The document provides information about the sun, including: - The sun is a star that is the center of our solar system and provides light, heat, and energy for life on Earth. - Ancient civilizations worshipped the sun as a god due to its importance. - The sun is powered by nuclear fusion in its core and has a diameter of over 432,000 miles. - Other topics covered include the sun's luminosity, solar constant, astronomical units, structure including the photosphere and chromosphere, sunspots, solar cycle, solar wind, and more.

1. Astronomy Chapter 04 : The Sun Sunset over the Giza pyramids

2. Sun & Earth The sun is the star closest to earth The sun provides light, heat, and energy for life The sun is powered by nuclear fusion, the conversion of hydrogen into helium inside the sun Ancient peoples such as the Egyptians, Greeks, and Aztecs worshipped the sun as a god

3. Mythology of the Sun Clockwise from left: Egyptian, Greek, Aztec

4. Luminosity Luminosity is the sun’s total energy output L = 3.85x10 26 watts (joules of energy per second) Sun’s energy output is truly enormous and nearly inexhaustible

5. Solar Constant Solar Constant : Amount of solar energy that falls per second on earth’s outer atmosphere Solar Constant = 1400 watts/m 2 One week of solar output equals the energy of all reserves of fossil fuels

6. Astronomical Unit Astronomical Unit (AU) is equal to the average earth-to-sun distance 93 million miles (150 million km) Used to measure distances in the solar system

7. Solar System (AU) 39.5 Pluto 30 Neptune 19.2 Uranus 9.5 Saturn 5.2 Jupiter 1.5 Mars 1.0 Earth 0.7 Venus 0.4 Mercury Distance in AU Planet

8. Size of Sun Radius = 432,000 miles (696,000 km) Sun = 99% of solar system mass Angular Size = ½ degree, same as full moon Sun is 400 times larger than the moon, yet 400 times more distant, so they appear about the same angular size Remarkable coincidence allows solar eclipses

9. Observing the Sun Safely Solar projection through a telescope, the sun is observed safely on a screen Never look at the sun through an unfiltered telescope!

10. Solar Projection Solar projection during an eclipse Toward sun

11. Origin of the Sun Nebular Theory : Proposed by Immanuel Kant (1724-1804) Sun and Planets formed together from a rotating cloud of gas and dust called the solar nebula about 5 billion years ago Such nebulas are observed around young stars such as Beta Pictoris

12. Nebular Theory Diagram The sun is believed to have been formed from a rotating disk of gas and dust, similar to the one observed around Beta Pictoris Beta Pictoris

13. Elements in the Sun Sun contains about 70 chemical elements 73% Hydrogen 25% Helium 2% Other elements

14. Sun’s Structure Atmosphere Corona Chromosphere Photosphere Interior Convection Zone Radiation Zone Core

15. Sun Diagram

16. Photosphere The photosphere is the sun’s visible “surface” Edge is termed the limb Limb is darker than center, termed limb darkening (visible in telescope) Photosphere can have a grainy appearance (granulation) in a good telescope Sunspots visible 10,000 K Granulation Photosphere “live”

17. White Light Photosphere The safe solar filter covers the entire aperture of the telescope The solar photosphere can appear granular in a good telescope—note the limb darkening

18. Limb Darkening The photosphere appears slightly dimmer near its limb This effect, limb darkening, is evident in this photograph of the sun http://users.otenet.gr/~a_evagel/Essay/faculae.htm

19. Granulation Grainy appearance of photosphere Individual cells around 600 miles in diameter, represent rising and sinking columns of gas

20. Sunspots Sunspots are temporary, dark, cool patches on the sun’s bright photosphere Sunspots can last from hours to months Some visible to unaided eye at sunset or through hazy clouds Chinese astronomers recorded sunspots as early as 800 BC.

21. Sunspot Details Range from the size of the earth to many earths Umbra : Dark core Penumbra : Grayish outer zone Often appear in groups called active regions Sunspots are created by variations in the sun’s magnetic field First studied in west by Galileo (1610), he was able to deduce the sun’s rotational period

22. Galileo’s Sunspot Observations A sketch of the sun by Galileo Galileo was the first astronomer to observe the sky with a telescope He used small, simple refracting telescopes that are crude by modern standards

23. Identify umbra, penumbra, granules Sunspots http://www.spaceweather.com/swpod2006/14aug06/roel.jpg

24. Chromosphere The chromosphere is a thin, transparent layer about 6000 miles above photosphere Visible during total eclipses, h-alpha telescope filters Red layer, prominences visible Prominence, not the size of the earth for comparison Chromosphere “live”

25. Chromosphere during Eclipse The chromosphere can be seen during eclipses as a red rim around the eclipsed sun

26. Coronado PST Photo & Chromosphere A hydrogen-alpha filtered telescope, such as the Coronado PST, allows observation of the chromosphere at any time

27. Prominences Prominences resemble fiery outbursts along the edge of the sun Represent gases held above solar surface by magnetic fields Last for days to months Named filaments when viewed on face of sun

28. Prominences

29. More Prominences http://www.spaceweather.com/swpod2006/09apr06/Chatman.jpg

30. Filaments Dark filament, a prominence on the solar disk Note prominences around the solar limb in this H-Alpha photo

31. Solar Features

32. Corona Corona is Latin, “crown” Visible during total eclipses as a spiky, jagged white halo around the eclipsed sun, one of the most awesome sights in nature Outermost atmosphere, extends millions of miles into space High temp, up to 2 million K Corona “live”

33. Totality, Awesome

34. March 2006 http://www.buytelescopes.com/gallery/view_photo.asp?pid=8652&sg=1 Note the face of the moon is faintly visible in this highly processed image

