, but ultraviolet hot!!


There are two quite different Wikipedia suggestions about the bolometric (total) luminosity of EZ Canis Majoris. According to the "Properties" Wikipedia entry, a star of spectral class WN4 is likely to have a bolometric luminosity of some 200,000 L_☉. But according to the "EZ Canis Majoris" entry, this particular star has a bolometric luminosity of 620,000 L_☉! Take your pick!

The reason why EZ Canis Majoris is so terrifically luminous, if you count all wavelengths, is precisely because it is so "ultraviolet hot" and emits such enormous amounts of invisible ultraviolet light. At visual wavelengths, it is much more modest.

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What about the Dolphin Head?


The Dolphin Head nebula is clearly produced by EZ CMa itself, because like all Wolf-Rayet stars EZ CMa is blowing a strong and steady wind that has removed all the outer layers from it.

So how did the Dolphin get its snout? Well, how about EZ CMa had an outburst of some sort and sent a pair of jets in opposite directions, "puffing out" its nice bubble shape particularly where the dolphin's snout is now located?


What about the bright orange star to the right of EZ CMa in Nik Szymanek's image? That star is Omicron 1 Canis Majoris, and it is a supergiant star of spectral class K2Iab. This makes Omi 1 CMa a cool red supergiant star, but not as cool or as red or as bright as, say, Antares. Because Antares is a supergiant star of spectral class M1.5Iab-Ib, so it is so much cooler than Omi 1 CMa, and Antares therefore produces so much more infrared light.

So it would seem that, appearances notwithstanding, EZ CMa really is a lot brighter than Omi 1 CMa, because the former is "so much more ultraviolet" than the latter is infrared. According to somewhat conflicting information at Wikipedia, the bolometric luminosity of Omi 1 C is "only" 16,000 L_☉, making it way way fainter than EZ CMa at, possibly, 620,000 L_☉.

I take it that both these stars are physically relatively close to one another, because it would be too much of a coincidence if two very massive and unusual stars (which show almost the same proper motion in space, too), and which are seemingly, from our point of view, located inside the same nebula, would actually be separated by hundreds or thousands of light-years. I don't believe they are, so they are a kind of neighbors.

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And what neighbors they are! You wouldn't want to live next door to them!!

Ann

[orin stepanek]

file.jpg (61.79 KiB) Viewed 9315 times

Good moniker of a dolphin! One of best named Nebulae!

[neufer]

Click to play embedded YouTube video.

Top: Skeleton of an adult and calf hippopotamus (Hippopotamus amphibius), Bottom: A blue whale calf (Balaenoptera musculus)

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<<Whippomorpha is a group of animals that contains all living cetaceans (whales, dolphins, etc.) and hippopotamuses, as well as their extinct relatives. The name Whippomorpha is a combination of English (wh[ale] + hippo[potamus]) and Greek (μορφή, morphē = form). Both hippos and whales have an unusually large and strangely-shaped larynx, which enables the booming calls of whales underwater and the unique noises produced by hippos to communicate while submerged.

The placement of Whippomorpha within Artiodactyla (even-toed ungulates) is a matter of some contention, as hippopotamuses were previously considered to be more closely related to Suidae (pigs) and Tayassuidae (peccaries). Most contemporary scientific phylogenetic and morphological research studies link hippopotamuses with cetaceans, and genetic evidence has overwhelmingly supported an evolutionary relationship between Hippopotamidae and Cetacea. Modern Whippomorphs all share a number of behavioural and physiological traits; such as a dense layer of subcutaneous fat and largely hairless bodies. They exhibit amphibious and aquatic behaviors and possess similar auditory structures.

Both whales and hippos must surface in order to breathe. This can pose problems for sleeping Whippomorphs. Cetaceans overcome this problem by exhibiting unihemispheric sleep, meaning they rest one side of their brain at a time, allowing them to swim and surface during rest periods. Hippopotamuses surface to breathe every three to five minutes. This process is partially subconscious, allowing hippopotamuses to surface and breathe whilst sleeping. Both whales and hippos exhibit symbiotic relationships with smaller fish, which they use as cleaning stations, allowing the smaller organisms to feed on parasites that enter the creature's mouth.

