Transcript The Sun - Ccphysics.us

The Sun
A Model Star
Factoids to Start:
• Official name: Sol (as in Solar System)
• 1.5 X 1011 meters distant (93 million miles, 8.3
light minutes)
• Size: 6.96 X 108 meters (432,000 miles) in radius;
about 54 times the Earth's radius
• Mass: 2 X 1030kg; about 1 million Earths
• 74% H, 24% He, 2% all other materials
• A Variable Star: output +/- 0.07%
Present age: ~4.567 billion years
old
• How do we know this?
– Stellar models (another slide show for another day)
– Luminosity rate
• t = M/L
– Chondrite meteorites
• Unchanged since they formed 1 million years (or less) after the Sun
– Time to condense
• Radiometric dating/half lives/proportions
– K-Ar
– Sm-Nd
– U-Th-Pb
• Three methods return very similar results
But factoids say so little!
• Let's use the Sun as a sample, a basis to
compare to all the other stars
• Many say the Sun is an average star, but you
can't take that meaning too seriously
– It’s just not an unusual star
• There are many, many smaller, cooler stars.
• There are some stars that are much, much
brighter, much, much bigger, and much,
much more massive than the Sun
Stages of the *Sun’s Life
•
•
•
•
•
•
•
•
Globule
Protostar
Main Sequence
Hydrogen Core Exhaustion
Red Giant
Helium Flash
Horizontal Branch
Asymptotic Giant
– AKA Double shell burning
• Planetary Nebula
• White Dwarf
*Other stars have different later-life experiences!
A Star's Beginnings
• What the Sun has in
common with all stars
is its origins, a large
dark, cold cloud of
interstellar gas and dust
• This is a so-called Bok
globule, named after
UAz astronomer Bart
Bok
Qualities of a star-forming region
• Must be cold: too warm, and the speed of the atoms
and molecules overwhelms the forces trying to
compress them
• Therefore, what we see in visible light is a blocking
of light, at least in the very early stages
• Must be dense enough for gravity to pull atoms and
molecules together
– Still not very dense by Earth standards!
• Must not have any great rotation, or it will fly apart
• One Bok globule can produce a single to maybe a
dozen stars, and there can be many Boks in a nebula
• The stars form all at one time
– Once bigger stars form they inhibit the formation of
new stars, and may affect smaller stars' evolution as
well
• The population of stars formed from a nebula is
called a cluster
• The stars are composed of the same stuff that made
up the nebula: H, He, and small amounts of other
materials
• Let's just think about the Sun, for now, a
glimmer in the nebula's eye
• Over a period of hundreds of thousands to a
million years, the region of the nebula that will
become the Sun slowly does three related
things:
– It contracts
– It gets hotter
– It spins faster
Why those three things?
• It contracts because of gravity
• It heats up because of thermodynamic
pressure
• It spins faster due to conservation of angular
momentum
• Let’s look at these concepts one at a time
Gravity
• Isaac Newton developed a simple working
definition for gravity 300 years ago
– Einstein refined it 100 years ago, but we can
use Isaac’s for now: it is much simpler
• “Every mass in the Universe attracts every
other mass with a force proportional to the
product of their mass and inversely
proportional to the square of the distance
between their centers”
Simple, he said?
• Yes, it truly is:
– Bigger masses attract each other more
– A small change in distance means a big change
in the gravitational attraction
• So each little bit of the globule attracts every
other little bit, which pulls them closer,
which further increases the attraction, and so
on.
Thermodynamic Pressure
• As the globule gets smaller, each particle is more
likely to bang into another one
• The consequence of all this banging increases the
pressure, much like people squeezing into the
exits after a concert
• As pressure increases so does the temperature,
like squeezing air in a bicycle pump
• Ultimately, the release of gravitational energy
from the contraction provides the energy to heat
up the gases
Conservation of Angular
Momentum
• Momentum is a concept from
Newton’s First Law of Motion:
– An object in constant in constant
motion in a straight line tends to
stay in constant motion
(including spinning) in a straight
line (or spinning in a circle)
unless acted upon by an external
force
But why was the globule spinning
in the first place?
• It spins for roughly the
same reason a hurricane
spins
• Part of the globule is
closer to the center of a
spinning galaxy
• In a hurricane, part of the
cloud mass is closer to the
equator which has a faster
speed than northern
latitudes
This part of the globule (hurricane) is closer…
…than this part to the galactic center.
