Download The Sun: Our Star

The Sun: Our Star
The Sun is an ordinary star and shines the same way other stars do.
Announcements
q Homework # 4 is due today!
Assigned Reading
q  Units 49 and 51
Today’s Goals
q  Today we start section 3 of our Course: STARS.
q We begin by investigating the closest star to us:
the Sun:
q The Sun
q  The Sun provides virtually all of the energy
necessary for life on Earth: food, weather, etc.
q  Our main experience with the Sun is through the
heat and light it provides (`sunshine ), and visually,
as a yellow/whit-ish sphere in the sky
The Sun’s ID
v Radius: 700,000 km (109 times the Earth)
v Mass: 2 x 1030 kg (330,000 times the Earth)
v Luminosity: 4 1026 W
v  Average density: 1.4 kg/l (a little over water); however:
v  Central density: 160 kg/l (20 times the density of
steel)
v  Photospheric density: ~1/3000 that of air (and
about 1/2,400,000 that of water)
v  Surface Temperature: 5,800 K
v  Central Temperature: 15,000,000 K
The Sun is hotter towards the
center:
the limb is slightly darker
The Solar Constant
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The S.C tells us the amount of energy the Sun
produces:
•  It is 1360 Joules per second per square meter…
•  …or about the luminosity (=ENERGY PER SECOND) of 60
W bulb every square millimeter.
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A stable change of ~1% in the S.C. would change
Earth’s temperature by 1-2 degrees
Random variation ~0.1% over days or weeks
Long-term, cyclical variation 0.018% per year
(period longer than the 22 years of the sunspots)
Small random fluctuations do not affect Earth’s
climate
What is the Sun?
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The Sun is a star, a fairly common and
typical star
A star is a ball of gas held in hydrostatic
equilibrium against its own self-gravity by
the thermal pressure of the hot gas,
cooling from center to the surface
This energy comes from nuclear fusion
reactions at the center of the Sun (we will
expand on this in the next lecture)
What is the Sun made of
It is made of hot gas; there is no solid material in the
sun!
The gas is mostly hydrogen (71%) and helium (~27%),
plus traces of heavier elements (2%)
Why does the Sun have such a sharp edge?
Because it is made of dense gas; it takes 100,000 years
for a photon to travel from the Sun’s center to the
photosphere (and only 8.3 minutes to come to us!).
Hot+dense gas = plasma
What is that Yellow-ish Sphere
The bright part normally
seen is called the
photosphere, which is
about 500 km deep.
It is an almost perfect
black body with a surface
temperature of 5800 K.
Structure of the Sun
The Surface and Atmosphere of
the Sun
Although the sun appears to have sharp edge, its
“surface” (Photosphere) actually has a complicated
structure.
Its atmosphere is complex, too.
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Name
Temperature
Spectrum
Photosphere: 5,800 K,
continuum + abs.
Chromosphere: 4,500-500,000 K,
emiss.+abs.
Corona:
1-2 106 K, continuum+emiss.+abs.
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Chromosphere and Corona are the `Atmosphere’
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Each square millimeter of the
Photosphere emits the same
Energy as a ~60 Watt light bulb
Corona (1-2,000,000 K)
spicules
The Sun surface (photosphere)
would cool off very quickly if Energy
were not produced continuously from
below
(4500-500,000 K)
(5,800 K)
The light we see originates from the thin photosphere
Spectrum of the Photosphere
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The photosphere: the region where the
black body radiation at 5,800 Kthat we
see is made
Thin gas: about 3,000 times thinner
than air (you need to travel about
7,000 km into the Sun to find a density
similar to that of air)
Its upper layer produces the absorption
spectrum (Fraunohfer spectrum)
Fraunhofer Spectrum
The Photosphere is NOT Perfect:
Granulation of the Photosphere
Each Granule is about the size of Texas and lasts for only
5-10 minutes before fading away! It is due to the convection
zone
Granules: convective cells
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The granules are just densely packed
convective cells
Convective cells are very similar to
thunderstorms
Sunspots
Sunspots, about 1000-1500 K
cooler than the rest of the Sun's
photosphere, appear as dark
spots. Size range ~500-5,000 km
Sunspots come and go with
time. Big sunspots can live for
several weeks.