Hippos are herbivores, normally their diet consists entirely of short grasses that they graze on. Some hippos have been observed consuming animals such as zebra and even other hippo carcasses. A hippo normally spends up to five hours a day grazing. They normally feed only on land, though occasional consumption of aquatic vegetation has been observed. By contrast, cetaceans are all carnivores, feeding on fish and marine invertebrates, with some individual species feeding on larger mammals and birds (such as seals and penguins).

All members of the suborder Whippomorpha share some anatomical similarities. Hippopotamus stomachs are multi-chambered as with all ruminants, however they do not regurgitate food. Instead, the hippopotamus stomach contains two preliminary chambers, which acts similarly to a compost bin, allowing foodstuffs to ferment before entering the animal's main stomach. All whale species possess similar stomach structures. Additionally, both animals bear single-lobed lungs (similar to other aquatic mammals), which allow to be filled with air more rapidly. This is a critical adaptation for both amphibious and aquatic organisms, as it reduces the frequency of dangerous trips to the water surface, where such organisms are more vulnerable to predation.

Hippos’ bodies contain a layer of dense fat, reminiscent of a whales’ blubber, and situated between skin and muscle. Hippos and whales both possess thick bones, which aid in rapid descent into water, have minimal hair (to aid in hydrodynamics) and a lack of sweat glands. Webbing is also present between the toes of hippopotamuses; a more land-suitable version of a whale's flippers. Hippos possess unique hind-limb musculature that provides them with powerful propulsion capabilities, rather than fine-tuned control. These features are characteristic of other ungulates.

There is strong resemblance between the dentition of primitive cetaceans and primitive ungulates, which seemingly cements the position of Cetacea within Artiodactyla. In addition, both Cetaceans and Artiodactyls possess two distinct components in their ears, the involucrum and sigmoid process. Similar features are considered responsible for the ability of cetaceans to hear underwater. The skeletons of prehistoric whales also contain uniquely shaped ankle bones, including a double-pulley system found only in even-toed ungulates and crucially not present in odd-toed ungulates.>>

[UncleJeff]

How soon might Sh2-308's central star go supernova, and at about what magnitude would that appear here on Earth?

[Ann]

How soon might Sh2-308's central star go supernova, and at about what magnitude would that appear here on Earth?

Unfortunately, I think it's anybody's guess when EZ CMa will explode as a supernova. It will explode, however, because it is clearly far far too massive to settle quietly down as a white dwarf. According to Wikipedia, the mass of EZ Canis Majoris is 23 M_☉, which is well over the limit for stars to go supernova.

Wolf-Rayet stars are highly evolved, which is to say that they will go supernova "soon". However, in the Universe, a time scale of, say, 10,000 years is like nothing. So if EZ CMa is going to explode in 10,000 years, that means that it is happening really soon. Of course it could happen tomorrow, too. Or in 100,000 years. We just don't know!

However, when EZ CMa does explode, it will go out as a type Ib/Ic supernova. Ib supernovas are core-collapse supernovas that lack hydrogen in their spectra (because they have shed their outer layer for one reason or another), and a Ic supernova lacks both hydrogen and helium in its spectrum because it has shed even more of its outer layers.

One Wolf-Rayet star that exploded as a supernova was SN 2015dj in the galaxy NGC 7371. According to arXiv, SN 2015dj peaked at an absolute V magnitude of MV = −17.37±0.02 mag. That's bright all right, but actually a lot fainter than supernovas type Ia, which (if I remember correctly) typically peak at magnitude −19 or so.

We should remember that when we talk about absolute magnitudes, we mean how bright a light source will look from about 30 light-years. So while the Sun is overwhelmingly bright as seen from the Earth (apparent magnitude −26.74 from 8 light-minutes away), the Sun's absolute magnitude, as seen from 30 light-years away, is only 4,83. That would make it a really faint star in the night sky. But we should also remember that the apparent magnitude of the full Moon (from 1.5 light-seconds away) is only −12.74. If we could have seen SN 2015dj from 30 light-years away, it would have been way way brighter than the full Moon in the night sky.

