APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep 08)

APOD Robot · Post by **APOD Robot** » Wed Sep 08, 2010 4:01 am

NGC 4911: Spiral Diving into a Dense Cluster

Explanation: Why are there faint rings around this spiral galaxy? Possibly because the galaxy, NGC 4911, is being pulled at by its neighbors as it falls into the enormous Coma Cluster of Galaxies. If NGC 4911 ends up like most of the galaxies in the central Coma cluster, it will become a yellowish elliptical galaxy, losing not only its outer layers, but dust, gas, and its cadre of surrounding satellite galaxies as well. Currently, however, this process is just beginning. Visible in the above deep image from the Hubble Space Telescope are NGC 4911's bright nucleus, distorted spiral arms laced with dark dust, clusters of recently formed stars, unusual faint outer rings, dwarf companion galaxies, and even faint globular clusters of stars. Far in the distance many unassociated galaxies from the early universe are visible, some even through NGC 4911 itself. The Coma Cluster contains over 1,000 galaxies making it among the most massive objects known. NGC 4911 can be found to the lower left of the great cluster's center.

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bystander · Post by **bystander** » Wed Sep 08, 2010 4:08 am

Observation Deck: 'Island Universe' in the Coma Cluster

neufer · Post by **neufer** » Wed Sep 08, 2010 4:35 am

APOD Robot wrote: NGC 4911: Spiral Diving into a Dense Cluster

Explanation: Why are there faint rings around this spiral galaxy?
Possibly because the galaxy, NGC 4911, is being pulled at by its neighbors
as it falls into the enormous Coma Cluster of Galaxies.

Who breaks a butterfly upon a wheel?

http://en.wikipedia.org/wiki/The_Diving_Bell_and_the_Butterfly wrote:
Jean-Dominique Bauby:
___ <<A poet once said, "Only a fool laughs when nothing's funny">>

<<The Diving Bell and the Butterfly is a translation of the French memoir Le scaphandre et le papillon by journalist Jean-Dominique Bauby. It describes what his life is like after suffering a massive stroke that left him with a condition called locked-in syndrome. It also details what his life was like before the stroke.

On December 8, 1995, Bauby, the editor-in-chief of Elle magazine, suffered a stroke and lapsed into a coma. He awoke 20 days later, mentally aware of his surroundings but physically paralyzed with the exception of some movement in his head and eyes. The entire book was written by Bauby blinking his left eyelid, which took ten months (four hours a day). Using partner assisted scanning, a transcriber repeatedly recited a French language frequency-ordered alphabet (E, S, A, R, I, N, T, U, L, etc.), until Bauby blinked to choose the next letter. The book took about 200,000 blinks to write and an average word took approximately two minutes. The book also chronicles everyday events for a person with locked-in syndrome. These events include playing at the beach with his family, getting a bath, and meeting visitors.

The French edition of the book was published on March 6, 1997. Its total sales are now in the millions. Bauby died two days after the book was published, on March 9, 1997, of pneumonia.>>

Ann · Post by **Ann** » Wed Sep 08, 2010 4:59 am

Today's APOD is a nice image, with many interesting details. I very much enjoy being shown the globular clusters of NGC 4911, white compact little blobs of an almost uniform brightness. And I really like all the little blue contorted background galaxies which are bursting with star formation. Are they as irregular in shape as they appear to be here? Or could it be that most of them have an underlying smooth population of yellow star whose light has been redshifted clear out of the range of the filters used to produce this image?

Filters, yes. I really appreciate the fact the use of the filter that brings out the pink emission nebulae in NGC 4911. But I question the overall blue/yellow color balance. Nearly all the large-looking and therefore nearby dwarf companions of NGC 4911 are smooth in texture and show little or no evidence of star formaiton. They should be yellowish in color, though admittedly some smooth dwarf galaxies are neutral-colored or whitish. But here they all look more or less blue. Only some of the distant background galaxies, whose light has been reddened due to the expansion of the universe, look yellow here.

