Starship Asterisk*

Leon1949Green wrote: ↑Fri Mar 26, 2021 12:51 pm So, does this mean that after our sun has gone into the planetary nebula phase, that the radius of it will reach halfway to Alpha Centauri?!

APOD Robot wrote: ↑Fri Mar 26, 2021 4:05 am The Medusa Nebula

Explanation: Braided and serpentine filaments of glowing gas suggest this nebula's popular name, The Medusa Nebula. Also known as Abell 21, this Medusa is an old planetary nebula some 1,500 light-years away in the constellation Gemini. Like its mythological namesake, the nebula is associated with a dramatic transformation. The planetary nebula phase represents a final stage in the evolution of low mass stars like the sun as they transform themselves from red giants to hot white dwarf stars and in the process shrug off their outer layers. Ultraviolet radiation from the hot star powers the nebular glow. The Medusa's transforming star is the faint one near the center of the overall bright crescent shape. In this deep telescopic view, fainter filaments clearly extend above and right of the bright crescent region. The Medusa Nebula is estimated to be over 4 light-years across.

When a star like our Sun comes to age, having longly burned away all the hydrogen to helium in its core in its main sequence phase, and in a later evolutionary state following the red giant stadium (the "Horizontal Branch" state, for their places in the Color-Magnitude Diagram (CMD) or Hertzsprung-Russell Diagram (HRD)), also the helium to carbon and oxygen, its nuclear reactions come to an end in its core, while helium burning goes on in a shell. This process makes the star expanding, and causes its outer layers to pulsate as a long-periodic Mira-type variable, also called AGB star (for "Asymptotic Giant Branch" star, also for their places in the CMD or HRD), which becomes more and more unstable, and loses mass in strong stellar winds.

Notes
[1] Counterintuitively, the stellar core of the Medusa Nebula is not the bright star in the centre of this image — this is a foreground star called TYC 776-1339-1. Medusa’s central star is a dimmer, bluish star lying just off-centre of the crescent shape and in the right-hand part of this image.

Fred the Cat wrote: ↑Fri Mar 26, 2021 3:45 pm Inhabitants of Earth may need to catch a wave and sail on out of here.

Monque wrote: ↑Fri Mar 26, 2021 5:59 pm ...Also trying to figure out which star is being referred to...

johnnydeep wrote: ↑Fri Mar 26, 2021 4:34 pm

Fred the Cat wrote: ↑Fri Mar 26, 2021 3:45 pm Inhabitants of Earth may need to catch a wave and sail on out of here.

Very interesting extension of the light sail idea! But still a bit impractical for us to have to wait until our Sun goes nova

Wikipedia wrote:

A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star, that slowly fades over several weeks or many months. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars.

Ann wrote: ↑Fri Mar 26, 2021 6:11 pm

johnnydeep wrote: ↑Fri Mar 26, 2021 4:34 pm

Fred the Cat wrote: ↑Fri Mar 26, 2021 3:45 pm Inhabitants of Earth may need to catch a wave and sail on out of here.

Very interesting extension of the light sail idea! But still a bit impractical for us to have to wait until our Sun goes nova

Our Sun will not go nova.

Wikipedia wrote:

A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star, that slowly fades over several weeks or many months. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars.

Our Sun is a singleton, or at least it has left its birth siblings far behind (or in front of it, or sideways, or above or below it), so it has no nearby companion star that can feed matter onto it after it becomes a white dwarf and make it have a nova explosion.

Therefore it definitely can't go supernova, either. Unless, that is, a (preferably evolved and swollen) star was to cross paths with the white dwarf Sun and become locked in a sufficiently tight binary that it can feed mass onto the degenerate "skeleton" of our star.

But you've got to remember that we expect a white dwarf to be as massive as circa 1.4 Solar masses if it is going to be able to explode. Our Sun weighs just 1.0 Solar masses, and it is likely to shed quite a lot more mass during its red giant stage before it becomes a white dwarf. Perhaps it will weigh just 0.5 Solar masses when it settles down into white dwarfhood.

