APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

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APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by APOD Robot » Wed Dec 07, 2022 5:07 am

Image NGC 7293: The Helix Nebula

Explanation: A mere seven hundred light years from Earth, toward the constellation Aquarius, a sun-like star is dying. The dying star's last few thousand years have produced the Helix Nebula (NGC 7293), a well studied and nearby example of a Planetary Nebula, typical of this final phase of stellar evolution. Combining narrow band image data from emission lines of hydrogen atoms in red and oxygen atoms in blue-green hues, it shows tantalizing details of the Helix, including its bright inner region about 3 light-years across. The white dot at the Helix's center is this Planetary Nebula's hot, central star. A simple looking nebula at first glance, the Helix is now understood to have a surprisingly complex geometry.

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Wed Dec 07, 2022 5:14 am

APOD Robot wrote: Wed Dec 07, 2022 5:07 am The white dot at the Helix's center...
Except it isn't white. Indeed, most of the stars in the image are similarly colored to the gaseous region they are seen in (in, in front of, or behind). Which shouldn't be the case, and which makes me a bit skeptical about how the color was processed in this image.
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by AVAO » Wed Dec 07, 2022 6:31 am

Chris Peterson wrote: Wed Dec 07, 2022 5:14 am
APOD Robot wrote: Wed Dec 07, 2022 5:07 am The white dot at the Helix's center...
Except it isn't white. Indeed, most of the stars in the image are similarly colored to the gaseous region they are seen in (in, in front of, or behind). Which shouldn't be the case, and which makes me a bit skeptical about how the color was processed in this image.
The text was taken from the APOD of 2021 October 14. The central star there is white.
http://www.star.ucl.ac.uk/~apod/apod/ap211014.html

Image
Image Credit & Copyright: Ignacio Diaz Bobillo

...The comparison with the APOD from 2019 February 13 is also interesting with regard to the details in the iris...

Image
https://apod.nasa.gov/apod/ap190213.html

Image Credit & Copyright: Andrew Campbell

Image
Image Credit & Copyright: Tommaso Stella

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Ann » Wed Dec 07, 2022 4:27 pm

AVAO wrote: Wed Dec 07, 2022 6:31 am
Chris Peterson wrote: Wed Dec 07, 2022 5:14 am
APOD Robot wrote: Wed Dec 07, 2022 5:07 am The white dot at the Helix's center...
Except it isn't white. Indeed, most of the stars in the image are similarly colored to the gaseous region they are seen in (in, in front of, or behind). Which shouldn't be the case, and which makes me a bit skeptical about how the color was processed in this image.
The text was taken from the APOD of 2021 October 14. The central star there is white.
http://www.star.ucl.ac.uk/~apod/apod/ap211014.html

Image
Image Credit & Copyright: Ignacio Diaz Bobillo

...The comparison with the APOD from 2019 February 13 is also interesting with regard to the details in the iris...

Image
https://apod.nasa.gov/apod/ap190213.html

Image Credit & Copyright: Andrew Campbell

Image
Image Credit & Copyright: Tommaso Stella
The central star of the Helix Nebula isn't white. We know that it must be a very hot star, because otherwise it wouldn't be able to ionize a planetary nebula. I tried to find info on the central star of the Helix Nebula on the net, but the only thing I could find was an old paper from 2001. According to that paper, the effective temperature of the ionizing star of the Helix Nebula is ~100,000 K.

I don't guarantee that the central star of the Helix is as hot as that, but it is hot, no question. Therefore, it is a blue star.

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Wed Dec 07, 2022 5:28 pm

Ann wrote: Wed Dec 07, 2022 4:27 pm The central star of the Helix Nebula isn't white. We know that it must be a very hot star, because otherwise it wouldn't be able to ionize a planetary nebula. I tried to find info on the central star of the Helix Nebula on the net, but the only thing I could find was an old paper from 2001. According to that paper, the effective temperature of the ionizing star of the Helix Nebula is ~100,000 K.

I don't guarantee that the central star of the Helix is as hot as that, but it is hot, no question. Therefore, it is a blue star.

