Let’s review some important things we want to know about stars…

1. Let’s review some important things we want to know about stars… • Given enough time and information, we can figure out their… • Brightness - easily observed • Parallax to measure distance • Spectral type - can get from the spectrum • Brightness + Distance = Luminosity • Temperature - can get from spectrum • Temperature + distance = Size • Mass - hard to figure out, but there are binary stars • Age - exact age is hard, but can estimate

2. What do you do when you have data and you don’t know what to do with it and you don’t understand it? CLASSIFY! HOPE: We just might get to know THE universe better?

3. Stars can be arranged into categories based on the features in their spectra… by the “strength” (depth) of the absorption lines in their spectra by their color as determined by their blackbody curve by their temperature and luminosity This is called “Spectral Classification” How do we categorize stars? A few options:

4. Much of the work in classifying and explaining stellar spectra and brightness was done by women at Harvard around the turn of the century. Harvard Computers (1890)

5. Annie Jump Cannon (1863-1941) • Single-handedly classified more than 250,000 stellar spectra. Henrietta Leavitt (1868-1921) Stars are classified by their spectra as O, B, A, F, G, K, and M spectral types

6. OBAF GKM • hottest to coolest • bluish to reddish • An important sequence to remember: • Oh Boy, An FGrade Kills Me

7. The Spectral Sequence

8. Eventually, the connection was made between the observables and the theory. • Observable: • Strength of Hydrogen Absorption Lines • Blackbody Curve (Color) • Theoretical: • Using observables to determine things we can’t measure: Temperature and Luminosity Cecilia Payne

9. Categorizing the stars… Hertzsprung-Russell (H-R) Diagram • done independently by Enjar Hertzsprung and Henry Norris Russell • graph of luminosity versus temperature • (or spectral class)

10. Shematic H-RDiagram OB AFGKM BRIGHT FAINT – 10 SUPER GIANTS 106 – 5 104 GIANTS 5 102 MAIN SEQUENCE L/LΘ 0 10 WHITE DWARFS 10-2 15 10-4 20,000 40,000 10,000 5,000 2,500 Temperature HOTCOOL

11. the stars aren’t randomly scattered on this graph-- they form a line! WHAT IS AMAZING: Stars of different masses fall along a narrow path in L/T diagram Same temperature, but much brighter than MS stars →Must be much larger ► Giant Stars “Red Giants” “Supergiants” Most stars are found along the Main Sequence Stars spend most of their active life time on the Main Sequence (MS). None of these “extra” stars are Hydrogen burning! Same temp., but fainter → Dwarfs

12. If a random star falls on the Main Sequence, you also know that it’s Hydrogen burning! If you measure the luminosity and the color of a star, you know its mass!!!

13. The more massive a star is, the more luminous it is… • Hotter • Brighter • Bigger • Shorter-lived But a higher rate of fusion means it’s burning its fuel faster! More massive stars are… Low mass stars have lifetimes comparable to the Age of the Universe High mass stars have very short lifetimes, and disappear quickly!

14. Same Temperature & Surface Brightness Hot. Cool. Hot, but tiny Faint. Cool, but big Luminous! Same Luminosity The Mystery of Red Giants and White Dwarfs… Many of these stars have the same temperature as normal Main Sequence stars, but they’re muchbrighteror fainter! How is this possible??? If the size of the star changes, its luminosity changes L = b x Area

15. After time passes… The high mass stars are gone! Only long-lived low mass stars are left on the main sequence!

16. Red Giants: • Cool, but bright. • Same temp as some main sequence stars  same surface brightness! Must be biggerAREABIGGERstar! (and thus the name, red giant)

17. First, there was a nebula. Or: last, there was a nebula. as gravity caused the collapse Stars are formed by a cloud of gas and dust that collapsed inward and began to spin. These clouds are called nebula. About 30 million years after the cloud collapsed, its center has reached 15 million kelvin and has become a protostar. As stars continue to go through nuclear fusion from hydrogen gas combining to make deuterons and then two deuterons making helium, the star will eventually run out of hydrogen.

20. The birth of stars in the M16 Eagle Nebula and the cycle starts again

21. RED GIANT PHASE of star’s existance A star experiences an energy crisis and its core collapses when the star's basic, non-renewable energy source - hydrogen - is used up. A shell of hydrogen on the edge of the collapsed core will be compressed and heated. The nuclear fusion of the hydrogen in the shell will produce a new surge of power that will cause the outer layers of the star to expand until it has a diameter a hundred times its present value.

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