Why Are U-B and B-V Values Important in Studying Stars and Clusters?

[randa177]

How can I get a sense of the numbers of (U-B) and (B-V)...
if a value of U-B for a star is larger than the other what does it mean? What about B-V... and what about the negative values...

 

[ideasrule]

U, B, and V represent a star's brightness in the ultraviolet, blue, and visible ranges, respectively. Here, brightness is measured on a logarithmic scale where lower values mean higher light output. So if U-B is low or negative, that means the star is bright in the ultraviolet compared to blue. If B-V is high, that means the star is outputting very little light in blue, so the star is likely to look reddish.

For some typical values, try looking up some famous stars on Wikipedia and looking at the sidebar. Here are some nice examples:

Betelgeuse (very red)
Rigel (very blue)
Altair, Vega (somewhere in between)

 

[randa177]

U, B, and V represent a star's brightness in the ultraviolet, blue, and visible ranges, respectively. Here, brightness is measured on a logarithmic scale where lower values mean higher light output. So if U-B is low or negative, that means the star is bright in the ultraviolet compared to blue. If B-V is high, that means the star is outputting very little light in blue, so the star is likely to look reddish.

For some typical values, try looking up some famous stars on Wikipedia and looking at the sidebar. Here are some nice examples:

Betelgeuse (very red)
Rigel (very blue)
Altair, Vega (somewhere in between)

By looking at the values of the U-B and B-V that you suggested it seems that there is a corelation between U-B and B-V... for a really red star both values are large and positive, for a very blue star both values are small and negative, and for stars in between theu have smaal positive number... is this a general rule?
So If for example I want to order a set of colors from the most red to the most blue, do I compare the U-B or the B-V or balance them both?

 

[harcel]

These are 'colors' over some specified range. For stars, it mostly means that the value of any of these combinations is very high or low, it will be so for about every combination (within the convention that in X-Y A is a bluer band than Y).

This is not necessarily true though, and the colors of galaxies do reverse sometimes, due to dust, starbursts and so on. If you look at 'color color selection criteria' for galaxies you will find some plots where galaxies are 'high' (we call it blue) in some color, but 'low' (i.e. red) in another.

 

[randa177]

These are 'colors' over some specified range. For stars, it mostly means that the value of any of these combinations is very high or low, it will be so for about every combination (within the convention that in X-Y A is a bluer band than Y).

This is not necessarily true though, and the colors of galaxies do reverse sometimes, due to dust, starbursts and so on. If you look at 'color color selection criteria' for galaxies you will find some plots where galaxies are 'high' (we call it blue) in some color, but 'low' (i.e. red) in another.

I am not sure I understand what you mean, so if these values of U-B and B-V are not fixed then why are they so important in the study of stars and clusters... etc?