Starship Asterisk*

Scorpius in Red and Blue

Explanation: Cosmic dust clouds dim the light of background stars. But they also reflect the light of stars nearby. Since bright stars tend to radiate strongly in the blue portion of the visible spectrum, and the interstellar dust scatters blue light more strongly than red, the dusty reflection nebulae tend to be blue. Lovely examples are the wispy blue reflection nebulae near bright, hot stars Pi and Delta Scorpii (upper left and lower right) in this telescopic skyscape from the head of the constellation Scorpius. Of course, the contrasting red emission nebulae are also caused by the hot stars' energetic radiation. Ultraviolet photons ionize hydrogen atoms in the interstellar clouds producing the characteristic red hydrogen alpha emission line as the electrons recombine. About 600 light-years away, the nebulae are found in the second version of the Sharpless Catalog as Sh2-1 (left, with reflection nebulae VdB 99) and Sh2-7. At that distance, this field of view is about 40 light-years across.

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Actually when you add the white starlight, it's kinda patriotic. Bucksnort Observatory Well, at least they have a UFO monitor.

It's a lovely picture!

As the "Pi and Delta Scorpii" link pointed out, Delta underwent a brightening in the year 2000, which peaked in 2003-2004. It is likely that we are still seeing some of the consequences of that outburst in this picture. For example, my software clearly shows the reflection nebulosity around Pi Scorpii, but very little nebulosity is recorded by my software near Delta Scorpii. I know that the original graphics for my software was made before the year 2000. That's why I wonder if the nebulosity around Delta has become brighter after the outburst of that star.

Ann

The blue nebula in the lower left should be called, "The Cobra Nebula"...

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Is it just me, or is the color-enhancement on a lot of recent astronomical photos *really* over-the-top?

Joe L wrote:Is it just me, or is the color-enhancement on a lot of recent astronomical photos *really* over-the-top?

You and I look at these things differently, Joe, since I love this color enhancement.

However, I must point out that the colors are real, just hard or impossible for us humans to see with the naked eye, or when we look at this part of the sky through a telescope.

I so rarely hear people complain about pictures of emission nebulae, where the emission nebulosity is red. The truth is that the emission nebulosity is red, but the wavelength of Ha light is 656 nm, which is far into the red part of the spectrum, where the sensitivity of the human retina is low. Also the surface brightness of most emission nebulae is also low. The fact is that it is impossible for humans to see the red colors of emission nebulae with the naked eye. No emission nebula in the sky, not even the Orion Nebula, looks red to the naked eye or when seen through a telescope.

What about the blue reflection nebulae? They, too, are almost always faint, and because of that their blue color is also impossible for us to see. However, many reflection nebulae are intrinsically very blue. This is particularly true of reflection nebulae surrounding hot blue stars, which is the case of the reflection nebulae in today's APOD.
What about O- and B-type stars? Are they really blue? Yes, they are, although they emit light of all the colors of the rainbow.

Here on the left you can see the spectrum of an O-type star. Blue starlight has a wavelength of about 450 nm. Red starlight has a wavelength of about 650 nm. As you can see, the O-type star emits a lot more 450 nm light than 650 nm light. Therefore the star is blue. B-type stars too, particularly B0, B1 and B2-type stars like Pi and Delta Scorpii, also emit a lot more blue than red light.

As a matter of fact, it is possible to spot the bluish color of some stars. Personally I have seen O-type star Lambda Orionis look obviously blue through a telescope, and other O- and B-type stars have looked blue to me, too. But few if any stars look obviously blue to the naked eye.

So this image does not so much enhance the color of the nebulosity as shows us what it would look like to us if the sensitivity of our eyes was greater.

Ann

When it comes to the "proper" color balance of space images, there are basically two ways to look at it. We may prefer images that are as "true color" as possible, or we may prefer images that are as "beautiful" as possible.

Each person must decide for himself or herself what is beautiful. Each person may find the current fashion in astrophotography esthetically pleasing or esthetically disappointing.

However, it is more interesting to discuss "true color".

