Understanding Stars: Composition, Distance, and Classification

1. STARS 101 Chapter 30

2. You can see over 6000 stars with the unaided eye

3. What is a Star? • a self-luminous ball of hot gas that generates its own energy

5. Stars vary in : • COLOR • SIZE • MASS • COMPOSITION • TEMPERATURE • DISTANCE FROM EARTH • BRIGHTNESS

6. Stars are composed of the same elements found on Earth: • Hydrogen (most common element) • Helium (second) • Carbon • Nitrogen • Calcium

7. Temperature is indicated by Color • BLUE Stars 35,000°C-50,000°C • YELLOW Stars 5,000°C • RED Stars 3,000°C

8. Distance to Stars • Measured in light years • The distance light travels in one year • Light moves at 300,000 km/sec 186,000 miles/sec

9. What does this mean? • The sun is 93 million miles away or 8 light minutes • Light from the sun reaches us in 8 minutes • The closest star is Proxima Centauri, over 4.2 light years away • Sirius is 9 light years away • Polaris is 700 light years away

11. ONE LIGHT YEAR- • 9.5 trillion km • 5.865 trillion miles

12. STELLAR MAGNITUDES • APPARENT- how bright a star seems from Earth • ABSOLUTE- how bright the stars would seem if they were all set at the same distance its TRUE brightness

13. APPARENT MAGNITUDE • The brightest stars have the lowest numbers • The dimmest stars have the highest numbers • The faintest star visible with the naked eye is ranked a 6 on the apparent magnitude scale. • Sirius is ranked a -1.46 • The sun has an apparent magnitude of -26.8

15. ABSOLUTE MAGNITUDE • Imagines all stars set at 32.6 light years from Earth • Scale ranges from -5 to +15 • The Sun is ranked +5

16. Spectral Analysis of Stars Astronomers learn about stars by analyzing their LIGHT SPECTROGRAPHS (define – use p 775) Separate light into colors / wavelengths SPECTRUM( define use p 775) Reveals the star’s composition and temperature

19. MOTION OF STARS • APPARENT- due to mvt of Earth • A) move west (counterclockwise) due to rotation • B) shift to west a little earlier each night due to earth’s revolution

21. Motions of Stars (continued) • ACTUAL • 1- rotate on their axis • 2-may revolve around another star • 3-move toward or away from the Solar System

22. DOPPLEREFFECT- shifting of wavelengths of light toward the RED end of the spectrum as an object moves away from the observer

24. How do astronomers measure distance? • PARALLAX-the apparent shift in position of a star (object) when viewed from a different angle • The closer a nearby star- the greater the shift • The farther away a star is , the less it will shift • Try it:

25. Demonstrating Parallax Raise your finger to your nose and view a nearby object. Blink your eyes alternately. Now move your finger a little further away and blink again. The shift is less than before.

27. Spectral Classification of Stars Astronomers group stars by their surface temperature Use letters : O B A F G K M Hottest coolest O Be A Fine Girl Kiss Me ! See reference table 1.1

28. Spectral Classification continued 10 Sub categories : by number (0 to 9) For ex : an M2 star is hotter than an M7 star Our Sun is a G 2 star Betelgeuse is an M 2

29. Luminosity Another way to classify stars By width of spectral lines Uses Roman numeral I,II,III,IV,V Betelgeuse is an M2 I star See reference table 1.2

31. Hertzsprung-Russell Diagram • A pattern revealed by plotting the temperature vs. the absolute magnitude of stars • Shows that brightness increases as temperature increases • Reveals “life cycle” of most stars