Transcript lecture15

The Sun: Our Star
The Sun is an ordinary star
and shines the same way other
stars of its type do.
The bright part normally seen
is called the photosphere,
which is about 500 km deep.
It is an almost perfect black
body with a temperature of
5800 K.
Assigned Reading
• Chapter 8.
Why is the Sun important
for life?
Provides virtually all of the energy necessary for
life on Earth: food, weather, etc.
What is the Sun?
• The Sun is a star, a failry common and typical star
• Its spectral type is G2
• A star is a ball of gas held in equilibrium against
its own self-gravity by the thermal pressure and
outflow of energy from cener to the surface
• This energy comes from nuclear fusion reactions
at the center of the Sun
Balance in the stars
Thermal
Pressure
Gravitational
Contraction
Pressure and Temperature
of a Gas
How does a
star hold itself?
This balance between
weight and pressure is
called hydrostatic
equilibrium.
The Sun's core, for
example, has a
temperature of about 16
million K.
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 (~80%) and
helium(~20%), as is in the whole universe
Why does the Sun have such a sharp edge?
An optical illusion
Structure of the Sun
The Atmosphere of the Sun
Although the sun appears to have sharp edge, its
“surface” actually has complicated structure.
Its atmosphere, too, is complex
It consists of:
"
"
Photosphere: 5,800 K,
continuum + abs.
Chromosphere: 4,500-500,000 K,
emiss.+abs.
Corona:
1-2 106 K, continuum+emiss.+abs.
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
What would you expect to see if
you looked at the Sun’s
photosphere now?
• The photosphere: the region where the black body
radiation that we see is made
• Thin gas: 3,400 times thinner than air
• It is the transition region from a black-body
emitting plasma to a transparent gas
• Its upper layer produces the absorption spectrum
• Limb darkening: proof that the Photosphere has a
thickness and that energy comes from below
• How does the photosphere look like if we close up
on it?
The limb is slightly fainter
Granulation of the
Photosphere
Each Granule is about the
size of Texas and lasts for
only 10-20 minutes before
fading away!
Granulation of the Photosphere
Each Granule is about the size of Texas and lasts for only
10-20 minutes before fading away!
Granules: convective
cells
• The granules are just densely packed
convective cells
• Convective cells are very similar to
thunderstorms
Sunspots
Sunspots, about 1000 K
cooler than the rest of
the Sun's photosphere,
appear as dark spots.
Sunspots come and go
with time. Big sunspots
can live for several
weeks.
Chromosphere
Reddish in color, which
is the origin of its name
(chromos meaning
``color'')
2000-3000 km thick
Faint relative to the
photosphere
From 4,500 up to
500,000K: hotter than the
photosphere and much
less dense.
Absorption + Emissionline spectrum
Corona
- the outermost layer
- T~1 million K
-made up of very diffuse
(but extremely hot gas)
- coronal emission is
dominated by X-rays
X-ray emission from the
Sun's corona
The corona is so hot that it emits
X-ray radiation
Rotation period: about a month
The middle rotates faster than the
north or south.
Corona has an emission line spectrum and a continuum spectrum
(dust scattering of photospheric black body light).
Some of the continuum spectrum has no absorption lines,
because of the dilution of the absorption lines by
very hot electrons
The heating of the
corona
The heating of the corona has
puzzled scientists for a long time.
The corona is heated by energy
outflowing from the sun's interior
not as heat but as magnetic energy.
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 gas is heated by the motion
through it of long lines of magnetic
force, which act like whips.
The gas is being “whipped”.
UltraViolet (UV) image of the Sun.
It shows regions of the
Photosphere above Sunspots
are more energetic than
elsewhere.
It also shows magnetic
link between Sunspots
extending into the
Cromosphere and Corona
Solar Prominence
Composed of hot gas
trapped in magnetic
fields extending from
one sunspot to another.
Prominence
Solar Flare
A solar flare is a violet outburst that lasts in an hour
or less. It radiates X-ray, ultraviolet, and visible
radiation, plus streams of high-energy protons and
electrons.
A large flare can be a billion times more energetic
than a large hydrogen bomb.
Solar Wind
Sun loosing mass, but only a small fraction: 10-14 Mo/yr
Mass loss will intensify later, and will much larger (~1/3)
Auroras: the Northern
and Southern Lights
May 11, 2002
• Auroras seen as far south as New England
• Caused by a gust of solar wind
• These powerful gusts are guided by Earth’s
magnetic field and can excite gases in the
upper atmosphere, causing the air there to
glow
The key point here is that nearly all “solar
weather” is a result of changes in the magnetic
fields that penetrate the Photosphere.