So the prospects of out Sun ever accreting so much mass from a possible future companion that it can ever become massive enough to explode as a supernova, or undergo a milder nova explosion, seem to be dim.

Ann

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star such as a white dwarf, or the sudden gravitational collapse of a massive star's core. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a binary companion through accretion, or a stellar merger. In the massive star case, the core of a massive star may undergo sudden collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics are established and accepted by the astronomical community.

johnnydeep wrote: ↑Fri Mar 26, 2021 6:10 pm

Monque wrote: ↑Fri Mar 26, 2021 5:59 pm ...Also trying to figure out which star is being referred to...

Got it! And my prior guess was wrong. From another APOD - https://apod.nasa.gov/apod/ap150612.htm - it points to this pic that has an arrow:

Central Star of The Medusa Nebula

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Ann wrote: ↑Fri Mar 26, 2021 6:56 pm

johnnydeep wrote: ↑Fri Mar 26, 2021 6:10 pm

Monque wrote: ↑Fri Mar 26, 2021 5:59 pm ...Also trying to figure out which star is being referred to...

Got it! And my prior guess was wrong. From another APOD - https://apod.nasa.gov/apod/ap150612.htm - it points to this pic that has an arrow:

Central Star of The Medusa Nebula

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I found the arrow pointing to that star too, Johnny. It is here.

But I don't feel convinced that the annotation is correct. The picture that was an APOD on June 12, 2015, is an ESO picture. But if you go to ESO's original page about the Medusa Nebula, you can't find the arrow pointing out that star on that page, or at least I can't.

My guess is that it is another star that is the culprit:


Central star of Medusa Nebula ESO.png

I have marked the star that I consider to be the most likely candidate as the central star of the Medusa Nebula. I have picked it because it is the bluest star in the picture, and we expect central stars o planetary nebulas to be very hot and therefore blue.

The star that is the ionizing star of the Medusa Nebula is TYC 776-1339-1. But I have to admit that I could be wrong in identifying TYC 776-1339-1 as the bluest star in the ESO image.

There is a picture of TYC 776-1339-1 on a SIMBAD page, and I don't think that the star that we see at SIMBAD really looks like the very blue star in the APOD. The nebulosity around the star in the APOD doesn't look the way it does at SIMBAD.

So maybe it really is the star by the arrow in the picture you posted, Johnny.

Ann

johnnydeep wrote: ↑Fri Mar 26, 2021 6:46 pm 'Tis a shame that good ol' Sol won't go out with a bang...just a slow red bloating.

Still not sure of the difference between a nova and a supernova. I gather that a single very massive star can (always?) die in a supernova, but in the wikipedia article for supernova, it also says that a white dwarf and accrete enough matter to go supernova. From https://en.wikipedia.org/wiki/Supernova:

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star such as a white dwarf, or the sudden gravitational collapse of a massive star's core. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a binary companion through accretion, or a stellar merger. In the massive star case, the core of a massive star may undergo sudden collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics are established and accepted by the astronomical community.

But the nova article says that a nova is caused by a white dwarf accreting matter. So, when does that process result in a supernova rather than a nova?

johnnydeep wrote: ↑Fri Mar 26, 2021 7:26 pm

Ann wrote: ↑Fri Mar 26, 2021 6:56 pm

johnnydeep wrote: ↑Fri Mar 26, 2021 6:10 pm

Got it! And my prior guess was wrong. From another APOD - https://apod.nasa.gov/apod/ap150612.htm - it points to this pic that has an arrow:

Central Star of The Medusa Nebula

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I found the arrow pointing to that star too, Johnny. It is here.

But I don't feel convinced that the annotation is correct. The picture that was an APOD on June 12, 2015, is an ESO picture. But if you go to ESO's original page about the Medusa Nebula, you can't find the arrow pointing out that star on that page, or at least I can't.

