Lab 8

1 Name:__________________________________________ Date:_________________ PHYS 110L Lab # 8 Sloan Digital Sky Survey Stellar Spectra Lab 1 Instructions: Please read carefully and follow the steps described below and answer all questions. If confused, please ask your instructor for help. Part #1 Introduction Classification lies at the foundation of nearly every science. Scientists develop classification systems based on the patterns they see. Classifying stellar spectra was a key step for astronomers in discovering how stars work. Thus, in astronomy as well as other sciences, the seemingly ordinary step of classifying things eventually yields critical insights into our world. Useful Definitions: • Absorption line ‒ A “ valley ” that appears in a spectrum due to light being absorbed by energy level transitions in atoms • Angstrom ‒ A unit of length measurement used for spectral wavelengths; one Angstrom is one ten-billionth of a meter (1 Å = 10 − 10 m) • Balmer series ‒ The series of absorption lines resulting from transitions from low to high energy starting at the second energy level ( n =2) in the hydrogen atom • Continuum ‒ The smooth curve of a spectrum • Continuum peak wavelength ‒ The wavelength at which the spectral continuum is greatest; it appears as the top of a broad hill in the spectrum • Emission line ‒ A “ peak ” that appears in a spectrum due to light being emitted from high to low energy level transitions in atoms 1 Modified from Sloan Digital Sky Survey III, http://skyserver.sdss.org/dr10/en/proj/teachers/basic/spectraltypes/lesson.aspx Learning Objectives: In this lab assignment you will conduct a series of inquiries to identify key features of spectra, including the continuum peak and absorption lines. You will also explore how atomic energy level transitions explain spectral absorption lines.

2 • Energy level ‒ An amount of energy that an electron can have in an atom • Hydrogen alpha (Hα) line ‒ An absorption line associated with the transition from the second energy level ( n =2) to the third energy level ( n =3) in the hydrogen atom • Noise ‒ Random variations in a spectrum that are not associated with the continuum or emission or absorption lines • Spectrum ‒ A graph of the amount of light given off by an object at different wavelengths • Thermal radiation ‒ Radiation given off by hot objects, such as stars Part #2 Spectra of Stars A spectrum (the plural is spectra) is a graph of the amount of light something gives off (how bright the object is) at different wavelengths. In analyzing the spectra of stars, we frequently do not know the distances to the stars, so a star ’ s spectrum shows how bright it appears from Earth. The Sloan Digital Sky Survey (SDSS) measures wavelength in units of Angstroms (symbol Å), 1 Angstrom = 10 − 10 meters. The wavelengths of SDSS spectra go from around 4000 Å (just into ultraviolet light) to 9000 Å (just into infrared light). The scale for the amount of light is complicated, but higher numbers are brighter. The image below shows a typical SDSS spectrum with some labels to point out several features. Study the image; the text on the next page describes some of its features.