Ann
Well, a star that is 100,000 K will appear visually white. The peak is so far off in the UV that the intensity across visual wavelengths is nearly flat.
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by orin stepanek » Wed Dec 07, 2022 6:29 pm

STScI-01EVT8HWJ754VADGE57NY9VWXP.jpg
Nice detailed picture of the Helix Nebula1 8-)
NGC7293-TommasoStella2022WEB1024.jpg
Helix Nebula; bright and beautiful! 8-)
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Ann » Wed Dec 07, 2022 6:47 pm

Chris Peterson wrote: Wed Dec 07, 2022 5:28 pm
Ann wrote: Wed Dec 07, 2022 4:27 pm The central star of the Helix Nebula isn't white. We know that it must be a very hot star, because otherwise it wouldn't be able to ionize a planetary nebula. I tried to find info on the central star of the Helix Nebula on the net, but the only thing I could find was an old paper from 2001. According to that paper, the effective temperature of the ionizing star of the Helix Nebula is ~100,000 K.

I don't guarantee that the central star of the Helix is as hot as that, but it is hot, no question. Therefore, it is a blue star.

Ann
Well, a star that is 100,000 K will appear visually white. The peak is so far off in the UV that the intensity across visual wavelengths is nearly flat.
I knew you would say that, Chris. So let me clarify: When I say that the central star of the Helix Nebula is blue, I mean that it would photograph as blue with RGB photography.

Many years ago, Adam Block took a lot of LRGB pictures of planetaries. The images are nowhere near his current standard of excellence, and I'm not at all sure I'm allowed to post them here. Nevertheless, I hope I'm allowed to give you links to the pages where he originally posted these pictures.

So here you are, here is a number of old Adam Block pictures of planetaries with blue central stars:

Abell 39

Abell 43

Abell 78

Abell 84 - yes, you can spot it!

IC 1454

Jones 1

M27, the Dumbbell Nebula - yes, you can spot it!

M57, the Ring Nebula. It just barely looks blue, but it is bluer than any other (similarly small) star in the picture.

M76, the Little Dumbbell Nebula. If you look at the largest version of the picture, you can see a tiny blue central star right next to an equally tiny yellow star.

M97, the Owl Nebula

NGC 40

NGC 1535

NGC 2022

NGC 2392, the Eskimo Nebula

NGC 2438

NGC 6058

NGC 6543, the Cat's Eye Nebula

NGC 6563

NGC 6772 - look at the largest version

NGC 6781

NGC 7094

NGC 7293, the Helix Nebula


I rest my case.

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by VictorBorun » Wed Dec 07, 2022 7:27 pm

Chris Peterson wrote: Wed Dec 07, 2022 5:28 pm
Ann wrote: Wed Dec 07, 2022 4:27 pm The central star of the Helix Nebula isn't white. We know that it must be a very hot star, because otherwise it wouldn't be able to ionize a planetary nebula. I tried to find info on the central star of the Helix Nebula on the net, but the only thing I could find was an old paper from 2001. According to that paper, the effective temperature of the ionizing star of the Helix Nebula is ~100,000 K.

I don't guarantee that the central star of the Helix is as hot as that, but it is hot, no question. Therefore, it is a blue star.

Ann
Well, a star that is 100,000 K will appear visually white. The peak is so far off in the UV that the intensity across visual wavelengths is nearly flat.
Do I get it wrong, that the colour of thermal radiation,
for a trichromatic human adapted to Sun's ~5777°K illumination,
is black to dark red to red to pale orange to white to slightly bluish:
Image

Does it, from 12 thousands °K to 100 thousands °K, lose the bluishness and get white again?

Planck's law gives a spectrum of
Image
where ν is the frequency of a photon in the thermal spectrum and T is the absolute temperature of the source of the thermal radiation.

