What color is the Milky Way, and what does this color mean?

[Ann]

A few days ago, Noel C wrote this:

I have always wondered why the dust reflects so well the blue light near a bright star, but when reflecting the collective light of all the stars (e.g., the parts of the nebula not near the bright star) it looks brown. Does that say that the ambient starlight in general has little blue in it because it has already filtered through much dust? Certainly the Milky Way looks yellowish overall...

Indeed, what color is the dust that is not near bright stars in the Milky Way? Is it brown? And if so, why is it brown?

Click to view full size image

Take a look at this image by Rogelio Bernal Andreo:


You can see M81 near the center of the image, and M82 to the right of it. NGC 3077 is to the upper left of M81, and NGC 2976 is at lower left. Please note, however, the billowing "veils" of beige-colored nebulosity. This is the so-called "Integrated Flux Nebula".




Steve Mandel, who has coined the phrase "integrated flux nebula" defines it like this:

Integrated Flux Nebulae is the term I've coined to describe high galactic latitude nebulae that are illuminated not by a single star (as most nebula in the plane of the Galaxy are) but by the energy from the integrated flux of all the stars in the Milky Way.

I will refer to the integrted flux nebula as the IFN.

If the IFN reflects the light from all the stars in the Milky Way, then we must say that the Milky Way is quite yellowish. We may compare the color of the IFN with the color of the spiral arms of M81, and we can see that the spiral arms of M81 are much bluer than the IFN. The disk of M82 is also noticeably bluer that the Milky Way's high-latitude clouds. The disk of NGC 2976 is also bluer than the IFN. Only NGC 3077 (and the bulge of M81) look yellower than our own galaxy's beige-colored clouds.

Let's repeat why galaxies are blue, and why they are red.

M87. Photo: Hubble Space Telescope.

Basically galaxies are red if practically all their stars are old, and they will be redder still if they have gone through multiple generations of star formation. Recycling gas through many generations of stars will make the gas ever more metal-rich, and metal-rich stars are redder than metal-poor ones. Galaxies mostly made up of old metal-rich stars will be quite red. Giant elliptical galaxy M97 is a prime example. Its B-V color is 0.94, so if the B-V of the Milky Way is 0.85 (which isn't certain), then M87 is redder than the Milky Way. And really it ought to be. It would be truly surprising if it wasn't. (It is possible that M87 would have been even redder if it hadn't had its bright bluish jet, which however looks yellow in this particular image.)

M82. Credit: The Electronic Universe Project.

But galaxies can be red for other reasons, too. They can be red because they are dusty. An interesting example is M82. M82 is known as a starburst galaxy, because it has formed many large clusters of young stars near its nucleus. The starburst has led to a fantastic outflow of ionized red hydrogen from the nuclear region. But interestingly, we can't see the young blue clusters, because they are hidden behind thick curtains of dust. Because of the dust, M82 is very bright in far infrared light. The outer, unreddened disk of M82 contains no obvious young clusters, but it is rich in intermediate stars of spectral classes A and F. The outer disk of M82 has almost certainly a bluer color index than 0.85, which is why the disk of M82 looks blue in Rogelio Bernal Andreo's picture of M81, M82 and the integrated flux nebula of the Milky Way. But the overall color index of M82 is 0.87, marginally redder than the putative integrated color index of the Milky Way, 0.85.

M81. Photo: Dietmar Hager.

What about M81? It is not as dusty as M82, and we can see many young blue star clusters and pink star formation regions in its spiral arms. The arms of M81 are indeed blue, which is why they looked so blue in Rogelio Bernal Andreo's image of M81, M82 and the integrated flux nebula.






But what about the overall, integrated B-V index of M81? It is 0.93. That is almost exactly as red as the color index of M87! How can that be possible? Well, the yellow bulge of M81 is very large, very bright and very dominant, much more so than we can see in this image. Most modern astrophotography emphasizes the outer blue regions of galaxies like M81, which is why we tend to think that the blue arms are brighter than they really are.Yes, there is a lot of star formation in M81 - but there are more old yellow stars in M81 than we can imagine. (And there is a bit of dust there, too, which causes reddening.)

M31, the Andromeda galaxy. Photo: Don Goldman.

What about our neighbour, the Andromeda galaxy? Many pictures of M31, like this one, emphasize this galaxy's blue outer arms.

But this image, taken by Jean-Charles Cuillandre and the Canada-France-Hawaii Telescope, emphasizes the brown dust of the arms of the Andromeda Galaxy rather than its young blue clusters, even though they are visible here, too.






So what about the B-V index of M31, the Andromeda Galaxy? It is 0.92, very similar to M81 and M87. The Andromeda galaxy definitely has a lot of star formation, but above all it has a large, bright, yellow bulge.

