Starship Asterisk*

by **starsurfer** » Tue Dec 17, 2013 12:52 pm

Chris Peterson wrote:
Boomer12k wrote:Not even a good comparison...my thinking....not the same type of object....Tarantula is not a Planetary Nebula....though further away by far...is more massive, contains more stars, is a total region in itself, etc....should really be compared to a Planetary Nebula of the same distance....
I think the comparison was meant to be of interest to imagers, who have some idea about the brightness of the Tarantula. The comparison was purely one of how many photons are received at the camera, nothing to do with the intrinsic brightness of the objects. So the types of objects aren't really important, only that they are extended and of similar scale.

A more useful comparison would have been between the separate Ha and OIII.

by **NGC3314** » Mon Dec 09, 2013 2:49 pm

Chris Peterson wrote: The reason why an 18,000 K blackbody is generally considered the most blue (and why most people see it that way) is that this is the temperature where you have the most "blue" (450 nm) energy in comparison with longer visual wavelengths. As you get hotter, the energy across the visual spectrum gets flatter, with relatively more energy at longer wavelengths compared to blue (you always have more in total at the short end, it's just that the distribution gets flatter).

I have to differ. The form of the blackbody function shows that its slope across the visible (or any other range longward of the peak) saturates toward high temperatures, so that arbitrarily hot blackbodies are no redder than cooler ones. This plot uses the standard IDL astro library function Planck, and compares blackbodies at 10,000, 25,000, 50,000, and 100,000 K. They are normalized to the same observed flux at 10,000 Angstroms = 1 micron to show the optical slopes more clearly. This continues to steepen to shorter wavelengths (become more blue, if I take the shorthand astronomer's parlance and sidestep perceptual nuances), albeit with diminishing returns at high temperatures.

by **Chris Peterson** » Mon Dec 09, 2013 2:37 pm

starsurfer wrote:When Don Goldman processes his images, the colour balance for the stars is based on a G2V star.

Yes, but keep in mind that this still only leads to approximately accurate star colors. It also fails when you have a mixture of continuum and emission sources, since the nature of the filters used requires different color balance techniques on the two. So you end up doing a lot of masking or other isolation processes to separate stars from nebula, or you allow one or the other to have different colors.

Finally, it is critically important to keep in mind the difference between color and hue. Simply changing the brightness of an image changes all the colors. Most astronomical images have their saturation pushed in a way that shifts the colors outside of what our eyes would see, even if they were more sensitive. Accurate hues does not mean accurate colors.

by **geckzilla** » Mon Dec 09, 2013 2:09 pm

by **Ann** » Mon Dec 09, 2013 8:10 am

Starsurfer wrote:
When Don Goldman processes his images, the colour balance for the stars is based on a G2V star.

David Malin, too, used the color of the Sun as the definition of white for his color pictures.

Ann

by **starsurfer** » Mon Dec 09, 2013 7:17 am

geckzilla wrote:
starsurfer wrote:
geckzilla wrote:Starsurfer: Of course it shows it to be blue. After you do the color balancing, things which are more strongly emitting shorter wavelengths have nowhere else to go. And colors presented are most certainly influenced by saturation and sharpening, especially when a star is in the middle of a faint planetary nebula that someone is paying special attention to and possibly giving it selective treatment over the rest of the stars in the image. Anyway, I'm not saying that they aren't bluer, just that things are not necessarily blue. The stars in the middle of planetaries are definitely special stars.
For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.
Incorrect. Raw => Color balance => Saturation increase
The color balance is where the blue is coming from.

When Don Goldman processes his images, the colour balance for the stars is based on a G2V star.

by **Chris Peterson** » Sun Dec 08, 2013 2:48 pm

starsurfer wrote:For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.

"Blue in the raw unprocessed data" isn't a meaningful concept. "Blue" is a physiological response. The raw data contains multiple channels each containing different spectral data, modified by filter widths, filter shapes, and sensor sensitivity.

From the image alone, we really don't have enough information to draw any accurate conclusions about the hue this star would appear given sufficient intensity (the color would change simply by changing the intensity).

