AST LABS

Distances to Clusters - 1

DISTANCES TO CLUSTERS Purpose: To study Hertzsprung-Russel diagrams and see how we can find the distance and age of star clusters. Introduction: Star clusters provide a laboratory for studying stars. We can learn about the distance, color, and evolution of stars. Because the life spans on stars are so long compared with human life spans, we must look at the stars as they exist today. Then by looking at several different stars, we can classify these stars like we classify a group of people. Some people have wrinkles and are stooped. We tend to call these people old. Some people have smooth skin and are upright. We call these people young. We can do these things with stars. Some are large and red. These are older stars called giants. Some stars are bright and blue. These are the younger stars. This is the science of population study. If the population contains older people/stars than younger people/stars we called the group/cluster old. If the population contains the reverse population, we call the population young. We tend to want to study these populations as close to each other because outside effects tend to skew your results. In the case of people, effects like birth rates and sun exposure could make the population seem younger (in the case of increased birth rates) or older (in the case of increased sun exposure). Therefore you pick your population clusters in close proximity to each other so that the conditions are the same. We use the same principal with the star cluster. Because we know they were the born the same time, we do not worry about the birth rate issue with stars. Since they are at the same distance we can ignore reddening within the cluster so we don’t worry that different stars are looking older than their true age, we only have to worry about the reddening between us and the cluster. When light travels through dust some of it can be scattered causing the light to look redder, thus older.

In this lab, we are going to look at the magnitude (brightness) of stars in the cluster and use this to tell us about the age of the cluster and the distance of the cluster. We will do this through a program called AstroImageJ, an image processing program. We will find the magnitude of the stars in two different filters, the red (R) and green (V). Then we will find the color magnitude diagram, a diagram similar to Hetzsprung-Russel diagram. We will then find the age and the distance of the cluster.

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Figure 1 a Hertzsprung-Russel Diagram and Stellar Isochrones (Wiki)

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Part I: AstroImageJ

1) Down AstroImageJ from https://www.astro.louisville.edu/software/astroimagej/installation_packages/. For mac download the x64_mac package and unzip into your applications folder. For pc download x64_windows and unzip into your programs folder. Follow any instructions from your instructor for further information.

2) Download Cluster Charts from D2L from Laboratory Manual >> Distance to the Clusters. Your instructor will assign your group a cluster, download R.fit and V.fit for your cluster to your computer.

3) Open AstroImageJ. The window should look like this:

4) Open the R.fit and V.fit file for you cluster using the File >> Open from the Main Menu. You should have two image windows that look like this:

5) Flip each image on the image windows using View >> Invert Y to align with the Cluster Chart for your cluster.

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6) Select Analyze >> Multi-aperture on the image window. The dialogue box should look like:

7) On the dialogue box that pops up enter 6, 8, and 10 in the first three boxes (object radius, inner radius, and outer radius). Select Use single step mode checkbox to save the apertures for future images.

8) In the Cluster Charts, there are fifteen green apertures and one red apertures for each cluster. The green apertures are the target stars and the red aperture is the comparison star. Now click Place Apertures to start the process.

9) Left click on the first star (any green star) to create a target. Now use Shift- Left Click to place the other green apertures. Now Left click to place the final aperture which will be red. The cluster image should look like:

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10) Now hit Enter to start the analysis process in which several windows will open. The most important window is Multi-Plot Reference Star Settings. Click on the Send to Multi-Aperture. Now start the analysis for the other image and click on the first star to place the apertures for the second image. Press enter to start the analysis for the second image. One you have analyzed the other image, click Save to Table to save both sets of images. Here is the window:

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Part II: Excel and Distances.

1) Download the Cluster Analysis excel worksheet from D2L. Open up the worksheet and this what you see:

2) Open up the excel worksheet(s) for your clusters that received from Part I and you should have one or two lines. Note which image each line is from. Selected the rel_flux_T1 column for each image in the excel file and select until the you reach rel_flux_T15 column and copy the numbers like this:

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3) Now select your cluster analysis worksheet and click under either R diff or V diff depending on the image. Now click on Paste in the upper left-hand corner or left-click under R diff or V diff and use the Transpose function to rotate the data from a row format to a column format. Values is another helpful option.

4) Do this for both images for each cluster (M 36, M 37, and M 38) by selecting the tabs at the bottom of the work sheet. Once you copy this data please follow the instructions in the image below.

5) Once you have entered the data into the worksheet please use File >> Save As >> Save as Pdf to submit this worksheet on D2L. Only submit this with your cover sheet, you do not need to submit the individual excel spreadsheets for each cluster.