One interesting dwarf galaxy shows up only in the enlarged picture. It is a dwarf galaxy near the bottom of the large size image. What makes this galaxy interesting is that it has a largish smooth "envelope" surrounding a brighter, mostly smooth concentration of stars. But at the very center are some definite concentrations and condensations of bright points, which ought to be large associations of young stars. At least two such associations are definitely blue. But due to the iffy color balance of this image, do I dare to interpret these blue knots of light as truly young associations of stars, surrounded by a smooth envelope of much older stars? If so, could this dwarf galaxy be similar to NGC 2976?

NGC 2976 by SDSS. This is a g-r-i (green-red-infrared) image. The brown envelope is made up of older stars, while the blue areas correspond to young stars and the green blobs to red emission nebulae.

Ann

bystander · Post by **bystander** » Wed Sep 08, 2010 6:24 am

Ann wrote:Today's APOD is a nice image, with many interesting details. I very much enjoy being shown the globular clusters of NGC 4911, white compact little blobs of an almost uniform brightness. And I really like all the little blue contorted background galaxies which are bursting with star formation. Are they as irregular in shape as they appear to be here? Or could it be that most of them have an underlying smooth population of yellow star whose light has been redshifted clear out of the range of the filters used to produce this image?

Filters, yes. I really appreciate the fact the use of the filter that brings out the pink emission nebulae in NGC 4911. But I question the overall blue/yellow color balance. Nearly all the large-looking and therefore nearby dwarf companions of NGC 4911 are smooth in texture and show little or no evidence of star formaiton. They should be yellowish in color, though admittedly some smooth dwarf galaxies are neutral-colored or whitish. But here they all look more or less blue. Only some of the distant background galaxies, whose light has been reddened due to the expansion of the universe, look yellow here.

http://heritage.stsci.edu/2010/24/caption.html wrote:This natural-color Hubble image, which combines data obtained in 2006, 2007, and 2009 from the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys, required 28 hours of exposure time.

http://heritage.stsci.edu/2010/24/fast_facts.html wrote:Color: The image is a composite of separate exposures made by the ACS and WFPC2 instruments on the Hubble Space Telescope. Four filters were used to sample broad and narrow wavelength ranges. The color results from assigning different hues (colors) to each monochromatic image. In this case, the assigned colors are:

F814W (I), red
F673N (redshifted H-alpha), red
F606W (V), green
F450W (B), blue

orin stepanek · Post by **orin stepanek** » Wed Sep 08, 2010 1:11 pm

APOD Robot wrote: NGC 4911: Spiral Diving into a Dense Cluster

Explanation: Why are there faint rings around this spiral galaxy? Possibly because the galaxy, NGC 4911, is being pulled at by its neighbors as it falls into the enormous Coma Cluster of Galaxies. If NGC 4911 ends up like most of the galaxies in the central Coma cluster, it will become a yellowish elliptical galaxy, losing not only its outer layers, but dust, gas, and its cadre of surrounding satellite galaxies as well.

Is this a fact or speculation? Why should 4911 lose all it's dust and satellite galaxies? Is the Coma Cluster some sort of energy eating anomaly?
BTW; I did enjoy the Hubble movie in today's APOD http://www.youtube.com/watch?v=SpkrVw_E6Nw

Ann · Post by **Ann** » Wed Sep 08, 2010 3:18 pm

bystander wrote:

http://heritage.stsci.edu/2010/24/caption.html wrote:
This natural-color Hubble image, which combines data obtained in 2006, 2007, and 2009 from the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys, required 28 hours of exposure time.

I know that the Heritage people describe this image as "natural color", but that doesn't mean that I necessarily believe them.

This link shows that the Hubble people choose the color balance of their images in a rather arbitrary way:

http://heritage.stsci.edu/1999/16/n4650araw_color.html

At least they chose their color balance arbitrarily in 1999. Maybe they have gotten better since then. I don't know.