What we do know, based on physical understanding, is that in the human colorspace a star this temperature should appear a cool white- that is, white with a higher intensity at the blue end of the spectrum. If there is any significant intervening dust (unlikely, but possible) the star could be slightly reddened- that is, it could drift towards a warmer white.

by **geckzilla** » Sun Dec 08, 2013 2:21 pm

starsurfer wrote:
geckzilla wrote:Starsurfer: Of course it shows it to be blue. After you do the color balancing, things which are more strongly emitting shorter wavelengths have nowhere else to go. And colors presented are most certainly influenced by saturation and sharpening, especially when a star is in the middle of a faint planetary nebula that someone is paying special attention to and possibly giving it selective treatment over the rest of the stars in the image. Anyway, I'm not saying that they aren't bluer, just that things are not necessarily blue. The stars in the middle of planetaries are definitely special stars.
For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.

Incorrect. Raw => Color balance => Saturation increase
The color balance is where the blue is coming from.

by **starsurfer** » Sun Dec 08, 2013 12:56 pm

geckzilla wrote:Starsurfer: Of course it shows it to be blue. After you do the color balancing, things which are more strongly emitting shorter wavelengths have nowhere else to go. And colors presented are most certainly influenced by saturation and sharpening, especially when a star is in the middle of a faint planetary nebula that someone is paying special attention to and possibly giving it selective treatment over the rest of the stars in the image. Anyway, I'm not saying that they aren't bluer, just that things are not necessarily blue. The stars in the middle of planetaries are definitely special stars.

For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.

by **Chris Peterson** » Sun Dec 08, 2013 3:18 am

johnemac wrote:since the final phase of the planetary nebula phenomenon for the star itself
is collapse to the white dwarf stage (immediately following the generation of
the planetary nebula), how can the white dwarf be 10 billion years old, and
the nebula be only 20 thousand ?? am i missing something ?

The star formed some 10 billion years ago. It's only been in its white dwarf phase a short time.

by **johnemac** » Sun Dec 08, 2013 1:05 am

since the final phase of the planetary nebula phenomenon for the star itself
is collapse to the white dwarf stage (immediately following the generation of
the planetary nebula), how can the white dwarf be 10 billion years old, and
the nebula be only 20 thousand ?? am i missing something ?

by **Anthony Barreiro** » Fri Dec 06, 2013 9:16 pm

Chris Peterson wrote:
Ann wrote:
Why do stars look their bluest at 18,000 K? And if so, why did 35,000 K Lambda Orionis look so strikingly blue to me?
I can't say why things do or do not look certain colors to you. While it's possible your eyes are physiologically different from those of most people, I suspect it's simply how your brain processes color. Certainly, color perception is far more complex than just the chemistry of the retina would suggest. Different people certainly see color differently. I'll say that I've never seen a star that I'd call strikingly blue. In fact, I'd describe stars that are classified as blue as visually being cold white.

The reason why an 18,000 K blackbody is generally considered the most blue (and why most people see it that way) is that this is the temperature where you have the most "blue" (450 nm) energy in comparison with longer visual wavelengths. As you get hotter, the energy across the visual spectrum gets flatter, with relatively more energy at longer wavelengths compared to blue (you always have more in total at the short end, it's just that the distribution gets flatter).

I've been enjoying this discussion of color. It sort of brings things full circle to think that very hot stars putting out lots of ultraviolet radiation give us beautiful red emission nebulae!

by **Ann** » Fri Dec 06, 2013 7:54 pm

The reason why an 18,000 K blackbody is generally considered the most blue (and why most people see it that way) is that this is the temperature where you have the most "blue" (450 nm) energy in comparison with longer visual wavelengths. As you get hotter, the energy across the visual spectrum gets flatter, with relatively more energy at longer wavelengths compared to blue (you always have more in total at the short end, it's just that the distribution gets flatter).

Thanks, Chris. That makes sense.

