Download The Sun The Chromosphere of the Sun

Key Concepts: Lecture 22: The Sun
Basic properties of the Sun
The outer layers of the Sun: Chromosphere, Corona
Sun spots and solar activity: impact on the Earth
Nuclear Fusion: the source of the Sun’s energy
The Surface of the Sun
• Visible surface is called
photosphere
• Temperature of visible surface
is about 6000 K
• Granular structure - gas
motion (convection) carries
energy to surface
• Dark cool regions are called
Sun Spots
Structure of the Sun (and other stars)
The Sun
• The Sun is huge compared
to the Earth
– 110 times bigger in diameter
– 300,000 times as massive
– Slightly denser than water, 1400
kg/m3 - mostly H & He
– Contains 99.9% of the mass in
the solar system
– Emits 3.9x1026 watts of power
– Large flux at Earth. e.g. an area
equivalent to Florida absorbs
~5x world energy usage
• G 2 Main Sequence Star
The Chromosphere of the Sun
• Outside photosphere
• Cooler than photosphere
hydrogen atoms exist
here
• Causes absorption lines
in the spectrum of the
Sun (recall Kirchoff’s
3rd law)
Sun Spots
Solar Activity
• Sun spots
have an 11
year cycle
–Number of
spots
changes
–Position of
spots
changes
Two sun spot cycles make up a solar cycle of 22 years
(the magnetic field reverses twice to return to its
original orientation).
Solar Activity
• Sunspots are regions where the solar
magnetic field breaks through surface
• Cycle due to twisting of magnetic field
–
–
–
–
Equator rotates faster than pole
Fields twist up like rubber bands
When fields cross they connect
Field flips every 11 years
Sun Spots and the Earth
• The amount of energy the Sun releases is not
perfectly constant
• The number of Sun spots may be related to the
amount of energy the Sun emits
–More spots = more energy
–1% more UV at solar maximum
The Maunder Minimum
• From 1650-1700 almost no Sun spots occurred
• Known as “Little Ice Age in Europe”
–Thames river in London froze 11 times
–Ice seen off the coast of England
• Another similar minimum may have occurred in
the 1400s
Where does the Sun get its Energy?
• Lifetime for the Sun to shine is related to source of
energy:
lifetime = (energy available) / (luminosity)
• Gravitational Contraction?
– Contraction due to gravity releases energy
• Total energy due to contraction 1042 Joules (from Newton’s Laws)
– lifetime = (1042 Joules)/(4x1026 Joules/sec) = (1/4)x1016sec ~ 108 years (short
compared to age of solar system)
– So gravitational contraction cannot be supplying Sun’s energy.
• Radioactive decay, similar to heating of Earth?
– Sun is mostly H and He, which do not decay. Need heavy nuclei
– Even if solid uranium it would only produce 1/2 of energy seen. So
not enough radioactivity to supply Sun’s power.
The Corona of the Sun
• Corona extends far out
– Visible only during eclipses
– Very tenuous
• Much hotter
– 500,000 to 2,000,000 K
– Emits mostly in X-rays
• How does it get so hot?
– Puzzle - Heat only flows from hot to cold
– Clue - Corona is hottest near sun spots
Corona is fed by sporadic flares and
prominences from the Sun’s surface, which
can lead to coronal mass ejections
The Corona is the source of the Solar Wind.
Strong ejection events in the wind can affect
communications on Earth.
Nuclear Fusion
• Several light nuclei (H) collide and combine
to form a heavier nucleus (He)
• Energy is released
• Need high temperatures and densities
–Must overcome electrostatic repulsion of
positively charged atomic nuclei
–Core of Sun is 15,000,000 K
–Density is 150,000 kg/m3
1st step towards He
Nuclear Fusion: The Proton-Proton Chain
Energy From Fusion
• Overall, fusion in stars involves destruction of about 1%
of the total mass, but this is still enough to make fusion
one of the most efficient means of energy generation
(much more energy can be released compared to
chemical reactions or radioactive decay)
• If 10% of the Sun’s Hydrogen is converted into Helium,
the Sun will shine at its current rate for 10 Billion Years
= 10 x 109yr
Note this process also produces neutrinos.
