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.

4 Part #3 Exploring Spectra Step 1. The table below lists 14 stars. Click on each of the star numbers in the table below and open the hyperlink to go to the Quick Look tool entry for that star. You will see a picture of the star and its spectrum. Click on the spectrum for a larger view. Star Star 1 8 2 9 3 10 4 11 5 12 6 13 7 14 Step 2. Examine each star and its spectrum and answer the questions below. Question 1: What do you notice about the spectra? All the spectra have absorptions line, noise, continuum peak which are on different sides. Also, #8 only has noise. Some look similar, but most of them look different. Question 2: What are the most important features they all have in common? The most important features are absorption lines, noise, and continuum peak. Question 3: What differences do you notice among the spectra? The continuum peak is almost always in a different spot. Some of them have it on the left side, and some have the right side. Question 4: How do the features you identified in Question 1 and 2 change among the 14 spectra? The continuum peak is on different sides, and they have different absorption lines.

5 When the continuum peak on the left side means the stars are hot and when the continuum peak is on the right side it means the stars are cold. Most of the stars are hot in given spectrum. Part #4 Absorption and Emission Lines In Part #3 you examined the spectra of 14 stars. You probably noticed two features that all the spectra had in common. All the spectra have similar overall shapes, and all have peaks and valleys of different heights. These are the very same features astronomers use to classify stars. In fact, it was through classifying stars that astronomers eventually realized what those features mean, and how they relate. The Hydrogen Atom To begin to understand what the peaks and valleys mean, take a close look at a hydrogen atom. A hydrogen atom has one proton and one electron. Its electron can only occupy certain energy levels; think of energy levels as unequally spaced steps of a ladder. The higher up an electron is on the ladder, the more energy it has. Astronomers use the letter “ n ” and a number to designate each energy level. The lowest energy level is called the “ n =1 ” level, the second lowest level “ n =2 ,” the third “ n =3 ,” and so on. Electrons can move from one level to another by absorbing energy or releasing energy. For example, if an electron moves down from the 2 nd energy level to the 1 st ( n =2 to n =1), then the atom emits a photon of light. The emitted photon has an energy equal to the difference between the 2 nd and 1 st energy levels. An electron can only move up from the 1 st to 2 nd energy level if it gains the right amount of energy. The electron typically gains this energy if the atom absorbs a photon of light with the right energy. The animation link to the right shows a hydrogen atom that starts with its electron moving from the n =1 energy level to n = 2 after absorbing a photon. The electron then emits a photon of the same energy to go back to n =1. Since a photon ’ s wavelength is determined by its energy, if you know the energy a photon has, you know its wavelength. To go up from n =1 to n =2, an electron must absorb a photon with an energy of 10.2 electron-volts (1.63 × 10 -18 Joules) ‒ this photon has a wavelength of 1216 Angstroms. To go down from n =2 to n =1, the atom must emit a photon of 1216 Angstroms. Question 4: To go up in energy level from n =1 to n =2, what wavelength of light must a hydrogen atom absorb? It must absorb 1216 Angstroms of light.

7 Question 5: The spectrum below is the spectrum of a typical star. Below the spectrum is a zoomed-in view of the area around 6560 Å. You saw in the graph previously that the Hα transition is at 6563 Å. So, the zoomed- in view shows the Hα line for the spectrum. Is the Hα line in this spectrum an emission line or an absorption line? It is an absorption line. How do you know? Because it has a valley which dips down. We know this is an absorption line because of that.

8 Conclusion. Please provide feedback regarding the lab assignment. Are there things that you liked or disliked? Thanks!!