NGC 253. Photo: R. Jay GaBany.

Can't we find a galaxy with the same color index as the Milky Way, 0.85 (if this is indeed the color index of the Milky Way)? There is indeed a relatively nearby galaxy whose color index is exactly 0.85, and that is NGC 253. NGC 253 is actually slightly similar to M82, in that it is a dusty, reddened starburst galaxy where the young bright clusters are mostly hidden and located close to the nucleus.

A massive star forming in the Milky Way. Photo: Spitzer and the GLIMPSE team.

Personally I doubt that the Milky Way is as dusty as NGC 253. I'm convinced that its yellow bulge is not as large as the bulge of the Andromeda galaxy. Astronomers searching the midplane of the Milky Way with infrared telescopes have found a lot of star formation there, so I don't think our galaxy has a shortage of star formation.










But it could be, nevertheless, that our galaxy is more dominated by its yellow bulge than we think it is. Perhaps the beige-colored Integrated Flux Nebula really says something important about the overall color of the Milky Way.

Finally, if the B-V index of the Milky Way really is 0.85, does that make our galaxy "typical" when it comes to spiral galaxies? Actually, I don't think so, although admittedly it depends on what you compare it with. But many well-known galaxies are somewhat bluer or a lot bluer than the value I've been comparing everything with, 0.85. M33, M51, M61, M63, M66, M74, M83, M88, M90, M94, M95, M98, M99, M100, M101: they all have bluer color indexes than 0.85. I don't think 0.85 is really "typical" for spiral galaxies, but instead I think it is a bit red.

Perhaps our galaxy is a bit "quiet" when it comes to star formation, just as it is quiet when it comes to its black hole.

Ann

[rstevenson]

Hi Ann,

Referring to Andreo's image, you say...

Please note, however, the billowing "veils" of beige-colored nebulosity. This is the so-called "Integrated Flux Nebula".

I hauled out my Digital Color Meter and had a close look. There is a cloud of rather reddish material just below M82. But all the rest of the cloudy material is about as close to gray as you can get, being less than 1% different in any of the RGB percentage values, with no discernible pattern to which of the three is slightly higher. So I'm not sure you can conclude that we have an overall beige IFN. (If you see a much browner image than I'm describing, is your monitor calibrated? Mine is. It's a Good Thing.)

Can this be determined by measurement or calculation rather than observation? Can we perhaps average the radiation from the Milky Way's stars and "project" this average onto an averaged dust cloud and actually calculate the colour of the reflected photons?

Rob

[Ann]

Good question, Rob.

Let's assume that these clouds are really as close to gray as you can get (which assumes that Rogelio managed to get a perfect color balance in his picture, which is not absolutely certain). Let's assume, too, that "perfect gray" is a faint shade of the color index of the Sun, which is 0.62.

If the color index of the integrated flux nebula is 0.62, and if it really reflects the overall color of the Milky Way, then our galaxy is blue as galaxies go. My impression is that spiral galaxies "typically" have a color index of about 0.70 to 0.75.

0.62 is not an extremely blue color index by any means. It is not as blue as M33, for example, whose color index is 0.55, and it is nowhere near as blue as M101, whose color index is as low as 0.46. But M101 is not your "typical" spiral galaxy by any means.

It is certainly not inconceivable that the color index of the Milky Way is around 0.62. If so, then we have to ask ourselves why it is so blue. Does it have more star formation than we realize? Does it have less dust? Does it have a larger than expected population of moderately blue intermediate-aged stars? Is its yellow halo smaller or fainter than we think?

Click to view full size image

NGC 157. Credit: Erica and Dan Simpson/Adam Block/NOAO/AURA/NSF

This is galaxy NGC 157. The UBV portrait of this galaxy in James D Wray's The Color Atlas of Galaxies shows that NGC 157 is dominated by an intermediate, moderately blue population, whereas its amount of very blue star formation is not remarkably high. The yellow bulge of NGC 157 is small. This stellar mix gives NGC 157 a color index of 0.61. It seems almost certain that the yellow bulge of the Milky Way is much more dominant in our galaxy than the bulge of NGC 157 is dominant in that galaxy (it isn't). To me it seems pretty clear that the Milky Way does not resemble NGC 157 very much. However, that in itself does not make it inconceivable that the color index of the integrated flux nebula of the Milky Way is close to 0.62, and that this color index indeed reflects the overall color of our galaxy.




If, on the other hand, the overall color index of our galaxy is close to 0.85, then we have to ask ourselves why our galaxy is so red. Does it have less star formation than we realize? Does it have more dust? Does it have a smaller than expected population of moderately blue intermediate-aged stars? Is its yellow halo bigger or brighter than we think?

Or perhaps we don't understand how dust reflects the overall light of a galaxy.

