Download The Sun—Our Star

Chapter 7
The Sun—Our Star
The sun is a normal star
The Sun—Our Star
The Sun—Our Star
General Properties of Stars
Stars are very simple--balls of hot gas held together
by their own gravity
their gravity would collapse them if they were not so hot
the hot gas inside stars has very high pressure but their
gravity confines them and keeps them from exploding
A star is a simple structure like a soap bubble. It is
balanced between opposing pressures .
We study the sun because it is a close-up example
of a star
Life on earth is depends critically on the sun
observations and phenomena found on the sun are
also found on other more distant stars
by our study of our sun, we can make some
assumptions for studying other stars
7-1 The Solar Atmosphere
The Solar Atmosphere
Sun is hot gas from highest layers down to its center
More dense than water but less dense than rock
Atmosphere is divided into three layers
Photosphere
Chromosphere
Corona
The Solar Atmosphere
Below the Surface
Stars make their energy near their centers so all
are dominated by the outward flow of energy.
Evidence of this outflow is found in the form of
hot and cool regions, gas motions, and magnetic
fields.
The Solar Atmosphere
The Photosphere (visible surface)
Not solid surface
Thin layer of gas from which we receive most of
the sun’s light
Dense enough to emit plenty of light—not so dense
that the light cannot escape
Less than 500km deep and has an average
temperature of about 6000K.
Image from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole
The Solar Atmosphere
Visible-light
photo of sun's
surface showing
granulation
The Photosphere (visible surface)
Photosphere in good photographs has a mottled
appearance because it is made up of dark-edged
regions.
Regions are called granules. Each granule is about
the size of Texas and lasts between 10 and twenty
minutes.
Centers are a few hundred degrees hotter than edges.
Granules are thought to
be the tops of rising
convection currents just
below the photosphere.
Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole
The Solar Atmosphere
The Photosphere (visible surface)
What is convection?
Convection is what happens when hot fluid rises and
cool fluid sinks.
Rising currents of hot gas heat small regions of the
photosphere which then emit more black body
radiation and look brighter. Cool sinking gas on the
edges emits less light and looks darker.
The Solar Atmosphere
The Chromosphere
Produces an emission spectrum
Kirchoff's laws says it is an excited, low-density
gas
Temperature ranges from
10,000 K to 1,000,000 K
or more
Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole
The Solar Atmosphere
The Chromosphere
The chromosphere is roughly 1000 times fainter
than the photosphere
Can only be seen with specialized telescopes
except during a total solar eclipse
During an eclipse (moon completely covers the
bright photosphere), the chromosphere appears as
a narrow layer of pink gas
The Solar Atmosphere
The Chromosphere
Source of Balmer lines from the sun
filtergrams reveal spicules – extensions of
relatively cool chromspheric gas up into the much
hotter corona
spicules spring up around the edges of
supergranules twice the diameter of Earth
An H filtergram showing
H filtergram
spicules and supergranules
Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole
The Solar Atmosphere
The Corona
Located above the chromosphere
Extends 12 solar radii or more
From the Latin word for crown
Best studies of the corona have been made from
spacecraft above Earth’s atmosphere.
Temperature of the corona rises with altitude
ranges from 50,000 K to 3,000,000 K
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
The Solar Atmosphere
The Corona
Density is 1 to 10 atoms/cm3
High temperature of corona has been a long
standing mystery, but it has been partially solved
by SOHO (Solar and Heliospheric Observatory).
The Solar Atmosphere
The Corona
The Solar Atmosphere
The Corona
Astronomers have mapped small loops of
magnetic field scattered all over the solar surface.
The loops of this magnetic carpet constantly
thrash around breaking and reconnecting. This
agitates the atoms above the photosphere and
deposits heat in the upper chromosphere and
corona.
The upper corona is so hot that the sun cannot
contain it. High-velocity gas atoms stream away
from the sun in a continuous breeze called the
Solar Wind.
Solar Wind blows past Earth at a speed of 300 to
800 km/sec continuing out past the planets to mix
with the gases between the stars.
The corona is heated by energy flowing from the
sun’s interior not as heat but as magnetic energy.
SOHO Images
Photosphere with
sunspots
Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole
Far ultraviolet
images from: Foundations of Astronomy, 7 th Edition; Seeds, Michael A.; Brooks/Cole
Corona
The Solar Atmosphere
Shorter wavelengths do not penetrate as deeply as longer ones.