Getting from Here to Here
Stellar Eggs, and the “Yolk”
•
The globule can be up to a light
year across
– Very cold ~100K, 106
particles/cm3
•
After it contracts it is about the
size of the Solar System
–
–
–
–
•
10,000 years have passed
1012 particles/cm3
105K core
IR
It becomes a protostar when it
is about the size of Mercury’s
orbit
– 100,000 years after starting
– 106K core
• (surface 4000K)
– Visible light
Ready to Ignite
• Material above the
rotation plane falls in
• Disk material moving too
fast will condense into
planets, asteroids, and
cometary mass (ice)
• When the core
temperature rises to 107K
fusion begins
Nuclear Fusion:
• E = mc2 keeps the Sun going
• 5 X 106 tons of H ‘burned’ every second
• Yields 4 X 1026 W!
– The Solar Constant
• Remember, +/- 0.07%
– Irradiance: ~750W/m2
• Requires the proper pressure and temperature
– 1.5 X 107 K
– 1.3 X 109 ATM
+
+
p -p
chain
Interior View
• We divide the Sun’s
interior into three
broad categories:
– Core (where fusion
happens)
– Radiative Zone
– Convective Zone
• The surface of the Sun
is called the
Photosphere
In Balance
• The balance between
the crushing force of
gravity and the
explosive force of
fusion is called
Hydrostatic
Equilibrium
– Hydro for fluid
– Static for not moving
– Equilibrium for balance
Energy Flow
• Understand this and you’ll understand much about
not just the Sun but all stars
• Thermodynamic Law states that heat flows from a
high level to a low level
– Don’t mistake heat for temperature!
• Heat is a form of energy
• Temperature is the level of the heat, not how much there is
• The outward flow of energy establishes
hydrostatic equilibrium
• The names radiative and convective zones give a
clue about how heat flows in the Sun
Surface Features
•
•
•
•
•
Granules
Spicules
Sunspots
Prominences
Flares
Helioseismology
• Pulsations in the
Sun cause it to
ring with
hundreds of
modes of
vibration
• Three prominent
modes are shown
Granules
• The tops of convection
rolls
• The yellow areas are the
hotter upwelling plasma
• The orange regions are
the cooler material
starting to sink
• There are some sunspots
in this picture
Sunspots
Early Records
•
•
As early as 28 BC, Chinese
Astronomers recorded dark
patches on the Sun
Galileo was the first to see
sunspots through a telescope
– Much consternation over an
‘imperfect’ Sun
•
•
German Astronomer Johann
Hevelius made this map in 1644
Maunder Minimum
– 1645-1715
– Virtually no sunspots seen
Cause
• Convection currents
carry huge amounts of
charge
• Electric current in a
loop makes a looping
magnetic field
• The sunspots are
evidence of that field
Rotational Influence
• The Sun is not a solid but
instead is a sphere of fluid
plasma
• Different latitudes rotate
at different speeds
• The magnetic field lines
stretch, pinch off, break
the surface, and dissipate
– and the cycle starts again
Cycles
• 11 or 22 year cycle, depending on how you count
– 11 peak to peak
– 22 for polarity change
Long Term Changes
Recent Data
An aside: correlation vs
causation
• In the case of causation, one parameter is the cause of
another
– Smoking can cause lung disease, not the reverse
• In the case of correlation, two or more parameters appear to
track but are actually under the influence of an external,
untracked variable
– Gamblers often smoke, but gambling doesn’t cause lung disease
• One must examine graphs carefully to determine if the data
depicted are just correlations or are actually causation. In
the latter case, which is the cause and which is the effect is
critical.
Climatic
Effects
14C
concentrations
• "The solar cycle may be going into a hiatus," Frank
Hill, associate director of the National Solar
Observatory's Solar Synoptic Network, said in a
news briefing today (June 14, 2011). "This is highly
unusual and unexpected," Hill said. "But the fact
that three completely different views of the sun
point in the same direction is a powerful indicator
that the sunspot cycle may be going into
hibernation.“
• “…[T]he recent findings indicate that the activity in
the next 11-year solar cycle, Cycle 25, could be
greatly reduced. In fact, some scientists are
questioning whether this drop in activity could lead
to a second Maunder Minimum, which was a 70year period from 1645 to 1715 when the sun
showed virtually no sunspots…”
Space.com article 6/14/2011, Denise Chow, author
Frost Fairs
• Popular from 17th
until the early 19th
century
• Thames froze every
winter from 14th to
19th century
• Coldest during the
Maunder Minimum
Spicules
• Spikes of superheated
plasma
– 5 minute cycle
– 100,000 on the surface
at any one time
• Caused by standing
waves at the solar
surface
– Remember
helioseismology?
