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Color Color diagram (U-B) (B-V) how to read it?

U-B means luminosity of Ultraviolet minus luminosity of blue
B-V means luminosity of blue minus lumionsity of Visible

If so, reading the diagram for all main types of galaxies:

I can get to the conclusion that Elipticals (B-V > 0.8) are the bluest and Irregulars are the last ones of it comes to being blue (B-V = 0.4).
It kind of contradicts the fact that Elipticals are full of old red giants, that should be RED, not BLUE, doesn't it?

2. Greetings,

I believe that such plots represent the luminosities in terms of magnitudes such that more negative numbers denote greater luminosity.

Best regards,
EigenState

3. First, you should find a diagram which clearly labels the axes.

Second, please note that colors with small numerical values mean "blue", while those with large numerical values mean "red". So the diagram you posted does show irregular galaxies with blueish colors, and ellipticals with reddish.

There are much better diagrams showing the integrated colors of galaxies. See, for example, figure 1 of
Strateva et al., AJ 122, 1861 (2001).
You can grab a PDF copy of the paper from the URL below.

http://arxiv.org/abs/astro-ph/0107201

4. Originally Posted by tu144
U-B means luminosity of Ultraviolet minus luminosity of blue
B-V means luminosity of blue minus lumionsity of Visible

If so, reading the diagram for all main types of galaxies:

I can get to the conclusion that Elipticals (B-V > 0.8) are the bluest and Irregulars are the last ones of it comes to being blue (B-V = 0.4).
It kind of contradicts the fact that Elipticals are full of old red giants, that should be RED, not BLUE, doesn't it?
In these color index values, the more strongly positive ones are redder, and the negative ones in the U-B axis are bluer. The appearance of being backward is an artifact of the archaic practice of using the lower magnitude number for brighter objects.

Yes, the ellipticals are reddest, while the irregulars and the loose spirals with little or no bulge are bluest.

Edit: I see two posters got theirs up while I was typing mine.

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Ok, that makes sense. Thanks.
2 more questions:
1. B-V means blue minus visible besides blue, right?
2. Where do they use the U-B parameter? I mean what it actually says, what are the applications of it?
I understand that B-V can be used to determine how bluish something is.
The U-B says how ultravioleted something is? Where is it generally used?

6. Greetings,

Originally Posted by StupendousMan
First, you should find a diagram which clearly labels the axes.
Always good practice to do exactly that!

Best regards,
EigenState

7. Originally Posted by tu144
Ok, that makes sense. Thanks.
2 more questions:
1. B-V means blue minus visible besides blue, right?
2. Where do they use the U-B parameter? I mean what it actually says, what are the applications of it?
I understand that B-V can be used to determine how bluish something is.
The U-B says how ultravioleted something is? Where is it generally used?
If I am not mistaken, the V refers to the remainder of the visible spectrum. I am not sure how wide a band is typically used.

My hunch is that U-B originated in the early days of photospectroscopy, when the emulsions were sensitive only to the blue to ultraviolet range. They would have been nearly blind in the part of the spectrum where differences in B-V are meaningful.

8. (U-B) measures the difference between the light emitted in the U-band and the light emitted in the B-band. You can find graphs showing the passbands of these filters at

http://spiff.rit.edu/classes/phys445...rs/colors.html

The previous poster had it right: U, B and V, all in the blueish half of the optical spectrum, were devised in the 1950s, when the solid-state detectors were most sensitive to photons of high energy.

The U-band is mostly to the short side of the 4000-Angstrom break in stellar spectra, while the B-band is mostly to the long side. Therefore, the (U-B) color is a quick way to estimate the strength of the 4000-Angstrom break, which in turn is a correlated with the temperature of the star.

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Clear. Thank you.

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