# Why Do We Use Different Pairs of Filters to Determine the Temperature of Stars?

• B
• heavystray
In summary, when determining the temperature of stars using color index, it is important to use appropriate pairs of filters based on their range of temperature. For cooler stars, R-I filters are used to allow more visible light to be measured, while B-V filters are used for hotter stars. This is because the radiation curve of blackbodies with different temperatures have different shapes, resulting in different values for B-V, U-B, and V-R. Using different pairs of filters affects the accuracy of temperature calculation because it can result in fewer photons being measured, leading to a smaller number of photons and a higher margin of error. Therefore, it is necessary to use the appropriate filters to ensure accurate temperature calculations for stars.
heavystray
Hi, in determining the temperature of stars using colour index (U-B,B-V,V-R, etc), why do we need to use the appropriate pairs of filters based on their range of temperature? (this is what i read from wiki)

For cool stars, we use R-I, and for hotter stars, we use B-V. I don't understand how using different pairs of filters make a difference because isn't radiation curve of blackbodies with different temperature have different shape and thus, the values of (B-V, U-B, V-R) will also be different? (I hope you understand what I'm trying too say)

My idea is this has to do with the star's peak wavelength...but i still don't see the justification to that...
your help would be greatly appreciated

heavystray said:
For cool stars, we use R-I, and for hotter stars, we use B-V. I don't understand how using different pairs of filters make a difference because isn't radiation curve of blackbodies with different temperature have different shape and thus, the values of (B-V, U-B, V-R) will also be different? (I hope you understand what I'm trying too say)

I'm betting that astronomers use the R-I filters instead of the B-V filters for cooler stars so that you don't block out most of the light. Cool stars don't put out much visible light in the higher frequencies, so using a bandpass in the red-infrared range makes them easier to see and measure.

Drakkith said:
I'm betting that astronomers use the R-I filters instead of the B-V filters for cooler stars so that you don't block out most of the light. Cool stars don't put out much visible light in the higher frequencies, so using a bandpass in the red-infrared range makes them easier to see and measure.

Right. Look at this plot showing the filter bandpasses with a 3000 K (red) blackbody and a 10,000K (blue) blackbody. You can see that the hot blue star doesn't have much light in the I band, and the cool red star doesn't have much in the B band.

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phyzguy said:
Right. Look at this plot showing the filter bandpasses with a 3000 K (red) blackbody and a 10,000K (blue) blackbody. You can see that the hot blue star doesn't have much light in the I band, and the cool red star doesn't have much in the B band.
View attachment 219165
but the question is, why does it affect the accuracy of temperature calculated based on the star's U-/B/B-V/V-R?? because we're just going to get either lower or higher values of B-V, V-R, R-I which correspond to temperatures. I mean for cooler stars, we already expected their U-B will be low. so, it shouldn't be a problem to determine their temperature based on the U-B value. Isn't the accuracy going to be just the same if we determine the temperature using R-I filters? (i hope you understand my question)

thanks for your answer

Drakkith said:
I'm betting that astronomers use the R-I filters instead of the B-V filters for cooler stars so that you don't block out most of the light. Cool stars don't put out much visible light in the higher frequencies, so using a bandpass in the red-infrared range makes them easier to see and measure.
yes it's easier to measure, but the question is, why does it affect the accuracy of temperature calculated based on the star's U-/B/B-V/V-R?? because we're just going to get either lower or higher values of B-V, V-R, R-I which correspond to temperatures. I mean for cooler stars, we already expected their U-B will be low. so, it shouldn't be a problem to determine their temperature based on the U-B value. Isn't the accuracy going to be just the same if we determine the temperature using R-I filters? (i hope you understand my question)

thanks for your answer

heavystray said:
yes it's easier to measure, but the question is, why does it affect the accuracy of temperature calculated based on the star's U-/B/B-V/V-R?? because we're just going to get either lower or higher values of B-V, V-R, R-I which correspond to temperatures. I mean for cooler stars, we already expected their U-B will be low. so, it shouldn't be a problem to determine their temperature based on the U-B value. Isn't the accuracy going to be just the same if we determine the temperature using R-I filters? (i hope you understand my question)

The lower the light intensity, the fewer photons there are to measure and the lower the accuracy. If you take a star, which is dim to begin with, then add a filter that removes most of the photons, you can end up with a surprisingly small number of photons. Since the photons are distributed according to a Poisson distribution, the error goes down as 1/(sqrt(N). So the more photons you have, the smaller the error.

## 1. What is the purpose of using a pair of filters?

A pair of filters is used to refine and control the light that passes through an optical system. They work together to block or transmit specific wavelengths of light, allowing for the selection of desired colors or wavelengths in an image.

## 2. How do I choose the right pair of filters for my experiment?

The choice of pair of filters depends on the specific requirements of your experiment. Consider the wavelengths of light you want to block or transmit, the intensity of light needed, and the type of optical system you are using. Consult with an expert or refer to literature for guidance on selecting the appropriate filters.

## 3. Can I use any two filters together as a pair?

No, not all filters can be used together as a pair. The filters must have complementary properties, such as one being a bandpass filter and the other being a longpass filter. Using incompatible filters can result in inaccurate or distorted results.

## 4. How do I maintain the quality of my pair of filters?

To maintain the quality of your filters, it is important to handle them carefully and avoid touching the surface of the filter. Keep them clean and free from dust or debris by storing them in a protective case when not in use. Regularly check for any damage or scratches that may affect their performance.

## 5. Can I reuse my pair of filters for different experiments?

Yes, as long as they are properly maintained and stored, a pair of filters can be used for multiple experiments. However, if the filters become damaged or their properties change, they may need to be replaced. It is also important to ensure that the filters are appropriate for the specific wavelengths and intensities needed for each experiment.

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