This past month (June 24, 2012- July 21, 2012), I attended the COSMOS (California State Summer School for Mathematics and Science) at the University of California, Irvine, with brilliant minds from Northern and Southern California, as well as other states. The 152 students were divided into 8 clusters. I was part of Cluster 2: Astronomy and Astrophysics. With 22 other COSMOS students, I ventured into the world of astronomy and astrophysics unraveled by UCI professors Dr. Tammy Smecker- Hane, Dr. James Bullock, Dr. Aaron Barth, Dr. Erik Tollerund, TAs John Phillips, Liuyi Pei, and Shea Garrison-Kimmel, and Teacher Fellow Lisa Taylor. I discovered that all students shared a strong passion for astronomy and high aptitudes for learning. It has been my honor to learn with the students, listen to the professors’ lectures, and follow the TAs’ instructions for CLEA (Contemporary Laboratory Experiences in Astronomy) Labs.
For a cumulative final, the TAs divided the class into 8 groups for Project Labs:
1. Deriving the Mass of Saturn
2. Finding the Angular Velocity of Asteroids
3. Properties of an Eclipsing Binary Star System
- By: Carlin Liao, Matthew Thibault, Sara Sampson; Directed By: Shea Garrison-Kimmel
4. Determining Stars’ Properties Using Stellar Spectra
- By: Tina Liu, Noemi Urquiza, John Cabrera; Directed By: John Phillips
5. Determining the Properties of Open Cluster M11
6. Determining Properties of Globular Cluster M13
7. Dark Matter in the Universe: Measuring the Rotation of Spiral Galaxies
- By: Emma MacKie, Danny Tuthill, Michael Cox; Directed By: Shea Garrison-Kimmel)
8. Number Counts of Distant Galaxies and the Shape of the Universe
- By: David Wong, Thomas Purdy, Joshua Heck; Directed By: Liuyi Pei
– The red, white, yellow, and blue dots in the background represent stars of the H-R Diagram, including main sequence stars, red giants, and white dwarfs.
My Project: Determining the Properties of Stars Using Stellar Spectra (By: Tina Liu, John Cabrera, and Noemi Urquiza; Directed By: John Phillips)
ABSTRACT: Stellar spectra are fundamental in understanding properties — temperature, spectral type, chemical composition, and mass — of stars. A spectrum is the amount of light that a star emits through narrow slit about 1 Angstrom in width. With the UCI Observatory’s 24-inch telescope and its photograph and ST-8 CCD camera, images of stars’ spectra — those of Arcturus, Vega (HD172167), and HD142780— were taken. Using the software program IRAF, the spectra were extracted, calibrated, and analyzed. Since stars are classified by spectral types, stellar spectra help distinguish a more massive and hotter star from a less massive and cooler star. Analyzing the strengths of absorption lines shows the stars’ compositions of elements such as hydrogen, helium, and calcium. While hotter stars such as Vega are more massive and have strong hydrogen absorption lines, cooler stars such as Arcturus are less massive and have strong neutral metals lines. Understanding stars’ properties leads to a better grasp of the past, present, and future of the Universe.
QUESTION: How can we use stellar spectra to determine the properties of stars such as spectral type, temperature, mass, and chemical composition?
BACKGROUND INFORMATION: Stars, actually infinitesimally small points of light, appear to twinkle because light refracts at Earth’s atmosphere. Held by gravity, stars shine due to nuclear fusion, its source of fuel. Their lifetimes depend primarily on mass; for their prime of life, stars, travel along the main sequence on the Hertzsprung- Russell, or Color- Magnitude Diagram. A stellar spectrum is the amount of light a star emits at a narrow wavelength interval (about 1 Angstrom, or 10^-10 meters). Each element has a distinguishable pattern of absorption lines (dark bands along the spectrum). Spectral types are a classification scheme developed by Annie Jump Cannon in the late 1800s and early 1900s. The spectral types are ordered in decreasing surface temperatures: O, B, A, F, G, K, M. Originally the classification scheme was A, B, C, D, etc. and stars were ordered according to the strengths of their Balmer (hydrogen) lines. Since stars with the strongest Balmer lines are not necessarily the hottest stars (hotter temperatures caused electrons to be excited and the atom to be ionized- lose electrons), the scheme was rearranged. The two stars analyzed were two variable stars: Arcturus of the constellation Boötes and Vega of the constellation Lyra.
24-inch telescope, ST-8 CCD camera
Needed Files on Linux (software):
- uciobs_fear_lowres.dat (from observatory website): List of arc lines used for wavelength calibration
- arc_red.jpg (from website): Plots of arc images, with wavelengths of prominent lines
- plotspec.pro : Plots the reduced spectrum and marks absorption lines that are found in LINES.UCI file
- wave : File containing wave limits of reduced spectra. Needed to run plotspec.pro
- LINES.UCI : Input file for plotspec.pro that contains prominent absorption lines to be marked on the final, reduced spectrum
PROCEDURES: Independent Variable: Wavelength (Angstroms); Dependent Variable: Intensity
Take pictures of stars using a 24 inch telescope and ST-8 CCD camera
Use DS9 on Linux to analyze and crop the portion of the image planned on using
Label absorption lines according to reference, with each element specific to its wavelength
Graph the spectrum
Change “pixel” on the x-axis to “wavelength”
Analyze the star’s properties by comparing them to predetermined spectral types.
HD142780: “M” type star, 3,000 K, 0.2 solar masses, absence of hydrogen lines and abundance of neutral metal lines.
CONCLUSION: By analyzing the absorption lines on the stars’ spectra, we determined the spectral types of each, thus allowing us to find their respective properties. The absence of hydrogen lines and prevalence of neutral metals in Arcturus’ spectrum allowed us to identify it as a K type star (Figure 1) . Vega’s spectrum contained strong hydrogen and ionized metal lines. Therefore we classified it as an A type star (Figure 2). Because the spectrum revealed absent hydrogen lines and visible neutral metals, we classified it as a M type star (Figure 3).
DISCUSSION: WHY DO STELLAR SPECTRA MATTER FOR THE FUTURE OF ASTRONOMY?
- Map galaxies
- Map the Universe
- Learn about the lifetimes of different stars
- Use information on old stars to learn about conditions after the Big Bang
- Learn about what has happened in the Universe since the Big Bang
Blumenthal, G., Burstein, D., Greeley, R., Hester, J., Smith, B., & Voss, H. G. (2007). Light, The Tools of the Astronomer, Taking the Measure of Stars. In 21st Century Astronomy. (2nd ed.). (pp. 92-128, 134-158, 380-385). New York, New York, U. S. A.: W. W. Norton & Company.
Kaler, J. B. (2010, July 30). Spectra. University of Illinois. Retrieved July 13, 2012, from http://stars.astro.illinois.edu/sow/spectra.html
Special Thanks to: COSMOS, UCI Professors and Graduate Students, our Teacher Fellow, and Cluster 2: Astronomy and Astrophysics!