Where Does the Energy Come From?
• Mass is converted directly into energy
– Einstein’s famous relationship
– Mass and energy are the same thing!
– The speed of light, c, is very big so you get a lot of energy
from a little bit of matter
E=mc2
• 4 H → 1 He
–
–
–
–
Mass of H = 1.67252x10-27 kg
Mass of He = 6.64258x10-27 kg
4xH - He = 0.04750x10-27 kg
E = mc2 = 4.75x10-29 kg (3x108 m/sec)2
= 4.3x10-12 Joules
[1 watt = 1 Joule/sec]
How much mass is destroyed each second to keep the Sun shining with
its luminosity of 4x1026Watts?
Question
• Remember that the luminosity of a main sequence star
was found be to related to its mass (L∝M4).
• Now we know that stars get their energy by converting
their mass directly into energy so the total amount of
energy a star has is proportional to its mass (E∝M).
• Will a massive star live a longer or shorter time than a low
mass star?
• Answer given and described in class lecture.
The Lifetime of Stars on the
Main Sequence
• tlifetime depends on amount of fuel (M), divided
by rate of using fuel (L ∝ M4)
• Therefore tlifetime ∝ M / (M4) ∝ M-3
• So how long will a star that is twice the mass
of the Sun be able to shine on the main
sequence?
• And a star 10x the mass of the Sun?
• Or a star 0.1x the mass of the Sun?
Solar Neutrinos
• Neutrinos
– Very low-mass particles produced as a
side product of nuclear fusion.
– They hardly interact with matter so
they can travel completely out of the
Sun undisturbed. Millions pass
through you every second!
• Detection of Neutrinos
– Difficult since they interact so weakly
with matter.
– Takes very large detectors.
– Several have been built to detect
different types of neutrinos from
inside the Sun.
>Confirms fusion is occurring in
center of Sun
The Structure of the Sun
Journey to the Stars, part 1
SAME BASIC IDEAS ALSO APPLY TO OTHER STARS
• Stars arrange themselves to balance
the forces within them
– Gravity tries to pull all matter
together
– Pressure of the gas prevents
collapse
– Pressure of escaping radiation also
helps to prevent collapse
• If not enough pressure, star will
contract until pressure increases to
balance gravity
• If too much luminosity and pressure,
star will expand until pressure drops
Gas Pressure
• The pressure of a gas is due to motion of gas atoms
–Higher temperature - faster speeds - higher pressure
–Higher density - more atoms - higher pressure
• Compressing a gas
increases pressure
• Heating a gas
increases pressure
Interior Structure of the Sun
• Pressure increases as you move
deeper into the Sun
– Must hold up all the overlaying material
– Density and temperature increase
• Only in core is it hot and dense
enough for nuclear fusion. Fusion
needs very hot conditions because the
protons need to move fast enough to
overcome their electrostatic
repulsion.
– All energy is generated in core
Energy Escaping From the Sun
Gas Pressure
• Energy always flows from hot regions to cold regions. So heat
flows from the core to the outer layers and then into space
• Two major ways to transport energy out
– Photons (Radiation Transport)
• Deep interior relatively transparent to light so photons can
carry the energy out
– Currents of hot gas (Convection)
• Outer parts are opaque to light so gas motions must move the
energy out
Random Walking
• Even in the relatively transparent interior of the Sun a photon
only travels about 1 cm before it is absorbed
• A photon is absorbed and re-emitted billions of times before it
wanders out of the Sun
–Re-emitted in random directions each time
• It takes about 1 million years for a photon to work its way out
Question
• In the Star Trek movie “Generations”, The evil
Dr. Zorron launches a missile into a star. The
missile stops fusion inside the star so it blows
up during a life and death battle between
Zorron, Kirk and Picard. How long would
they really need to be fighting before they
would notice fusion had stopped in the star?
The Main Sequence Revisited
• Like the Sun, all stars arrange themselves
to balance the force of gravity and their
interior pressure
–As mass increases gravitational forces
increase
• Pressure increases inside the star
• Stars generate more energy and are
more luminous
• Stars are hotter and larger to let the
energy out
• This equilibrium sequence of mass is the
Main Sequence