When T = 100,000°K and the temperature of the photon hν/k_Boltzmann that a human can see is 20 times smaller than T, we can approximate Planck's law by Rayleigh–Jeans law

Image

I think this means that the spectrum function is ~T and do not change the shape (for visible range photons) at all; the star just gets brighter with no change in its hue, the same slightly bluish all the way up to T→+∞

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Wed Dec 07, 2022 7:47 pm

Ann wrote: Wed Dec 07, 2022 6:47 pm
Chris Peterson wrote: Wed Dec 07, 2022 5:28 pm
Ann wrote: Wed Dec 07, 2022 4:27 pm The central star of the Helix Nebula isn't white. We know that it must be a very hot star, because otherwise it wouldn't be able to ionize a planetary nebula. I tried to find info on the central star of the Helix Nebula on the net, but the only thing I could find was an old paper from 2001. According to that paper, the effective temperature of the ionizing star of the Helix Nebula is ~100,000 K.

I don't guarantee that the central star of the Helix is as hot as that, but it is hot, no question. Therefore, it is a blue star.

Ann
Well, a star that is 100,000 K will appear visually white. The peak is so far off in the UV that the intensity across visual wavelengths is nearly flat.
I knew you would say that, Chris. So let me clarify: When I say that the central star of the Helix Nebula is blue, I mean that it would photograph as blue with RGB photography.
A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.

The central star in today's image is not accurately represented in terms of color, regardless of its temperature, however.
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by johnnydeep » Wed Dec 07, 2022 8:43 pm

Chris Peterson wrote: Wed Dec 07, 2022 7:47 pm
Ann wrote: Wed Dec 07, 2022 6:47 pm
Chris Peterson wrote: Wed Dec 07, 2022 5:28 pm

Well, a star that is 100,000 K will appear visually white. The peak is so far off in the UV that the intensity across visual wavelengths is nearly flat.
I knew you would say that, Chris. So let me clarify: When I say that the central star of the Helix Nebula is blue, I mean that it would photograph as blue with RGB photography.
A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.

The central star in today's image is not accurately represented in terms of color, regardless of its temperature, however.
So as far as all the examples Ann posted links to that do have a central progenitor star photographing as blue in LRGB, does that imply that all of those central stars are cooler than 100,000 K?
Last edited by johnnydeep on Wed Dec 07, 2022 9:13 pm, edited 1 time in total.
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Wed Dec 07, 2022 9:09 pm

johnnydeep wrote: Wed Dec 07, 2022 8:43 pm
Chris Peterson wrote: Wed Dec 07, 2022 7:47 pm
Ann wrote: Wed Dec 07, 2022 6:47 pm

I knew you would say that, Chris. So let me clarify: When I say that the central star of the Helix Nebula is blue, I mean that it would photograph as blue with RGB photography.
A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.

The central star in today's image is not accurately represented in terms of color, regardless of its temperature, however.
So as far as all the examples Ann posted links to that do have a central progenitor star photographing as blue in LRGB, does that all of the central stars are cooler than 100,000 K?
Most very hot stars are more in the 25,000 K range, which output quite a bit more blue than they do longer wavelengths. I think that's a more typical temperature for stars in PNs.
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Ann » Wed Dec 07, 2022 9:12 pm

Chris Peterson wrote: Wed Dec 07, 2022 7:47 pm
Ann wrote: Wed Dec 07, 2022 6:47 pm
Chris Peterson wrote: Wed Dec 07, 2022 5:28 pm

Well, a star that is 100,000 K will appear visually white. The peak is so far off in the UV that the intensity across visual wavelengths is nearly flat.
I knew you would say that, Chris. So let me clarify: When I say that the central star of the Helix Nebula is blue, I mean that it would photograph as blue with RGB photography.
A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.
That's not true, Chris, and the blackbody curves prove it.

Blackbody spectra for different temperatures.png

And it is also not true because the human impression of white is not a spectrum composed of equal amounts of all wavelengths. White, to us, is mainly the blackbody curve of the Sun, which peaks in the green part of the spectrum. That makes it very different from O-type stars, which peak in the ultraviolet part of the spectrum.

