Starship Asterisk*

[quote=MarkBour post_id=337169 time=1708136775 user_id=141361]
[quote=Ann post_id=337084 time=1707770683 user_id=129702]
[/quote]

1. Yes, that does look like a cat's paw! I can even imagine a claw or two showing in it.
2. Got it. As you indicate, no red giant currently in the nebula.

Interestingly, the system does not settle for one famously confusing term ("planetary"), but is described as a "cometary" system as well.
In
[url]https://ui.adsabs.harvard.edu/abs/2016MNRAS.457....9C/abstract[/url],
Chiotellis, Boumis, Nanouris, Meaburn, and Dimitriadis model that the long, red tail came from the progenitor of the white dwarf half of this pair.
By their modelling, we do see the [i]left-overs[/i] of a red giant in the image, but it is smeared out behind HFG1.
I guess the rapid travel through the interstellar medium created both the bow shock and the cometary tail at the same time.
[/quote]

Wow, Mark. We see the [i]leftovers[/i] of a red giant in this system? Or at least the long red tail is the leftovers of a red giant?

If so, that suggests to me that the non-degenerate star in the HFG1 system is not a main sequence star at all, but a catastrophically withered-down, faint and sorry version of its former bright and ruddy self, like the puny red giant of the Algol system?


[img3="The red giant star of the Algol system has had much of its gas stolen from it by its blue main sequence companion, and the red giant is now a shadow of its former self. Illustration: Mark Garlick/Science Photo Library"]https://images.fineartamerica.com/images-medium-large-5/algol-binary-star-mark-garlickscience-photo-library.jpg[/img3]


But if the red giant of the HFG1 system has lost so much of its gas, shouldn't it have interacted with its white dwarf companion and caused the white dwarf to act up?

Ann

[quote=Ann post_id=337084 time=1707770683 user_id=129702]
[/quote]

1. Yes, that does look like a cat's paw! I can even imagine a claw or two showing in it.
2. Got it. As you indicate, no red giant currently in the nebula.

Interestingly, the system does not settle for one famously confusing term ("planetary"), but is described as a "cometary" system as well.
In
[url]https://ui.adsabs.harvard.edu/abs/2016MNRAS.457....9C/abstract[/url],
Chiotellis, Boumis, Nanouris, Meaburn, and Dimitriadis model that the long, red tail came from the progenitor of the white dwarf half of this pair.
By their modelling, we do see the [i]left-overs[/i] of a red giant in the image, but it is smeared out behind HFG1.
I guess the rapid travel through the interstellar medium created both the bow shock and the cometary tail at the same time.

[quote=Ann post_id=337084 time=1707770683 user_id=129702]

(And of course, as the Color Commentator I wonder why the shock front of HFG1 is blue in almost all pictures of this nebula. I guess that the blue color could possibly be OIII, mapped as blue.)

[/quote]

It does say so on the page behind the ‘bluish’ link in the text :ssmile:

[float=left][img3="HFG1 & Abell 6: Planetary Nebulae.
Image Credit & Copyright: Julien Cadena & Mickael Coulon; Text: Natalia Lewandowska (SUNY Oswego)"]https://apod.nasa.gov/apod/image/2402/Hfg1Abell6_CadenaCoulon_1080.jpg[/img3][/float][float=right][img3="A cat's paw. Credit: I don't know."]https://www.petsmont.com/cdn/shop/articles/photo-1570579380989-4ad216439577_1024x1024.jpg?v=1596407005[/img3][/float]
[clear][/clear]


Am I the only one who can see a similarity between planetary nebula HFG1 and a cat's paw? :wink:

Since I am the Color Commentator, I wondered about this part of the caption:

[quote]APOD Robot wrote:

HFG1 was created by the binary star system V664 Cas, which consists of a white dwarf star and [b][color=#FF0000]a red giant star[/color][/b]. [/quote]

A red giant star? Really? Red giants are typically quite bright (and reddish), and white dwarfs are typically quite faint. Shouldn't the visual light output from this binary system be strongly dominated by the reddish light from the red giant? I don't see any signs of that in planetary nebula HFG1!

I know of only one example of a red giant star with a white dwarf companion, and that is the famous Mira. In visible light, the red giant component outshines its white dwarf companion completely, even though in ultraviolet light the appearance of the binary pair changes dramatically!


[float=left][img3="The visible light output of binary star Mira is completely dominated by the red giant. The white dwarf companion is invisible in optical images like this one. Credit: ESA."]https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2004/08/mira/9751798-3-eng-GB/Mira_pillars.jpg[/img3][/float][float=right][img3="In invisible ultraviolet light, the appearance of Mira changes dramatically, and it sports a fantastic ultraviolet tail as well as an ultraviolet bow shock."]https://i.ytimg.com/vi/rl5XZErFtEk/maxresdefault.jpg[/img3][/float]
[clear][/clear]


Also, and even though this isn't a certainty, isn't there a fairly good chance that the HFG1 system would be a cataclysmic variable, if one of the components is a red giant star? If the white dwarf and the red giant are close enough, matter can start flowing from the red giant onto the white dwarf, which can then have violent nova-like eruptions.