35. America’s Next Total Solar Eclipse-2017 Observers inside the red line will see a total eclipse Observers within the shaded area will see a partial eclipse

36. Interior Below photosphere, temp and density increase with depth Up 15 million degrees K inside, density 100 times that of water Pressure = 200 billion earth atmospheres Core, nuclear fusion powers the sun Heat & energy of the sun provides pressure to balance the inward, crushing pull of gravity, keeps star intact as a sphere

37. Equilibrium Balance of two forces Gravity-pushes inward Gas pressure-pushes outward Hydrostatic Equilibrium : Star’s radius represents a compromise between two forces

38. Light from Within Energy slowly transmitted out (radiative diffusion) Takes about 20 million years for light from the core to reach the surface and become sunshine

39. Sun’s Rotation Sun rotates on an axis from west to east, just as the earth does Sun is not a rigid body, different parts rotate at different speeds Period of rotation = 25 days at equator, 35 days at poles Termed differential rotation

40. Solar Rotation Note that sunspot groups on the sun appear to change position when observed for a few days in a row The motion of sunspots reveals the sun’s rotation

41. Rotation This animation shows the growth and rotation of sunspot 848, Jan. 19 th -21 st 2006 http://www.spaceweather.com/index.cgi

42. Differential Rotation Note the different rotation rates of the equator and poles of the sun The interior rotates at a different rate also The sun is a complex place!

43. 11-Year Cycle Number of sunspots rises and falls over an 11-year cycle The highest part of the cycle is termed solar maximum, the lowest solar minimum At solar max, the sun can display nearly 200 sunspots per year, at solar min it can approach zero

44. Solar Cycles Present

45. Cycle 23 (as of 2006) In what year did solar maximum occur? Minimum?

46. Current Cycle http://www.swpc.noaa.gov/SolarCycle/

47. Maunder Minimum Period of low solar activity from 1645-1715, virtually no sunspots were visible for many years Coincided with cold period in Europe (“Little Ice Age”) River Thames froze solid Apparently sunspots have a relationship to earth’s climate

48. Butterfly Diagram Sunspots tend to appear at high latitudes at the start of a solar 11-yr cycle Latitudes shift toward solar equator through the rest of the cycle Graph: Butterfly diagram

49. Solar Magnetism Sunspots act as huge magnets, have N and S poles Entire sun exhibits a weak magnetic field offset 15 degrees from sun’s axis of rotation Entire field extends out beyond Pluto Magnetic polarity reverses every 11 years shortly after solar maximum, requires 22 years for a complete cycle

50. Sunspot Magnetism Sunspots often appear in pairs displaying a north and south pole

51. Flares Solar Flare : A sudden, tremendous, explosive outburst of light, invisible radiation, and material from the sun One great flare can equal the energy that would be used by the entire world in 100,000 years Short, last a few minutes to a few hours Occur near sunspot groups (active regions)

52. Flare Effects Can cause power outages, radio blackouts, electrical power outages, radiation hazard to astronauts Cause of Northern Lights on earth (Aurora Borealis)

53. Solar Flare This image was taken by the Soho satellite, a satellite devoted to observing the sun at various wavelengths

54. Solar Flare Effects

55. Aurora Borealis Northern Lights, Southern Lights (Aurora Australis) Bands of light visible from earth’s high latitudes, Arctic, and Antarctic; Occasionally lower (We can see them here sometimes) Occur about 2 days after major solar flares Occur when high-energy particles from the sun ionize gases in earth’s atmosphere

56. Northern Lights

57. Solar Wind Solar Wind : A plasma, or stream of charged particles that flows outward from the sun at all times Much faster, hotter, and thinner than earth wind Solar wind escapes from the sun through “gaps” in the corona named coronal holes Solar wind takes 4 days to reach earth, about 1 million miles/hour Strongest during solar maximum

58. Coronal Mass Ejections CME : Coronal Mass Ejections, particularly large bursts of solar wind Solar flares & CMEs emit blasts of solar wind, causing auroras and earth disturbances

59. CME In this image from a coronagraph, a cme appears to the right The sun’s disk is represented by the small, central circle Movie: http://sohowww.nascom.nasa.gov/bestofsoho/Movies/C3_Apr01/C3_Apr01sm.mpg http://solar-center.stanford.edu/magnetism/magnetismsun.html

60. Earth’s Protective Shield Earth’s magnetic field shields us from harmful bursts of solar radiation (solar wind and cmes) Solar radiation is deflected around our planet Earth

61. Heliopause Heliopause : Edge of solar wind, lies beyond the orbit of Pluto The heliopause represents the boundary marking the edge of the sun’s direct influence Spacecraft from the 1970s and 80s, the Pioneers and Voyagers, will soon cross the heliopause

62. Heliopause The heliopause marks the edge of the sun’s influence

63. Motion in Space Sun, like other stars, is racing through space Apex of the Sun’s Way : Sun is speeding towards the star Vega at 45,000 mi/hr carrying the nine planets along with it In consequence, the planets move in a spiral path through space as they orbit the moving sun

64. Vega, the Apex of the Sun’s Way The direction of the sun’s motion through the galaxy is located near the star Vega on the celestial sphere

65. Earth’s Spiral Motion Vega-Apex of the Sun’s Way Earth Sun

66. Galactic Revolution Sun & planets orbit the center of the Milky Way Galaxy at about 563,000 mi/hr One revolution takes 230 million years The sun is located about 28,000 ly from the Milky Way’s Center

67. Sun’s Revolution in Milky Way