Ann
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Wed Dec 07, 2022 9:18 pm

Ann wrote: Wed Dec 07, 2022 9:12 pm
Chris Peterson wrote: Wed Dec 07, 2022 7:47 pm
Ann wrote: Wed Dec 07, 2022 6:47 pm

I knew you would say that, Chris. So let me clarify: When I say that the central star of the Helix Nebula is blue, I mean that it would photograph as blue with RGB photography.
A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.
That's not true, Chris, and the blackbody curves prove it.


Blackbody spectra for different temperatures.png


And it is also not true because the human impression of white is not a spectrum composed of equal amounts of all wavelengths. White, to us, is mainly the blackbody curve of the Sun, which peaks in the green part of the spectrum. That makes it very different from O-type stars, which peak in the ultraviolet part of the spectrum.

Ann
That curve only goes up to 10,000 K. By 100,000 K the intensity is nearly flat across the entire visible spectrum. Which pretty much defines white.
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by johnnydeep » Wed Dec 07, 2022 9:32 pm

Chris Peterson wrote: Wed Dec 07, 2022 9:18 pm
Ann wrote: Wed Dec 07, 2022 9:12 pm
Chris Peterson wrote: Wed Dec 07, 2022 7:47 pm

A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.
That's not true, Chris, and the blackbody curves prove it.


Blackbody spectra for different temperatures.png


And it is also not true because the human impression of white is not a spectrum composed of equal amounts of all wavelengths. White, to us, is mainly the blackbody curve of the Sun, which peaks in the green part of the spectrum. That makes it very different from O-type stars, which peak in the ultraviolet part of the spectrum.

Ann
That curve only goes up to 10,000 K. By 100,000 K the intensity is nearly flat across the entire visible spectrum. Which pretty much defines white.
Is that really true? I used the calculator at https://www.spectralcalc.com/blackbody_ ... ckbody.php to create this graph of radiation of a BB at 100,000 K from 350 nm to 800 nm:

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Wed Dec 07, 2022 9:36 pm

johnnydeep wrote: Wed Dec 07, 2022 9:32 pm
Chris Peterson wrote: Wed Dec 07, 2022 9:18 pm
Ann wrote: Wed Dec 07, 2022 9:12 pm

That's not true, Chris, and the blackbody curves prove it.


Blackbody spectra for different temperatures.png


And it is also not true because the human impression of white is not a spectrum composed of equal amounts of all wavelengths. White, to us, is mainly the blackbody curve of the Sun, which peaks in the green part of the spectrum. That makes it very different from O-type stars, which peak in the ultraviolet part of the spectrum.

Ann
That curve only goes up to 10,000 K. By 100,000 K the intensity is nearly flat across the entire visible spectrum. Which pretty much defines white.
Is that really true? I used the calculator at https://www.spectralcalc.com/blackbody_ ... ckbody.php to create this graph of radiation of a BB at 100,000 K from 350 nm to 800 nm:

Look at the wavelength scale. That is micrometers, not nanometers! Here is the visible spectrum, 0.3 to 0.7 um. That is not going to look blue! Not to your eyes, not to a camera.
_
100000K.png
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by johnnydeep » Wed Dec 07, 2022 9:57 pm

Chris Peterson wrote: Wed Dec 07, 2022 9:36 pm
johnnydeep wrote: Wed Dec 07, 2022 9:32 pm
Chris Peterson wrote: Wed Dec 07, 2022 9:18 pm
That curve only goes up to 10,000 K. By 100,000 K the intensity is nearly flat across the entire visible spectrum. Which pretty much defines white.
Is that really true? I used the calculator at https://www.spectralcalc.com/blackbody_ ... ckbody.php to create this graph of radiation of a BB at 100,000 K from 350 nm to 800 nm:

Look at the wavelength scale. That is micrometers, not nanometers! Here is the visible spectrum, 0.3 to 0.7 um. That is not going to look blue! Not to your eyes, not to a camera.
_
100000K.png
Ah - thanks for catching that. I had a suspicion I was misinterpreting something incorrectly!
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by VictorBorun » Thu Dec 08, 2022 1:23 am

