Beyond wrote:The remarkable wide field view link is in spanish--i think. I can almost see Ann going for the vacuum to try to get a better view of all that nice blue.
Hah! Yes, almost!
So why is the Iris Nebula so blue? For two reasons. Do follow the link provided by today's APOD to get
here. You'll come to a text explaining how you can create your own reflection nebula by mixing water with dilute sulfuric acid (H2SO4) and sodium thiosulfate (Na2S2O3). You'll get a solution which is at first colorless, then
light blue, then
bright blue, and then it fades and turns
white. What's happening?
Well, the mixing of the two chemicals produce a solution of tiny grains of sulphur, which however gradually grow in size. When the grains are just the perfect size, they will scatter blue light about 4.4 times more efficently than red light. Or as the article puts it:
This means that blue light with the wavelength of 450 nm is preferred against red light, which typically has 650 nm, by a factor (650/450)to the power of 4, roughly 4.4.
The article goes on:
When dimensions of the motes become comparable with the wavelength, the scattering is still selective but not so much. Approximately, intensity of the scattered light is now inversely proportional to the wavelength itself. This applies to dust particles in reflection nebulae or cigarette smoke.
I must admit that I don't quite understand the "inversely proportional to the wavelength itself" bit, but although I'm an avid non-smoker, I do know that cigarette smoke can look very blue. Apparently that is because the grains of cigarette smoke are about 450 nm in diameter, just like the wavelength of blue light. This appears to be the typical size of the grains in dusty reflection nebulae, too, which is why most reflection nebulae are blue. But not all reflection neulae. If the dust grains in the nebula become too big, the nebula will become increasingly non-blue.
However, when it comes to the Iris Nebula, there is another reason why it is blue. The central star of the nebula, HD 200775, is of spectral class B2. That means that its temperature is about 22,000 Kelvin, almost four times hotter than the Sun. Such a star will produce a lot more blue than red light, so the reflection nebula around HD 200775 has far more blue than red light to scatter.
But if you look at the Iris Nebula, you can see that not all parts of it are equally blue. This has to do with how the dust is distributed. If the dust is primarily behind the star, then the (blue) light of the star will be scattered back in our direction, making the dust look bright and blue. But if the dust is in front of the star, then the blue light of the star will have to pass through the dust in order to reach our eyes. But what happens when the blue light is trying to pass through the dust cloud? Well, it is scattered away.
Take a look at today's APOD again. You can see that there are parts of the nebula that are very blue. Here there is a lot of dust behind the star, scattering its blue light back at us. But there are also parts where there is dust in front of the star. These parts show various shades of "diluted blue", "dirty blue" or "brownish blue", because only some of the blue light manages to pass through the dust and some of it doesn't.
But a reflection nebula doesn't have to be blue in the first place. There are two reasons why it may not be blue. First, the grains of dust making up the nebula may be so large that they don't preferentially scatter blue light. Second, the star producing the light might be very cool and red, so that there is almost no blue light for the nebula to scatter. A perfect example of a non-blue (in fact yellow) reflection nebula is the nebula around Antares.
The yellow reflection nebula in this picture is produced by the yellow-orange supergiant Antares. The red nebula at lower left is an emission nebula produced by B0 star Tau Scorpii (not seen here), and the red emission nebula at far right is caused by B2 plus O9 binary star Sigma Scorpii. The large blue reflection nebula at top is caused by a small group of B-type stars, the brightest of which is B2 star Rho Ophiuchi. (The difference between a reflection and an emission nebula is that a reflection nebula just reflects the light of a star, whereas an emission nebula produces its own light.)
But let's return to today's APOD. There is a small non-blue reflection nebula in it. At upper left of the bright blue Iris Nebula you can see a small pale brown reflection nebula surrounding a small star, which appears to be bisected by dust. Why is that reflection nebula not blue? Well, since the star is clearly involved with the nebulosity it is obviously at the same distance from us as HD 200775, the star producing the blue Iris Nebula. Therefore we can compare the brightness of that small star with the brightness of HD 200775, and we can see that the small star is comparatively very small and faint. That also means that this star is cool and produces very little blue light. It is actually also possible that the dust grains surrounding this small star are larger than the dust grains surrounding HD 200775, because hot stars tend to "cook" the dust surrounding them to a smaller size.
Today's APOD shows that the Iris Nebula is just a small part of a large dust complex. The general color of the dust is brown, because the dust filters away most of the light that tries to penetrate it and only lets through a bit of red and orange light. Interestingly, you can see that the thickest dust pockets are the reddest as well as the darkest parts of the dust. Where the dust is not so thick it takes on a less red shade of brown, almost with a tinge of dull green. Where stars peek through the thick dust, they take on a dust-reddened hue themselves.
There are some faint hints of pink in the dust, particularly along the edges of the "Africa-shaped" opening in the dust just below the Iris Nebula. Other pictures of the Iris Nebula have shown the dust lanes immediately to the right and left of the star to be pink. What we are seeing are small faint pink emission nebulae, where the hydrogen in the dust cloud has become ionized by the ultraviolet radiation of the 22,000 degrees Kelvin star. This temperature is only enough to produce a little bit of emission nebulosity.
You can see that there are large dense pockets of dust scattered in the much larger, thinner dust cloud, which is oblong in shape. Note how the dust stretches away to the left of the Iris Nebula, but thins out and ends to the right of of it. We can be sure that the part of the dust cloud that gave rise to HD 200775 was once really very dark and dense. It is typical that star formation begins in one part of a dense dust cloud and then progresses, maybe, along the dust. It is possible that the dust to the left of the Iris Nebula will become sufficiently compressed and dense to give rise to more star formation.
This is the Cocoon Nebula. The picture was taken by the signature RStar, and you can read more about the picture
here.
Please note that the Cocoon Nebula, which is an emission nebula surrounding a star that must be of spectral class O, is situated at one end of a long dust feature. It is conceivable that star formation may progress along this dust feature in the future.
Ann
A Dusty Iris Nebula
