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Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthFebruary 2, 2009
Dr. Uwe Trittmann
Today’s Topics
• The closest Star: Our Sun
• The Night Sky in February
The Sun – A typical Star
• The only star in the solar
system
• Diameter: 100  that of Earth
• Mass: 300,000  that of Earth
• Density: 0.3  that of Earth
(comparable to the Jovians)
• Rotation period = 24.9 days
(equator), 29.8 days (poles)
• Temperature of visible surface
= 5800 K (about 10,000º F)
• Composition: Mostly hydrogen,
9% helium, traces of other
elements
How do we know the Sun’s Diameter?
• Trickier than you might think
• We know only how big it appears
– It appears as big as the Moon
• Need to measure how far it is away
– Kepler’s laws don’t help (only relative
distances)
• Use two observations of Venus transit in
front of Sun
– Modern way: bounce radio signal off of Venus
How do we know the Sun’s Mass?
• Fairly easy calculation using Newton law of
universal gravity
• Again: need to know distance Earth-Sun
• General idea: the faster the Earth goes around
the Sun, the more gravitational pull  the
more massive the Sun
• Earth takes 1 year to travel 2π (93 million
miles)  Sun’s Mass = 300,000  that of
Earth
How do we know the Sun’s Density?
• Divide the Sun’s mass by its Volume
• Volume = 4π × (radius)3
• Conclusion: Since the Sun’s density is so low,
it must consist of very light materials
How do we know the Sun’s Temperature?
• Use the fact that the Sun is a “blackbody”
radiator
• It puts out its peak energy in visible light,
hence it must be about 6000 K at its surface
Black Body Spectrum
• Objects emit radiation of all frequencies,
but with different intensities
Ipeak
Higher Temp.
Ipeak
Ipeak
Lower Temp.
fpeak<fpeak <fpeak
Why does the Sun appear yellow?
• Tricky question. It is actually whitishyellow.
• The blackbody curve peaks at green.
• Impression we get is a mix of all colors but
with different intensities
• Caveats:
– Eye’s receptors are not equally sensitive to all
colors
– Atmosphere scatters away short wavelengths
– When you can conveniently observe it
(sunrise/set), it appears yellow
Color of a radiating blackbody as a
function of temperature
• Think of heating an iron bar in the fire: red
glowing to white to bluish glowing
How do we know the Sun’s rotation
period?
• Crude method: observe sunspots as they
travel around the Sun’s globe
• More accurate: measure Doppler shift of
spectral lines (blueshifted when coming
towards us, redshifted when receding).
– THE BIGGER THE SHIFT, THE HIGHER
THE VELOCITY
How do we know the Sun’s
composition?
• Take a spectrum of the Sun, i.e. let sunlight
fall unto a prism
• Map out the dark (Fraunhofer) lines in the
spectrum
• Compare with known lines (“fingerprints”)
of the chemical elements
• The more pronounced the lines, the more
abundant the element
Spectral Lines – Fingerprints of the Elements
• Can use spectra
to identify
elements on
distant objects!
• Different
elements yield
different
emission spectra
• The energy of the electron depends on orbit
• When an electron jumps from one orbital to another, it
emits (emission line) or absorbs (absorption line) a
photon of a certain energy
• The frequency of emitted or absorbed photon is related
to its energy
E=hf
(h is called Planck’s constant, f is frequency, another word for color
)
Sun 
Compare Sun’s
spectrum (above)
to the fingerprints
of the “usual
suspects” (right)
Hydrogen: B,F
Helium: C
Sodium: D
“Sun spectrum” is the sum of many
elements – some Earth-based!
The Sun’s spectrum in some detail
The Sun’s Spectrum
• The Balmer
line is very
thick  lots
of Hydrogen
on the Sun
• How did
Helium get its
name?
How do we know how much energy
the Sun produces each second?
• The Sun’s energy spreads out in
all directions
• We can measure how much
energy we receive on Earth
• At a distance of 1 A.U., each
square meter receives 1400 Watts
of power (the solar constant)
• Multiply by surface of sphere of
radius 149.6 bill. meter (=1 A.U.)
to obtain total power output of the
Sun
Energy Output of the Sun
• Total power output: 4  1026 Watts
• The same as
– 100 billion 1 megaton nuclear bombs per
second
– 4 trillion trillion 100 W light bulbs
– $10 quintillion (10 billion billion) worth of
energy per second @ 9¢/kWh
• The source of virtually all our energy
(fossil fuels, wind, waterfalls, …)
– Exceptions: nuclear power, geothermal
Where does the Energy come from?
