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Star Stuff

Star Stuff. Hot solid, liquid, dense gas: no lines, continuous spectrum. Hot object through cooler gas: dark lines in spectrum. Cloud of thin gas: bright lines in spectrum. What is the physical explanation for these different spectra?. The energy levels of the Hydrogen atom:.

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Star Stuff

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  1. Star Stuff

  2. Hot solid, liquid, dense gas: no lines, continuous spectrum Hot object through cooler gas: dark lines in spectrum Cloud of thin gas: bright lines in spectrum What is the physical explanation for these different spectra?

  3. The energy levels of the Hydrogen atom: • Places where the electron is located • Fixed levels by quantum mechanics • Energy levels depend on atomic make-up

  4. Spectral lines originate from electrons moving in atoms In order to go UP a level, electron must absorb energy In order to go DOWN a level, electron releases energy • ENERGY takes • the form of • EM radiation, or • “photons” absorption of a photon to go up energy levels • photons have • wavelength which • corresponds to the • energy change emission of a photon to jump down levels

  5. Measuring the “spectrum” of light from a star -divides the light up into its colors (wavelengths) - use a smaller range of wavelengths than entire EM spectrum because instrumentation is different “spectrometer” Filters the light into its different parts

  6. The “spectrum” of a star in the visible part of EM spectrum A plot of INTENSITY vs. WAVELENGTH

  7. Features of stellar spectra: Blackbody objects (wavelength of peak intensity) Have additional features – “spectral lines” no lines bright lines dark lines

  8. A much closer look at the spectrum of the Sun • Wollaton (1802) discovered dark lines in the solar spectrum. • Fraunhofer (1817) rediscovered them, and noted some • were not present in stars - but other stars had more. Image courtesy of the McMath-Pierce Solar Observatory

  9. A much closer look at the spectrum of the Sun Things to note in solar spectrum:  brightest intensity at green/yellow wavelengths  presence of many dark lines and features Image courtesy of the McMath-Pierce Solar Observatory

  10. Spectra for a variety of stars 1 • Some stars have • fewer dark lines • in their spectra • than the Sun • and others have • more dark lines • than the Sun • The dark lines are • also at different • positions than • the Sun’s 2 3 4 5 6 7

  11. Real Data Simulated Data Both of these plots show wavelength vs. intensity

  12. Experiments on Spectra of the early 1900’s Burned different elements over a bunsen burner  glow different colors!!

  13. Scientists discovered that the bright lines also correspond to the dark lines

  14. The three types of spectra: no lines bright lines dark lines

  15. Hot solid, liquid, dense gas: no lines, continuous spectrum Hot object through cooler gas: dark lines in spectrum Cloud of thin gas: bright lines in spectrum

  16. Identifying the spectral lines in the Sun’s spectrum There are many dark absorption lines – what does this mean?? The Sun’s cooler gaseous outer layers are absorbing the photons arising from the hotter inside ! Mainly hydrogen absorption lines, but over 60 different elements identified in small quantities

  17. Identifying the spectral lines in the Sun’s spectrum O2 at 759.4 to 726.1 nm (A) O2 at 686.7 to 688.4 nm (B) O2 at 627.6 to 628.7 nm (a) H at 656.3 nm – Hydrogen alpha line Ha (C)  electron moves between n=3 and n=2 H at 486.1, 434.0 and 410.2 nm (F, f, h) Ca at 422.7, 396.8, 393.4 nm (g, H, K) Fe at 466.8, 438.4 nm (d, e) Terrestrial Oxygen – in Earth’s atmosphere!

  18. Identifying chemical composition of the Sun’s spectrum

  19. Procyon (F5) Arcturus (K1)

  20. Spectra for a variety of stars 1 • Some stars have • fewer dark lines • in their spectra • than the Sun • and others have • more dark lines • than the Sun • The dark lines are • also at different • positions than • the Sun’s 2 3 4 5 6 7

  21. Using spectra to identify chemical compositions

  22. How many different kinds of spectral “signatures” are there? What determines “signatures” of different kinds of stars? Major research effort at Harvard in the 1920’s Need to inspect many, many different stellar spectra look for categories, patterns among them

  23. The Harvard College Observatory: female “computers”  under direction of Professor Henry N. Russell

  24. Figuring out the various types of stars Cecelia Payne-Gaposchkin (1900-1979) Annie Jump Cannon (1863-1941) 1918-1924: she classified 225,000 stellar spectra! PhD 1925 Harvard (first Astronomy PhD) Figured out that different spectra were due to TEMP.