[img3="Illustration of a cataclysmic variable. It is a binary star, where one component is a red giant and the other own a white dwarf. If the two stars are close enough, matter can stream from the red giant to the white dwarf, causing the white dwarf to have outbursts. Image credit: Grace Treanor."]https://www.gaia.ac.uk/sites/default/files/media/images/alerts/cataclysmic-variable.jpg[/img3]


In short, I don't believe that the HFG1 consists of a white dwarf and a red giant. And indeed, the [url=https://www.sternwarte-baerenstein.de/hfg1-abell6-en.html]Heckathorn-Fesen-Gull 1 link[/url] in today's APOD takes us to a page where yu can read the following:

[quote]HFG1 was created by the central star V664 Cas. This is not a single star, but a dense binary star system consisting of a white dwarf and [b][color=#FF8000][size=120]a sun-like star[/size][/color][/b], which are only a few million kilometres apart and orbit every 14 hours.[/quote]


So the white dwarf doesn't have a bright red giant companion, and instead its partner is a relatively faint sunlike star. And even though the two stars are very close together they don't yet interact, because the sunlike star is small and holds its outer atmosphere close to itself.

But they will interact in the future! Because the sunlike star will turn into a red giant! And then matter will stream from the red giant onto the white dwarf, and then this stellar pair will become a cataclysmic binary.


[img3="GK Persei, a cataclysmic binary star, had a brilliant outburst in 1901. The fading X-rays from this large explosion are still visible to Chandra. Credit: X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA"]https://upload.wikimedia.org/wikipedia/commons/thumb/1/11/GKPersei-MiniSuperNova-20150316.jpg/1280px-GKPersei-MiniSuperNova-20150316.jpg[/img3]


But so far, HFG1 is a pre-cataclysmic binary. Everything is calm and peaceful in this planetary nebula binary star system! I'm reminded of the medieval guardians who would keep watch from a tower over their cities at night, and blow their horns at night at regular intervals all night, and shout the following soothing words into the night: [b][i][size=130]Everything is calm in the streets of .....[/size][/i][/b] (fill in the name of a suitable city).


[img3="Everything is calm. BWWAAHHH! So far."]https://imengine.gota.infomaker.io/?uuid=39bde841-7232-5fdc-a835-dd4077eb0739&width=960&height=640&type=preview&source=false&q=90&z=100&x=0.070&y=0.000&crop_w=0.930&crop_h=1.000&function=cropresize[/img3]


(And of course, as the Color Commentator I wonder why the shock front of HFG1 is blue in almost all pictures of this nebula. I guess that the blue color could possibly be OIII, mapped as blue.)

Ann

This image is unique and fantastic - thanks so much for submitting to APOD!

Regards,
Doug Z

AAVSO
TWAN

[url=https://apod.nasa.gov/apod/ap240212.html] [img]https://apod.nasa.gov/apod/calendar/S_240212.jpg[/img] [size=150]HFG1 & Abell 6: Planetary Nebulae[/size][/url]

[b] Explanation: [/b] [url=https://en.wikipedia.org/wiki/Planetary_nebula]Planetary nebulae[/url] like [url=https://www.sternwarte-baerenstein.de/hfg1-abell6-en.html]Heckathorn-Fesen-Gull 1[/url] (HFG1) and [url=https://in-the-sky.org/data/object.php?id=Abell_6]Abell 6[/url] in the constellation [url=https://en.wikipedia.org/wiki/Cassiopeia_(constellation)]Cassiopeia[/url] are remnants from the last phase of a [url=https://imagine.gsfc.nasa.gov/educators/lifecycles/SC_main_p1.html]medium sized star[/url] like our [url=https://science.nasa.gov/sun]Sun[/url]. In spite of their shapes, planetary nebulae have [url=https://images.squarespace-cdn.com/content/v1/5b631cba5b409b413bb3a633/1533669968141-15YL8HF098WAOR8RYKFD/A-dog-looking-confused-and-surprised.jpg]nothing in common with actual planets[/url]. Located in the bottom left part of the [url=https://www.facebook.com/ZwoDesignAstronomyCameras/posts/802324015256448]featured photo[/url], HFG1 was created by the [url=https://science.nasa.gov/universe/stars/multiple-star-systems/]binary star system[/url] V664 Cas, which consists of a [url=https://apod.nasa.gov/apod/ap231224.html]white dwarf star[/url] and a [url=https://apod.nasa.gov/apod/ap110408.html]red giant star[/url]. Both stars orbit their [url=https://spaceplace.nasa.gov/barycenter/en/]center of mass[/url] over about [url=https://www.astro.oma.be/en/gaia-observes-a-cosmic-clock-inside-a-large-planetary-nebula/]half an Earth day[/url]. Traveling with the entire nebula at a speed about 300 times faster than the fastest train on Earth, V664 Cas generates a [url=https://www.astronomersdoitinthedark.com/index.php?c=145&p=571]bluish[/url] arc shaped [url=https://apod.nasa.gov/apod/ap240104.html]shock wave[/url]. The wave interacts most strongly with the surrounding [url=https://en.wikipedia.org/wiki/Interstellar_medium]interstellar medium[/url] in the areas where the arc is brightest. After roughly [url=http://koreascience.or.kr/article/JAKO200502637762045.page]10,000 years[/url], planetary nebulae become invisible due to a lack of [url=https://science.nasa.gov/ems/10_ultravioletwaves]ultraviolet light[/url] being emitted by the [url=https://en.wikipedia.org/wiki/Star]star[/url]s that create them. Displaying [url=https://apod.nasa.gov/apod/ap240107.html]beautiful shapes[/url] and structures, planetary nebulae are [url=https://skynews.ca/hfg1-abell-6-by-dan-kusz/]highly desired object[/url]s for astrophotographers.


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