Solar Prominences
Plasma caught in sunspot magnetic
field loop
Solar Flares
• When the magnetic loop twists and breaks, the
plasma is released into space
Flare Power
• A magnetic field can only
store a limited amount of
energy
• When the energy density
in the field exceeds
confinement it is suddenly
and violently released
• The energy released is
equivalent to millions of
100-megaton hydrogen
bombs exploding
simultaneously
Chromosphere
• The lower atmosphere of
the Sun
• Low density hot gas
• ~ 8000 miles thick
– Visible only during eclipse
• Varies from 4000K at the
photosphere interface to
10,000K at the coronal
interface
Corona
• Extremely hot outer
atmosphere
• Up to 1 million K,
heated by magnetic
fields
• Very rarified
• Merges into the solar
wind, 300-1000kps
Corona visible during eclipse
Solar Wind
Coronal Mass Ejection
(Earth-Sun distance not to scale)
The Sun Does Affect the Earth
• Coronal Mass Ejections,
Flares, radiation—all
affect modern life
• Satellites
– Radiation damage
– Atmospheric drag
• Power Grids
– Induced currents
• Aurorae
Changes
• The output of the Sun changes on long time scales
as well as short ones
• The Sun is about 25% more luminous than when
its fusion engine first turned on
– Due to convective mixing
– The ambient temperature of the Earth has not changed
25% because the Earth also changes
• The Sun will continue to heat up as it evolves but
will remain essentially the same for another 5
billion years
Timeline
ZAMS
Main Sequence
Late Age and Death of the Sun
•
•
•
•
•
•
•
•
Hydrogen Core Exhaustion
Red Giant
Helium Flash
Horizontal Branch
Asymptotic Branch
Planetary Nebula
White Dwarf
Black Dwarf?
Hydrogen Core Exhaustion
• Eventually (~5 Gyr from now) there will not be enough
hydrogen in the Sun’s core to maintain hydrostatic
equilibrium
• Gravity wins temporarily, core collapses
–
–
–
–
Temperature increases
Radiant pressure pushes solar envelope outward
Surface cools
Outer layers expand, keeping luminosity more or less constant
• The Sun will expand, filling half the sky
– One side of the Earth will see sunset as the other sees sunrise!
– At noon the Sun will fill the sky
Red Giant Phase
• About 1 Gyr has passed since HCE began
• As the helium rich core contracts it heats up
enough for an inner shell of hydrogen to
start fusing
• Surface temperature stays the same
• Further expansion
– Expands out to Venus
• Luminosity increases
Helium Flash
• The contracting helium rich core grows hotter,
reaching 108K
• Density up to 106 g/cc
• Gas can no longer contract
• Increased pressure without possibility of
expansion further heats the gas
• Helium ignites throughout the core more or less
simultaneously
Horizontal Branch
• On the Horizontal
Branch
• Fusion continues
– He inside
– H in shell
• Very inefficient
process
• HB lasts for 100Myr
Asymptotic Giant Branch
• Eventually, the He in
the core is exhausted
– C, O left
– Not hot enough for C,
O to fuse
•
•
•
•
Double Shell burning
More expansion
Outer cooling
Increased luminosity
Planetary Nebula
• First seen by William
Herschel in 1784
• Not planets!
– It fit in with solar system
formation theories of the
day
• See the naked C-O core?
• Super energetic photons
from the interior
disassociate molecules in
the ejecta, causing
unusual spectral lines
– Misinterpreted in the 19th
century as a new element!
Mass Ejection
• Outer layers of the
Sun will be blown off
by the superwind
created by the double
shell burning in about
10,000 years
• The very hot (but not
enough for further
fusion) core is
revealed
White Dwarf
• Naked, the C-O core cools
and becomes less luminous
• Shrinks down to roughly the
size of the Earth
• No more fusion
• Surface temperature due to
gravity: 10,000K
• Hugely dense: 1 tbls = 10
tons!
Black Dwarf
• The theoretical conclusion to the Sun’s
existence
• The state after the white dwarf cools to
ambient temperature (3K)
• No black dwarfs have been seen because:
– They are black
– It would take longer than the age of the
Universe to make one!
Remember:
• This is how our Sun will expire
• Different stars will end up differently,
depending largely on their initial mass and
how much mass they can shed
The End!