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Stars. Physics 360 - Astrophysics. Brightness. Different brightness. Different color. How bright are they really? What is due to distance? What is due to luminosity?. Spectral Classification. 0.005 arcsec. Mass. 50 mas. Stellar Radii. How big are stars? Stars have different sizes.
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Stars Physics 360 - Astrophysics
Brightness • Different brightness. • Different color. • How bright are they really? • What is due to distance? • What is due to luminosity?
0.005 arcsec Mass
50 mas Stellar Radii How big are stars? • Stars have different sizes. • If you know: • Distance • Angular size • Learn real size.
Sizes of Stars Supergiants, Giants and Dwarfs
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.
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)
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
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.
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?)
Density = 20 x density of Iron Temperature = 15,000,000 K Hydrogen atoms fuse together. Create Helium atoms. In The Core
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
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).
Solar Interior • Core • Only place with fusion • Radiation Zone • Transparent • Convections Zone • Boiling hot
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.
Solar Cross-Section • Progressively smaller convection cells carry the energy towards surface. • See tops of these cells as granules.
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.
11-year sunspot cycle. • Center – Umbra: 4500 K • Edge – Penumbra: 5500 K • Photosphere: 5800 K Sunspots
Magnetic fields and Sunspots • At kinks, disruption in convection cells. • Sunspots form.
Magnetic fields and Sunspots • Where magnetic fields “pop out” of Sun, form sunspots. • Sunspots come in pairs.
Prominences Hot low density gas = emission lines