Here are a pair of Hubble Heritage spiral galaxies, whose bulges look strikingly yellow:

http://heritage.stsci.edu/1999/41/index.html

Here is a Hubble Heritage disk galaxy which somewhat resembles M104, and whose bulge is white, almost bluish:

http://heritage.stsci.edu/2001/23/index.html

I don't believe in the bluish color of the bulge of the latter galaxy. Sorry.

This is a Hubble Heritage image of galaxy M104. Although this galaxy has very little if any star formation and an unusually "red" color index, it looks decidedly bluish here. Is that natural color?

http://heritage.stsci.edu/2003/28/index.html

I don't believe it.

Ann

hstarbuck · Post by **hstarbuck** » Wed Sep 08, 2010 8:12 pm

Beautiful picture--I never get tired of Hubble. If I were to guess at the number of stars contained in the visible objects of this picture I would say.........2 trillion give or take.

bystander · Post by **bystander** » Thu Sep 09, 2010 2:34 am

Biosphere 1 split to new topic: http://asterisk.apod.com/vie ... 23&t=21018

Chris Peterson · Post by **Chris Peterson** » Thu Sep 09, 2010 2:45 am

Ann wrote:I know that the Heritage people describe this image as "natural color", but that doesn't mean that I necessarily believe them.
This link shows that the Hubble people choose the color balance of their images in a rather arbitrary way...

Do you mean arbitrary or do you mean inconsistent? It is worth remembering that the only natural color this object has is gray. When they use the term "natural color" all it means is that their intent is to approximate how this object would appear to our eyes if they were more sensitive. This can never be more than an approximation, because the color gamut of the sensor does not match that of our eyes. Things are further complicated because the images are seldom collected with the intent of producing "natural color" images. They are collected for some particular scientific project, and that project will require a set of exposures through very specific, and usually narrow band, filters. Taking such data and actually performing the transforms necessary to approximate the human color response is part science, part art. It is, in fact, somewhat arbitrary- something there is no getting around.

In comparing images for consistency it is necessary to look at the filters used for the images you are comparing. If they were different, the final color balance is going to be different as well, regardless of any intent to render the images in "natural color".

Ann · Post by **Ann** » Thu Sep 09, 2010 3:38 am

Chris wrote:

Do you mean arbitrary or do you mean inconsistent? It is worth remembering that the only natural color this object has is gray. When they use the term "natural color" all it means is that their intent is to approximate how this object would appear to our eyes if they were more sensitive.

Good question, Chris. I mean that the Hubble people tend to be inconsistent when they create what they call "natural color images". Each single image may not be that much off from some sort of standard, but the problem is that I can't see that they have such a standard. If they did, then virtually all galactic bulges would look yellowish, because virtually all galactic bulges are yellowish.

But the Hubble images don't tend to look like that. My impression is that Hubble images of spirals often have a relatively "yellow" color balance, wheras Hubble images of elliptical galaxies or galaxies dominated by their bulges tend to have a "blue" color balance. As a consequence, Hubble images of spirals dominated by their arm structure seem to have yellow bulges (which is correct), while spirals such as M104, which are dominated by their bulges, seem to have bluish bulges, which is flat wrong.

The problem is that it becomes very hard to assess the stellar content of galaxies in Hubble images when there is no standard for how color is presented in these images. To me it is obvious that the APOD of Sep 08, 2010 is insufficiently yellow. If the yellow color of bulges had been stronger in this image, then it would have been much easier to assess the stellar content of the dwarf galaxies surrounding NGC 4911. It is probable that these dwarfs are made up of an old population of metal-poor stars, which would give these galaxies a smooth texture and an intermediate color, neither really yellow nor really blue. But also, a stronger yellow component in this picture would single out the dwarf galaxy that really does seem to have star formation.

To me, the inconsistent use of color in "natural color images" of galaxies by Hubble makes it harder to "read" those images. At least for me.

Ann

Chris Peterson · Post by **Chris Peterson** » Thu Sep 09, 2010 4:13 am

Ann wrote:The problem is that it becomes very hard to assess the stellar content of galaxies in Hubble images when there is no standard for how color is presented in these images...