Ann

by **Chris Peterson** » Fri Dec 06, 2013 7:28 pm

Ann wrote:
Why do stars look their bluest at 18,000 K? And if so, why did 35,000 K Lambda Orionis look so strikingly blue to me?

I can't say why things do or do not look certain colors to you. While it's possible your eyes are physiologically different from those of most people, I suspect it's simply how your brain processes color. Certainly, color perception is far more complex than just the chemistry of the retina would suggest. Different people certainly see color differently. I'll say that I've never seen a star that I'd call strikingly blue. In fact, I'd describe stars that are classified as blue as visually being cold white.

The reason why an 18,000 K blackbody is generally considered the most blue (and why most people see it that way) is that this is the temperature where you have the most "blue" (450 nm) energy in comparison with longer visual wavelengths. As you get hotter, the energy across the visual spectrum gets flatter, with relatively more energy at longer wavelengths compared to blue (you always have more in total at the short end, it's just that the distribution gets flatter).

by **Chris Peterson** » Fri Dec 06, 2013 7:21 pm

Ann wrote:(Since many of these pastels are very pale, we might just say that most of the cubes in this work of digital art are white, for all intents and purposes. To save time and effort, we might just call them "white".

Or not.) :wink:

Not. But neither, perhaps, would we call them red, or green, or blue.

In reality, almost all stars are pretty accurately considered white with various casts. They are yellowish-white, reddish-white, or bluish-white. The only exception I know of is carbon stars, which are deeply red... but that's because of absorption processes that make them deviate far from classical blackbodies.

by **Ann** » Fri Dec 06, 2013 7:15 pm

: Click to view full size image

Cubed Pastels by Amanda Moore.

Finally, in defense of pastels...

(Since many of these pastels are very pale, we might just say that most of the cubes in this work of digital art are white, for all intents and purposes. To save time and effort, we might just call them "white".

Or not.)

Ann

by **geckzilla** » Fri Dec 06, 2013 7:08 pm

Starsurfer: Of course it shows it to be blue. After you do the color balancing, things which are more strongly emitting shorter wavelengths have nowhere else to go. And colors presented are most certainly influenced by saturation and sharpening, especially when a star is in the middle of a faint planetary nebula that someone is paying special attention to and possibly giving it selective treatment over the rest of the stars in the image. Anyway, I'm not saying that they aren't bluer, just that things are not necessarily blue. The stars in the middle of planetaries are definitely special stars.

Ann: I try not to make judgements on what color things really are. I just look at them as being bluer or redder relative to one another. Just from looking at the image you linked it looks like there is a lot of brightening going on but I can't tell exactly what went on just by looking at the end product. When he writes that he was heavy handed that probably just means he just had to brighten a very dark image significantly along with the usual color balancing. I won't presume that anyone does or does not look at B-V indexes when doing their images. There are probably hundreds of ways to go about processing and everyone has their own unique and subtle take on things.

by **Ann** » Fri Dec 06, 2013 6:26 pm

Chris, you wrote:
there is a temperature where we see the most blue, which is about 18,000 K

Why do stars look their bluest at 18,000 K? And if so, why did 35,000 K Lambda Orionis look so strikingly blue to me?

On a few occasions, before I knew so much about blue stars, I "discovered" a few of them because they looked so strikingly blue in what was apparently RGB (or HaRGB) images. One example is Beta Cephei. I saw a color picture of constellation Cepheus, and Beta Cephei was just so much bluer than any other star in the picture. I looked it up, and it was indeed blue, with a color index of -0.20 (Johnson) or -0.23 (Tycho). On another occasion I saw a color picture of IC 405 and IC 410 and the asterism "The Leaping Minnow" in between, and one the stars was just so much bluer than the others. This turned out to be IQ Aurigae, with a color index of -0.167 (Johnson) or -0.194 (Tycho), even though it is classified as a peculiar A-type star.

In this picture Beta Cephei is at about 9 o'clock (or 9.30) and looks rather unremarkable. In this picture, IQ Aurigae looks blue, but not obviously bluer than one other star in the picture. But in the pictures I saw, the difference was very striking.

