Hi Kim, I think what Chuck is saying that the early stage of the very hot young white dwarfs, that is heating the gas, is really an intermediate stage between the atypical red giant and a white dwarf. We can observe the nebula until the gas from the red giant "evaporates" I agree with what you are saying, in that the conventional thought is that it is "heat" from the white dwarf that makes the nebula visible. The article is saying that once it is not a red giant it is a white dwarf. I would be interested to see an article that explains that intermediate stage more clearly. It is likely a matter of semantics. I have also observed side by side with the "2 Daves" as much as anyone, I would agree most of the time I have seen the same thing, it gets sketchier with galaxy clusters. It would be a mistake to disagree, some people get very angry if you don't. At public star parties I keep my descriptions to the obvious, to avoid same kind of disappointment that comes from over hyping comets. If someone needs to describe what you are seeing are you really seeing it. I have processed about dozen Messier logs for the Astronomical League and no one describes every object the same, the same goes for double star logs, even the non colored ones. Erik --- kimharch@cut.net wrote: From: "Kim" <kimharch@cut.net> To: "'Utah Astronomy'" <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] White dwarfs and YOU Date: Sun, 25 Jan 2009 17:36:31 -0700 I've enjoyed reading y'alls comments. As I mentioned some days ago regarding stellar colors, I rarely trust what I've read that others see - so it is with planetary nebula, central stars and other phenomena. When I've been observing side-by-side with keen observers like Dave Bernson and Dave Bennett, I've never had difficulty seeing what they see, but I admit I may not do so well on my own. I've always understood that the very hot radiation from a pn's central star was responsible for the excitement, and hence visibility, of the shell of matter surrounding the star, and that (only?) a white dwarf can generate the conditions necessary for that to occur. Hence, no white dwarf, no visible nebula. Have I been wrong? Also, since I don't have time to do some serious web-surfing right now, can anyone succinctly describe for me the internal processes that cause white dwarves to "burn" if it isn't nuclear fusion? Kim -----Original Message----- From: utah-astronomy-bounces+kimharch=cut.net@mailman.xmission.com [mailto:utah-astronomy-bounces+kimharch=cut.net@mailman.xmission.com] On Behalf Of zaurak@digis.net Sent: Sunday, January 25, 2009 4:38 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] White dwarfs and YOU Hi Chuck, I am far from expert, but that seem contrary to what Professor Ryden (Ohio State) says. She indicates the White Dwarf outlives the planetary nebula. As far as what can be seen visually, Zwicky seemed to push the envelope when he claimed he could visually detect motion in galaxy clusters. Most contemporaries thought him crazy, although time proved they do have motion. I am not sure this proves he could see the motion though. I think it easy to convince yourself you are seeing a central star after you have seen it once. I agree with Daniel that I have seen many stars in the center of many planetary nebula, with the Grim and they seem likely to be White Dwarfs. Bernson can describe many objects from memory, in detail, without looking through an eyepiece, I find myself not always seeing the detail he describes. I have noticed many at public star parties scratching their heads over details they are told they "should" be seeing. I tell people that the more you observe the more you will see, past experiences and views do matter. The 20" dob I observed with also yielded many central stars in planetary nebula. I have only observed the Messier Planetary Neb with smaller scopes, under very dark skies I recall seeing the central star of M57 with a 10-12" scope. Erik Quoting Ryden: "An asymptotic giant branch star (a red giant star which is about to run out of fuel) is not very stable. It undergoes thermal pulses during which the outer layers of the star are ejected into space. Finally, when the star totally exhasts it fuel supply, its core collapses and heats up. The core becomes a very hot white dwarf, with a surface temperature of 100,000 Kelvin, or more. The ejected outer layers, heated by the hot new white dwarf, form an emission nebula. An emission nebula of this sort - ejected gas which is being excited by a hot white dwarf - is called a planetary nebula. (This confusing name goes back to the 18th century; viewed through a small telescope, the fuzzy disk of a planetary nebula looks a bit like the fuzzy disk of a planet like Uranus. After the planetary nebula fades, the white dwarf will still be visible. White dwarfs shine because they are hot; although a white dwarf has no internal power source, it takes billions of years for a white dwarf to cool down. Thermal energy in the interior of a white dwarf is carried to the surface by conduction, then radiated away." --- chuck.hards@gmail.com wrote: From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] White dwarfs and YOU Date: Sun, 25 Jan 2009 16:06:40 -0700 The same can be said of your "central star" statement. From what I've been reading, now stellar evolutionists believe that the white dwarf stage isn't clearly reached until the planetary nebula has ceased to exist- an incredibly short lifespan of only a few tens to a hundred thousand years is typical for a planetary nebula. Central stars of visible planetaries are considered pre-white dwarf stars. The knowledge base continues to be refined as time passes. It may be that Bishop knows this but wanted to point out examples of stars unambiguously settled in the white-dwarf stage. Firmly in the stellar grave yard, with no evidence of recent nuclear reactions in the immediate environ. Bishop never made an absolute statement such as you imply; he mentioned no apertures at all. A 10" telescope isn't considered a small telescope by the general public- or with many serious amateurs. It's pretty big to me, and I've been doing this since 1968. Hair-splitting, semantic games can always be played with someone's words trying to communicate astronomy to a wide audience. Only a tiny fraction of people would jump on a soapbox and find issue with Bishop's exact choice of words. I wholeheartedly approve of Bishop's piece, and your statement does nothing to provide contrary evidence. On Sun, Jan 25, 2009 at 3:36 PM, daniel turner <outwest112@yahoo.com> wrote:
The problem comes when someone writes with a voice of authority about what "can't be done". This causes people to immediately try to prove them wrong. Bishop may regret his choice of words but it's best to remember that just because you haven't seen something, doesn't mean the other people haven't either or that next year they might be able to.
I look at dozens of planetary nebula every year, and a large fraction of these have an easily seen central star. Most can be seen with a 10" which is no longer considered a particularly large telescope anymore. The standards for an "amateur" or "backyard" or "average" telescope may continue to change change with time.
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