by Ann » Fri Sep 27, 2013 12:51 am
geckzilla wrote:Blown out, not burnt out. It makes no sense to me to compare the "color" of the Sun (or any star) to the dynamic range of both tone and hue to an entire galaxy. What is the point of that, anyway? I could understand comparing galaxies to galaxies but galaxies to the Sun? I don't get it.
The light from galaxies comes mostly from stars. Other factors contribute, such as jets and bright nebulae. The presence or absence of dust will affect the light we can see from a particular galaxy.
Take a look at
this picture of the Milky Way. At far right, we can see a bright, yellowish area. That is a part of our galaxy's bulge. Galactic bulges are typically bright, because they are so rich in stars, and they are typically yellow, because almost all the stars that are found in bulges are old and yellow, yellower than the Sun. Therefore galactic bulges are almost always yellow.
We can see a lot of dark dust in the picture. At upper right, the dust takes on a strange orange color. In my personal opinion, the orange color here is more pronounced than it would be in most other pictures of the same area. Nevertheless, this is a clear example of "dust reddening". Dust dims and reddens the light of stars behind it. Where the dust is sufficiently thick, it blots out all optical light behind it, and the area looks black to our eyes.
Between the bright yellow bulge and the orange dust-reddened area is a length of dust which is bright from star light and nebulae. This is an example of star formation in the dust. Almost all stars are born in dusty areas, and in some patches a dust lane can be full of star formation.
We can spot a few reddish patches in this "bright dust lane". One of the prominent nebulae is the
Lagoon Nebula.
The brightest part of the "bright dust lane", some distance to the left of the Lagoon Nebula, is a whitish patch made up of mostly young stars. This is
M24, the Small Sagittarius Star Cloud. The overall color of this star cloud is probably somewhat bluer than the color of the Sun, because most of the light here comes from stars that are bluer than the Sun. Certainly this area also contains many stars that are yellower than the Sun, but the light from the blue stars is likely to be dominant.
This is my point. If most of the light from a galaxy comes from stars that are yellower than the Sun, then the overall color of that galaxy is going to be yellower than the color of the Sun. If most of the light comes from stars that are bluer than the Sun, then the overall color of that galaxy is probably going to be bluer than the Sun. Admittedly dust reddening could be a factor that makes such a galaxy less blue than we might expect it to be.
I should add that I define the light of the Sun as white. The Sun is the source of all our natural daylight, and our eyes have evolved in such a way that we see daylight (direct sunlight plus light from the clear sky, or light from an overcast sky) as neutral, or white.
The B-V index of the Sun is 0.656 ± 0.005, which, as I said, I define as white. The B-V index of the Andromeda galaxy is yellower, 0.950, comparable to well-known star
Pollux. The B-V index of M33 is 0.550, a bit bluer than the Sun, and somewhat comparable to Mirfak,
the brightest star in this picture.
Ann
[quote="geckzilla"]Blown out, not burnt out. It makes no sense to me to compare the "color" of the Sun (or any star) to the dynamic range of both tone and hue to an entire galaxy. What is the point of that, anyway? I could understand comparing galaxies to galaxies but galaxies to the Sun? I don't get it.[/quote]
The light from galaxies comes mostly from stars. Other factors contribute, such as jets and bright nebulae. The presence or absence of dust will affect the light we can see from a particular galaxy.
Take a look at [url=http://upload.wikimedia.org/wikipedia/commons/thumb/7/78/Dark_Rift_2012.jpg/1024px-Dark_Rift_2012.jpg]this picture of the Milky Way[/url]. At far right, we can see a bright, yellowish area. That is a part of our galaxy's bulge. Galactic bulges are typically bright, because they are so rich in stars, and they are typically yellow, because almost all the stars that are found in bulges are old and yellow, yellower than the Sun. Therefore galactic bulges are almost always yellow.
We can see a lot of dark dust in the picture. At upper right, the dust takes on a strange orange color. In my personal opinion, the orange color here is more pronounced than it would be in most other pictures of the same area. Nevertheless, this is a clear example of "dust reddening". Dust dims and reddens the light of stars behind it. Where the dust is sufficiently thick, it blots out all optical light behind it, and the area looks black to our eyes.
Between the bright yellow bulge and the orange dust-reddened area is a length of dust which is bright from star light and nebulae. This is an example of star formation in the dust. Almost all stars are born in dusty areas, and in some patches a dust lane can be full of star formation.
We can spot a few reddish patches in this "bright dust lane". One of the prominent nebulae is the [url=http://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/M8HunterWilson.jpg/300px-M8HunterWilson.jpg]Lagoon Nebula[/url].
The brightest part of the "bright dust lane", some distance to the left of the Lagoon Nebula, is a whitish patch made up of mostly young stars. This is [url=http://www.astrosurf.com/jwisn/m24.jpg]M24, the Small Sagittarius Star Cloud[/url]. The overall color of this star cloud is probably somewhat bluer than the color of the Sun, because most of the light here comes from stars that are bluer than the Sun. Certainly this area also contains many stars that are yellower than the Sun, but the light from the blue stars is likely to be dominant.
This is my point. If most of the light from a galaxy comes from stars that are yellower than the Sun, then the overall color of that galaxy is going to be yellower than the color of the Sun. If most of the light comes from stars that are bluer than the Sun, then the overall color of that galaxy is probably going to be bluer than the Sun. Admittedly dust reddening could be a factor that makes such a galaxy less blue than we might expect it to be.
I should add that I define the light of the Sun as white. The Sun is the source of all our natural daylight, and our eyes have evolved in such a way that we see daylight (direct sunlight plus light from the clear sky, or light from an overcast sky) as neutral, or white.
The B-V index of the Sun is 0.656 ± 0.005, which, as I said, I define as white. The B-V index of the Andromeda galaxy is yellower, 0.950, comparable to well-known star [url=http://scienceblogs.com/startswithabang/files/2010/01/Pollux-2006-01-27-120s-f20-joeyd.jpg]Pollux[/url]. The B-V index of M33 is 0.550, a bit bluer than the Sun, and somewhat comparable to Mirfak, [url=http://cdn.astrobin.com/images/2b509160-6693-4d3b-8b6f-846890367087_resized.jpg]the brightest star in this picture[/url].
Ann