My guess is that it is another star that is the culprit:

Click to view full size image

I have marked the star that I consider to be the most likely candidate as the central star of the Medusa Nebula. I have picked it because it is the bluest star in the picture, and we expect central stars o planetary nebulas to be very hot and therefore blue.

The star that is the ionizing star of the Medusa Nebula is TYC 776-1339-1. But I have to admit that I could be wrong in identifying TYC 776-1339-1 as the bluest star in the ESO image.

There is a picture of TYC 776-1339-1 on a SIMBAD page, and I don't think that the star that we see at SIMBAD really looks like the very blue star in the APOD. The nebulosity around the star in the APOD doesn't look the way it does at SIMBAD.

So maybe it really is the star by the arrow in the picture you posted, Johnny.

Ann

The SIMBAD pic (which is zoomable!) identifies the brightest star in today's APOD as TYC 776-1339-1:

Click to view full size image

As for the progenitor of the Medusa Nebula, it's either the one pointed to by the white arrow in the ESO pic, or it's the one pointed to by your blue arrow and my red arrow (which point to the same star),... or, it's a different star.

Ann wrote: ↑Fri Mar 26, 2021 9:39 pm

johnnydeep wrote: ↑Fri Mar 26, 2021 7:26 pm

Ann wrote: ↑Fri Mar 26, 2021 6:56 pm


I found the arrow pointing to that star too, Johnny. It is here.

But I don't feel convinced that the annotation is correct. The picture that was an APOD on June 12, 2015, is an ESO picture. But if you go to ESO's original page about the Medusa Nebula, you can't find the arrow pointing out that star on that page, or at least I can't.

My guess is that it is another star that is the culprit:

Click to view full size image

I have marked the star that I consider to be the most likely candidate as the central star of the Medusa Nebula. I have picked it because it is the bluest star in the picture, and we expect central stars o planetary nebulas to be very hot and therefore blue.

The star that is the ionizing star of the Medusa Nebula is TYC 776-1339-1. But I have to admit that I could be wrong in identifying TYC 776-1339-1 as the bluest star in the ESO image.

There is a picture of TYC 776-1339-1 on a SIMBAD page, and I don't think that the star that we see at SIMBAD really looks like the very blue star in the APOD. The nebulosity around the star in the APOD doesn't look the way it does at SIMBAD.

So maybe it really is the star by the arrow in the picture you posted, Johnny.

Ann

The SIMBAD pic (which is zoomable!) identifies the brightest star in today's APOD as TYC 776-1339-1:

Click to view full size image

As for the progenitor of the Medusa Nebula, it's either the one pointed to by the white arrow in the ESO pic, or it's the one pointed to by your blue arrow and my red arrow (which point to the same star),... or, it's a different star.

If the star at the green cross in the interactive AladinLite view is TYC 776-1339-1, then I'd say that TYC 776-1339-1 is definitely not the ionizing star of the Medusa nebula.

For one thing, it is too bright, bright enough to have a Tycho designation. And it's not particularly blue, and, worse, it's not strikingly ultraviolet.

I checked what Wikipedia would write about the well-know Helix Nebula, to see if it would mention the Helix's central star. Bear in mind that the Helix Nebula is at a distance of some 700 light-years (or even a bit closer), making the Medusa Nebula more than twice as distant from us as the Helix.

So you would expect the central star of the Helix Nebula to be a bit brighter, or at least as bright, as the central star of the Medusa Nebula.

Well, lo and behold. My software, Guide, identifies a stellar source at the center of the Helix Nebula, which is faint 14th magnitude. Bear in mind that TYC 776-1339-1 is an 11th magnitude star.

I tried to find out some more about the Helix's central star. Wikipedia has extremely little to say about it. But I found this paper that said that the designation of the central star of the Helix Nebula is WD 2226-210. Thanks to that designation, I could find the star on Simbad.

Well, the star is a little brighter than my software gives it credit for, but it is still much fainter (in visible light) than TYC 776-1339-1. Also note that Simbad calls WD 2226-210 a planetary nebula, but it has nothing special to say about TYC 776-1339-1. In short, TYC 776-1339-1 is just an ordinary star, nothing special.

