Astronomy 122 First Homework Assignment: Measuring stellar brightnesses and stellar colors with virtual simulators.

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This assignment consists of two parts that each make use of a JAVA based simulator.

Note the following:

In January 2014 Oracle broke Java in a major way and some of you may have difficulty getting the simulations to open on your personal machine. The main problem involves a security setting. In general, if you open the Java Control Panel to the security tab, simply set your security down to medium (the default is medium high which disables these simulations).

You may have to visit java.com to get the latest java release.

Open these simulations on any of the public/student computers in the Knight Library. They have all been fixed to handle this JAVA problem so if you can't get the simulations to launch on your personal machine, go to the library.

In general the above difficulties only apply to the second simulation (Blackbody Radiation) and not the First Simulation (Detector).

Tutorial on how to use the Detector Simulation for Part I below: READ IT. Also note that the virtual detector for this homework assignment is a NON-JAVA version of the one used in the lecture pages (e.g. Module 1 Lecture D).

The Assignment:

Part I:

Launch the detector.

Increase the exposure time to 40 seconds and measure the net counts (using the procedure shown in the tutorial) for Star 1 and Star 3. Report those values on the template worksheet.

Set the exposure to 1 second. Report the counts in the background at 1 second. Then measure the background counts at 10 seconds and 100 seconds and report all those values on the tempalte worksheet.

Explain what you notice in the background counts as a function of exposure time.

Does it make a difference where you place the aperture on the image for the backgound counts? If not, what does this tell you?

Now set the exposure back to 1 second and measure and report the net counts for Star 4.

Explain why you can barely detect Star 4 at an exposure time of 1 second.

Launch Simulation A Make the exposure time 25 seconds and report the net count value.

Launch simulation B Again make the expoure time of 25 seconds and report the net count value.

The star is the same between simulation A and B. Suggest what might be different about the observing conditions for these two cases and why this star is harder to detect in one of these cases.

Launch simulation C Make the expoure time of 100 seconds and report the net count value.

Suggest in this case why this same star is almost impossible to detect under the observation conditions associated with simulation C.

Launch simulation D Make the expoure time of 100 seconds.

From this exercise it should be apparent to you that for any given exposure of the sky with any given telescope plus detector there will be many stars that are simply too faint to register on the detector and different detectors will require different amounts of exposure time to produce similar quality data in terms of net counts of signal.

Part II
Download the blackbody simulator for this part
This simulator will reproduce 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 is superposed on the blackbody curve to ease in identification of color. That represents the wavelenght limits of human vision. At the long wavelength end, our vision dies out around 8000 angstroms as our eyes, compared to nocturnal predators, are not sensitive to infrared radiation. Light with wavelength shorter than about 3200 angstroms does not penetrate our 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 numercial 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.
Answer the following questions in the worksheet for this assignment.

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?

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

What is the ratio of the two wavelengths? (a ratio is two numbers divided by each other). Is this ratio what you expect? Explain why or why not.

What color would a star appear to be which has a temperature of 7000?
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. 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.

What temperature produces a B-V value of 0.5?

Suppose that I can measure B-V to an accuracy of 10% (.1 in B-V). For the case of B-V = 0.5, what percentage change in temperature produces this 10% change in B-V? (i.e. B-V moves to 0.6 or 0.4 when you change the temperature)

Set T to 15000. At this temperature, what temperature change is required to change B-V by +/- 0.1?

Explain why B-V loses temperature sensitivity for these hot stars.

What kind of observations would you need to do perform in order to accurately measure the temperatures of very hot stars?