True color might be a description of the number and wavelengths of the photons that are emitted or reflected by objects in space.

True color might be a description of the ability of the human eye and brain to detect color in light from objects in space.

True color might possibly be a description of the ability of cameras and other photon detectors to adequately describe the the number and wavelengths emitted or reflected by objects in space, although the sensitivities of various photon detectors vary so much that such a discussion becomes difficult. Bear in mind, too, that the photographer makes a decision as to how long his exposures for the image will be, often through different filters. After that, the photographer will probably want to process the image until it looks "good" or "right". Therefore it is difficult to talk about "true color" in astroimages.

To ilustrate what I mean, I want to show you two images of the emission nebula IC 1396. This one, from 2007, was Astronomy Picture of the Day on December 24 that year. It was Christmas Eve when this picture was the APOD, and perhaps in honor of the season, when red is the dominant color, the color balance of the picture is quite red. This is because the filter that the photographer used for luminance, which is often aquired through a clear filter, was an Ha filter for this image. Therefore the photographer gathered a lot of red Ha light for his picture, which is indeed quite red.

But take a look at this picture from 2008 by Michael Theusner. Clearly Michael Theusner has not used a Ha filter at all to produce his image. I wasn't able to find out if he had used any filters at all, or if his image was taken with some sort of automatic "color recording" camera. As you can see, the red nebulosity in Theusner's image is quite faint. Personally I believe that the nebula is relatively faint, and I also think that there is some faint blue reflection nebulosity near the central O-type star. That reflection nebulosity doesn't show up in Kent Wood's image. So which of these pictures is the best "true color" one?

You decide.

Ann

Hi all,
I would like to know, please:
Does a dust cloud block Halpha light ?
Does a Hydrogen cloud block blue reflection light ?
It seems to me that dust clouds around Delta Scorpii are in front of hydrogen clouds because they seem to dim the red light.... Am I right ?

Last but not least question :
why is there such a sharp border between blue and red below Pi Scorpii ?

APOD editor and astronomers, thanks for all the beauty that you share with us.

epitalon wrote:Does a dust cloud block Halpha light ?

Dust clouds attenuate all light in the visible spectrum. The attenuation is generally a bit higher for shorter wavelengths, so H-alpha will make it through just a little better than blue light. But probably not significantly so.

Does a Hydrogen cloud block blue reflection light ?

Not much. Blue light mixes with the deep red light of ionized hydrogen to produce a magenta color.

It seems to me that dust clouds around Delta Scorpii are in front of hydrogen clouds because they seem to dim the red light.... Am I right ?

I don't know. But I don't think it's a question easily answered by simply studying this image.

Last but not least question :
why is there such a sharp border between blue and red below Pi Scorpii ?

I don't think it's quite as sharp as it appears; there is obviously a shock front there, which probably creates an illusion of a very sharp boundary. But in any case, gas and dust clouds frequently have well defined edges due to shock effects and the dynamics of how they form, and how they disperse.

Ann wrote:

Joe L wrote:Is it just me, or is the color-enhancement on a lot of recent astronomical photos *really* over-the-top?

You and I look at these things differently, Joe, since I love this color enhancement.

However, I must point out that the colors are real, only just hard or impossible for us human to see with the naked eye, when we look at this part of the sky through a telescope.

Wow! Ann, thanks for your fascinating and very en-*light*-ening answer! You made my Earth-rotation, and I hope you have a great one too.

Ann wrote:To ilustrate what I mean, I want to show you two images of the emission nebula IC 1396. This one, from 2007, was Astronomy Picture of the Day on December 24 that year. It was Christmas Eve when this picture was the APOD, and perhaps in honor of the season, when red is the dominant color, the color balance of the picture is quite red. This is because the filter that the photographer used for luminance, which is often aquired through a clear filter, was an Ha filter for this image. Therefore the photographer gathered a lot of red Ha light for his picture, which is indeed quite red.

