Wikipedia - Stellar classification:

Transcription

1 Stars and Hertzprung-Russell Diagram Introductory Astronomy laboratory exercise with Stellarium Mike Chu Name Stellarium is an open source and cross-platform application from A star is mainly categorized by its spectral class and absolute magnitude. The spectral type also is referred to as color and it is an indication of the surfacetemperature of the star. Absolute magnitude tells information about the luminosity, or the rate of energy output, of a star. The term spectral type originally pertains to the method of observation in which the spectra of a star is recorded and then the star type is categorized according to the pattern shown in the spectra. Now astronomers have a solid understanding of the physical mechanism to and connection between spectral patterns and surface temperature. There are seven major spectral classes, from hotter classes to colder classes, denoted by the letters O, B, A, F, G, K, M. Each spectral class is furthered subdivided into 10 smaller subclasses indicated by a number from 0 to 9. For example: an A5 star is colder than A0 but hotter than A9 furthermore A9 is hotter than the first subclass of next spectral class F0, and so on. Below is an excerpt from Wikipedia listing the common information about each spectral class. There are additional spectral classes not shown below and are not needed for this work. Wikipedia - Stellar classification: 1

2 The Hertzsprung-Russell (HR) diagram is simply a scatter plot of Absolute Magnitude versus the Spectral Type that exhibits very tell-tale patterns about stars and stellar evolution. The plot is easily constructed from a sufficiently large set of data of absolute magnitude and spectral type of stars. In this lab, the application Stellarium will be used to obtain data from the built-in stellar data. Suggestion for instructor: 1) This exercise may be conducted as an individual exercise where each student is asked to map out the diagram with enough selected star information. 2) The exercise can also be conducted in collaboration where each student is to focus on a particular constellation 3) As this exercise is simple and that Stellarium is highly flexible, creativity is encouraged to make this lab fun and illuminating. Stellarium Instruction: 1) Open up Stellarium, and toggle off Atmosphere, Ground and Nebulae on the bottom menu. 2) Click the search button on the left icon bar (or F3, or ctrl-f3) and search for a common bright star such as Sirius, Betelgeuse, or Rigel or simply find any bright named star on the screen and right-click on the star - the star information will be displayed on the top left corner which will include Absolute Magnitude and Spectral Type. 3) In Stellarium, the spectral class of a star typically is given by more than three letters or characters, such as F0lb. The extra characters or numbers beyond the first 2 places provide additional information such as luminosity class but will not be needed. Make sure that the stars of higher magnitude, or lower visibility, are turned on. This gives a better representation of the available stars, and not just the bright ones, and a more correct look of the HR diagram. Go to Sky View and options, select Sky tab, and find relative scale. Set relative scale from 1.0 to about 0.5. You can turn off all labels of planets, stars and other unnecessary markings for a cleaner visual. If later you find yourself not able to obtain enough faint stars then lower the relative further to see more even fainter stars. (To be improved) 2

3 Examples of stars and their corresponding spectral type: Betelgeuse M2lb Sirius A0M Rigel B8la HIP GIV 4) a. If necessary to show constellations and boundaries, go to Sky and viewing option window [F4] on the left menu, select the Markings tab: Check Show lines, Show labels and Show Boundaries b. Zoom into the constellation of your choice by using mouse or touch pad scroll. 6) Find 5 to 10 stars of each of 7 spectral types, and fill up a table or spreadsheet with enough star information. Anywhere between 45 to 75 stars may be sufficient to illuminate the main features of the HR diagram (you will need to determine the diagram quality visually). Be sure to include some of the very faint stars not easily visible on the screen or the resulting HR diagram may not correctly demonstrate some features. For each star, obtain the following information: Absolute magnitude Spectral type Constellation (for plotting purpose spectral type represented by numbers. See below) 7) Complete your spreadsheet. An example spreadsheet with only 3 stars is shown below: Illustration 1: Example spreadsheet with star information including Absolute Magnitude, Spectral Type, Constellation and the ad-hoc Converted Spectral Type The Converted Spectral Type is an ad-hoc creation for the purpose of plotting Absolute Magnitude versus Spectral Type. The spreadsheet plotting-function will not be able to recognize characters (such as M2 or K1 of spectral type) as numerical input, thus for plotting purpose the ad-hoc conversion is made to convert spectral 3

4 types into numbers but following the same order. This ad-hoc conversion is done by replacing the character O-B-A-F-G-K-M with the number : O 1 B 2 A 3 F 4 G 5 K 6 M 7 In the example spreadsheet, M2 is turned into 72 by replacing M with 7, B0 is converted into 20 by replacing B with 2, and K1 is changed to 61 by replacing K with 6. With both Absolute Magnitude and Spectral Type represented by numbers, a scattered plot then can be generated using the spreadsheet plotting function. 7) Plot Absolute Magnitude on the y-axis versus Spectral Type* Converted on the x-axis. (Instruction for Libre Office is to be given in Spring, 2015) Here is a quick and dirty instruction for Excel plot. Simply use your mouse and select all your input on the spreadsheet. Then go to Insert on the top menu bar (next to Home ) select Chart select Scatter Choose Marked Scattered (Scatter with only Markers) type, which is the first choice. You will see a plot but do not worry about the first appearance. It will be fixed via some adjustments. Under Design (make sure you click on the plot first), click Select Data icon in the icon bar (not the Data tab on top). (You will need to remove Series 1, which we do not want). Under Legend Entrees (Series) click Add. Here you will select the data for x-axis and y-axis. Click inside the entry row of Series X values: to make sure the cursor is active within (a flash vertical bar). Now you are ready to select x-axis. Use your mouse to select all data under the Converted Spectral Type. Then hit Enter. (The simple idea is to use Converted Spectral Type for x-axis). For Series Y values:, click on the entry row and then select all data under Absolute Magnitude. Hit Enter. 4

5 Now that you see a plot and think you are done. Not quite! Your nice looking HR diagram may still look nothing like ones shown in text or online resources. Do recall that the magnitude scale, for seemingly strange but sound historical development, is one scale in which the brighter the star the more negative the number. That means the Absolute Magnitude as labeled on the y-axis that you are seeing, is plotted in reverse. So let's revers it. Right click (or maybe just click) on y-axis area > select Format Axis then finally under Axis Options, check the box of Values in reverse order. You should now see a correct HR digram representation of your work. Do check to see if your dataset is sufficient to display the main features of the HR diagram. Consult any reputable online resource. See below link with Excel plotting function: 8) (Per Instruction) your spreadsheet to your instructor. Be sure to include your name, the selected constellation and other important information. A follow up exercise can be built from the overall class result. List of Constellation (Wikipedia): You will be assigned a constellation region. 5