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Lab 8: Stellar Classification and the H-R Diagram
1 Introduction
Stellar Classification
As early as the beginning of the 19th century, scientists have studied absorption spectra in an effort to classify stars. At first, spectra were divided into groups by general appearance; however, in the 1930’s and 1940’s, astronomers realized that spectral type was mainly determined by temperature instead of chemical composition. As a result, spectral types were refined and renamed with letters, leading to the familiar “O, B, A, F, G, K, M” system. Each spectral class is then divided into tenths, so that a B0 star follows an O9, etc. In this scheme, our Sun would be labeled a type G2 star. The classification system we use today, called the MK system, adds a Roman numeral to the end of the spectral type, indicating the so-called luminosity class (ex. a I indicates a supergiant, a III a giant star, and a V a main sequence star). In this new system, our Sun (a typical main-sequence star) becomes a G2V. Spectral type allows astronomers to know not only the temperature of the star, but also its luminosity and color. These properties can help in determining the distance, mass, and other physical quantities associated with the star, its surrounding environment, and its past history, making spectral classification a fundamental piece of our understanding of the nature and evolution of stars.
Hertzsprung-Russell Diagrams
One of the most useful tools an astronomer has for studying the evolution and age of stars is the Hertzsprung-Russell (or H-R) Diagram, a graph of surface temperature versus luminosity, on which is
1 plotted characteristic values for a single star or a group of stars. A star’s luminosity (sometimes written as absolute magnitude) and temperature (often denoted by spectral type) determine its position on the HR diagram. The hottest, most luminous stars lie at the upper left of the diagram, and the coolest, dimmest stars lie at the lower right. Note that the stars with lower temperatures are placed on the right-hand side of the HR Diagram, so that temperature increases towards the left. Stars at the upper right are extraordinarily luminous despite having low surface temperatures, and so they must have huge surface areas. These are called red giants. Stars at the lower left of the diagram are exceptionally faint even though they are very hot, so they must be small. They are called white dwarfs. Most stars are found along a line running from the lower right to the upper left of the HR diagram, a region called the main sequence. In this lab, we will explore the relationship between stellar classification and stellar properties to develop our own H-R Diagram. We will also use a pre-existing H-R Diagram to explore the properties of the different classes of stars.
2 Instructions
For this lab, you may find the following formulas useful:
L T 4 R 2 = (1) L T R B T 4 = (2) B T T λ = 500 nm (3) max T
1. Open the NAAP page entitled “Stellar Classification of Stars,” and scroll down to the last simulation on the page. There will be an image of a star, with a slider for spectral type and information about the effective temperature at a given spectral type.
2. For each star in Table 1, set the slider to the appropriate spectral type and record the effective temperature in Table 1. Also, record the color of the star you see in the image. 3. Using the temperatures you’ve just written down, calculate the max wavelength of the light from each star.
4. Copy the temperatures from Table 1 to Table 2, and use them with the radii in Table 2 to calculate the brightness (B) and luminosity (L) of the stars in Table 2. 5. For question 2, use the data in Table 1 and Table 2 to construct your own H-R Diagram. Remember to label your axes!
6. Close the simulation on Spectral Type and open the second link from the NAAP simulations entitled “Hertzsprung-Russell Diagram Explorer.” Click “show main sequence,” “show luminosity classes,” and “show isoradius lines.” The x-axis scale should be set to “spectral type,” and the y-axis scale set to “luminosity.” Answer questions 3-5 on your lab worksheet. 7. For questions 6 and 7, use the options underneath “Plotted Stars” to help identify both the stars nearest to us and the brightest stars nearest to us.
2 Name:
Section:
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Lab 8 Worksheet: Stellar Classification and the H-R Diagram
1. (9 points) Table 1
Star Spectral Class Temperature Color λmax
Our Sun G2
Betelgeuse M2
Arcturus K0
Spica B1
Procyon A F5
Sirius A A1
ζ-Puppis O4
Table 2
Star Temperature R(R/R ) B(B/B ) L(L/L )
Our Sun 1
Betelgeuse 1000
Arcturus 35
Spica 7.4
Procyon A 2.6
Sirius A 1.9
ζ-Puppis 20
3 2. (3 points) Using the two tables above, draw your own HR diagram using the luminosities and temper- atures you’ve identified. Label each star on the diagram, and remember to label your axes!
4 3. Our Sun has T = 5700 K and L = 1L . Star A has T = 21, 000 K and L = 200L .
(a) (2 points) What is its radius (in units of solar radii)? Show your work.
(b) (2 points) Which star is brighter?
(c) (2 points) What is the (approximate) spectral class of Star A? Use the H-R Diagram in the simu- lation to help with your estimate.
(d) (2 points) Given your answers to the two questions above, how can you explain Star A’s radius using the NAAP simulation and the trends seen on the H-R diagram?
4. Exploring the Properties of White Dwarf Stars (a) (2 points) What is the radius, in kilometers, of a white dwarf star with T = 23, 000K and L = 5 0.01L ?(Hint: The radius of the sun is 7 × 10 km.)
(b) (2 points) Where does this white dwarf star lie on the H-R Diagram? Is it more or less hot than a red dwarf star? Approximately how large is its radius compared to our sun? Use the H-R Diagram in the simulation to help with your estimate. You may want to change the x-axis scale to “temperature.”
5 (c) (2 points) The length of the continental United States is approximately 4300 km. Compare this to the radius of the white dwarf star in part a.
5. (2 points) Describe the temperature, radius, and luminosity of the nearest stars using the H-R diagram simulation. Where are these stars clustered on the H-R diagram?
6. (2 points) Use the “overlap” button in the H-R Diagram simulation to view the stars that are both nearest and brightest. Compare these stars’ luminosities, radii, and temperatures to those of our Sun. Are they similar?
This introductory text in this lab has been edited from its original form, as published by Project CLEA. See Snyder, Glenn, and Laurence Marschall. “HR Diagrams of Star Clusters.” Project Clea Windows Documentation. Ed. Brad Knockel. Project CLEA: Gettysburg College, May 2015. Web. 24 Aug. 2015. See also “The Classification of Stellar Spectra.” Project Clea Windows Documentation. Ed. Brad Knockel. Project CLEA: Gettysburg College, May 2015. Web. 24 Aug. 2015. NAAP simulations and materials can be found at
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