The strong red in the image has nothing at all to do with using a red filter for collecting the luminance data. The luminance channel only provides intensity, not color. The color in this image was derived from the data collected through conventional wide band red, green, and blue filters, all with equal exposure times. The reason the image is so red is because the source is an H-alpha emission nebula. We don't know what weighting factors were applied to each color channel during processing, but they probably were not extreme.

This image could be constructed without using the L data at all, and its color would look exactly the same. All that would be different would be that the signal/noise ratio would be reduced.

several APODs about constellations had the outlines when you passed the cursor over the picture -- can that return as a regular feature?

I love APOD, it's my home page -- marvelous

lsbrandon wrote:several APODs about constellations had the outlines when you passed the cursor over the picture -- can that return as a regular feature?

That feature is not provided by the APOD editors. It has occasionally been provided by forum members, but most often by the original image author. So expect it to remain hit-or-miss.

lsbrandon wrote:Several APODs about constellations had the outlines when you passed the cursor over the picture.

Today’s APOD shows a small segment of Scorpius. If constellation lines were added, then only a line between the brightest stars, Delta Scorpii en Pi Scorpii, would be shown.

Ann wrote:When it comes to the "proper" color balance of space images, there are basically two ways to look at it. We may prefer images that are as "true color" as possible, or we may prefer images that are as "beautiful" as possible.

Each person must decide for himself or herself what is beautiful. Each person may find the current fashion in astrophotography esthetically pleasing or esthetically disappointing.

...

To ilustrate what I mean, I want to show you two images of the emission nebula IC 1396. This one, from 2007, was Astronomy Picture of the Day on December 24 that year. ... which is indeed quite red.

But take a look at this picture from 2008 by Michael Theusner. ... As you can see, the red nebulosity in Theusner's image is quite faint. Personally I believe that the nebula is relatively faint, and I also think that there is some faint blue reflection nebulosity near the central O-type star. That reflection nebulosity doesn't show up in Kent Wood's image. So which of these pictures is the best "true color" one?

You decide.

Ann

I prefer Theusner's picture. Paradoxically, picking out the faint red nebulosity actually allows me to see it more clearly. I might turn up the red a notch or two, but not too much. Like a spice in a dish of food, the right amount stands out, but too much just overwhelms the palate.

This is a subjective aesthetic judgment on my part. I find it helpful to remember that every picture is a representation of reality, not reality itself -- a finger pointing at the Moon, not the Moon herself. I guess this is why I love looking through telescopes. Even though you see much less detail, much less color, etc. than in a photograph, you're seeing the actual photons that have been travelling through space for hundreds or millions of years. Fortunately we don't have to choose. I can look through my little telescope at night, and visit apod during the day.

You may possibly want to compare today's APOD with a wide-angle picture of the Milky Way and Scorpius.

This spectacular, and large, picture shows a part of the Milky Way running vertically down the picture from top to bottom. At far upper right is a red emission nebula made by the runaway star Theta Ophiuchi, as it ran into some interstellar gas. At about three o'clock you can see a bright white "star", which is in fact no star at all, but giant planet Jupiter. Below Jupiter you can see many brightly colored patches of nebulosity. The yellow patch is a rare yellow reflection nebula made by red supergiant Antares. To the left of Antares and seemingly attached to it is a moderately large red emission nebula, made by B0-type star Tau Scorpii. A smaller red emission nebula is seen to the right of the yellow Antares nebula, and this red nebula is made by B2III + O9.5V double star Sigma Scorpii. To the upper right of the small red nebula is blue reflection nebula, madeby B-type multiple star Rho Ophiuchi. To the right of the blue reflection nebula around Rho Ophiuchi is another, fainter and more elongated blue reflection nebula, centered on B-type star Nu Scorpii.

Now go down from the blue nebulosity around Nu Scorpii until you come to a fairly large, pink nebula. That is the nebula surrounding Delta Scorpii. Personally I believe that this emission nebula has become larger, or at least slightly brighter, since Delta had its outburst in 2000-2004.

To the lower left of Delta is Pi Scorpii, surrounded by a smaller pink nebula. But you can also faintly make out a blue reflection nebula to the right of Pi Scorpii.

Ann