Finally, Rob, you asked:

Can this be determined by measurement or calculation rather than observation? Can we perhaps average the radiation from the Milky Way's stars and "project" this average onto an averaged dust cloud and actually calculate the colour of the reflected photons?

The problem is that we don't know what the Milky Way really looks like in the first place, let alone exactly how bright its old, intermediate and young populations are in relation to each other. We also don't know how much dust it has got. That's why I doubt that we can come up with an average of the radiation from the Milky Way stars, so that we can "project" it onto an average dust cloud.

But it would be a most interesting experiment, I have to say.

Ann

[neufer]

what color is the dust that is not near bright stars in the Milky Way?
Is it brown? And if so, why is it brown?

It is the color of chocolate-malt nougat topped with caramel:

The US & UK Milky Way bars feature two entirely different types of filling

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<<The Milky Way bar is a chocolate bar distributed by the Mars confectionery company. The American version of the Milky Way bar is made of chocolate-malt nougat topped with caramel and covered with milk chocolate and is very similar to the Mars bar sold in other countries. The non-US Milky Way bar, on the other hand, is not topped with caramel and is therefore similar to the American 3 Musketeers bar.

The Milky Way bar was created in 1923 by Frank C. Mars and originally manufactured in Minneapolis, Minnesota. It was the first commercially distributed filled chocolate bar. The name and taste was taken from a famed malted milk drink (milkshake) of the day – not the Earth’s galaxy, as many contend.

By 1926 it had two flavors, chocolate and vanilla, each for a nickel. In June 1932, the Milky Way bar was sold as a two piece bar, but just four years later, in 1936, the chocolate and vanilla flavors forever parted. The vanilla Milky Way bar, which was covered in dark chocolate rather than milk chocolate, was renamed the Forever Yours bar. The Forever Yours bar remained available until 1979 when it was discontinued. Due to popular demand, the Forever Yours bar returned in 1989 and renamed as the Milky Way Dark bar. In 2000, it was renamed again, creating the now-popular Milky Way Midnight bar. In 2010, the Milky Way Simply Caramel bar went on sale. This version has no nougat and is made of caramel covered in chocolate.

The European version of the bar has no caramel topping, and consists of a nougat centre that is considerably lighter than that of the Mars bar. Because of this low density (0.88 g/cm³), it floats. This rare attribute was used for an advertising campaign in Germany, France, Russia, Republic of Ireland, Poland, the Netherlands and the United Kingdom.

A popular child-oriented derivative of the Milky Way bar known as 'Milky Way Magic Stars' is also sold in the UK and consists of small aerated chocolate star shapes. Originally, every star was engraved with a different smiley face, each representing one of the magic star characters portrayed on the packaging. The characters were: Pop Star, Jess Star, Bright Star, Super Star, Happy Star, Sport Star and Baby Star.>>

[Ann]

I have to agree that the US and UK Milky Way bars are made of partly different populations, stellar or otherwise. Is it fair to say that the Milky Way in the US is yellower than the Milky Way in the UK? And if so, does that mean that the US is located in a yellow-mellower sweet spot of the bar of the Milky Way than the UK is?

Ann

[Chris Peterson]

What color is a zebra when you are standing 5 meters away from it? 10 km away? If you are a flea in its coat?

[rstevenson]

What color is a zebra when you are standing 5 meters away from it? 10 km away? If you are a flea in its coat?

I suspect the Zebra would remain black and white, no matter the distance from which it was viewed. The difficulty of determining that fact is all that changes with scale.

Rob

[Chris Peterson]

I suspect the Zebra would remain black and white, no matter the distance from which it was viewed. The difficulty of determining that fact is all that changes with scale.

Sorry, "black and white" isn't a color. It is multiple colors. I want to know what one color you would use to describe a zebra, since when galaxies are described with color (in the context of this discussion) it is a single color being considered.

[neufer]

After the riots on Tottenham High Road on August 7, 2011.

I have to agree that the US and UK Milky Way bars are made of partly different populations, stellar or otherwise. Is it fair to say that the Milky Way in the US is yellower than the Milky Way in the UK? And if so, does that mean that the US is located in a yellow-mellower sweet spot of the bar of the Milky Way than the UK is?

[c]Yes. [/c]

[bystander]

The US & UK Milky Way bars feature two entirely different types of filling

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The UK Milky Way looks more like a 3 Musketeers, also from Mars.