To me, the inconsistent use of color in "natural color images" of galaxies by Hubble makes it harder to "read" those images. At least for me.

Well, that makes sense. But as I noted, these images were, in many cases, taken with different filter sets and the resulting approximations to "natural color" can't help but to be different.

The thing is, the sort of evaluation you are attempting by looking at apparent color is much better achieved by looking at specific ratios of intensity through specific filters. That is, it is a much more quantitative approach than you are using by simply assessing the color. "Natural color" isn't usually very useful scientifically, so the production of such images in often secondary to their original intent.

In many cases, the raw images used are publicly available. Have you ever considered downloading some of these and attempting your own color versions? Sounds like a project you might enjoy.

bystander · Post by **bystander** » Thu Sep 09, 2010 5:25 pm

Ann wrote:I know that the Heritage people describe this image as "natural color", but that doesn't mean that I necessarily believe them.

I think you are reading too much into what is meant by natural color. As Chris has said many times, and as is obvious if you look at the original images, it's all gray. Natural color using wide band filters means that red filters are mapped to red, green to green, and blue to blue, with some effort applied to match the proper intensity of each. Contrast that with false color using narrow band filters, such as the Hubble palette, where the color mapping is chosen to highlight particular features, or where the image portrays colors outside the visible light band. The color balance is more a matter of aesthetics. Obviously yours aren't the same as the professionals at Hubble Heritage.

I agree with Chris, maybe you should try it yourself. The data and software is available. As NoelC explained here, achieving the right balance is not as easy, nor as straight forward as you seem to think. I would be interested in seeing your efforts.

Ann · Post by **Ann** » Fri Sep 10, 2010 12:23 am

Chris wrote:

The thing is, the sort of evaluation you are attempting by looking at apparent color is much better achieved by looking at specific ratios of intensity through specific filters. That is, it is a much more quantitative approach than you are using by simply assessing the color. "Natural color" isn't usually very useful scientifically, so the production of such images in often secondary to their original intent.

In many cases, the raw images used are publicly available. Have you ever considered downloading some of these and attempting your own color versions? Sounds like a project you might enjoy.

Thanks for the suggestion, Chris. I can see that for a person other than myself, looking at the raw black and white images might be a very good idea. Even for a person like me it may be useful, but not at all very enjoyable.

The thing is that to me, colors that are both "beautiful" (to me) and "true" (to me) are exquisitely enjoyable. I guess you might compare it with listening to music. Personally I care less for music than most people, which is not to say that I don't enjoy certain songs, certain compositions, certain voices, certain harmonies, certain sounds. But if I haven't listened to music for a whole day, I don't miss it. Color, on the other hand, is vital to me. I would never surround myself with objects and images that are not beautiful in their colors. I would certainly never put a black and white image on one of my walls, so that I would have to look at it every day.

Now consider people who truly love music. Some of them can appreciate musical notation as much as they can enjoy the music itself, because when they see the notation they immediately "translate" that notation into sounds "in their heads". Beethoven may have had that ability. Others, however, can't read musical notation at all. Swedish composer Benny Andersson, composer of all of ABBA's music, may be one of those. Others can read musical notation and actually "translate" it to sounds and actually sing that tune from the musical sheet alone, but with some effort, and they don't get the same enjoyment out of the notation as they would get out of the actual music if they could hear it played. My mother is one of those.

To me, black and white images are depressing in themselves. So are obviously false color images. Or rather, the black and white images make me "sad", while the obviously false color images make me "angry". I guess that to me black and white images are what "no music" (or even "the negation of music") would be to a person who craved music, whereas "obviously false color" is what "deliberately ugly music" would be to a person who loves "beautiful" music.

So there is no way I'm going to download "raw" Hubble images and spend hours looking at black and white or false color images. Even if I could "read" them.

Ann

owlice · Post by **owlice** » Fri Sep 10, 2010 12:55 am

Ann wrote: Others can read musical notation and actually "translate" it to sounds and actually sing that tune from the musical sheet alone

Most people can be taught to do this.

rstevenson · Post by **rstevenson** » Fri Sep 10, 2010 1:12 am

Ann wrote:... I would certainly never put a black and white image on one of my walls, so that I would have to look at it every day.

