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04 The Sun Mc Neely

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Astronomy Chapter 04 : The Sun Sunset over the Giza pyramids
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Page 1: 04 The Sun Mc Neely

AstronomyChapter 04: The Sun

Sunset over the Giza pyramids

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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

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Mythology of the Sun

Clockwise from left: Egyptian, Greek, Aztec

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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

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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

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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

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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

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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

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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!

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Solar Projection

Solar projection during an eclipse

Toward sun

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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

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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

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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

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Sun’s Structure

Atmosphere Corona Chromosphere Photosphere

Interior Convection Zone Radiation Zone Core

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Sun Diagram

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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”

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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

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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

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Granulation

Grainy appearance of photosphere

Individual cells around 600 miles in diameter, represent rising and sinking columns of gas

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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.

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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

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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

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Identify umbra, penumbra, granules

Sunspots

http://www.spaceweather.com/swpod2006/14aug06/roel.jpg

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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”

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Chromosphere during Eclipse

The chromosphere can be seen during eclipses as a red rim around the eclipsed sun

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Coronado PSTPhoto & Chromosphere

A hydrogen-alpha filtered telescope, such as the Coronado PST, allows observation of the chromosphere at

any time

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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

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Prominences

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More Prominences

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

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Filaments

Dark filament, a prominence on the solar disk

Note prominences around the solar limb in this H-Alpha photo

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Solar Features

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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”

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Totality, Awesome

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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

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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

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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

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Equilibrium

Balance of two forcesGravity-pushes inwardGas pressure-pushes outwardHydrostatic Equilibrium: Star’s radius

represents a compromise between two forces

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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

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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

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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

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Rotation

This animation shows the growth and rotation of sunspot 848, Jan. 19th-21st 2006

http://www.spaceweather.com/index.cgi

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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!

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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

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Solar Cycles

Present

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Cycle 23 (as of 2006)

In what year did solar maximum occur? Minimum?

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Current Cycle

http://www.swpc.noaa.gov/SolarCycle/

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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

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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

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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

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Sunspot Magnetism

Sunspots often appear in pairs displaying a north and south pole

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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)

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Flare Effects

Can cause power outages, radio blackouts, electrical power outages, radiation hazard to astronauts

Cause of Northern Lights on earth (Aurora Borealis)

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Solar Flare

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

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Solar Flare Effects

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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

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Northern Lights

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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

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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

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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

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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

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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

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Heliopause

The heliopause marks the edge of the sun’s influence

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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

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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

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Earth’s Spiral MotionVega-Apex of the Sun’s Way

Earth

Sun

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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

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Sun’s Revolution in Milky Way


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