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Observing >> Deep Sky Observing

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Otto Piechowski
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Reged: 09/20/05

Loc: Lexington, KY
Helix 11/10/12 new
      #5513747 - 11/10/12 10:12 PM

I saw the Helix and its central star this evening. It was the second time I have seen it, but it was more nicely framed this time.

Conditions: 12 inch dob, with an OIII filter, 24mm Baader Hyperion giving a FOV of about 1 degree. This combination placed the Helix in the middle 50% of the eyepiece; the focusing sweet spot of the 24mm Baader in an F5 system.

The sky was transparent, ambient temperature, unsteady, clear, suburban sky (ZULM magnitude 4.5).

With the OIII, the nebula was easily seen and traceable, but no detail could be seen.

Without the OIII, I could not distinguish the Helix from the background glow; i.e. I could not see it; well, maybe a hint.

With a 12mm plossl I was easily able to see the magnitude 13.9 central star.

I am curious...the central star of planetary nebula; what are they? Are they still doing nuclear fusion, or are they a glowing remnant?


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Helix 11/10/12 new [Re: Otto Piechowski]
      #5513764 - 11/10/12 10:21 PM

White dwarfs that have shed their outter layers revealing their hot core. LOL, I think thats it.

Pete


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Otto Piechowski
Pooh-Bah


Reged: 09/20/05

Loc: Lexington, KY
Re: Helix 11/10/12 new [Re: azure1961p]
      #5513768 - 11/10/12 10:27 PM

Pete,

Is the white dwarf just a glowing cinder...or is it still fusing something?

Otto


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Otto Piechowski
Pooh-Bah


Reged: 09/20/05

Loc: Lexington, KY
Re: Helix 11/10/12 new [Re: Otto Piechowski]
      #5513785 - 11/10/12 10:46 PM

Another question I have regards the OIII aspect of planetary nebulae. I have read that 90 to 95% of the visible light emitted by planetaries is in the OIII line. Why is this?

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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Helix 11/10/12 new [Re: Otto Piechowski]
      #5513911 - 11/11/12 01:27 AM Attachment (16 downloads)

Otto Piechowski asked:


Quote:

I am curious...the central star of planetary nebula; what are they? Are they still doing nuclear fusion, or are they a glowing remnant?




They are the cores of red giant stars that have run out of fuel and puffed off their outer atmospheres to form the planetary nebulae. The cores generally stop fusing material and have contracted, shrinking considerably in size under their immense mass, so they continue to be hot and shine, although they are fading somewhat. They produce a lot of ultraviolet light which excites the gas in the planetary nebulae's expelled shells, making them glow. The central stars will eventually become a class of star known as a white dwarf which will shine faintly for billions of years, long after the planetary nebula has faded into invisibility. Clear skies to you.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Helix 11/10/12 new [Re: David Knisely]
      #5515930 - 11/12/12 12:36 PM

The exposed stellar core illuminating the planetary is already by that point a white dwarf, being typically about the size of Earth. Fusion has ceased (or at least soon will?), and the object will cool over many billions of years to become an invisible 'black' dwarf.

The reason for the dominance of O-III emission in most planetaries is the combination of oxygen produced as a byproduct of fusion, the very high UV flux from the hot stellar core, and the just right density of the gas.

The planetary phase is extremely brief, cosmically. After only a few tens of thousands of years the nebula will puff up and fade to invisibility.


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Otto Piechowski
Pooh-Bah


Reged: 09/20/05

Loc: Lexington, KY
Re: Helix 11/10/12 new [Re: GlennLeDrew]
      #5516075 - 11/12/12 02:10 PM

Pete, David, thank you very much for the information.

Would you mind, be able to take on one last question...how is an OIII filter made? What causes it to allow only one thin line of the electromagnetic spectrum to pass. The other day I took my good 5mw green laser and pointed it through the OIII....as you guess, nothing came through.

How is that done?

Otto


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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Helix 11/10/12 new [Re: GlennLeDrew]
      #5517217 - 11/13/12 02:38 AM

Quote:

The exposed stellar core illuminating the planetary is already by that point a white dwarf, being typically about the size of Earth. Fusion has ceased (or at least soon will?), and the object will cool over many billions of years to become an invisible 'black' dwarf.