... To me, black and white images are depressing in themselves.

I enjoy your colour commentary Ann, because I've never before met (not even virtually) anyone as opinionated about colour as you are. I would hate to do without the exquisite black and white photography of Ansel Adams, Minor White, the Westons, Robert Frank, Eugene Atget, ... . Or black and white drawings in various media. I have an extraordinary drawing in pen and ink with gray water-colour wash, which I would miss greatly if I couldn't see it every day. Colour would be a distraction to my appreciation of that image. (But there's lots of colour in other images I have.)

Often, after downloading a large astronomical image for my desktop wallpaper collection, I reduce the saturation of the colours, sometimes going all the way to monochromatic. I find I prefer a black and white night sky, even when the image is of something I can't see with my naked eye.

Rob

Ann · Post by **Ann** » Fri Sep 10, 2010 1:21 am

owlice wrote:
Ann wrote: Others can read musical notation and actually "translate" it to sounds and actually sing that tune from the musical sheet alone
Most people can be taught to do this.

I agree. But most people can't be taught to consider spending a lot of time reading musical sheets a truly enjoyable experience.

Ann

Ann · Post by **Ann** » Fri Sep 10, 2010 1:24 am

Rob, thanks for your comment! I'm not questioning the ability of other people to truly, truly enjoy black and white images! (Hey, even I can sometimes spot a good black and white image when I see it. Which is not to say that I could stand having that gray splotch on my wall.)

Ann

Chris Peterson · Post by **Chris Peterson** » Fri Sep 10, 2010 1:26 am

Ann wrote:Thanks for the suggestion, Chris. I can see that for a person other than myself, looking at the raw black and white images might be a very good idea. Even for a person like me it may be useful, but not at all very enjoyable.

I think there might be some confusion. When I said the natural color is gray, I was not referring to individual grayscale images made through filters. I meant that the actual object, viewed with the human eye, either nearby or through a telescope, is gray. That is the only color any human can ever see these things. The idea behind "natural color" images is to try and approximate what these objects would look like to us if they were only brighter, or if our eyes were more sensitive. A color image of a galaxy or nebula is, fundamentally, unnatural. It is only made possible by technology.

I'm not suggesting looking at the images in B&W, only pointing out that the "color" isn't usually important to scientists. If I were making an image like this, I'd have a plan for collecting my data through a certain set of filters, which would give me intensity information I could use to assess something like star formation. I could display the image using a particular color mapping that makes it easy for people to see some particular feature of interest, like the distribution of oxygen, or of hot stars, or whatever.

Since it is fairly common for the filters used to provided a rough analog to the behavior of human retina pigments, it is often possible to construct an image that could be considered "natural color". But that can never be done very accurately, because the camera sees colors that we can't, and we see colors the camera can't. The camera and our eyes are literally operating in different color spaces. I expect this, more than anything else, explains the inconsistency seen between different images. Unless the raw data for two images is collected through exactly the same filters, and with the same exposure ratios, they aren't going to look the same- even if both have been manipulated to approximate "natural color".

Ann · Post by **Ann** » Fri Sep 10, 2010 2:55 am

Chris wrote:

I think there might be some confusion. When I said the natural color is gray, I was not referring to individual grayscale images made through filters. I meant that the actual object, viewed with the human eye, either nearby or through a telescope, is gray. That is the only color any human can ever see these things. The idea behind "natural color" images is to try and approximate what these objects would look like to us if they were only brighter, or if our eyes were more sensitive. A color image of a galaxy or nebula is, fundamentally, unnatural.

That is a reasonable postion, Chris, but when it comes to myself I don't agree.

I do think I have a good grasp of the "natural" color of galaxies. That is because during the time I spent actually observing the sky, I spent 99% of that time looking up bright stars and assessing their colors. First, second and even third-magnitude stars can look truly colored when you look at them through a 14-inch telescope, which is what I had access to.

