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IC 4603: Reflection Nebula in Ophiuchius

Explanation: Why does this starfield photograph resemble an impressionistic painting? The effect is created not by digital trickery but by large amounts of interstellar dust. Dust, minute globs rich in carbon and similar in size to cigarette smoke, frequently starts in the outer atmospheres of large, cool, young stars. The dust is dispersed as the star dies and grows as things stick to it in the interstellar medium. Dense dust clouds are opaque to visible light and can completely hide background stars. For less dense clouds, the capacity of dust to preferentially reflect blue starlight becomes important, effectively blooming the stars blue light out and marking the surrounding dust. Nebular gas emissions, typically brightest in red light, can combine to form areas seemingly created on an artist's canvas. Photographed above is the central part of the nebula IC 4603 surrounding the bright star SAO 184376 (actually 8th magnitude) which mostly illuminates the blue reflection nebula. IC 4603 can be seen near the very bright star Antares (1st magnitude) toward the constellation of Ophiuchus.

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Hello Mods,
http://apod.nasa.gov/apod/ still points to yesterday's APOD.
To access this photo I had to visit http://apod.nasa.gov/apod/ap140723.html

Yup, we're aware of it and so are the editors. Unfortunately, there's nothing we can do about it from here.

The image is linking OK now.

But can someone help me with the text? I'm confused. It speaks of "cool, young stars" and then says "as the star dies".

A SIMBAD search for SAO 184376 describes it as pre- main sequence and classification B2III/IV .

( A separate query: How does a pre main sequence star reach the temperature implied by a B2 classification?)

Astrostudio.org refers to the star as a "Blue Giant", which confused me even more.

I checked http://en.wikipedia.org/wiki/Blue_giant#Evolution, but they confirmed that

Stars found in the blue giant region of the HR diagram can be in very different stages of their lives, but all are evolved stars that have largely exhausted their core hydrogen supplies.

I feel that I ought to be able to work out what is intended by the text, ("hot" young star rather than "cool" young star: star "ages" rather than "dies"?), but I have had to admit that I can't. Can anyone help me?

Many thanks
Margarita

Awesome!

:---[===] *

Good questions, Margarita.

There are two kinds of cool young stars. One kind is the small, low-mass young star. Such low-mass stars are born in the Rho Ophiuchi region. In the upper right quadrant of today's APOD there is yellow-orange light penetrating the thick dust. At least one source (near a particularly dark cloud) is undoubtedly the light of (low-mass) star formation. (Small yellow-orange stellar objects are scattered throughout most of the picture, although many of these sources might easily be background objects strongly reddened by dust.)

Low mass stars, M- and K-type dwarfs, don't create an appreciable amount of dust, as far as I know. But medium-mass stars die moderately young (say, at the age of 6-7 billion years) as Mira variables, and they produce a lot of dust as they die. Even younger, more massive stars, that started out as blue main sequence stars but have evolved into cool red giants or supergiants, produce a lot of dust towards the end of their lives. (On the other hand, still very hot evolved young stars also produce a lot of dust at the end of their lives.) Note that today's caption talks about "large, cool, young stars", so it clearly refers to evolved giants.

Perhaps today's caption refers to the fact that dust is scattered when a star dies violently in a supernova explosion. But even stars that die more gently to produce planetary nebulas create and scatter dust.

Note that supernovas themselves not only scatter but apparently create dust:

http://www.sciencealert.com.au/news/20141107-25863.html wrote:
"At day 868, the last time Gall’s team observed the supernova, the amount of dust had increased to 0.04 of the Sun’s mass, or 830 Earth masses,” says Cowen at Nature Magazine. "If the increased dust production continues, in 20 years SN 2010jl will have produced the equivalent of half the Sun’s mass in dust particles, similar to the amount observed in the widely observed supernova SN 1987A.”
This means that if there were a number of supernovae active around the time our Universe was still young, and they were producing star dust at the same rate as SN 2010jl and supernova SN 1987A, they could certainly have provided all the dust we know was present in the formation of our Universe.

As for the confusion of what is a pre-main sequence star and what is an evolved giant, no star can, to the best of my understanding, be a pre-main sequence star and an evolved giant at the same time. A star whose classification is B2III/IV is an evolved, aging giant and can't possibly be a stellar fledgling that has barely begun its life as a star.

However, a blue giant star which is old enough to have exhausted its core hydrogen has one thing in common with a pre-main sequence hot star. Both are larger than a main sequence star of the same spectral classification. Therefore a pre-main sequence star may be misclassified as giant star. An example is NGC 6727 and new-born, possibly still pre-main sequence star HD 176386. You can see it here (at 6 o'clock) with another star totally engulfed in blue nebulosity. At the upper right of the binary baby star there is some very dark dust and low-mass star formation. (Here is a large picture of the entire area of star formation around HD 176386.)