  25. The categories of stars: O B A F G K M Differences are due to the TEMPERATURE of star • TEMPERATURE can determine: • where the electrons are located (which energy levels) • which elements have absorption, emission lines -- an O-star has a temperature of ~50,000 K -- an A-star has a temp of ~10,000 K, enough for hydrogen to be ionized (spectral lines in the UV) -- a G-star (like our Sun) has a temperature of ~6,000 K

  26. Different stars have different spectral “signatures” • All stars fall into several categories: O-B-A-F-G-K-M Hot 50,000 K  Our Sun Cool 4,000 K

  27. Stellar Evolution is the study of - how stars are born - how stars live their “lives” - how stars end their lives

  28. The Hertzsprung-Russell Diagram (H-R Diagram) Plots the relationship between TEMPERATURE (x) and LUMINOSITY (y) of different stars • not a star • chart (positions)! Sun  • shows that a • star’s T is related • to its Luminosity • in a certain way

  29. The Hertzsprung-Russell Diagram (H-R Diagram) Plots the relationship between TEMPERATURE (x) and LUMINOSITY (y) of different stars MAIN SEQUENCE Most stars fall along this line The MORE LUMINOUS the star, the HOTTER it is The LESS LUMINOUS the star, the COOLER it is

  30. The Hertzsprung-Russell Diagram (H-R Diagram) Plots the relationship between TEMPERATURE (x) and LUMINOSITY (y) of different stars color illustrates the main sequence (MS) BLUE MS stars are LUMINOUS, HOT RED MS stars are DIM, COOL

  31. The Hertzsprung-Russell Diagram (H-R Diagram) Properties of stars on the Main-Sequence • fusing H  He in their cores • the length of time fusion can • last depends on how much • “fuel” is there for fusion • and the rate at which fusion occurs • amount of fuel = star’s MASS • rate of fusion = star’s LUMINOSITY

  32. The Hertzsprung-Russell Diagram (H-R Diagram) Properties of stars ON the Main-Sequence The LUMINOUS stars are more massive 5 to 50 times solar mass The DIMMER stars are less massive 0.1 – 1 times solar mass

  33. Masses given in Solar Masses

  34. The Hertzsprung-Russell Diagram (H-R Diagram) Properties of stars on the Main-Sequence The more massive stars have MORE FUEL but also more LUMINOSITY  They fuse H  He faster! The LESS LUMINOUS stars have less fuel but they fuse H  He more slowly

  35. A relationship between MASS and LUMINOSITY For stars ON the MAIN SEQUENCE  direct relationship LARGER MASS Higher Luminosity SMALLER MASS Lower Luminosity

  36. The Hertzsprung-Russell Diagram (H-R Diagram) Two other categories of where stars are: SUPERGIANTS – COOL but very very LUMINOUS WHITE DWARFS – HOT but very very DIM

  37. Changes in a star’s physical state result in changes on the H-R diagram Red Giants are LARGER COOLER than the Sun UPPER RIGHT

  38. Examples of Red Giants: Arcturus, Betelgeuse

  39. What happens next depends on initial mass 1. Stars ~ 1 solar mass “Helium Flash” – explosive consumption of He fuel T ~ 300 million K L ~1014 solar luminosity 2. Stars > 2 solar masses Continue to fuse He and carbon to make core rich in oxygen and carbon

  40. Red Giants: instabilities & brightness variations • very delicate balance between pressure, gravity • easily offset – causing star to expand, contract • these changes can be observed as a variation in • star’s brightness as it “pulses”

  41. Instability of Red Giants – Variable Stars • brightness changes by many times • because of pulsations in the star •  factors of a few to 100 or more! • can see the “signature” of • changes over a few days to many • years • Long period variables: Miras • Shorter period variables: Cepheids Optical images of a variable star spaced over a few days

  42. Instability of Red Giants – Variable Stars • Red Giants are constantly • changing their relationship • between Luminosity and • Temperature “Instability Strip” • with every expansion, • some of the star’s outer • layers are lost into the • interstellar medium

  43. Red Giant expanding into the interstellar medium NGC 6826 3 minutes exposure 2.5 hours exposure

  44. Planetary Nebula (PN) • remains of star: very hot core T~100,000 K • surrounded by thin, hot layers of expanding star • symmetric shape • shows how gas ejected • bad name: no planets • spectra of PN: • emission lines of • H, Oxygen, Nitrogen • common in our • Galaxy ~50,000 ! • in the end, 80% of • star’s mass is lost

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