Astronomy 104: Stellar Astronomy Lecture 14: Thermal Spectra to the Stars Spring Semester 2013 Dr. Matt Craig 1 1
Next Time Read Chapter 10 Focus on How we know the distance, brightness, luminosity, temperature, and composition of stars. 2 2
Blurring the (Spectral) Lines For Dense Material the different atoms are close enough that the electron potential energy levels get randomly changed Spectral lines blend into a CONTINUOUS aka THERMAL spectrum. 3 3
Behavior of the Thermal Spectrum [in Real Life ] Imagine an electric range. What happens to the color and energy output of the range as it gets hotter? Color: Lower temperature: gives off infrared (you feel this)... Higher temperature: gets dull red, then red, then orange if you heat it more. Energy Output: Low temperature: gives off very little energy. Higher temperature: gives off much more energy! 4 4
Temperature, Size and Luminosity Do the Lecture Tutorial Luminosity, temperature and size, PART I ONLY, pages 37-39. Remember: Work in groups of 2-4 Each person should commit to an answer in writing before moving to the next question. 5 5
Key Behaviors of the Thermal Spectrum [in English with Sketch] As temperature increases 1.The wavelength at which most energy is given off shifts to shorter wavelengths. 2.Energy emitted goes up at all wavelengths. 6 6
Goals How does light interact with matter, both macroscopically and microscopically... [MOSTLY DONE] What exactly happens when an atom (really an electron) absorbs/emits light? [MOSTLY DONE] What happens inside dense material that is different and results in a continuous spectrum? [MOSTLY DONE] What are the properties of the continuous (aka thermal) spectrum? [MOSTLY DONE] 7 7
Key Point about Emission/Absorption Spectra Emission Spectra let you identify the elements the light was created in. Also tells you the electrons in those elements were energized to start with (likely hot). Absorption Spectra let you identify the elements the light passed through. Also tells you the electrons were not that energized to start with (likely cold). 8 8
Key Behaviors of the Thermal Spectrum [in English with Sketch] As temperature increases 1.The wavelength at which most energy is given off shifts to shorter wavelengths. 2.Energy emitted goes up at all wavelengths. 9 9
Clicker Question The graph shows the blackbody (a.k.a. thermal) spectra for three different stars. Which of the stars is at the highest temperature? (a)star A (b)star B (c)star C 10 10
Lecture Tutorial Another expression for a Thermal Spectrum is Blackbody Spectrum. Do Part II of "Blackbody Radiation" tutorial (starts on p. 42). 11 11
Clicker Question Use the graph to determine which of the following best describes how Star A would appear as compared with Star B? (a)star A would appear more red than Star B. (b)both stars would appear more red than blue. (c)both stars would appear more blue than red. (d)star A would appear more blue than Star B. (e)none of the above. 12 12
Clicker Question Use the graph to determine which of the following relationships is possible to infer about stars A and C based on the information provided in the graph at right. (a)star A is smaller than Star C. (b)star A is larger than Star C. (c)the stars are the same size. (d)all of the above are possible. (e)it is not possible to infer any of these relationships. 13 13
Key Point about Thermal Spectra Thermal Spectra let you determine the temperature of the object giving off the light. Also tells you the material you are looking at is sufficiently dense for the electron orbitals of adjacent atoms to interfere with each other. 14 14
The Properties of Stars, Part 1 a.k.a. Stamp Collecting becomes Science 15 15
Properties of stars Things we would like to know about other stars: How luminous? Need distance to get this! What temperature? Color or a detailed spectrum gives this. How massive? Can only directly measure this for binary stars. How large (in size)? Can get from temperature and luminosity. 16 16
Clicker Question Which star is larger? (a) A (b) B (c) Same size (d) Can t be determined 17 17
Organizing Stellar Info Are any stellar properties related to each other? How do we organize the data to show any patterns? One useful approach: Temperature versus Luminosity diagram Also called: Color-magnitude diagram Hertzsprung-Russel (H-R) diagram 18 18
Hertzsprung-Russell Diagramof all stars within 250 light years Luminosity DIM LUMINOUS HOT Temperature Color COOL Image adapted from: http://www.atlasoftheuniverse.com/hr.html 19 19
Clicker Question For the stars labelled A or B, which star is largest? (a) A (b)b (c) Can t Tell Luminosity DIM LUMINOUS B C HOT Temperature Color A D COOL Image adapted from: http://www.atlasoftheuniverse.com/hr.html 20 20
Clicker Question For the stars labelled B or D, which star is largest? (a) D (b) B (c) Can t Tell Luminosity DIM LUMINOUS B C HOT D Temperature Color A D COOL Image adapted from: http://www.atlasoftheuniverse.com/hr.html 21 21
Clicker Question For the stars labelled B or C, which star is largest? (a) Can t Tell (b) B (c) C Luminosity DIM LUMINOUS B C HOT D Temperature Color A D COOL Image adapted from: http://www.atlasoftheuniverse.com/hr.html 22 22
HR Diagram and Star Size Luminosity DIM LUMINOUS HOT Supergiants Main Sequence ("Most Stars") White Dwarfs Temperature Color Giants Larger Stars COOL Image adapted from: http://www.atlasoftheuniverse.com/hr.html 23
H-R Diagram Diagram represents snapshot of stars at many different ages Used as basis for understanding evolution of stars Most stars (90%) lie along diagonal band called the main sequence 24
H-R Diagram: A Snapshot of Stellar Properties Luminosity DIM LUMINOUS 90% of stars along Main Sequence HOT Temperature Color COOL Image adapted from: http://www.atlasoftheuniverse.com/hr.html 25 25
Clicker Question Most stars lie along the main sequence because (a) Stars spend most of their lifetime on the main sequence (b) Stars are brightest on the main sequence, and so easier to see (c) Stars can only exist on the main sequence 26 26
H-R Diagram Interpretation We suspect that most stars lie along the main sequence because stars spend most of their lifetime on the main sequence. The same way a photograph of a random crowd in New York City will show mostly adults because we spend most of our lives as adults. Photo Used Under Creative Commons, Photo by Divya Thakur 27 27
Next Time Read Chapter 10 Focus on How we know the distance, brightness, luminosity, temperature, and composition of stars. 28 28