1 / 27

HR Diagrams

HR Diagrams. AST 112. Measurements. We can measure: Temperature Mass Spectra Size Luminosity Distance We can make measurements on trillions of stars. A pattern? Seriously?.

yetta-chan
Download Presentation

HR Diagrams

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. HR Diagrams AST 112

  2. Measurements • We can measure: • Temperature • Mass • Spectra • Size • Luminosity • Distance • We can make measurements on trillions of stars.

  3. A pattern? Seriously? Given the many properties we can measureand the trillions of stars that we can observe,is there any hope of fitting it into a simplescheme? Or is it all just random?

  4. Classification Scheme • These stars at same distance • Brightest ones are redor bluish white • Less bright ones yellow white • Dimmest ones red

  5. Classification Scheme • Blue is hotter, red is cooler • So the photo suggests relation between luminosity (brightness) and color (due to temperature) • EjnarHertzsprung and Henry Russell graphed luminosity vs spectral type, found patterns

  6. Size, Temperature, Luminosity • Keep the following relations in mind: • For a given temperature, luminosity goes UP as size goes up • Temperature sets color • For a given size, luminosity goes UP as temperature goes up

  7. Hertzsprung – Russell (H-R) Diagram

  8. H-R Diagram • Very important in astronomy • Luminosity on vertical axis • Temperature (or spectral type) on horizontal axis

  9. H-R Diagram • Notice the star-size indicators on the graph • If it is a given temperature: • If it’s larger, it gives off more light • For a given size, if it’s hotter, it gives off more light

  10. H-R Diagram • Four major groups: • Main Sequence • Supergiants • Giants • White Dwarfs

  11. H-R Diagram • Luminosity Class: Where it lives on H-R Diagram • I: Supergiants • III: Giants • V: Main Sequence Stars • VII: White Dwarf Stars

  12. Stellar Classification • To completely classify a star, specify: • Spectral Type • Luminosity Class

  13. Stellar Classification • The Sun is G2 V • Spectral type is G2 (yellow-white), Luminosity Class is V (main sequence) • Betelgeuse is M2 I • Spectral type is M2 (red), Luminosity Class is I (supergiant) • Sirius is A1 V • Spectral type is A1 (bluish-white), Luminosity Class is V (main sequence)

  14. The Main Sequence • Most stars we observe live on the Main Sequence • Star’s position along Main Sequence depends on its mass • Remember that we know mass by looking at binary stars

  15. The Main Sequence • All stars on Main Sequence fusing H into He • As mass increases: • H fusion increases • Size increases • Temperature goes up • Luminosity goes up

  16. The Main Sequence • During a star’s life on the Main Sequence, it doesn’t move very far on the HR diagram.

  17. The Main Sequence • Stars have finite amount of H • More massive stars have higher fusion rates, shorter lifetimes • If we know the mass and know the nuclear reactions, we can estimate the lifetime

  18. The Main Sequence • Largest Main Sequence stars live 10 million years • Smallest ones live 1 trillion years • Larger stars therefore more rare • Their lifetimes are 10,000,000 years. The Universe is 14,700,000,000 years old. Why do these stars exist?

  19. Stellar Mass • Ultimately, a star’s mass determines -all- of its properties • Temperature • Spectral type • Luminosity • Size • Fusion rate • Lifetime

  20. Giants and Supergiants • These have exhausted their supply of hydrogen • At the end of their lives. Can pull some “tricks” before gravity wins and crushes the star inward. • They fuse heavier elements. • Fusion rate is very high • Star swells from releasing the energy of fusion

  21. Giants and Supergiants • Red giants and supergiants: • They are cooler but more luminous • They must be larger than the Sun • Betelgeuse is 500x larger than the Sun!

  22. White Dwarfs • If a giant (not a supergiant!) has a mass similar to the Sun: • It expels its outer layers • What’s left is a dense, hot core • Book page 535: White dwarfs are “remaining embers of former giants”

  23. Relative Sizes

  24. Variable Stars • Brightnesses of a star may vary with time • Outer layers may be opaque • Absorb energy, swell out • Becomes more transparent, lets energy out • Contracts • Repeats

  25. Variable Stars • Steady repetition of brightness variation • Period ranges from several hours to several years

  26. Variable Stars • Variable stars tend to live on a particular part of the H-R diagram • “Instability Strip”

  27. Cepheid Variable Stars • Tip of the Instability Strip has Cepheid variable stars • Period of pulsation closely related to their luminosities • We use these to measure the distances to other galaxies.

More Related