Download •TODAY •Chapter 5/10: The Sun Required: Sec. 1

•TODAY
•Chapter 5/10: The Sun
Required: Sec. 1-5, Sec. 7,
Also, Ast. Toolbox #1
Optional: Sections 8,9,10
•Next Week:
Read Ch. 6 (=Ch. 11) The Stars
(Sec. 12,13 optional)
HW #3 Due. Oct 11
•Solar Viewing
•See sunspots and flares!
WHERE: plaza in front of Thornton Hall (3rd floor entrance)
WHEN: Mondays 2:30-3:30
Wed: 11:30-12:30
Thurs: 10:30-11:30
Earth
The Sun – Our Star
The Sun: Our Nearest Star
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!
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The Sun is a star
Because it is so close, its very bright
The Sun is an “ordinary” star.
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Some stars are larger, some smaller
Some stars are brighter, some dimmer
Some stars are older, some younger
So, we can apply what we learn about the Sun
to other stars.
Observations Reveal The Sun’s Properties
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The Sun is 150 million km (93,000,000 miles) away=1AU
! Distance found during the Transit of Venus, 1761
The Sun 109 times larger than Earth
! ...from its angular size
It is 333,000 times more massive than Earth
! Newton’s Law of Gravity lets us measure mass
Its surface temperature is 5,800 Kelvin
! From Wien’s Law & star’s spectrum & λ
max
It is composed of Hydrogen & Helium
! Determined from spectral lines.
Composition of The Sun
!
!
!
The Sun is a plasma -- an extremely hot gas
Electrons in its atoms have been stripped away, or ionized
The Sun contains:
Hydrogen (~70%)
Atom
Ion
Helium (~30%)
To study the Sun, we consider its interior
and its atmosphere
The Outer Parts of the Sun
The outer layers of the Sun are called the Sun’s
Atmosphere.
It is usually divided into the 3 layers.
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!
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The photosphere is the layer we see
The chromosphere is just above the
photosphere
The corona is the outermost part of the Sun.
Convection & Granulation
Gas in the Sun is undergoing convection.
Cells of hot gas are bubbling up in the Convection Zone
The gas then cools and sinks down.
The top of the convection
zone is called the
photosphere of the
Sun.
Each “bubble”
is about 700
km across.
The Photosphere
• Is the apparent surface layer of the Sun
• Light from the interior leaves the Sun at the Photosphere
• Temperature ≈ 5800 K
• The Chromosphere is a thin
layer above the Photosphere
Image ©JulioC
Prominences Seen at the edge of the Sun during Eclipse
“Diamond ring” Effect: Total Solar Eclipse July 11, 2010
The Corona
!
Outside the chromosphere is the Sun’s Corona
Its temperature is even hotter than the
photosphere:
1 million Kelvins.
!
But, the density of the corona is very low.
!
Because it is so hot, it emits X rays
It has streamers that follow magnetic field lines
!
!
“Diamond ring” Effect: Total Solar Eclipse July 11, 2010
More recently....
March 8 2016
Solar Wind
!
The corona expands
outwards, becoming the
solar wind, which
eventually encounters Earth
!
Sometimes the Sun produces
Solar Flare, which can
bombard the Earth with
high-energy particles
Sun
“Coronal Mass Ejection” moves out at 4 million M.P.H.!
Eruption on the Sun seen in Ultraviolet light.
Anim.
Such a “solar storm” could endanger astronauts.
Fortunately, Earth’s magnetic field deflects the particles.
They strike Earth at the North & South Poles.
!
When they strike the Earth’s atmosphere, they produce the Aurora
Borealis, the Northern Lights.
Image from: http://astroguyz.com/
Oct. 2016 Aurora over Iceland
Sun’s Magnetic Field
Like the Earth, the Sun has a Magnetic Field.
As the Sun rotates, its magnetic field lines
become twisted.
When they reach the surface of the Sun,
they emerge as sunspots.
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!