Geckzilla or Chris, do you think that whoever took those pictures that alerted me to the very blue color of Beta Cephei or IQ Aurigae was aware of the very blue B-V indexes of these two stars and therefore manipulated their pictures so that the blue star would look that much bluer?

Ann

by **starsurfer** » Fri Dec 06, 2013 5:55 pm

geckzilla wrote:
Chris Peterson wrote:Certainly, the star is very blue in this image. But that is an imaging artifact. The color we see here is nowhere near what we'd expect for the object itself.
To further illuminate why this is an imaging artifact (specifically, a result of processing), I have attached an image of the red channel (cropped to just the nebula) so that anyone can see the dark black ring around the central star as well as some other small ones due to sharpening and saturation adjustments which adversely affect the integrity of the objects in favor of aesthetics. Note that the adjustments were applied locally to just the area of the nebula.

goldman_abell7_red_only.jpg

I like the point you make geckie. However the colour is unrelated to any saturation and sharpening, the colour is derived from the information present in the R(ed)G(reen)B(lue) data.
Another image of Abell 7 by Jim Shuder shows the star to be blue: http://www.pbase.com/jshuder/image/131285894
Another image that isn't available online anymore also showed the central star to be blue.
The RGB data is taken specifically for star colours.

by **Ann** » Fri Dec 06, 2013 5:42 pm

Geckzilla, thanks for your thoughtful post. Very interesting. Let me start by saying that I have certainly never believed that a star can be a saturated shade of blue. Of course I realize that every hot star that emits a lot of blue light must also emit respectable amounts of green and red light, and can therefore never be saturated. I don't believe that a picture that shows us an intensely blue star of any kind is really "true color". What I do believe is that hotter stars should emit a greater proportion of blue light compared with their total light output than cooler stars, and it should be at least theoretically possible for photographic or other techniques to detect the proportion of blue compared to other colors in a star's light output and show us the hot star's "degree of blueness" as different pastel shades of blue.

David Malin, who used glass plates for his color photography, has written about how he photographed a faint planetary nebula, Ack 277-03.1, and found that the central star was subtly bluer than any other star in the field.

Malin wrote in his book, A View of the Universe,

A much fainter but similarly beautiful example is seen in Fig 7.19. This is listed as Ack 277-03.1 (after Agnes Acker, the cataloguer) and is so faint that I had to use rather heavy-handed photographic amplification to extract a color picture, which is why the picture is so grainy.

In your opinion, geckzilla, would you say that the central star of Ack 277-03.1 looks bluer than the other stars because of stretching or other kinds of "manipulation"? David Malin himself wrote that he used "rather heavy-handed photographic amplification" to extract a color picture in the first place. If he hadn't done that, would the central star of Ack 277-03.1 have looked white instead of blue in his picture? (But the slightly yellow-looking stars would still have looked a bit yellow, because yellow color isn't a product of stretching or manipulation?)

Geckzilla, you wrote:
At this point, I assert that the blue is always getting a boost somehow for wideband RGB images (narrowband is another story). This happens either during processing or elsewhere.

The way I understand it, modern filters are typically less sensitive to blue light than to red or green light. The situation was quite the opposite when photography was young, when the photographic plates were primarily sensitive to blue and ultraviolet light. The dark sky and sea in this old negative might have shown a brilliantly bright blue summer sky and sea. (Be aware that the picture is large before you click on it.) In this old picture of the Rho Ophiuchi region, taken by Edward Emerson Barnard in 1892, blue B0-type star Tau Scorpii looks just as bright as red M1-type supergiant Antares, even though the V magnitude of Antares is 1.064 and the V magnitude of Tau Scorpii is 2.794.

What I'm saying is that the sensitivity of the equipment must be taken into account before we discuss if objects emit a little or a lot of certain colors.

Ann

by **neufer** » Fri Dec 06, 2013 5:33 pm

Chris Peterson wrote:
Ann wrote:
According to Jim Kaler, the photosphere of the Sun is 5800K. That means that the Sun is yellow? Because yellow is a factor that exists in nature independent of humans, and 5800K is yellow? Even if it doesn't look that way to us?
By classification, the Sun is a yellow star. Visually, it is white. Because "color" means something different in these two cases.