• Anaxagoras (500-428 BC): Sun a large hot
rock – No, it would cool down too fast
• Combustion?
– No, it could last a few thousand years
• 19th Century – gravitational contraction?
– No! Even though the lifetime of sun would be
about 100 million years, geological evidence
showed that Earth was much older than this
What process can produce so much
power?
• For the longest time we did not know
• Only in the 1930’s had science advanced to
the point where we could answer this question
• Needed to develop very advanced physics:
quantum mechanics and nuclear physics
• Virtually the only process that can do it is
nuclear fusion
Nuclear
Fusion
• Atoms: electrons orbiting nuclei
• Chemistry deals only with
electron orbits (electron exchange
glues atoms together to from
molecules)
• Nuclear power comes from the
nucleus
• Nuclei are very small
– If electrons would orbit the
statehouse on I-270, the nucleus
would be a soccer ball in Gov.
Strickland’s office
– Nuclei: made out of protons (el.
positive) and neutrons (neutral)
Atom: Nucleus and
Electrons
The Structure of Matter
Nucleus: Protons and
Neutrons (Nucleons)
Nucleon: 3 Quarks
| 10-10m |
| 10-14m |
|10-15m|
Nuclear fusion reaction
–
–
–
In essence, 4 hydrogen nuclei combine (fuse) to
form a helium nucleus, plus some byproducts
(actually, a total of 6 nuclei are involved)
Mass of products is less than the original mass
The missing mass is emitted in the form of energy,
according to Einstein’s famous formulas:
E=
2
mc
(the speed of light is very large, so there is a
lot of energy in even a tiny mass)
Hydrogen fuses to Helium
Start: 4 + 2 protons  End: Helium nucleus + neutrinos
Hydrogen
fuses to
Helium
Could We Use This on Earth?
• Requirements:
– High temperature
– High density
– Very difficult to achieve on Earth!
Fusion is NOT fission!
• In nuclear fission one splits a large nucleus
into pieces to gain energy
• Build up larger nuclei Fusion
• Decompose into smaller nuclei Fission
Harvesting Binding Energy
Small harvest by decay
Big harvest by fusion
Most stable element in the universe
The Standard Solar Model (SSM)
• Sun is a gas ball of hydrogen & helium
• Density and temperature increase towards
center
• Very hot & dense core produces all the
energy by hydrogen nuclear fusion
• Energy is released in the form of EM
radiation and particles (neutrinos)
• Energy transport well understood in physics
Standard Solar Model
How much energy does the Sun
produce in theory?
• Short answer: As much as it has to …
• Longer answer: … to maintain hydrostatic
equilibrium
Hydrostatic Equilibrium
• Two forces compete: gravity (inward) and energy
pressure due to heat generated (outward)
• Stars neither shrink nor expand, they are in
hydrostatic equilibrium, i.e. the forces are equally
strong
Gravity
Heat
Gravity
More Mass means more Energy
• More mass means more gravitational
pressure
• More pressure means higher density,
temperature
• Higher density, temp. means faster reactions
& more reactions per time
• This means more energy is produced
Does too much Energy lead to
Explosion?
• No, there is regulative feedback:
– More energy produced means more radiative
pressure
– This means the stars gets bigger
– This means density, temperature falls off
– This means less reactions per time
– This means less energy produced
How do we know what happens in
the Sun?
• We can’t “look” into the Sun
• But: come up with theory that explains all the
features of the Sun and predicts new things
• Do more experiments to test predictions
• This lends plausibility to theory
Example: Solar Neutrino Crisis
• We can detect the neutrinos
coming from the fusion reaction
at the core of the Sun
• Way to few are seen! (1/3 to 1/2
of the predicted value!)
