How can we determine the color of a star, color index and color-color diagram
In my last posts, I have been talked about stellar magnitudes (apparent and absolute ). But now when we measure those magnitudes we measure them in a particular wavelength range. This range define the sensitivity of a filter. But now for idealism, we also have magnitude which doesn't depend on a particular range of light wavelength, rather measured under all wavelength is called bolometric magnitudes. For apparent magnitudes we use m bol and for absolute M bol. Why this is important I will explain it in a while.
Wavelength filters
Is it possible to determine the color of a star? The answer to that question is yes, it is possible. Now, the next question arrives is, how? The answer is by using filters which are sensitive in a specific range of wavelengths. Practically there is a system known as UBV, where we measure a star's apparent magnitude through three filters.
By U filter we measure the star's apparen t magnitude in ultraviolet range. The filter is centered at 365 nm and has a wide range of 68 nm
B filter is used to measure star's blue magnitude. The filter is centered at 400 nm and has a wide range of 98 nm
V filter is used for measuring the star's visual magnitude. It has it's center at 550 nm and bandwidth of 89 nm.
What is color index and bolometric correction
Until now we talked about the bolometric and filter apparent magnitude of stars. But we can measure the absolute magnitude of them but for that we need to know the distance and known as M U, M B, M V for UBV filter system. U-B color index is known as the difference between the absolute magnitude through U and B filter and for other's are likewise.U-B = M U - M B
B-V = M B - M V
You may ask me now why i am bothering with all this equations? Well, we say mathematics is the language of nature. Every symbol we use have some meaning. As we know when the brightness of a star increases, the magnitude decreases. So, what is the application of B-V? Well, for example, if we measure two star's B-V magnitude, the star which has less B-V value indicates that it is more blue than the other. Fro, the equations we can also see that this color indexes are independent of the distance. We can also measure the difference between bolometric magnitude and visual magnitud and it is known as the bolometric correction.
BC= M bol - M V
Color-color diagram
Reference
1. Carroll and Ostlie, An Introduction to Modern Astrophysics
2. Olga Atanackovic, General Atrophysics
3. Mirjana Vukicevic, Theoretical Astrophysics
Wow, there are smartness in here! :D Not just star sign reading :D :D :D
It's great to know how we determine the colour of a star !!!
Hahahaha may be one day i will write about the star sign as well :D
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