2. 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

3. 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? HOPE: We just might get to know THE universe better?

4. Stars can be arranged into categories based on the features in their spectra… This is called “Spectral Classification” 1.by the “strength” (depth) of the absorption lines in their spectra 2.by their color as determined by their blackbody curve 3.by their temperature and luminosity How do we categorize stars? A few options:

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

6. Annie Jump Cannon (1863-1941) Single-handedly classified more than 250,000 stellar spectra.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

7. O B A F G K M hottest to coolest bluish to reddish An important sequence to remember: –Oh Boy, An F Grade Kills Me

8. The Spectral Sequence ClassSpectrumColorTemperatureO ionized and neutral helium, weakened hydrogen bluish 31,000-49,000 K B neutral helium, stronger hydrogen blue-white 10,000-31,000 K A strong hydrogen, ionized metals white 7400-10,000 K 7400-10,000 K F weaker hydrogen, ionized metals yellowish white 6000-7400 K 6000-7400 K G still weaker hydrogen, ionized and neutral metals yellowish 5300-6000 K 5300-6000 K K weak hydrogen, neutral metals orange 3900-5300 K 3900-5300 K M little or no hydrogen, neutral metals, molecules reddish 2200-3900 K 2200-3900 K L no hydrogen, metallic hydrides, alkalai metals red-infrared 1200-2200 K 1200-2200 K T methane bands infrared under 1200 K under 1200 K

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

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

11. Temperature 10 -4 10 -2 10 2 10 2 10 4 10 4 10 6 10 6 L/L Θ 40,000 20,000 10,000 5,000 2,500 WHITE DWARFS MAIN SEQUENCE SUPER GIANTS GIANTS OB AFGKM O B A F G K M Shematic H-R Diagram BRIGHTFAINT HOT COOL HOT COOL 15 15 10 10 5 – 5 – 10 0

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

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

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

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

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

17. Cool, but bright.Cool, but bright. Same temp as some main sequence stars  same surface brightness!Same temp as some main sequence stars  same surface brightness! Must be bigger AREA  BIGGER star! (and thus the name, red giant) Red Giants:

18. 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. First, there was a nebula. Or: last, there was a nebula.

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

22. 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.

25. 10 -4