I looked at bright colored stars over and over again, until I could truly "memorize" their colors. Betelgeuse. Arcturus. Pollux. Castor. Mirfak, Alpha Persei. Capella. Procyon. Rigel. Vega. Deneb. Altair. Mu Cephei, the "Garnet Star". Alcyone of the Pleiades. Alnilam and Alnitak of Orion's Belt. Dubhe of the Big Dipper. Alioth of the Big Dipper. And many others. Over and over again.

I have "color memories" of stars that I will never forget. The pale coppery color of Mu Cephei, unlike anything I've ever seen in any other star, and unlike anything I've ever seen in any photograph of Mu Cephei. The amazing tomato redness of V Aquilae, a carbon star, which was even more amazing because I could see some generally "white" stars in the same field of view. The pure, pure yellow color of Arcturus. The snow white color of Mirfak and the "warm white" color of Capella. The amazing blue color of Lambda Orionis, an O star, which was even more remarkable because there were some relatively "white" stars nearby. The amazingly colorful trio of 30 Cygni: the yellow-orange K-type primary, the aqua-colored A-type secondary and the strongly blue B-type third star. I'll also never forget how I noted that blue stars always seemed to have "blue haloes" around them, but non-blue stars never had those haloes. Vega always had a strong blue halo around it, and Deneb and even Altair also did, but F-type, white Mirfak never did. And the yellower stars never did, of course.

This is my point: I spent so much time learning the colors of stars that I feel confident that I know the color of stars. Therefore I am unable to think of galaxies as colorless. Galaxies are made up of stars. All stars are colored, even the F- and G-type white ones. But F- and G-type stars rarely dominate the light output of galaxies. Instead, it is the yellow light of K and M stars in galactic bulges and A-type stars in galactic arms that usually dominate the light of galaxies. In many galaxies there are also concentrated knots of star formation with bright blue B- and O-stars, pink emission nebulae, some bright yellow-orange "red" supergiants and dark dust clouds.

I know these things. I know what these kind of stars look like. To me it is impossible to think of a galaxy as grey. What I mean when I talk about natural color is that I want galaxies to look the way they have to look if they are made up of stars whose colors I know. And they are.

Let me add that I usually enjoy enhanced color, which emphasizes the color contrast between different kinds of stars, making the stellar content easier to read. That is another objection of mine against the APOD of August 08, 2010: The colors are very muted, with a very weak contrast between yellow and blue populations of stars.

Ann

Chris Peterson · Post by **Chris Peterson** » Fri Sep 10, 2010 4:06 am

Ann wrote:I know these things. I know what these kind of stars look like. To me it is impossible to think of a galaxy as grey. What I mean when I talk about natural color is that I want galaxies to look the way they have to look if they are made up of stars whose colors I know. And they are.

All I can say is that this is a remarkable skill you have. I know what star colors look like, but I can't construct in my mind what a galaxy made of billions of such stars, all having different colors, is going to actually look like. Of course, I do know what that galaxy looks like with unresolved stars: gray. Add to this glowing ionized gases, which are also gray to the eye, and the situation becomes even more confused.

To me, being able to picture the colors of a galaxy because you know what the colors of stars are is something like picturing the colors of an image on your monitor, because you know what red, green, and blue pixels look like.

Ann · Post by **Ann** » Fri Sep 10, 2010 5:59 am

Chris wrote:

All I can say is that this is a remarkable skill you have. I know what star colors look like, but I can't construct in my mind what a galaxy made of billions of such stars, all having different colors, is going to actually look like. Of course, I do know what that galaxy looks like with unresolved stars: gray. Add to this glowing ionized gases, which are also gray to the eye, and the situation becomes even more confused.

I can't picture the nebulae. I have never seen the color of nebulae; I have certainly never seen the red color of nebulae. The best I can do is picture the color I have seen in the Orion Nebula, which I have found to be grey with a very faint tint of green. I'm sure that there is a bit of green in the Orion Nebulae because of the presence of OIII and Hβ emission, both of which are blue-green in color. The green color I thought I could discern in the Orion Nebula was more blue-green than yellow-green, but in any case it was very faint. (When I looked at famous globular cluster M13 through a telescope, I found it to have an unpleasant off-white to yellow-green color, and I have never wanted to look at globular clusters again. Clearly the stars of M13 were too faint for me to detect true color in them.)