Sunspots appear on the Sun, and
last for a few weeks.
!
The number of spots on the Sun
changes from day to day.
A sunspot
Iron Filings
Sunspots
…are cooler regions of the
photosphere (T ≈ 4240 K).
Sunspot image
from SOHO
satellite visible
The Solar-Activity Cycle
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The sunspot cycle: every 11 years the number of sunspots increases
Every 11 years the north and south magnetic poles reverse!
So the “solar activity cycle” is actually a 22 year cycle.
In 2007 there were no spots on the sun (Solar Minimum)
The number of spots has increased (Maximum ~2014)
Sunspots
Sun just after Solar Minimum (2008)
The 11-year sunspot cycle has been observed for centuries.
However, from 1645 to 1715, very few sunspots were observed.
This lack of spots is called the “Maunder Minimum”
On Earth, extreme cold temperatures were reported...
The Interior of the Sun
!
!
All solar energy is generated in the core
The core is about 10% of the Sun’s diameter
!
The temperature of the
core is about 15 million K!
!
It also has very high
density.
!
Under these conditions,
nuclear reactions occur,
which power the Sun.
Energy from the Sun
!
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Chemical reactions (like fire, metabolism) involve an
atom’s electrons. Eg.
2 H2 + O2 = 2 H2O
Nuclear reactions involve an atom’s nucleus....
They are much more powerful.
They create new elements!
All of the Sun’s energy comes from nuclear reactions.
(so the Sun is not “on fire”)
Since the Sun is mostly made of Hydrogen (H), the
reactions involve H nuclei.
Nuclear Fusion
!
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!
!
Normally protons (+) will repel each other.
But at very high temperature and pressure protons will
collide and stick together or “fuse”
“Nuclear Fusion”
They are held together by the strong nuclear force
!
Magnet Demo
Energy from the Sun
!
Atoms in the Sun’s core have lost their
electrons. (they are ionized)
!
A Hydrogen nucleus = 1 proton
Atom
Ion
!
If four Hydrogen nuclei come together,
they can form a Helium nucleus.
!
The Hydrogens are fused together into
something larger.
!
This fusion reaction creates a
tremendous amount of energy
Nuclear Power
can be generated two ways
Fusion
Fission
!
Breaking apart a big
nucleus, (like Uranium) to
produce smaller nuclei (like
Lead)
!
Combining small nuclei
(like Hydrogen) to form
larger (like Helium)
Nuclear Fission on Earth
Nuclear Fission involves breaking large nuclei to produce energy.
A Uranium nucleus can break in half, releasing energy.
This is what happens in Nuclear Power Plants.
Fission of Uranium & Plutonium can also be used for bombs.
San Onofre Fission Plant, CA
Trinity Test, New Mexico, 1945
First Nuclear Bomb
Nuclear Fusion on Earth
Nuclear Fusion involves combining small nuclei.
Hydrogen can be combined to make Helium.
This is difficult to do in a controlled way.
An Out-of-control fusion reaction is what happens
In a Hydrogen Bomb
If we can figure out how to
make controlled fusion,
we will have a reliable,
and “clean” source of
energy.
“Tokamak”, an experimental
nuclear fusion reactor
Hydrogen (fusion) bomb,
Pacific Ocean, 1962
Fusion:
How it happens
!
For Fusion to happen, 4 Hydrogen nuclei (protons) must
combine to become Helium
!
But Helium has 2 protons and 2 neutrons.
!
Can protons turn into neutrons?
!
Yes!
!
Positive electrons, called “positrons” are an example of
antimatter!
!
The sequence of steps to produce Helium is called the
proton-proton chain
... if they also produce a positively charged particle.
The Proton-Proton Chain
!)
r
e
is tr att
po ntim
(a
on
)
y
n erg
o
ot t en
h
p igh
(l
Energy from the Sun:
Nuclear Fusion
Start with:
End up with:
4 Hydrogen
nuclei
1 Helium
nucleus
4 Hydrogens weigh more
than one Helium….
