Yes Dan, I know what you are saying, you can't expect a novice to see the same detail as an experienced observer. It seems white dwarfs have a long and varied existence (avoiding saying "life") they start out hot and end up very cool. I guess PHd candidates need something to write their thesis on, redefining white dwarfs is surely fair game. Erik --- outwest112@yahoo.com wrote: From: daniel turner <outwest112@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Subject: Re: [Utah-astronomy] White dwarfs and YOU Date: Sun, 25 Jan 2009 16:11:49 -0800 (PST) Erik: I guess the status of central stars is like the status of Pluto. It depends on where you draw the lines and both points are currently under debate. This is good and even healthy. We don't yet have a good definition of "precursor white dwarves" the separates them from Wolf Rayet stars and OVI classic white dwarves. It's a minor quibble and more a study of human nature than astronomy. My take on what advanced amateurs like Dave Bernson can see that the rest of us cannot is that it's a training issue. The first time I saw the cc in M57 was an extaordinary night at Wolf Creek with an 18" dob. After that I found it easier to see it each time I spend the time to look. Dave sees things because he knows where and how to look. Jay Eades described using averted vision when he looked at M74 and he was not completely satisfied with the view. To guys like Dave and Stephen James O'meara averted vision is second nature. The more I look the more I see and the better I get at using averted vision. I'm not through learning stuff in the hobby yet. DT --- On Sun, 1/25/09, zaurak@digis.net <zaurak@digis.net> wrote:
From: zaurak@digis.net <zaurak@digis.net> Subject: Re: [Utah-astronomy] White dwarfs and YOU To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Date: Sunday, January 25, 2009, 3:37 PM 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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