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AstronomyChapter 04: The Sun
Sunset over the Giza pyramids
Sun & Earth
The sun is the star closest to earthThe sun provides light, heat, and energy
for lifeThe 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
Mythology of the Sun
Clockwise from left: Egyptian, Greek, Aztec
Luminosity
Luminosity is the sun’s total energy outputL = 3.85x1026 watts (joules of energy per
second)Sun’s energy output is truly enormous and
nearly inexhaustible
Solar Constant
Solar Constant: Amount of solar energy that falls per second on earth’s outer atmosphere
Solar Constant = 1400 watts/m2
One week of solar output equals the energy of all reserves of fossil fuels
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
Solar System (AU)
Planet Distance in AU
Mercury 0.4
Venus 0.7
Earth 1.0
Mars 1.5
Jupiter 5.2
Saturn 9.5
Uranus 19.2
Neptune 30
Pluto 39.5
Size of Sun
Radius = 432,000 miles (696,000 km)Sun = 99% of solar system massAngular Size = ½ degree, same as full
moonSun 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
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!
Solar Projection
Solar projection during an eclipse
Toward sun
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
Nebular Theory Diagram
Beta Pictoris
The sun is believed to have been formed from a rotating disk
of gas and dust, similar to the one observed around
Beta Pictoris
Elements in the Sun
Sun contains about 70 chemical elements 73% Hydrogen 25% Helium 2% Other elements
H y d r o g e n , 7 3
H e liu m , 2 7 . 4
O t h e r , 2
Sun’s Structure
Atmosphere Corona Chromosphere Photosphere
Interior Convection Zone Radiation Zone Core
Sun Diagram
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 visible10,000 K
Granulation
Photosphere “live”
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
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
Granulation
Grainy appearance of photosphere
Individual cells around 600 miles in diameter, represent rising and sinking columns of gas
Sunspots
Sunspots are temporary, dark, cool patches on the sun’s bright photosphere
Sunspots can last from hours to monthsSome visible to unaided eye at sunset or
through hazy cloudsChinese astronomers recorded sunspots
as early as 800 BC.
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
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
Identify umbra, penumbra, granules
Sunspots
http://www.spaceweather.com/swpod2006/14aug06/roel.jpg
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”
Chromosphere during Eclipse
The chromosphere can be seen during eclipses as a red rim around the eclipsed sun
Coronado PSTPhoto & Chromosphere
A hydrogen-alpha filtered telescope, such as the Coronado PST, allows observation of the chromosphere at
any time
Prominences
Prominences resemble fiery outbursts along the edge of the sun
Represent gases held above solar surface by magnetic fields
Last for days to monthsNamed filaments when viewed on face of
sun
Prominences
More Prominences
http://www.spaceweather.com/swpod2006/09apr06/Chatman.jpg
Filaments
Dark filament, a prominence on the solar disk
Note prominences around the solar limb in this H-Alpha photo
Solar Features
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”
Totality, Awesome
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
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
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
Equilibrium
Balance of two forcesGravity-pushes inwardGas pressure-pushes outwardHydrostatic Equilibrium: Star’s radius
represents a compromise between two forces
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
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
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
Rotation
This animation shows the growth and rotation of sunspot 848, Jan. 19th-21st 2006
http://www.spaceweather.com/index.cgi
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!
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
Solar Cycles
Present
Cycle 23 (as of 2006)
In what year did solar maximum occur? Minimum?
Current Cycle
http://www.swpc.noaa.gov/SolarCycle/
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
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
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 PlutoMagnetic polarity reverses every 11 years
shortly after solar maximum, requires 22 years for a complete cycle
Sunspot Magnetism
Sunspots often appear in pairs displaying a north and south pole
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 hoursOccur near sunspot groups (active
regions)
Flare Effects
Can cause power outages, radio blackouts, electrical power outages, radiation hazard to astronauts
Cause of Northern Lights on earth (Aurora Borealis)
Solar Flare
This image was taken by the Soho satellite, a satellite devoted to observing the sun at various wavelengths
Solar Flare Effects
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
Northern Lights
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
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
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
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
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
Heliopause
The heliopause marks the edge of the sun’s influence
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
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
Earth’s Spiral MotionVega-Apex of the Sun’s Way
Earth
Sun
Galactic Revolution
Sun & planets orbit the center of the Milky Way Galaxy at about 563,000 mi/hr
One revolution takes 230 million yearsThe sun is located about 28,000 ly from
the Milky Way’s Center
Sun’s Revolution in Milky Way