Where Stars Get Their Energy
!
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!
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The end product of nuclear fusion is the Helium nucleus
This Helium is about 1% lighter than the 4 Hydrogens that
made it up.
! (exact factor: 0.007 = 0.7 % )
Where did that mass go?
Albert Einstein discovered that mass (m) can transform into
energy (E).
E =mc2
!
!
!
c is the speed of light
c = 3 x 10 8 m/s so c2 = 9 x 10
16
m2/s2!!!!!
A small amount of mass can produce a lot of energy
Energy from Mass Example
!
!
When Hydrogen fuses into Helium, 1% of the mass becomes energy.
Question: If 1 gram of matter disappears, how many Joules of Energy will
be produced.
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1 Joule = kg m2 / s2 .
(eg: a 100 Watt light emits 100 Joules per second)
!
In nuclear reactions, mass is converted into energy
E =Mc2
M is the mass lost. (not the total mass), in kilograms (kg)
E is the Energy produced, in Joules (J)
!
M =1 gram = 0.001 kg
c2 = 9 x 10
16
m 2/s
2
(Note: use m/s here!)
E
2
=Mc
E = (0.001 kg) x ( 9 x 10
16
m 2/s 2)
E = 9 x 1013 kg m2 / s2 = 9 x 1013 Joules of energy
This is equal to the energy:
!
!
of burning 1 million gallons of gasoline!
exploding 25,000 tons of dynamite (25 kilotons of TNT)
All from one gram of mass disappearing!
US: 100 exajoules/year=10^20
Nuclear Fusion in the Sun
!
The Sun converts 4 million tons of matter to energy
every second!
!
This energy is equal to 2 billion hydrogen bombs
! (1 Hydrogen bomb ~ 2 Megatons of TNT)
!
The Sun has been shining for 5 billion years, all this
time converting Hydrogen to Helium.
!
Fortunately, the Sun has enough Hydrogen “fuel” to
keep shining for 5 billion more years.
Fusion… powers every star
The Sun: Summary
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Size & Mass
Interior: Core, Convection Zone
Atmosphere: Photosphere, Corona, Chromosphere
Sunspots: 11 year cycle.
Energy source: nuclear fusion of hydrogen to
helium
Chapter 6:
Stars
Optional: 6:10-6:13
We already know how to determine a star’s:
• surface temperature
• chemical composition
• motion
Next, we will learn how we can determine its:
• distance
• brightness
• mass
Measuring the Distance to Stars
!
!
Measuring distances is
difficult
However, several reliable
methods can be used.
The best method for measuring distances of nearby
stars is called parallax.
It relies on observing a star from two different places.
Field of background stars: Parallax test
Parallax
!
We obtain a different perspective on a
star by observing it at different times of
the year.
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The star is compared to distant
background stars
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In 6 months the Earth has moved 2 AU
!
The parallax method lets us measure the
distance to stars that are closer than
about 1000 light years away.
Background Stars
Nearby
Star
Measuring Distances: Parallax
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The larger the star’s distance, d, the smaller its parallax p.
!
So distance and parallax are inversely related.
d= 1/p
!
Measuring Angles & Distances
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Most stars have a parallax angle, p, which is very small…
…. much smaller than 1 degree.
So parallax angles are measured in units of arc seconds
!
1/60 of one degree = 1 arc minute
!
1 /60 of one arc minute = 1 arc second.
!
Distances to stars are measured in either:
light years, or parsecs.
!
1 parsec = 3.2 light years
!
!
(parsec = PARallax
of one arcSEC)
Parallax Example
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If a star has a parallax 1 arcsecond, then its distance
is 1 parsec.
Suppose a star has a parallax 0.1 arc seconds
Question: what is its distance in parsecs?
Answer: d = 1 / p
d = 1 / 0.1
d = 10 parsecs = 32 light years.
(note: “p” = parallax angle, not parsecs)