Waves reflecting off underside of surface can cause a pattern of
rising and falling regions.
Helioseismology
Helioseismology is the study of the modes of
vibration of the sun.
Solar astronomers explore the sun’s interior by
studying how sound waves travel through the
sun.
Solar astronomers can determine temperature,
density, pressure, composition and motion of the
inner layers.
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
One of 10 million possible patterns of oscillation
WARNING!
DO NOT LOOK DIRECTLY AT SUN WITH
EITHER AIDED OR UNAIDED EYE
THE INFRARED RAYS COULD
COOK YOUR RETINA
Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole
7-2 Solar Activity
Sunspots
Dark spots on the
photosphere
Dark center called the
umbra
Lighter region around
center called penumbra
Usually occur in pairs
and last up to two
months
Image from: Horizons, 6 th Edition; Seeds, Michael A.; Brooks/Cole
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
Image from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
Solar Activity
Sunspots
Temperature of umbra about 4240 K
Number of sunspots visible is not constant and
varies with a period of about 11 years
This is known as the sunspot cycle.
Another
maximum
cycle is
expected in
the year 2012.
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
Solar Activity
Sunspots
images from: Foundations of Astronomy, 7 th Edition; Seeds, Michael A.; Brooks/Cole
Solar Activity
Active Regions and Sunspots
Maunder Butterfly Diagrams
Plots of sunspot activity during sunspot cycles
Sunspots appear at about 35˚ latitude at the beginning
of the cycle and at about 5˚ latitude at the end of the
cycle.
Plots of
latitude
over time
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
What are sunspots?
Visible traces of great magnetic
storms on the solar surface
This is backed up by the
Zeeman Effect
The splitting of single spectral
lines into multiple components
through the influence of
magnetic fields
Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole
Solar Activity
Active Regions and Sunspots
The magnetic field in a typical sunspot is 1000 times
stronger than the sun’s average field.
Apparently,the strength of the magnetic field inhibits
gas motion below the photosphere and the rising gas
cannot deliver its heat to the surface. In the cooler
area we see a sunspot.
Infrared observations show the area outside a sunspot
is brighter than the surrounding photosphere,
suggesting the heat rising from the interior is
deflected and emerges around the sunspot
Solar Activity
The Sun's Magnetic Cycle
Helioseismology tells us that
deeper layers of gas rotate
slower than the surface layers,
and the gases near the poles
rotate slowest of all
This phenomenon is called
differential rotation and is
linked with the magnetic
cycle
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
image from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
Solar Activity
The Sun's Magnetic Cycle
The sun's gases are highly ionized and are,
therefore, good conductors of electricity.
When an electrical conductor rotates rapidly and
is stirred by convection, it converts some of the
energy flowing outward as convection into a
magnetic field.
This process is called the dynamo effect.
The Babcock Model
Solar Activity
The Sun's Magnetic Cycle
The sun's magnetic cycle is not fully understood,
but the Babcock model begins to explain it.
The theory behind the Babcock model is:
The differential rotation of the sun winds up the
magnetic field.
Tangles in the field rise to the surface and cause
active regions, visible as sunspot pairs.
When the field becomes too tangled, it reorders itself
into a simpler but reversed field and the cycle starts
over.
Solar Activity
Prominences and Flares
Prominences occur in the chromosphere
Have arched shapes
Shows they are formed of highly ionized gas trapped in a
magnetic field
Produce an emission spectrum
Two types:
eruptive – burst out of active regions and in a few hours
extend into the lower corona only to fall back
quiescent – sometimes hang in the lower corona for many
days, sometimes with gas streaming downward into the
active region along magnetic fields
images from: Foundations of Astronomy, 7th Edition; Seeds, Michael A.; Brooks/Cole
Solar Activity
Prominences and Flares
Flares are also related to the
magnetic field.
Sudden eruptions of X-ray,
ultraviolet, and visible radiation
and high-energy particles from the
twisted magnetic fields around
sunspot groups
Flares can have dramatic effects
on Earth, causing communication
blackouts and auroras.
Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole
Solar Activity
Coronal Activity
Magnetic activity also impacts the corona.
Corona seems to be composed of streamers of
thin hot gas escaping from the magnetic field.
In some parts of the corona, the magnetic field
does not loop back and the gases escape.
These areas are called coronal holes and are
thought to be the source of the solar wind.
Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole
Images from: Horizons, 6th Edition; Seeds, Michael A.; Brooks/Cole