Using Wien's law to approximate the thermal spectrum:

only normalized such that the peak intensity is B=1
and it is located at a frequency of ν=1:

B(ν)= ν³exp[3(1-ν)]

gives a weak peak negative curvature of:

d²B(ν)/dν² = -3

However, when this is Rayleigh scattered the normalized Rayleigh scattered spectrum:

R(ν)= ν⁷exp[7(1-ν)]

has a much sharper peak negative curvature of:

d²R(ν)/dν² = -7

Which is why one can observe an actual
yellow reflection nebula of a red giant (e.g., Antares)
but not an actual yellow Sun.

by **Chris Peterson** » Fri Dec 06, 2013 3:22 pm

Ann wrote:The Sun is classified as yellow because of its temperature, but that doesn't mean that it is visually yellow?

To me that suggests that color is an absolute quality, separate from human perception.

No, it just means that color has different meanings. Heck, the term is even applied to sound!

Visual color is a perceptual phenomenon. But the term is also applied to stars based on their temperature. Stars in a certain temperature range are called yellow. That doesn't mean they appear yellow visually (although at one end of the classification range, they do).

It is like saying that color is an aspect of the universe measured in temperature. According to Jim Kaler, the photosphere of the Sun is 5800K. That means that the Sun is yellow? Because yellow is a factor that exists in nature independent of humans, and 5800K is yellow? Even if it doesn't look that way to us?

By classification, the Sun is a yellow star. Visually, it is white. Because "color" means something different in these two cases.

by **Chris Peterson** » Fri Dec 06, 2013 3:16 pm

Ann wrote:
Chris wrote:
What is important in this case is that we know a star with a temperature as high as this one- given that it's an evolved white dwarf- will have a blackbody spectrum that makes it appear visually white.
To me that is mumbo jumbo, totally incomprehensible. The white dwarf has a very high temperature, so it will have a blackbody spectrum that makes it optically white?

I'm not sure why this should be confusing. If you look at the spectrum of what we call cool white light, its power peaks in the blue and drops steadily towards the red. Typically, for every unit of blue energy (450 nm) we would have 0.5 units of green (550 nm) and 0.2 units of red (650 nm). That is, a spectrum that is very blue-biased in terms of energy distribution is visually white.

A blackbody produces a spectrum that has a spectral peak that shifts with temperature. We see the typical color progression - red, orange, yellow, warm white, blue-white, cool white as the peak moves through our visual range with increasing temperature. But once that peak moves into the UV (at very high temperatures), just the long wavelength tail of the curve passes through our range of sensitivity, highest in the blue and descending slowly towards red. In short, we have precisely the same energy distribution as we see for a cool white light source.

That's why very hot bodies look white. Although we never see a saturated blue from blackbodies, there is a temperature where we see the most blue, which is about 18,000 K. Above that, the blue starts desaturating toward white.

by **geckzilla** » Fri Dec 06, 2013 2:00 pm

Ann wrote:
geckzilla wrote:
Chris Peterson wrote:Certainly, the star is very blue in this image. But that is an imaging artifact. The color we see here is nowhere near what we'd expect for the object itself.
To further illuminate why this is an imaging artifact (specifically, a result of processing), I have attached an image of the red channel (cropped to just the nebula) so that anyone can see the dark black ring around the central star as well as some other small ones due to sharpening and saturation adjustments which adversely affect the integrity of the objects in favor of aesthetics. Note that the adjustments were applied locally to just the area of the nebula.

goldman_abell7_red_only.jpg
Geckzilla, I don't question your skill an dyour ability to tell whether or not a picture has been somewhat manipulated for aesthetic or other reasons. But should we conclude, then, that the central stars of planetary nebulae don't typically stand out in LRGB images?

