APOD: M27: Not a Comet (2019 Aug 29)

[APOD Robot]

M27: Not a Comet

Explanation: While hunting for comets in the skies above 18th century France, astronomer Charles Messier diligently kept a list of the things he encountered that were definitely not comets. This is number 27 on his now famous not-a-comet list. In fact, 21st century astronomers would identify it as a planetary nebula, but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space, with a visible glow generated by atoms excited by the dying star's intense but invisible ultraviolet light. Known by the popular name of the Dumbbell Nebula, the beautifully symmetric interstellar gas cloud is over 2.5 light-years across and about 1,200 light-years away in the constellation Vulpecula. This impressive color composite highlights details within the well-studied central region and fainter, seldom imaged features in the nebula's outer halo. It incorporates broad and narrowband images recorded using filters sensitive to emission from hydrogen and oxygen atoms.

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[zendae1]

That is truly stunning. I like how it has such a depth of dimension quality to it.

[Ann]

That is truly stunning. I like how it has such a depth of dimension quality to it.

Bob Franke is a truly great astrophotographer. All his pictures are stunning.

I'm normally not all that interested in planetary nebulas, but I have to admit that I'm intrigued by this one, thanks to the details and colors of Bob Franke's image. Note how twin jets seem to emanate from the tiny but scorchingly hot central star. The jets are reddish because they are "thicker" than their surroundings, i.e., they contain more mass. Most of the inner part of the nebula, by contrast, is blue-green from OIII emission, which is primarily found in very energetic and extremely "thin" (extremely near vaccuum) conditions. Reddish clumps of matter is mixed into the blue-green near-vaccuum. Everything is pretty sharply delineated as well as quite chaotic in the inner part of the nebula.

By contrast, the outer part of the nebula is surrounded by a sort of barrier, almost as if the inner nebula was a cosmic baby in its amniotic sac. Outside this barrier the nabula is quite different and "soft".

Note that, apart from the twin jets emitted by the central star, the star also seems to have ejected two huge puffs of energy centered on about 11 o'clock and 5 o'clock. The inner wall of the "amniotic sac" is aglow with bright red nebulosity in these parts. The other parts of the inner wall, by contrast, seem "dormant".

Fascinating!

Ann

[Boomer12k]

Nice image. I like the inner part of the halo...the lighter gray-whitish areas... I have rarely seen that.

:---[===] *

[orin stepanek]

Definitely not a comet

[heehaw]

DARK skies! https://epod.usra.edu/blog/

[neufer]

<<The Dumbbell Nebula (also known as Apple Core Nebula) is a planetary nebula in the constellation Vulpecula. In 1970, Bohuski, Smith, and Weedman found an expansion velocity of 31 km/s. Given its semi-minor axis radius of 1.01 ly, this implies that the kinematic age of the nebula is some 9,800 years. This object was the first planetary nebula to be discovered; by Charles Messier in 1764. The central star, a white dwarf progenitor, is estimated to have a radius which is 0.055±0.02 R_☉ which gives it a size larger (6 times larger than the Earth ) than most other known white dwarfs. The central star mass was estimated in 1999 by Napiwotzki to be 0.56±0.01 M_☉.>>

Amateur image of LDN 778 (center) and Alpha Vulpeculae, (red giant, top center).

---

<<Alpha Vulpeculae (α Vulpeculae, abbreviated Alfa Vul, α Vul), officially named Anser, is the brightest star in the constellation of Vulpecula. It is approximately 297 light-years from Earth. It forms a wide optical binary with 8 Vulpeculae. Alpha Vulpeculae is a red giant of spectral class M1 and has apparent magnitude +4.4. It bore the traditional name Anser, derived from when the constellation had the name Vulpecula cum Ansere 'the little fox with the goose'.

