Our Star – the Sun

1. Our Star – the Sun

2. The study of the stars & nebulae provides information about the origin and history of the Earth and the solar system. Astronomers discover how stars like our Sun are born, grow old and die.

3. Review... • organize the following ... solar core...photosphere...radiative zone... ... chromosphere...convective zone...corona • the solar core is ...like the nuclear reactors on Earth because.... ...not like nuclear reactors on Earth because ...

4. lesson outcomes... • know, understand and explain: • the structure of the Sun • the process by which the Sun produces energy • the model for energy getting from the solar interior to the solar surface • some features of the Sun that affect the Earth

5. The Sun’s Structure

6. The Sun’s Structure • the Sun has no surface • gaseous • would only get denser gases as you go into greater depths • consists of a solar interior with an atmosphere that has three layers; atmosphere • increases in temperature & decrease in density • composed of a gaseous mixture known as a plasma • so hot that a lot of atoms are ionized (TBD unit 2: chemistry) • electrically charged ions and electrons which move freely • heated from below  photosphere granulation

7. The Sun’s Structure • the atmosphere of the Sun • the photosphere • visible surface of the Sun • all of the sun’s visible light emanates from this single thin layer of gas • convection of gases in this layer produces granulation, meaning the Sun is heated from the interior • the chromosphere • the corona • blends into the solar wind at great distances from the Sun

8. the Sun’s interior • theoreticaldescription of a sun’s interior • hydrogen burning takes place in a core extending from the sun’s centre • solar core is surrounded by a radiative zone in which energy travels outwards through radiative diffusion • radiative zone surrounded by an opaque convective zone of gas at relatively low temperature and pressure, where energy gravels outwards primarily through convection

9. The Solar Interior is a Model • stellar model is a theoreticaldescription of a sun’s interior derived from calculations based on the laws of physics • in science a model combines observed facts with accepted principles of nature to make new and quantitative predictions • a model of the Sun predicts what the pressure, temperature and density must be at various depths in its interior.

10. Hydrogen burning in the Sun’s core • The suns’ energy is produced by a thermonuclear reaction called hydrogen burning (nuclear fusion) • four H nuclei is slightly heavier than He nucleus • combine to form He nucleus • mass difference is converted to energy according to Einstein’s equation E = mc2 • energy is released • occur only at very high temperatures ( 107 K) such as in the dense and hot solar core.

11. Hydrogen burning in the Sun’s core • occur only at very high temperatures ( 107 K) such as in the dense and hot solar core. • otherwise positive electric charges on nuclei will keep hydrogen atoms apart because like charges repel

12. The Model of the Solar Interior – Solar neutrinos & Helioseismology • solar interior not visible to us so conditions in the solar interior inferred from measurements of : • solar neutrinos • neutrinos emitted from the nuclear reaction in the Sun detected but in smaller numbers than expected • solar vibrations • helioseismology (study of how the sun vibrates) infers pressure, densities, chemical composition and rotation of solar interior from the vibrations • cannot hear vibrations  0.003 Hz (too low for humans) so use technology e.g. SOHO; Earth telescopes

13. How energy gets to the Sun’s surface • 3 possible methods of energy transport from the solar core solar atmosphere • conduction (probably absent in the Sun) • requires materials of high density  efficiency decreases as materials become less dense  best in solids • stars are gases so density low so probably not inside Sun as too inefficient • convection (occurs in the Sun) • hot fluids (gas) rise and cold fluids sink • radiative diffusion(occurs in the Sun) • photons from star’s centre absorbed & reemitted by atoms & ē inside the star • so photons diffuse outward from the hot core, where photons are constantly created, toward the cooler surface where they escape into space

14. Sunspots – a feature of our Sun sunspot close-up sunspot

15. Sunspots – a feature of our Sun • dark regions on the surface of the Sun that are low-temperature regions in the photosphere • usually clusters called sunspots groups but sometimes in isolation • solar flare is a brief burst of hot ionized gases from sunspot groups • aurora borealis and aurora australis • interfere with satellites orbiting the Earth • movement of sunspots shows the sun has differential rotation • moves more rapidly at the equator (centre) than at the poles •  25 days at equator and 33 days at poles

16. Sunspots – a feature of our Sun • varies periodically with a 22 year solar cycle in the sun’s magnetic field • sunspots produced by areas of concentration of sun’s magnetic field • solar magnetic field increases, decreases and then increases again with opposite polarity • perhaps results from the effect of differential rotation and convection on the solar magnetic field • number of sunspots varies with a period of about 11 years variation in solar activity appears to affect Earth’s climate • a year with many sunspots = sunspot maximum  hotter • a year without sunspots = sunspot minimum  cool & wet

17. Sunspots & the Solar Cycle

18. Sunspots & the Solar Cycle

19. Solar Composition – the Spectrum • Hot objects like the Sun produce light • the kind and amount of light tells us how • how hot the object is • colour related to temperature • temperature: Kelvin (K) or degrees Celsius (°C) • how dense the object is • what it is made of (spectral lines)

20. K-W Chart Questions • does the sun have a solid surface? • where does solar wind come from? • what are sunspots? why are they dark? • how do we investigate the sun’s interior? • how do we know thermonuclear reactions are happening in the sun’s core?