They are `magnetic hurricanes ,
regions with magnetic fields
~100 times stronger than the rest
of the Sun.
Summary of Photosphere
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It is the `surface’ of the Sun, where most of the
light we observe is coming from
It is an almost perfect blackbody with
temperature 5,800 K
The photosphere shows `granularity’; the
granules are convective cells of cooling gas
Sun spots are magnetic disturbances (`magnetic
hurricanes’) on the surface of the Sun that can
live up to several weeks; they are regions about
1,000-1,500 K cooler than the surrounding
regions (hence they appear to be as dark spots).
The Atmosphere of the Sun
Atmosphere of the Sun:
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Name
Temperature
Spectrum
Chromosphere: 4,500-500,000 K,
emiss.+abs.
Corona:
1-2 106 K, continuum+emiss.+abs.
Chromosphere
Reddish in color, which
is the origin of its name
(chromos meaning
``color'')
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2000-3000 km thick
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Faint relative to the
photosphere
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From 4,500 up to
500,000K: hotter than the
photosphere and much
less dense.
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Contains thin columns of
gas called spicules
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Solar Prominence
Composed of hot gas
trapped in magnetic
fields extending from
one sunspot to another.
They jut from the
lower chromosphere
into the corona.
Prominence
UltraViolet (UV) image of the Sun.
It shows that regions of the
Photosphere corresponding to
Sunspots
are more energetic than
elsewhere.
It also shows magnetic
links (prominences) between
Sunspots
extending into the
Chromosphere and Corona
Solar Flare
A solar flare is a violet outburst that lasts an hour or less. It
radiates X-ray, ultraviolet, and visible radiation, plus streams
of high-energy protons and electrons.
Solar Flares originate in/near Sun spots and juts into the
chromosphere.
A large flare can be a billion times more energetic than a large
hydrogen bomb.
Solar flares originate in/near sunspots, and
are violent, but brief (~1 hour), outbursts of
energy.
Picture of a Solar Flare
Corona
- the outermost layer
- made up of very diffuse (tenuous) but
extremely hot gas (T~1 million K)
-  coronal emission is dominated by X-rays
-  carries very little energy
-  `stirred’ by the Sun’s magnetic field
The heating of the corona
The heating of the corona has
puzzled scientists for a long time.
How can heat go from cooler
regions to hotter ones?
It doesn’t! Radiant heat
penetrates the corona nearly
undisturbed
Corona heated by something
else:
whipping by magnetic force lines
The corona is heated by energy
outflowing from the sun's interior
not as heat but as magnetic energy.
The gas is heated by the motion
through it of long lines of magnetic
force, which act like whips.
The gas is being whipped .
The heating of the corona
The corona is heated by energy from the sun's interior not as heat
but as magnetic energy. Heated by the whipping motion of lines
of magnetic force
Solar Activity
Solar Wind
The corona’s high temperature gives sufficient kinetic energy to
the atoms that the gas `escapes’, creating the solar wind. The
Sun is loosing mass, at a rate of 1.5E6 tons/s, but is only a small
fraction of Sun’s mass: 10-14 Mo/yr
Auroras: the Northern and
Southern Lights
Auroras
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Auroras seen as far south as New
England
Caused by a gust of solar wind (coronal
mass ejections)
These powerful gusts are guided by
Earth’s magnetic field and can excite
gases in the upper atmosphere,
causing the air there to glow
Activity on the Sun
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Two key points to remember:
1.  nearly all “solar weather” is a result of changes in
the magnetic fields on the Photosphere.
2.  Magnetic Fields are the result of
convection in the Sun
The Sun’s Rotation and Magnetic Field:
Impact on Activity
The Sun is a `ball of gas’ that rotates on its axis; the equatorial
regions rotate faster (25 days) than the north or south (30 days).
Babcock Model
Sunspots are created at the `kinks’ of the magnetic field.
After the maximum of activity (flares, mass ejection, many
sunspots), the field relaxes again, and inverts polarity.
The differential rotation causes magnetic cycles:
appearance of sun spots, and their cycling through a
maximum every ~ 11 years
The Solar Cycle
Solar activity peaks
roughly each 11 years.
Survey Question
The bulk of the violent surface activity on the
Sun is due to “seasonal” variations in the
Sun’s _________.
1) energy output
2) radius
3) electric fields
4) magnetic fields