Anser has been analysed as a member of the Arcturus stream, a group of stars with high proper motion and metal-poor properties. The Arcturus stream of stars is not in the plane of the Milky Way galaxy, and has been proposed as a remnant of an ancient dwarf satellite galaxy, long since disrupted and assimilated into the Milky Way. It consists of old stars deficient in heavy elements. However, Bensby and colleagues, in analysing chemical composition of F and G dwarf stars in the solar neighbourhood, found there was no difference in chemical makeup of stars from the stream, suggesting an intragalactic rather than extragalactic origin. One possibility is that the stream appeared in a manner similar to the Hercules group, which is hypothesized to have formed due to Outer Lindblad Resonance with the Galactic bar .>>

A.C.Ne. (Arcturus/Neufer)

[OB1Kubota]

Which star is the origin for this image?

[neufer]

Click to play embedded YouTube video.

Which star is the origin for this image?

X marks the spot in the middle.

[MarkBour]

https://www.youtube.com/watch?v=uP6eChP7i44

Which star is the origin for this image?

X marks the spot in the middle.

Thanks for posting that video link, Art (Arcturus?), I enjoyed that video.

I wish I had a better understanding of why a star with a mass like our Sun, will produce a planetary nebula when it expires.

As its fuel runs out, the main thing that makes sense to me is that it quits producing the energy from fusion that has long been imparting kinetic energy to the star's atoms, to push the cloud outward. So, collapse makes sense ... and in the general description, it is part of what happens. But I haven't seen much of an explanation of a mechanism that would give energy in a huge quantity at the end that would propel the outer layers of the star away from this collapse.

[Chris Peterson]

https://www.youtube.com/watch?v=uP6eChP7i44

Which star is the origin for this image?

:arrow: X marks the spot in the middle.

Thanks for posting that video link, Art (Arcturus?), I enjoyed that video.

I wish I had a better understanding of why a star with a mass like our Sun, will produce a planetary nebula when it expires.

As its fuel runs out, the main thing that makes sense to me is that it quits producing the energy from fusion that has long been imparting kinetic energy to the star's atoms, to push the cloud outward. So, collapse makes sense ... and in the general description, it is part of what happens. But I haven't seen much of an explanation of a mechanism that would give energy in a huge quantity at the end that would propel the outer layers of the star away from this collapse.

Towards the end of its life, a solar mass star grows to be extremely huge, with a very small but very hot and dense core and a large, tenuous outer atmosphere. That outer shell simply gets blown away by the steady energy output of the core. There is no explosion or other rapid energy release that propels the material away. (There are explosive events before this, helium flashes, which do create a layered elemental structure within the core and atmosphere, however, and are probably important in defining the structure of the final planetary nebula.)

[neufer]

I wish I had a better understanding of why a star with a mass like our Sun, will produce a planetary nebula when it expires. As its fuel runs out, the main thing that makes sense to me is that it quits producing the energy from fusion that has long been imparting kinetic energy to the star's atoms, to push the cloud outward. So, collapse makes sense ... and in the general description, it is part of what happens. But I haven't seen much of an explanation of a mechanism that would give energy in a huge quantity at the end that would propel the outer layers of the star away from this collapse.

Towards the end of its life, a solar mass star grows to be extremely huge, with a very small but very hot and dense core and a large, tenuous outer atmosphere. That outer shell simply gets blown away by the steady energy output of the core. There is no explosion or other rapid energy release that propels the material away. (There are explosive events before this, helium flashes, which do create a layered elemental structure within the core and atmosphere, however, and are probably important in defining the structure of the final planetary nebula.)

The evolutionary track of a solar mass, solar metallicity, star from main sequence to post-AGB

---

I don't think it is quite as simple as that.

When the Sun has blown up to engulf the Earth the escape velocity from the photosphere will still be ~30 km/s. This corresponds to millions of degrees K not the ~3,000K of the photosphere. So it will still be the coronal gas that escapes (like it does today with the current ~400 km/s solar wind). However, it will then be a solar wind on steroids.

Note, however, that we really don't know for sure how the corona is heated to millions of degrees.