So presumably EZ CMa is about 5,000 light-years away. It is reddened by dust, of course, but the dust-reddening isn't too bad, since this star still has a "blue" (that is, negative) B-V index.

So you will have to ask the math geniuses here. If a star explodes at magnitude −17.37 from a distance of some 5,000 light-years, and dust-reddening isn't too much of an issue, how bright will it be?

Chris? Art? Anyone?

Ann

[neufer]

Click to play embedded YouTube video.

How soon might Sh2-308's central star go supernova, and at about what magnitude would that appear here on Earth?

... If a star explodes at magnitude −17.37 from a distance of some 5,000 light-years, and dust-reddening isn't too much of an issue, how bright will it be? Chris? Art? Anyone?

5,000 light-years = 1533 parsecs
Magnitude −17.37 at 10 parsecs
Magnitude = −17.37 + 5log₁₀(1533/10)
Magnitude = −17.37 + 10.93 = -6.44

[starsurfer]

Somewhere in this image is a planetary nebula.

[johnnydeep]

From the “sharpless” link, and the description of sharpless 2-308:

This ring nebula surrounds the Wolf-Rayet star WR 6 (EZ CMa), which is one of the closest Wolf-Rayet stars to our solar system and may also partially ionise Sh 2-303 and Sh 2-304.

This distance to WR 6 is a matter of some controversy. The Hipparcos astrometric satellite results were used in 1997 to determine a parallax suggesting a distance of about 575 parsecs. However, other researchers strongly disagreed with this estimate and proposed distances of up to three times this amount.

A revised Hipparcos analysis published in 2007 using the same data determined a very different parallax and a distance of about 1400 pc, much closer to other estimates.

A striking narrow-band image here shows the strong ionised oxygen emission from Sh 2-308, which the photographer calls the Gourd nebula.

“Dolphin Head Nebula”, “Gourd Nebula”, “Ring Nebula” - hmm

[Fred the Cat]

Click to play embedded YouTube video.

How soon might Sh2-308's central star go supernova, and at about what magnitude would that appear here on Earth?

... If a star explodes at magnitude −17.37 from a distance of some 5,000 light-years, and dust-reddening isn't too much of an issue, how bright will it be? Chris? Art? Anyone?

5,000 light-years = 1533 parsecs
Magnitude −17.37 at 10 parsecs
Magnitude = −17.37 + 5log₁₀(1533/10)
Magnitude = −17.37 + 10.93 = -6.44

That whale of an explosion could be a nasty one!

[Oldbear63]

Somewhere in this image is a planetary nebula.

I noticed the bubble at the bottom of the image that no one seems to comment on.

Is this a new discovery?

[johnnydeep]

Somewhere in this image is a planetary nebula.

I noticed the bubble at the bottom of the image that no one seems to comment on.

Is this a new discovery?

Pictures help, but I think I found your bubble anyway:

Tiny Bubble on the Edge of the Bubble Nebula

It does seem to be a separate tiny faint bubble that intersects the larger bubble nebula. Is it something special? I don't know.

[Ann]

Somewhere in this image is a planetary nebula.

I noticed the bubble at the bottom of the image that no one seems to comment on.

Is this a new discovery?

Pictures help, but I think I found your bubble anyway:

Click to view full size image

It does seem to be a separate tiny faint bubble that intersects the larger bubble nebula. Is it something special? I don't know.

It looks real, and its color is slightly different from the overall color of the Dolphin Head Nebula.

You can see it here, too: https://telescope.live/gallery/dolphin- ... la-sh2-308

If you ask me, the fact that it is less blue than the Dolphin Head suggests that it contains less doubly ionized oxygen and probably more Hα than SH2-308. To me, that suggests a relatively old, faint, perfectly spherical oxygen-poor but fairly hydrogen-rich planetary nebula.

The Soap Bubble Nebula. Credit & Copyright: T. Rector (U. Alaska Anchorage), H. Schweiker (WIYN), NOAO, AURA, NSF

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The tiny bubble at one end of the Dolphin Head Nebula is not too unlike the Soap Bubble Nebula, is it?