• Possible explanations:
1. Models of the solar interior are
incorrect
2. Our understanding of the physics
of neutrinos is incorrect
• Against all odds, #2 is the
answer – neutrinos “oscillate”,
they change their identity
Details
•
•
•
•
•
•
•
Radiation Zone and Convection Zone
Chromosphere
Photosphere
Corona
Sunspots
Solar Cycle
Flares & Prominences
Convection Zone
• The core generates all energy, which
propagates as particles of light, photons
• By the end of radiation zone (500,000 km)
all photons are absorbed by Sun’s gas
• Energy is then carried to the solar surface
by convection
Evidence: Granulation
• Bright granules
move up; dark one
move down
– About 1 km/sec
• Granules are about
the size of Earth’s
continents
A Puzzle
• Photons and Neutrinos move at light speed
• Photons take about 700,000 years to leave
the Sun
• Neutrinos make it in a few seconds
Chromosphere
• Above the photosphere
• Gas too thin to glow
brightly, but visible during a
solar eclipse
– Characteristic pinkish color is
due to emmision line of
hydrogen
• Solar storms erupt in the
chromosphere
• Spicules – thin jets of matter
thrown out from photosphere
Solar Corona
• Thin, hot gas above the
chromosphere
• High temperature
produces elements that
have lost some
electrons
– Emission in X-ray
portion of spectrum
• Cause of high
temperatures in the
corona is unknown
Sunspots
• Dark, cooler regions
of photosphere first
observed by Galileo
• About the size of the
Earth
• Usually occur in pairs
• Frequency of
occurrence varies
with time; maximum
about every 11 years
• Associated with the
Sun’s magnetic field
From the Roof: Sunspots in October ‘03
Sun
Spots
• 1/180
second
exposure
• With
Solar
Filter!
Sunspots and Magnetism
• Magnetic field lines
are stretched by the
Sun’s rotation
• Pairs may be caused
by kinks in the
magnetic field
The Solar Cycle
Prominences
• Loops or sheets of gas
• May last for hours to weeks; can be much larger
than Earth
• Cause is unknown
Solar Flares
• Like prominences, but
so energetic that
material is ejected
from the Sun
• Temperatures up to
100 million K
• Flares and
prominences are more
common near sunspot
maxima
Finally: Sizing Up the Sun
• Compared to other stars, is the Sun
– Very hot or cool?
– Very big or small?
– Very long lived or short lived?
Sizes of Stars
• Dwarfs
– Comparable in
size, or smaller
than, the Sun
• Giants
– Up to 100 times
the size of the Sun
• Supergiants
– Up to 1000 times
the size of the Sun
• Sun is average in size
Classification of the Stars:
Temperature
Class
O
B
A
F
G
K
M
Temperature
30,000 K
20,000 K
10,000 K
8,000 K
6,000 K
4,000 K
3,000 K
Color
blue
bluish
white
white
yellow
orange
red
Examples
Rigel
Vega, Sirius
Canopus
Sun,  Centauri
Arcturus
Betelgeuse
Sun is a bit below average in temperature
The Night Sky in January
• Long nights, but getting shorter!
• Winter constellations high up: Orion, Auriga,
Taurus, Gemini, Canis Major
• Saturn visible late at night
Moon Phases
• Today (First Quarter, 50%)
• 2 / 9 (Full Moon)
• 2 / 16 (Last Quarter Moon)
• 2 / 24 (New Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early
February
Saturn
Star
Maps
Celestial
North Pole –
everything
turns around
this point
Zenith – the
point right
above you &
the middle of
the map
40º
90º
Due
North
Big Dipper
points to the
north pole
High up
Perseus,
Auriga &
Taurus
with Plejades
and the
Double
Cluster
South
• Orion
• Canis
Major &
Minor
• Beautiful
open star
clusters
• Orion
Nebula
M42
South
-East
• Gemini
• Cancer
• M44
Beehive
(open star
cluster)
• Mars
East
Spring
constellations:
– Leo
– Hydra
M44 Beehive
(open star
cluster)
Saturn
Deep
South
Lepus
Columba
Puppis (part
of the former
Argo)
Horizon in Germany (50º lat.)
Mark your Calendars!
• Next Starry Monday: March 2, 2009, 7 pm
(this is a Monday
)
• Observing at Prairie Oaks Metro Park:
– Friday, March 6, 7:30 pm
• Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp
(Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
Mark your Calendars II
•
•
•
•
Physics Coffee is every Monday, 3:00 pm
Open to the public, everyone welcome!
Location: across the hall, Science 244
Free coffee, cookies, etc.