Also note how blue WD 2226-210 really is. Its brightness in ultraviolet is 11.894, its brightness in B is 13.158, and its brightness in V is 13.524. That's actually a whopping difference between B and V. No ordinary stars are that blue!

So, Johnny, I think you and I can agree that the central star of the Medusa Nebula is indeed the very blue star that both you and I singled out.

Ann

johnnydeep wrote: ↑Fri Mar 26, 2021 6:46 pm
'Tis a shame that good ol' Sol won't go out with a bang...just a slow red bloating.

Still not sure of the difference between a nova and a supernova. I gather that a single very massive star can (always?) die in a supernova, but in the wikipedia article for supernova, it also says that a white dwarf and accrete enough matter to go supernova. From https://en.wikipedia.org/wiki/Supernova:

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star such as a white dwarf, or the sudden gravitational collapse of a massive star's core. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a binary companion through accretion, or a stellar merger. In the massive star case, the core of a massive star may undergo sudden collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics are established and accepted by the astronomical community.

But the nova article says that a nova is caused by a white dwarf accreting matter. So, when does that process result in a supernova rather than a nova?

epitalon wrote: ↑Sat Mar 27, 2021 9:29 am Hi all,

would you think that one see a crescent shape because the nebula has this actual shape or because one cannot see another part of it, symetrical.
In other words, could the nebula have a rotational symmetry that we could not see or barely see ?

In order to find an answer, I processed the image this way : add both thee color channels, filter high frequencies and then amplify brightness.
The result is here https://postimg.cc/w7QJ9Z5Z:
I can see a bulging vase shape with a mouth at its top.

alter-ego wrote: ↑Sat Mar 27, 2021 4:35 am Yes, the identified blue star is the white dwarf. The linked image shows this star much bluer than the surrounding stars. However, the ID = WD0017+283 in the image description is not correct (does not match Medusa coordinates). The correct white dwarf ID is WD0726+133. Both coordinates and magnitude listings (WD table and Aladin) point to this star. Also, the Uranometria Field Guide lists the central star magnitude = 15.9, and to no surprise, if you look up "Medusa Nebula" in Aladin, the cursor is on the central star.
https://www.pbase.com/jshuder/image/122825657

Ann wrote: ↑Sat Mar 27, 2021 8:10 am

johnnydeep wrote: ↑Fri Mar 26, 2021 6:46 pm
'Tis a shame that good ol' Sol won't go out with a bang...just a slow red bloating.

Still not sure of the difference between a nova and a supernova. I gather that a single very massive star can (always?) die in a supernova, but in the wikipedia article for supernova, it also says that a white dwarf and accrete enough matter to go supernova. From https://en.wikipedia.org/wiki/Supernova:

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star such as a white dwarf, or the sudden gravitational collapse of a massive star's core. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a binary companion through accretion, or a stellar merger. In the massive star case, the core of a massive star may undergo sudden collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics are established and accepted by the astronomical community.

But the nova article says that a nova is caused by a white dwarf accreting matter. So, when does that process result in a supernova rather than a nova?

GK Persei, nova of 1901, a classical nova. X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA."

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Artist's impression of a supernova. Source: https://www.supernovae.net.

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A classical nova is a white dwarf that gets matter dumped on it by a companion until it blows off its outer layers. (More specifically, I think it is the newly acquired mass that gets blown off.) The white dwarf survives.

A supernova type Ia is a white dwarf that gets so much matter dumped on it that it exceeds its Chandrasekhar limit of 1.4 Solar masses. It then undergoes a "deflagaration" (don't ask me) where basically all of its matter is turned into energy (I think). Nothing remains of the white dwarf afterwards, although there will be a supernova remnant where the explosion took place. Which means, I guess, that not all of the white dwarf's mass turned into energy. Maybe. Or not.

Ask the math people to give you details!

Ann