The reason for the dominance of O-III emission in most planetaries is the combination of oxygen produced as a byproduct of fusion, the very high UV flux from the hot stellar core, and the just right density of the gas.

The planetary phase is extremely brief, cosmically. After only a few tens of thousands of years the nebula will puff up and fade to invisibility.




Actually, I refrained from calling the central stars "white dwarfs" because a few astronomers have called at least some of them "pre-white dwarf stars", as if the initial stage where a planetary nebula is forming means that the cores have not quite reached a true white dwarf stage. The central stars are certainly small and are no longer fusing, as they do not contain enough mass to fuse the Carbon in their interiors. As such, the cores have contracted and heat up even more as they do, reaching very high surface temperatures (sometimes over 170,000 K) and putting out a lot of ultraviolet light to excite the atoms in the expelled shells of gas around the star.

As for the OIII emission, that is not necessarily due to the gas being made of more oxygen. In fact, the shells of the planetary nebulae probably contain a lot more hydrogen than Oxygen, since they came from the outermost layers and atmosphere of the precursor star. The OIII emission is due to the excitation of the electrons in oxygen atoms in the planetary shells. They reach a "metastable" state and then decay into a lower energy state to emit the OIII lines, but *only* if they are "left alone" somewhat. We don't see the OIII lines in the laboratory because the gas in Oxygen discharge tubes is still too dense with the atoms colliding frequently. However, the gas is rarified enough in the planetary nebula that the "forbidden" OIII emission process can occur without the gas atoms being knocked around by collisions enough to cause the electrons in the Oxygen atoms to be ejected from that metastable state before they can decay and produce the OIII lines. The OIII lines appear so strongly to the eye because they are near the eye's peak sensitivity. Indeed, with some planetary nebulae, their H-alpha line is nearly as strong (or occasionally stronger) as the OIII lines are. We just don't see them as well due to the fact that our eyes don't have good sensitivity at H-alpha in low light levels. There are some exceptions to this like Campbell's Hydrogen Star or IC 418 (the "Raspberry" Nebula), but visually, for detecting the red H-alpha emission in planetaries, color imaging is often needed. Clear skies to you.


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Otto Piechowski
Pooh-Bah


Reged: 09/20/05

Loc: Lexington, KY
Re: Helix 11/10/12 new [Re: David Knisely]
      #5519659 - 11/14/12 03:04 PM

David,

Is it (ever) the case that the core temperature of stars which are no longer fusing, are hotter than the core temperature of stars which are still fusing?

Otto


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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Helix 11/10/12 [Re: Otto Piechowski]
      #5519727 - 11/14/12 03:46 PM

Quote:

David,

Is it (ever) the case that the core temperature of stars which are no longer fusing, are hotter than the core temperature of stars which are still fusing?

Otto




Well, not usually. The central temperature at the core of the sun is around 15 million Kelvin, while the temperature inside a white dwarf is only around 10 million K, and is remarkably uniform inside due to the star's internal high thermal conductivity. There may be some low-mass red dwarf stars that may have core temperatures somewhat lower than 15 million K, but usually they are at or above 10 million K which is approaching the threshold for the full proton-proton nuclear fusion cycle to be sustained. Clear skies to you.


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Otto Piechowski
Pooh-Bah


Reged: 09/20/05

Loc: Lexington, KY
Re: Helix 11/10/12 new [Re: David Knisely]
      #5520130 - 11/14/12 08:52 PM

One off-topic, but related question....is a Wolf Rayet star a star in which the nuclear-"furnace" is directly exposed to space; there is no "plasma cloud" surrounding the nuclear furnace? Is there any such object of which we know?

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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Helix 11/10/12 new [Re: Otto Piechowski]
      #5520524 - 11/15/12 03:27 AM

Quote:

One off-topic, but related question....is a Wolf Rayet star a star in which the nuclear-"furnace" is directly exposed to space; there is no "plasma cloud" surrounding the nuclear furnace? Is there any such object of which we know?




No, a Wolf-Rayet star is just a high mass high-temperature star that is rapidly losing mass due to its very intense solar winds. Some of their winds interacting with local gas clouds are even able to create their own expanding nebular shells (a few are even found in some planetary nebulae). One of the more prominent ones (HD 192163, mag. 7.4) sits in the middle of the "Crescent" nebula NGC 6888 in Cygnus which was formed from those stellar winds. Clear skies to you.


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