Chris Peterson wrote: Wed Dec 07, 2022 9:36 pm
johnnydeep wrote: Wed Dec 07, 2022 9:32 pm
Chris Peterson wrote: Wed Dec 07, 2022 9:18 pm
That curve only goes up to 10,000 K. By 100,000 K the intensity is nearly flat across the entire visible spectrum. Which pretty much defines white.
Is that really true? I used the calculator at https://www.spectralcalc.com/blackbody_ ... ckbody.php to create this graph of radiation of a BB at 100,000 K from 350 nm to 800 nm:

Look at the wavelength scale. That is micrometers, not nanometers! Here is the visible spectrum, 0.3 to 0.7 um. That is not going to look blue! Not to your eyes, not to a camera.
_
100000K.png
Well, I went to that online plotter, required 3000°K, 5777°K, 12,000°K, and 100,000°K in the visible range only.
Look, 100,000°K is a little more bluish that 12,000°K:
100000°K..jpg
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Ann » Thu Dec 08, 2022 5:17 am

VictorBorun wrote: Thu Dec 08, 2022 1:23 am
Chris Peterson wrote: Wed Dec 07, 2022 9:36 pm
johnnydeep wrote: Wed Dec 07, 2022 9:32 pm

Is that really true? I used the calculator at https://www.spectralcalc.com/blackbody_ ... ckbody.php to create this graph of radiation of a BB at 100,000 K from 350 nm to 800 nm:

Look at the wavelength scale. That is micrometers, not nanometers! Here is the visible spectrum, 0.3 to 0.7 um. That is not going to look blue! Not to your eyes, not to a camera.
_
100000K.png
Well, I went to that online plotter, required 3000°K, 5777°K, 12,000°K, and 100,000°K in the visible range only.
Look, 100,000°K is a little more bluish that 12,000°K:
100000°K..jpg
Thanks, Victor. Of course a 100,000 K star is bluish in the visible range.

However, Chris is semi-right. At higher temperatures still, I believe that the visible range of the electromagnetic curve becomes relatively flat. The gamma ray pulsar Geminga must be extremely hot, but I remember reading, after its visible spectrum had been detected, that the visible spectrum was quite flat.

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Thu Dec 08, 2022 5:22 am

Ann wrote: Thu Dec 08, 2022 5:17 am
VictorBorun wrote: Thu Dec 08, 2022 1:23 am
Chris Peterson wrote: Wed Dec 07, 2022 9:36 pm
Look at the wavelength scale. That is micrometers, not nanometers! Here is the visible spectrum, 0.3 to 0.7 um. That is not going to look blue! Not to your eyes, not to a camera.
_
100000K.png
Well, I went to that online plotter, required 3000°K, 5777°K, 12,000°K, and 100,000°K in the visible range only.
Look, 100,000°K is a little more bluish that 12,000°K:
100000°K..jpg
Thanks, Victor. Of course a 100,000 K star is bluish in the visible range.

However, Chris is semi-right. At higher temperatures still, I believe that the visible range of the electromagnetic curve becomes relatively flat. The gamma ray pulsar Geminga must be extremely hot, but I remember reading, after its visible spectrum had been detected, that the visible spectrum was quite flat.

Ann
I'm not seeing that. Of course, there are very, very few stars that reach 100,000 K.
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Ann » Thu Dec 08, 2022 5:43 am

Chris, I showed you all those LRGB images of planetary nebulas, where you could clearly see that the central star was bluer than the other stars in the picture. Are you saying that those images are lies?

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Ann » Thu Dec 08, 2022 7:46 am

Chris, if you argue that a 100,000 K star is white because its output of light in the visible part of the spectrum is (nearly) flat, than you can't possibly argue that Vega, at about 9602 ± 180 K, is a white star:

Spectrum of Vega David Haworth.png
Spectrum of Vega. Source: David Haworth.

For comparison, you may want to look at the spectrum of Betelgeuse:

Spectrum of Betelgeuse Christophe Pellier.png
Spectrum of Betelgeuse. Image: Christophe Pellier.


I'd say Vega is almost as blue as Betelgeuse is red.

Vega doesn't stand out in our night skies, because so many of the brightest stars that we can see are similar in color to Vega. Few bright stars are as red as Betelgeuse, so Betelgeuse stands out and looks golden-yellow to our eyes.