It is clear that HD 176386 has either just entered the main sequence, or else it is still a pre-main sequence star. But my software Guide classifies it as a B9 IV star. Obviously this classification is wrong. So I'd say that HD 147889 (SAO 184376), just like HD 176386, is a pre-main sequence star which is larger than a main sequence star (precisely because it hasn't yet entered the main sequence) and therefore has been misclassified as a giant star.

A pre-main sequence star can certainly have reached a temperature corresponding to a B2-type star. As for HD 147889 (SAO 184376) it is deeply buried in dust in such a way that it most certainly looks as if the star is in the process of being born in there. A pre-main sequence classification seems very reasonable to me. The star is deeply reddened, but it could well be quite hot.

Ann

I am confused by the phrase: ...effectively blooming the stars blue light out...

The text refers to the formation of dust which is still discussed among astropysicists. In my view, the text correctly describes the current understanding if the one typo is corrected: young is replaced by old . Atmospheres of stars late in the stellar evolution produce substantial amounts of dust, as do supernovae.
Please note that the text does not claim that the dust affecting the radiation in the image is actually formed near the stars that are displayed.
Btw.: APOD still rocks, so many nice images of our rich universe, congrats.

One thing is clear, this is a beautifully blue nebula and the overall Rho Ophiuchi region is one of my favourite areas of the sky!

Humbleastrophysicist wrote:The text refers to the formation of dust which is still discussed among astropysicists. In my view, the text correctly describes the current understanding if the one typo is corrected: young is replaced by old . Atmospheres of stars late in the stellar evolution produce substantial amounts of dust, as do supernovae.
Please note that the text does not claim that the dust affecting the radiation in the image is actually formed near the stars that are displayed.
Btw.: APOD still rocks, so many nice images of our rich universe, congrats.

Mistakes do happen. I emailed the editors for you.

Humbleastrophysicist wrote:The text refers to the formation of dust which is still discussed among astropysicists. In my view, the text correctly describes the current understanding if the one typo is corrected: young is replaced by old . Atmospheres of stars late in the stellar evolution produce substantial amounts of dust, as do supernovae.
Please note that the text does not claim that the dust affecting the radiation in the image is actually formed near the stars that are displayed.
Btw.: APOD still rocks, so many nice images of our rich universe, congrats.

Thanks for your comment, HumbleAstrophysicist. I have always pictured that the dust in this part of the sky was produced primarily in the atmospheres of stars that are younger than our Sun -- some much younger. These stars have already entered the red giant phase, for example, which our Sun has not. Yes, this occurs late in the evolutionary process, and so in that sense "old" is a better adjective, but "young" is not wrong in the other sense. I will therefore change this word to be neither "young" nor "old" but rather "evolved." Given recent caching anomalies, it may take some time for this change to show up.

- RJN

Thank you for the clarification. From what Humbleastrphysicist and RJN have said, my understanding is that the
star SAO 184376 illuminates dust which was produced by other more evolved stars. Is my understanding correct?

I'd be grateful for advice about the age of SAO 184376.


SIMBAD (link at bottom) classifies the star SAO184376 aka HD147889 as B2III/IV and described it as pre main sequence.

Ashland studio, (using the XHIP catalogue) gives the same spectral class but calls it a Blue Giant. (Link below) Which puts it the other end of the stellar lifetime...


Here are the search links.
• SIMBAD search link: http://simbad.u-strasbg.fr/simbad/sim-i ... 7%2056.572

• Ashland Studio search link: http://www.astrostudio.org/xhip.php?hip=80462

Many thanks
Margarita

MargaritaMc wrote:Thank you for the clarification. From what Humbleastrphysicist and RJN have said, my understanding is that the
star SAO 184376 illuminates dust which was produced by other more evolved stars. Is my understanding correct?

Young dust is mostly made up of silicates, mainly oxides. They can only be produced in an oxygen rich environment, which suggests a star well past its hydrogen fusion stage- primarily cool red giants (which also have the Si present). Some of the dust may be carbon particles, which probably comes from carbon stars, which are similar to red giants. So in both cases, what we could reasonably call stars well along their evolutionary path.

Old dust is different- larger particles that have grown as they encountered cold dust clouds over a long period. But the raw materials in either case are still from evolved stars or supernovas.

The dark arc feature in this interesting image seems like it could be at the top of a Pillar of Creation or globule type object. There are ripple-like features, particularly towards the bottom left and right corners. It's all very intriguing.