Blackbodies

Download the blackbody simulator for this part

This simulator reproduces the blackbody spectrum as a function of temperature. The X-axis is wavelength increasing to the right (decreasing energy per photon). The Y-axis is the amount of energy emitted at that wavelength. Clicking anywhere on the graph will indicate the wavelength (value of the X-axis) at location of the cursor. This is useful when you need to identify the wavelength of the peak emission. A background corresponding to the optical spectrum (the rainbow) is superposed on the blackbody curve to ease in identification of color. Black means that our eyes can't see the light and NOT that the color is black! Light with wavelength shorter than about 3200 angstroms does not penetrate our atmosphere. This is where the ultraviolet region of the electromagnetic spectrum begins.

As you change the temperature (T) you will see the curve changing but you will also see the numerical values in the B-V V-R U-B and T fields changing. For this exercise we will only care about the values in the T and B-V fields.

1. Set the temperature to approximately 8000 by using the slider bar (note the arrow keys on your keyboard can be used for fine temperature adjustment. What is the wavelength of the peak emission?

2. Now set the temperature to approximately 4000. What is the wavelength of the peak emission?

3. What is the ratio of the two wavelengths? (a ratio is two numbers divided by each other). Is this ratio what you expect?

4. What color would a star appear to be which has a temperature of 8000? 4000? 5500?

5. Do we really see stars with these colors?

   Now click on the box that says "Draw Limits of Integration" -the white lines that appear there represent filter band passes of standard astronomical filters. As you can see, filters measure a "band" of wavelengths, not just one wavelength. Their response isn't perfect either. See how they curve at the edges. This means that not all of the light at the edges of the filter bandwidth are 100% transmitted.
   To measure stellar temperatures, astronomers put filters in front of their digital cameras and measure the flux ratio between the two filters. For B (blue) and V (visual or green) this ratio is encoded as the index value B-V. The lower that number, the hotter the star (more flux is emitted in the B filter than the V filter. A value of B-V = 0.5 means that approximately the same amount of energy is emitted in the blue filter as the green filter.

6. What temperature produces a B-V value of 0.5?

7. What range in temperature corresponds to B-V = 0.5? Change the temperature until B-V = .49 and .51.

   As you can see, the B-V index is a sensitive indicator of stellar surface temperature for stars with temperatures of around 6000 (like our Sun). However, for very hot stars B-V starts to lose its sensitivity to temperature simply because the B and V wavelength regions contain very little of the total energy flux of the star. We can verify this by doing the last situations and compare that to what was just done.

8. Set T to 15000. What is B-V? What range in temperature corresponds to this B-V? Is this range larger than for B-V=0.5?

9. If measuring hot stars is highly inaccurate in comparison to stars like the sun, What kind of observations would you need to do to be able to accurately measure the temperatures of very hot stars? In other words, what is the peak wavelength of this kind of blackbody. What kind of light is it (IR, visible, ultra violet, etc). Can we see it from ground base telescopes?