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HR Diagram Report



  • Watch the Videos

  • read the pre-lab activity. It will give you some idea
  • read the lab answer sheet carefully, the Hipparcos catalogues and the Excel Workbook (uploaded)
  • please answer all questions that has [pts] in the lab report



Your name:

For this project we will use real astronomical data gathered by the Hipparcos spacecraft. You will find this project on the “Sloan Digital Sky Surveyor” website:

The questions in this lab exercise have been adapted from the above mentioned website.

The first H-R diagram you should try is a diagram for the brightest stars in the sky. The table below shows the 26 brightest stars in the sky. Instead of plotting their luminosities (which are so large that they’re hard to visualize), plot the stars’ absolute magnitudes. Absolute magnitude is defined as the magnitude that a star would have if you saw it from a distance of 10 parsecs (about 32 light-years). Stars with higher luminosities put out more light, so they are brighter – they have lower apparent magnitudes. Stars with lower luminosities put out less light, so they are dimmer – they have higher absolute magnitudes.

The table below shows the 26 brightest stars, giving their names, apparent magnitudes, absolute magnitudes, and b-v colors.

Star Name Apparent Magnitude Absolute Magnitude b-v
Sun -26.8 4.8 0.63
Sirius -1.46 1.4 0.0
Canopus -0.72 -2.5 0.15
Arcturus -0.04 0.2 1.23
Alpha Centauri -0.01 4.4 0.71
Vega 0.00 0.6 0.0
Capella 0.08 0.4 0.08
Rigel 0.12 -8.1 -0.03
Procyon 0.38 2.6 0.42
Betelgeuse 0.41 -7.2 1.85
Achernar 0.46 -1.3 -0.16
Hadar 0.63 -4.4 -0.23
Acrux 0.76 -4.6 -0.24
Altair 0.77 2.3 0.22
Aldebaran 0.85 -0.3 1.54
Antares 0.92 -5.2 1.83
Spica 1.00 -3.2 -0.23
Pollux 1.14 0.7 1.0
Formalhaut 1.16 2.0 0.09
Becrux 1.20 -4.7 -0.23
Deneb 1.25 -7.2 0.09
Regulus 1.35 -0.3 -0.11
Adhara 1.50 -4.8 -0.21
Shaula 1.60 -3.5 -0.22
Gacrux 1.63 -1.2 1.59
Castor 1.98 0.5 0.03

Table 1: The 26 brightest stars

In the table you have one column that shows the b-v values. B-v is called the color index and it is always plotted on the horizontal or x-axis. It is defined by taking the difference between the magnitudes in the blue and visual regions of the spectrum. Blue stars have a negative index and red stars have a positive index of approximately 1.41.

These data are also available in the Excel Workbook provided with this report.

If you want more stars, there is a list of the 314 brightest stars available here ( ).

Exercise 1: [15 pts.] Make an H-R diagram for the brightest stars by graphing b-v (on x-axis) and absolute magnitude (on y-axis) for the 26 stars above. Use the Microsoft Excel workbook named “H-R Diagram Exercise DATA” (Available on Blackboard together with this report) to make your diagram.

For help on how to make a graph using Microsoft Excel, see SkyServer’s Graphing tutorial.

Question 1: [6 pts.] Do you see any groups of stars that appear to have something in common? Sketch a box around those groups in your graph. (Hint: On your Excel Spreadsheet go to: insert – shapes – rectangle. Give the rectangle a certain color so that you can distinguish the different groups. As answer to this questions describe the commonalities for the different groups of stars you have found).

Question 2: [6 pts.]  The stars in the upper right of the diagram are very bright but are also very cool. If the stars are cool, why do you think they are so bright?

Question 3: [6 pts.]  Where does our sun plot on this diagram? Is it hotter or cooler than average? Does it emit more or less light than average?

Question 4: [6 pts.]  Do you think your diagram constitutes a good random sample of stars? Why or why not?

Next, you should make an H-R diagram using the nearest stars (Data are in your Excel workbook) rather than the brightest. This diagram will tell us what the stars closest to the Sun are like.

Question 5: [6 pts.]  What is the advantage of looking at the nearest stars rather than the brightest? Do you think the diagram for the nearest stars will look different from the diagram for the brightest stars? If so, how?

Exercise 2: [15 pts.] Now graph an H-R diagram for the nearest stars. For your convenience the data were copied into the excel workbook (Sheet 2 “Nearest Stars”)

Question 6: [6 pts.] How does this diagram differ from the diagram for the brightest stars?

Question 7: [4 pts.] How does our Sun compare to the other stars in our neighborhood?

In the following activities you will find out how to determine the data necessary to create an HR Diagram using real astronomical data bases.

Exercise 3: [15 pts.] Use the Hipparcos data from Hipparcos Catalog Volume 6 to find the distances to the following stars and record them in table 3.  The coordinates given are RA and Dec. Record the Visual magnitudes (V mag) of the stars as well. You will need them later. (In the Catalog provided the stars have been highlighted in yellow. You need to search for the highlighted objects. The data for each object go over two pages.) The distance d can be determined by calculating 1/parallax. Before you perform this math operation you need to convert the milli-arcseconds in to arcseconds.  The value for parallax as given in the catalog is in milli-arcsec. For example: a Parallax value of 2.5 milli-arcsec = 2.5 x 10^(-3) or 2.5 x 10 -3 arcsec. The distance would be 1/parallax = 1/(2.5 x 10-3) = 400 parsec

You need to fill out the table completely to get credit for it.

RA Dec Visual Magnitude Parallax
06 42 05.31 -15 12 53.8      
06 38 35.43 -16 52 24.6      
06 41 33.99 -17 32 01.0      
06 48 48.78 -16 12 41.2      

Table 3: Distance of  stars

Now that you know these stars’ apparent visual magnitudes and distances, you can find their absolute magnitudes. Absolute magnitude is defined as the magnitude a star would appear to have if it were 10 parsecs away from us. Our Sun’s absolute magnitude is 4.84 (compared to its visible magnitude of -26.2!).

The relationship between a star’s apparent or visible magnitude and absolute magnitude is given by the expression

M = m – 5*( log d) + 5,

where m is the star’s apparent magnitude, M is the star’s absolute magnitude, and d is the distance to the star in parsecs.

Let’s take the star Sirius as an example. It has a visual magnitude (V mag) of -1.44 and it is 2.637 parsecs away. Therefore, its absolute magnitude is

M = -1.44 – 5 log (2.637) + 5 = 1.45.

Exercise 4: [15 pts.] Use the visual magnitudes and distances you found earlier to find the absolute magnitudes of these stars and record them in table 4.

RA Dec Visual Magnitude Distance Absolute Magnitude
06 42 05.31 -15 12 53.8      
06 38 35.43 -16 52 24.6      
06 41 33.99 -17 32 01.0      
06 48 48.78 -16 12 41.2      

Table 4: Absolute magnitude of stars

Now you are ready to make an H-R diagram!

The next lab will build on this lab exercise and allow you do analyze a star cluster by doing a HR-Diagram fit.


Convert the file into a PDF. Submit the report AND Your Excel Workbook in the assignment drop box on Blackboard.  Make sure you have submitted the correct document.  Formatting the report is important. It counts 10 pts. You can lose up to 10 pts., if the report does not look professional.




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