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Individual Lab Exercise #3.A: Hertzsprung-Russell Diagrams

(Adapted from Univ. Nebraska – Lincoln)

Instructions: Place your answers in the appropriate location on this Word Document. When you are finished with the assignment, copy and paste this document (with your answers, obviously) into the appropriate individual Lab Journal on BBLearn (i.e., Stellar Zoo Lab #).

For this lab you will be using WorldWide Telescope & Wikipedia to find the spectral characteristics of select stars and explore Hertzsprung-Russell Diagrams using the HR Diagram Explorer from UNL: http://astro.unl.edu/naap/hr/animations/hr.html

In WWT, you will need to use the “Search” feature in order to locate these stars. By using the “Finder Scope” in WWT (that is, right-click or shift+left-click on the object) you will see a variety of physical properties associated with that object. By clicking on “Research” then navigating to “Information” “Look up on Wikipedia” you will see a list of the spectral information of that star. (This is one reasonably rare case where Wikipedia’s information is correct; many dedicated astronomers have contributed to vetting this class of information.)

Note: You may find it best to unselect the “View from this location” option under the “View” menu since this turns off the horizon which may obscure your view. This lab can be run under whatever date/time you wish since our only concern is the spectral characteristics.

Exercise 1: Fill out the table below to summarize the relationship between spectral type, temperature, and color for stars. Note that the surface temperature of the stars in the table increases.

Star

Surface Temperature (K)

Spectral Type

Color

Constellation

Betelguese

M2

Arcturus

4,300

G2

Yellow

N/A

Procyon A

F5

Yellow-White

Sirius A

A1

White

Rigel A

11,000

Delta Orionis

O9

HR Diagram Explorer

Open the HR Diagram Explorer at http://astro.unl.edu/naap/hr/animations/hr.html

· Begin by familiarizing yourself with the capabilities of the Hertzsprung-Russell Diagram Explorer through experimentation.

· An actual HR Diagram is provided in the upper right panel with an active location indicated by a red x. This active location can be dragged around the diagram. The options panel allows you to control the variables plotted on the x-axis: (temperature, BV, or spectral type) and those plotted on the y-axis (luminosity or absolute magnitude). One can also show the main sequence, luminosity classes, isoradius lines, or the instability strip. The Plotted Stars panel allows you to add various groups of stars to the diagram.

· The Cursor Properties panel has sliders for the temperature and luminosity of the active location on the HR Diagram. These can control the values of the active location or move in response to the active location begin dragged. The temperature and luminosity (in solar units) are used to solve for the radius of a star at the active location.

· The Size Comparison panel in the upper left illustrates the star corresponding to the active location on the HR Diagram. Note that the size of the sun remains constant.

· Check “show luminosity classes”. This green region (dwarfs V) is known as the main sequence and contains all stars that are fusing hydrogen into helium as their primary energy source. Over 90% of all stars fall in this region on the HR diagram. Move the active cursor up and down the main sequence and explore the different values of stellar radius.

Question: Describe the sizes of stars along the main sequence. What are stars like near the top of the main sequence, the middle, and the bottom?

Exercise 2: Finding the brightest, nearest stars

Make sure you have all four of the “show” buttons selected (e.g. “show main sequence,” “show luminosity classes”) and click on each of the options in the “Plotted Stars” portion of the simulator. Notice where the nearest stars and the brightest stars fall on the HR diagram.

Select “the overlap” in the HR Diagram Explorer to see only the brightest, nearest stars on the HR Diagram. I want you to use WWT and the stellar information on Wikipedia to identify four of the five brightest, nearest stars.

Tips on finding these stars: Explore the sky view with WWT to identify the brightest stars in the sky. By selecting each of those stars with the “Finder Scope” you can see the apparent magnitude of the object to verify that it is indeed a bright star; that is to say, the magnitude should be close to or less than zero. As we’ve seen from the text, the apparent magnitude is really a confluence of stellar luminosity and distance so this serves as a good indicator in your search for the bright, near stars. You can click on each of the plotted stars in the HR Diagram Explorer to see what luminosity & temperature you are looking for or refer to the table below. Do note that the values you find in the Wikipedia entries may not exactly match what is listed on the “Cursor Properties” but they should be close.

For example, I selected the star on the HR Diagram with a Temperature of 7600 K and Luminosity of 10. By searching the sky in WWT I narrowed down the possibilities by investigating the brightest stars first. After a few tries, I found that the star Altair has a luminosity of 10.6 and a temperature of 6,900K to 8,500K. In addition, the distance to Altair is 16.73 light-years. All of this together tells me that Altair is indeed one of the brightest, closest stars.

Complete this exercise for the four remaining stars in the table below:

Temperature (K)

Luminosity (in solar units)

Star Name

Distance from Earth (lyr)

7600

10

Altair

16.73

9600

24

6400

7

5800

1.6

5100

0.55