3 Musketeers split in two, exposing the fluffy filling.

---

[Beyond]

This thread is starting to turn into a confectioners convention. YUM But could we savor a little something that's NOT from Mars all the time

[neufer]

[size=150][b][color=#FF00FF]Ung par Tout, Tout par Ung[/color] or [color=#FF00FF]One for All, All for One[/color][/size] [url=http://en.wikipedia.org/wiki/Henry_Wriothesley,_3rd_Earl_of_Southampton][size=150]Henry Wriothesley[/size][/url][size=150], 3rd Earl of Southampton’s motto[/b][/size]

---

3 Musketeers split in two, exposing the fluffy filling.

Were you born yesterday

Don't you know that the PROPER way to break up
a 3-Musketeers is into 3 (or possibly 4) pieces.

[bystander]

Don't you know that the PROPER way to break up
a 3-Musketeers is into 3 (or possibly 4) pieces.

But you still have to start with two!

[neufer]

[youtube]http://www.youtube.com/watch?v=O3oURsGz ... re=related[/youtube]

Don't you know that the PROPER way to break up
a 3-Musketeers is into 3 (or possibly 4) pieces.

But you still have to start with two!

Not if you're the greatest with a blade

[Sam]

Don't you know that the PROPER way to break up
a 3-Musketeers is into 3 (or possibly 4) pieces.

But you still have to start with two!

Not necessarily:

Click to play embedded YouTube video.

[neufer]

Don't you know that the PROPER way to break up
a 3-Musketeers is into 3 (or possibly 4) pieces.

But you still have to start with two!

Not necessarily: http://www.lmm.jussieu.fr/spaghetti/short_story.html

But who really understands dynamic Kerchief equations

<<A kerchief (from the French couvre-chef, "cover the head") is a triangular or square piece of cloth tied around the head or around the neck for protective or decorative purposes. The popularity of head kerchiefs may vary by culture or religion, as among Amish women, Orthodox Jewish women, Muslim women, and older Orthodox Christian women.

A bandanna or bandana (from the Hindi: बन्धन bandhana, "to tie") is a type of large, usually colorful, kerchief, usually worn on the head or around the neck of a person or pet and is not considered to be a hat. Bandannas are frequently printed in a paisley pattern. Bandanas are most often used to hold hair back, either as a fashionable head accessory, or for practical purposes.

In the late 1980s/early 1990s, the Bloods and the Crips, wore red or blue paisley bandanas as a signifier of gang affiliation.>>

[BMAONE23]

Don't you know that the PROPER way to break up
a 3-Musketeers is into 3 (or possibly 4) pieces.

Absolutely, you stick the bar as far into your mouth as it will fit then bite, then hand the remainder to your 3 friends to split

[rstevenson]

Don't forget to save some for the Zebra. Poor thing doesn't know what colour he is.

Rob

[Ann]

I suspect the Zebra would remain black and white, no matter the distance from which it was viewed. The difficulty of determining that fact is all that changes with scale.

Sorry, "black and white" isn't a color. It is multiple colors. I want to know what one color you would use to describe a zebra, since when galaxies are described with color (in the context of this discussion) it is a single color being considered.

Well, I guess you would have to measure the zebra's integrated B-V index. My guess is that it would turn out close to 0.62, assuming the white parts of the zebra's coat are truly white. But if those parts are slightly yellow-white, which is likely, then the color index will be higher. Perhaps it will be close to 0.80 or even 0.85. (On the other hand, if it turns out that the black parts of the zebra's coat reflect slightly more blue than red and yellow light, then the integrated color of the zebra may be close to 0.62 after all. And maybe that's the integrated color of the Milky Way, too.)

So perhaps the integrated color of the zebra is close to the integrated color of the Milky Way, although the individual colored parts that together make up the total color index of the zebra and the Milky Way are very different.

(Of course, since the zebra does not emit its own light, its reflected color will be highly dependent on the color of the light that is illuminating it.)

A zebra...

...and the Milky Way.














Their integrated colors may be similar, at least as long as the zebra reflects the light of a B-V = 0.62 Sun.

Ann

[Ann]

[size=150][b][color=#FF00FF]Ung par Tout, Tout par Ung[/color] or [color=#FF00FF]One for All, All for One[/color][/size] [url=http://en.wikipedia.org/wiki/Henry_Wriothesley,_3rd_Earl_of_Southampton][size=150]Henry Wriothesley[/size][/url][size=150], 3rd Earl of Southampton’s motto[/b][/size]

---

3 Musketeers split in two, exposing the fluffy filling.

Were you born yesterday

Don't you know that the PROPER way to break up
a 3-Musketeers is into 3 (or possibly 4) pieces.

Click to view full size image

What are those eagles or whatever they are doing in the blue fields of the Swedish flag?











Ann

[neufer]

Edward VI (12 October 1537 – 6 July 1553)

What are those eagles or whatever they are
doing in the blue fields of the Swedish flag?

Pooping

Southampton (Wriothesley):
azure, a cross or between four falcons close argent.
Motto: ung par tout, tou par ung: one for all, all for one