Take a look at Adam Block's collection of planetary nebulae in RGB. (These images are rather old, and I don't know how proud Adam is that I call attention to them. Forgive me, Adam.)

Note that very many of these planetary nebulae have strikingly blue central stars. Check out, for example, PK205+14.1: The Medusa Nebula. The color of the central star is the only thing that gives it away.

Ann

Ok, full disclosure before you read my following post. A lot of these assertions are based on my experience with Hubble data and they are anecdotal. This is not something I have any formal training or education in. I hope if I am wrong that someone with such education will correct it (Hi, Chris!).

Those [Adam Block's PNs] all look like they are emphasized by similar processing. If you look at the red channel there is usually a black ring around the central star which indicates that the saturation has been increased substantially. I used to use saturation adjustments a lot when I first started processing but when I realized that it is destructive to the integrity of the image, I began avoiding it. Nowadays, if I decide it is necessary, I make a note of it in the description. If a saturation adjustment isn't to blame then some kind of sharpening algorithm is. I still use a couple of sharpening algorithms and I know that sometimes unnatural rings or halos can appear which can have a similar effect of creating color artifacts even though I do my best to avoid it.

Anyway, with a saturation modification you are seeing what was formerly a very slight blue turn into a very blue blue. Furthermore, in my experience with Hubble data, even though oftentimes the blue wavelengths have received more exposure time, I still have to increase the signal significantly to make the colors look balanced. I assume that you look at Adam Block's photos and see that he has given all of his channels a 1:1:1 and you think this means all of the colors are getting fair treatment. Right? Sort of.

What's happening during processing is something that you are not fully understanding, which is the reason I suggested months ago that you attempt processing in order for you to attain a less superficial understanding. At this point, I assert that the blue is always getting a boost somehow for wideband RGB images (narrowband is another story). This happens either during processing or elsewhere. For Adam Block's Abell 39, it seems to happen at the filter level. I took a look at his AstroDon Gen II filter set spectra. As you can see, blue is significantly wider. This means that during processing less adjustment (or none at all) needs to be made to the color balance but it's still happening at a lower level. Another feature mentioned for the AstroDon Gen II filter set is "Better color rendition for galaxies based upon color theory" which, as you should understand, is a human, subjective way of looking at colors and deciding that some combinations simply look better than others. This is awesome, but it is important to understand that this is an aesthetic technique. I mean, science and math is involved, of course, but there is bias for aesthetics.

What do things look like before blue gets a boost? Various shades of red, orange, yellow, and then up to white. The white or less yellow ones can turn blue once blue gets its signal increase. This is why it is more important to say that a star, relative to its surrounding stars, emits shorter wavelength light than its neighbors, and NOT that the star is truly blue. It might be blue. But the only thing you can gather for certain is that some things are redder and some things are bluer, not that they are necessarily blue. Blue things might actually be white and red things might actually be infrared depending on the imaging technique, but you knew about the infrared thing.

Anyway, that is my explanation and how I understand that an apparently blue star is actually white. Hopefully I didn't land too far off the mark.

by **Nitpicker** » Fri Dec 06, 2013 11:37 am

DavidLeodis wrote:The forecasts in the time line that is brought up through the "years hence" link are very interesting, of which I have selected these few:-

298,000 AD - Voyager 2 is approaching Sirius.
6,800,000 AD - DNA from the 21st century has completely decayed.
225,000,000 AD - Sol completes one galactic year.
600,000,000 AD - Total solar eclipses are no longer possible on Earth.
1,000,000,000 AD - Earth is becoming too hot to support liquid water.
100,000,000,000,000 AD - The end of the stellar era.
10,000,000,000,000,000,000,000,000,000,000,000,000 AD - The degenerate era of the universe.
10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 AD - The black hole era of the universe.
Beyond 10 to the power 100 - The dark era of the universe.

Shame I won't be there to see the end of the Universe, unless there is a Milliways restaurant at the end of the Universe.

I lost interest at 1,000,000,000 AD. Last drinks.

Starship Asterisk*

APOD: Planetary Nebula Abell 7 (2013 Dec 05)

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