I realize that galaxies are made up of mixtures of stars, so I try to take that into account when I picture what a galaxy would look like. To make that easier, I have also looked at nearby or moderately nearby open clusters. Most just look white to me. However, the Pleiades is definitely blue overall, and the Tau Canis Majoris cluster is also slightly blue. Both the Pleiades and the Tau Canis Majoris clusters are truly blue, because all the bright stars in them are blue. As for h and chi Persei, these rich clusters look white to me, but the prominent orange stars in them "jump out at me". For all of that, the overall impression of them is definitely white.

However, h and chi Persei are reddened clusters. The blue stars in them don't have very blue color indexes, due to dust reddening. I do take dust reddening into account when I picture the overall color of galaxies. But I know that in most spiral and starforming galaxies, dust reddening is important only in some parts of the galaxy. In the case of h and chi Persei, much of the reddening is due to the fact that we are situated square in the middle of the galactic plane, and anything that we can see in a spiral arm other than our own is likely to be dust reddened from our vantage point. When I picture a galaxy, I usually picture it so that clusters like h and chi Persei would be less reddened.

I generally picture areas of a galaxy which are dominated by stars of class A, such as our own neighbourhood, as blue. After all, most of the brightest stars that we can see in the sky are stars of class A. The three bright stars of the Summer Triangle are of class A, and five out of seven stars of the Big Dipper are class A. Apart from the class A stars there are also some prominent bright stars of class B and even O. All the bright stars of Orion except Betelgeuse are of class B or O. Large numbers of nearby bright stars of the Scorpius-Sagittarius association are of class B. Prominent open clusters in our neighbourhood are dominated by stars of class A or B: M6, M7, M36, M38, M46, M47 and many others. Certainly there are yellow and yellow-orange stars in our part of the galaxy as well, but these yellow stars are simply outnumbered by the blue stars and don't contribute that much to the overall color of of the stellar light around here. Therefore I think or our galactic neighbourhood as blue, about as blue as, perhaps, the star Altair, which is not impressively but definitely blue to me.

But I also know that most of the light of our galaxy comes from its bulge and nucleus. The bulge, in particular, is particularly yellow, since it almost certainly contains extremely few young stars. This part of our galaxy may not be as yellow as Arcturus, but it will be yellow enough, almost certainly yellower than Pollux. As for the nucleus, there will be a lot of yellow light here too, but the nucleus also contains some very bright young stars which are definitely blue in color.

So it is true that I can't picture the true color of red emission nebulae - or rather, I don't strictly know what they look like because I have never seen their red color, but apart from that I don't find it extremely difficult to "read" the stellar populations of galaxies if I'm shown good true-color images of these galaxies. And as for the red nebulae, well, I just trust the color images that I'm shown. More exactly I trust the color images of nebulae if I trust the star colors that I can see in the same image.

Ann

Matt B · Post by **Matt B** » Fri Sep 10, 2010 10:39 pm

Anyone know where I can get a copy of this image, especially without the overlays??

Chris Peterson · Post by **Chris Peterson** » Fri Sep 10, 2010 10:55 pm

Matt B wrote:Anyone know where I can get a copy of this image, especially without the overlays??

Almost every APOD has a link to the original image, or the source of the original image, in its caption. This one is no exception. Follow the link labeled "above deep image".

bystander · Post by **bystander** » Fri Sep 10, 2010 10:57 pm

Matt B wrote:Anyone know where I can get a copy of this image, especially without the overlays??

APOD: 2010 Sep 08
Hubble Heritage
NASA Image of the Day

Starship Asterisk*

APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep 08)

APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep 08)

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Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep

Re: APOD: NGC 4911: Spiral Diving into a Dense... (2010 Sep