Many years ago, I and an interested teenager showed Vega to a lot of people and asked them about its color. First we pointed out Vega in the sky, and then we showed it to them through a telescope. The response was always the same. Vega looked white to the unaided eye, but strikingly blue-white through a telescope.

(And Betelgeuse doesn't look red through a telescope. It looks golden-yellow. It doesn't photograph as red, either. As you may have noticed.)

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Thu Dec 08, 2022 1:58 pm

Ann wrote: Thu Dec 08, 2022 7:46 am Chris, if you argue that a 100,000 K star is white because its output of light in the visible part of the spectrum is (nearly) flat, than you can't possibly argue that Vega, at about 9602 ± 180 K, is a white star:
Do I argue that?
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Ann » Thu Dec 08, 2022 2:22 pm

Chris Peterson wrote: Thu Dec 08, 2022 1:58 pm
Ann wrote: Thu Dec 08, 2022 7:46 am Chris, if you argue that a 100,000 K star is white because its output of light in the visible part of the spectrum is (nearly) flat, than you can't possibly argue that Vega, at about 9602 ± 180 K, is a white star:
Do I argue that?
Perhaps not. :wink:

But I know I have argued, time and again, that Vega is not a white star, and I have been told, again and again, that it is white, because its fluxes are the same in the blue, the yellow-green and the red channel. That is so obviously not true.

Anyway, Vega has been used by the astronomical community as the very definition of a white star. I find that so very wrong.
Bob King of Sky & Telescope wrote about Vega:

Both its proximity and luminosity (40 times greater than the Sun) make it one of the brightest stars in the sky. It shines at magnitude +0.03 and for years was the standard reference for the magnitude-zero point used to calibrate the magnitude scale on photoelectric devices.
I take that to mean that Vega was defined as the "zero-point star", the one that is neither blue or red, but is the perfect example of whiteness, against which all other stars can be compared to see how much their fluxes deviate from this definition of white light.

As a consequence, the bluest stars in the sky have B-V indices of about -0.3 (compared with Vega, that is), whereas the reddest stars have B-V indices of about +2 (or in rare cases, for so called carbon stars, occasionally up to +5 or more), compared with Vega.

I find the scale skewed.

Ann
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VictorBorun
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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by VictorBorun » Thu Dec 08, 2022 11:04 pm

Ann wrote: Thu Dec 08, 2022 2:22 pm I take that to mean that Vega was defined as the "zero-point star", the one that is neither blue or red, but is the perfect example of whiteness, against which all other stars can be compared to see how much their fluxes deviate from this definition of white light.

As a consequence, the bluest stars in the sky have B-V indices of about -0.3 (compared with Vega, that is), whereas the reddest stars have B-V indices of about +2 (or in rare cases, for so called carbon stars, occasionally up to +5 or more), compared with Vega.

I find the scale skewed.

Ann
There is another redskew of the stellar colour hue range as well, an objective one:

thermal spectrum can be absolutely red

(at 1000°K. We have yet to find a way to make a colour snapshot of such quasi-stellar object, a small red dwarf or a young brown dwarf. So far we can search for them in IR; they are drowned in the noise in the visible range),

but not violet or even deep indigo, only slightly more bluish, as T→+∞ and the
Planck's law → the Rayleigh–Jeans law, B ~ ν²/T, a spectrum shape that is not dependent on T at all and therefore cannot get more bluish however you rise T.

I wonder what is the source of the myth of a Flat Thermal Spectrum at 100,000°K.
Definitely it's not what the Rayleigh–Jeans law says at all.

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Re: APOD: NGC 7293: The Helix Nebula (2022 Dec 07)

Post by Chris Peterson » Fri Dec 09, 2022 12:30 am

VictorBorun wrote: Thu Dec 08, 2022 11:04 pm I wonder what is the source of the myth of a Flat Thermal Spectrum at 100,000°K.
Definitely it's not what the Rayleigh–Jeans law says at all.
What myth of flatness?
Chris

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