Stars

1. Stars Physics 360 - Astrophysics

3. Brightness • Different brightness. • Different color. • How bright are they really? • What is due to distance? • What is due to luminosity?

4. Spectral Classification

5. 0.005 arcsec Mass

6. 50 mas Stellar Radii How big are stars? • Stars have different sizes. • If you know: • Distance • Angular size • Learn real size.

7. Sizes of Stars Supergiants, Giants and Dwarfs

8. The Sun, Our Star • The Sun is an average star. • From the Sun, we base our understanding of all stars in the Universe. • No solid surface.

9. Vital Statistics • Radius = 100 x Earth (696,000 km) • Mass = 300,000 x Earth (1.99 x 1030 kg) • Surface temp = 5,800 K • Core temp = 15,000,000 K • Luminosity = 4 x 1026 Watts • Solar “Day” = • 24.9 Earth days (equator) • 29.8 Earth days (poles)

10. Interior Properties • Core = 20 x density of iron • Surface = 10,000 x less dense than air • Average density = Jupiter • Core = 15,000,000 K • Surface = 5800 K

11. 1. The Core • Scientific Method: • Observations • Make hypothesis (a model) • Models make predictions • Test predictions • Compare results of predictions with observations • Revise model if necessary.

12. Testing the Core • Observe Sun’s: • Mass (how?) • Composition (how?) • Radius • Use physics to make a model Sun. • Predict: • Surface temp/density (how do you test?) • Surface Luminosity (how do you test?) • Core temp/density  Fusion Rate  neutrino rate (test?)

13. Density = 20 x density of Iron Temperature = 15,000,000 K Hydrogen atoms fuse together. Create Helium atoms. In The Core

14. Nuclear Fusion • 4H  He • The mass of 4 H nuclei (4 protons): 4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg • The mass of He nuclei: = 6.643 x 10-27 kg • Where does the extra 4.7 x 10-29 kg go? • ENERGY!  E = mc2 • E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2 • E = hc/l l = 4.6 x 10-14 m (gamma rays) • So: 4H  He + light

15. 2. Helioseismology • Continuous monitoring of Sun. • Ground based observatories • One spacecraft (SOHO) • Surface of the Sun is ‘ringing’ • Sound waves cross the the solar interior and reflect off of the surface (photosphere).

16. Solar Interior • Core • Only place with fusion • Radiation Zone • Transparent • Convections Zone • Boiling hot

17. Convection • A pot of boiling water: • Hot material rises. • Cooler material sinks. • The energy from the pot’s hot bottom is physically carried by the convection cells in the water to the surface. • Same for the Sun.

18. Solar Cross-Section • Progressively smaller convection cells carry the energy towards surface. • See tops of these cells as granules.

19. The Photosphere • This is the origin of the 5,800 K thermal radiation we see. l = k/T = k/(5800 K)  l=480 nm (visible light) • This is the light we see. • That’s why we see this as the surface.

20. 11-year sunspot cycle. • Center – Umbra: 4500 K • Edge – Penumbra: 5500 K • Photosphere: 5800 K Sunspots

21. Magnetic fields and Sunspots • At kinks, disruption in convection cells. • Sunspots form.

22. Magnetic fields and Sunspots • Where magnetic fields “pop out” of Sun, form sunspots. • Sunspots come in pairs.

23. Prominences Hot low density gas = emission lines