[Chris Peterson]

I wish I had a better understanding of why a star with a mass like our Sun, will produce a planetary nebula when it expires. As its fuel runs out, the main thing that makes sense to me is that it quits producing the energy from fusion that has long been imparting kinetic energy to the star's atoms, to push the cloud outward. So, collapse makes sense ... and in the general description, it is part of what happens. But I haven't seen much of an explanation of a mechanism that would give energy in a huge quantity at the end that would propel the outer layers of the star away from this collapse.

Towards the end of its life, a solar mass star grows to be extremely huge, with a very small but very hot and dense core and a large, tenuous outer atmosphere. That outer shell simply gets blown away by the steady energy output of the core. There is no explosion or other rapid energy release that propels the material away. (There are explosive events before this, helium flashes, which do create a layered elemental structure within the core and atmosphere, however, and are probably important in defining the structure of the final planetary nebula.)

I don't think it is quite as simple as that.

I don't mean to suggest it's simple, just that the mechanism is much more one of an expansion driven by a sustained source of energy, as opposed to some sort of explosive release.

[OB1Kubota]

Art, thank you so much. I did suspect that the one smack dab in the middle might be the one. It does look just right seeing as the expansion is quite symmetrical.It must be more difficult to find the star of origin in a nebula that are not symmetrical. Thanks again.

[neufer]

Towards the end of its life, a solar mass star grows to be extremely huge, with a very small but very hot and dense core and a large, tenuous outer atmosphere. That outer shell simply gets blown away by the steady energy output of the core. There is no explosion or other rapid energy release that propels the material away. (There are explosive events before this, helium flashes, which do create a layered elemental structure within the core and atmosphere, however, and are probably important in defining the structure of the final planetary nebula.)

I don't think it is quite as simple as that.

I don't mean to suggest it's simple, just that the mechanism is much more one of an expansion
driven by a sustained source of energy, as opposed to some sort of explosive release.

The mechanism is probably much more one of an expansion driven by a
AGB [helium shell flash] PULSATING source of energy, as opposed to some sort of explosive release.

Click to play embedded YouTube video.

Planetary Nebula Formation.

<<After the ejection of the envelope of the AGB (red giant, Asymptotic Giant Branch) star [through periodically] thermal [helium shell flash] pulsing or some other mechanism), the bare core of the AGB star (hot white dwarf, center) is revealed. The temperature of the core is over 100,000 Kelvins initially and is an intense source of Ultraviolet radiation and a high-speed stream of particles (fast stellar wind, light blue arrows). Fast wind moves away from the star at speeds of up to several thousand kilometers per second and easily overtake the ejected envelope of the AGB star. Fast wind compresses the ejected envelope of the star into a thin dense shell (cyan) The UV radiation strongly ionizes the gas in the shell, ionized shell emits visible light.>>

[Chris Peterson]

I don't think it is quite as simple as that.

I don't mean to suggest it's simple, just that the mechanism is much more one of an expansion
driven by a sustained source of energy, as opposed to some sort of explosive release.

The mechanism is probably much more one of an expansion driven by a
AGB [helium shell flash] PULSATING source of energy, as opposed to some sort of explosive release.

Maybe, but my understanding is that the helium flashes are earlier, and not a major driver of expansion.

[MarkBour]

...

...

Thank you sirs, for the explanations. I also enjoyed the youtube video, which helped me better understand this.
A planetary nebula looks so much like the aftermath of an explosion of unfathomable violence.
But a more gradual process of expiration is a sensible picture, and thanks to your efforts I can now imagine that this is what happens.

It seems that nuclear reactions, mostly fusion, convert some of the mass to energy, and this is the basic source of power that will eventually drive some of the mass away from the core. It would be interesting to determine a plot of mass versus radius from the center over time, to see how much of the mass is able to be ejected. I'm guessing that the majority of a planetary nebula's outer shell that we can observe is indeed given escape velocity.