An interesting question is if this tiny bubble is indeed really impinging on the Dolphin Head Nebula, and, of course, if it is really a planetary nebula. Because if it is, it must be very much smaller than the Dolphin Head. It must be much older, too, because it must be the product of a rather low-mass, long-lived star. Now why would such a star become a planetary nebula right next to a truly massive supernova progenitor?

Ann

[starsurfer]

Looks like people found the planetary nebula and now it's time to reveal its identity, which is PN G234.9-09.7
Both nebulae are at different distances and just happen to share the same line of sight.

[johnnydeep]

I noticed the bubble at the bottom of the image that no one seems to comment on.

Is this a new discovery?

Pictures help, but I think I found your bubble anyway:

Click to view full size image

It does seem to be a separate tiny faint bubble that intersects the larger bubble nebula. Is it something special? I don't know.

It looks real, and its color is slightly different from the overall color of the Dolphin Head Nebula.

You can see it here, too: https://telescope.live/gallery/dolphin- ... la-sh2-308

If you ask me, the fact that it is less blue than the Dolphin Head suggests that it contains less doubly ionized oxygen and probably more Hα than SH2-308. To me, that suggests a relatively old, faint, perfectly spherical oxygen-poor but fairly hydrogen-rich planetary nebula.

The tiny bubble at one end of the Dolphin Head Nebula is not too unlike the Soap Bubble Nebula, is it?

An interesting question is if this tiny bubble is indeed really impinging on the Dolphin Head Nebula, and, of course, if it is really a planetary nebula. Because if it is, it must be very much smaller than the Dolphin Head. It must be much older, too, because it must be the product of a rather low-mass, long-lived star. Now why would such a star become a planetary nebula right next to a truly massive supernova progenitor?

Ann

I had hastily assumed it intersected the bubble nebula, but now wonder if it could be either much farther away, or even much closer. The fact that its almost perfectly spheroidal shape doesn't seem to have been affected by the bubble nebula suggests to me that it is indeed NOT intersecting it. Also, I wonder if either of those faint twin stars at its almost dead center are the progenitor...

[Ann]

I had hastily assumed it intersected the bubble nebula, but now wonder if it could be either much farther away, or even much closer. The fact that its almost perfectly spheroidal shape doesn't seem to have been affected by the bubble nebula suggests to me that it is indeed NOT intersecting it. Also, I wonder if either of those faint twin stars at its almost dead center are the progenitor...

The one on the left looks very slightly bluer than the one on the right, and might be the progenitor. Of course, it would be much easier to make an educated guess at the color of this star in an RGB image.

Ann

EDIT: I was wrong. Check out this image (and click to enlarge it), and you can see the blue central star of the planetary.

[johnnydeep]

I had hastily assumed it intersected the bubble nebula, but now wonder if it could be either much farther away, or even much closer. The fact that its almost perfectly spheroidal shape doesn't seem to have been affected by the bubble nebula suggests to me that it is indeed NOT intersecting it. Also, I wonder if either of those faint twin stars at its almost dead center are the progenitor...

The one on the left looks very slightly bluer than the one on the right, and might be the progenitor. Of course, it would be much easier to make an educated guess at the color of this star in an RGB image.

Ann

EDIT: I was wrong. Check out this image, and yo can see the blue central star of the planetary.

Based on starsurfer's revealing the mysterious object as PN G234.9-09.7, you can see a much more clearly defined image of it from https://www.cloudynights.com/topic/6966 ... pless-308/

Click to view full size image

[lsheen]

LOVED the Douglas Adams reference - you guys are the BEST!

[illexsquid]

No one seems to be discussing the fact that the Wolf-Rayet star is noticeably offset from the center of the bubble. Does anyone know the explanation for this? High proper motion? Compression on one side? Something else? Or is this an open topic of research? Maybe it's not too late to start a doctoral thesis....

[neufer]

No one seems to be discussing the fact that the Wolf-Rayet star is noticeably offset from the center of the bubble. Does anyone know the explanation for this? High proper motion? Compression on one side? Something else? Or is this an open topic of research? Maybe it's not too late to start a doctoral thesis....

At ~60 light years in diameter the outflow has already run into hundreds of solar masses of interstellar medium.

Since the "offset" is also in the direction of apparent maximal compression it's all as one might imagine it to be.