Download Integrative Studies 410 Our Place in the Universe

Nuclear fusion reaction
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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)
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
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
Details
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Radiation Zone and Convection Zone
Chromosphere
Photosphere
Corona
Sunspots
Solar Cycle
Flares & Prominences
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
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
Understanding Stars
• “Understanding” in the scientific sense
means coming up with a model that
describes how they “work”:
– Collecting data (Identify the stars)
– Analyzing data (Classify the stars)
– Building a theory (Explain the classes and their
differences)
– Making predictions
– Testing predictions by more observations
Identifying Stars - Star Names
• Some have names that go back to ancient times
(e.g. Castor and Pollux, Greek mythology)
• Some were named by Arab astronomers (e.g.
Aldebaran, Algol, etc.)
• Since the 17th century we use a scheme that lists
stars by constellation
– in order of their apparent brightness
– labeled alphabetically in Greek alphabet
– Alpha Centauri is the brightest star in constellation
Centaurus
• Some dim stars have names according to their
place in a catalogue (e.g. Ross 154)
Classification by Star Properties
• What properties can we measure?
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distance
velocity
temperature
size
luminosity
chemical composition
mass
Distances to the Stars
• Parallax can be used out to about
100 light years
• The parsec:
– Distance in parsecs = 1/parallax (in
arc seconds)
– Thus a star with a measured
parallax of 1” is 1 parsec away
– 1 pc is about 3.3 light years
• The nearest star (Proxima
Centauri) is about 1.3 pc or 4.3
lyr away
– Solar system is less than 1/1000 lyr
Homework: Parallax
• Given p in arcseconds (”), use
d=1/p to calculate the distance
which will be in units “parsecs”
• By definition, d=1pc if p=1”, so
convert d to A.U. by using
trigonometry
• To calculate p for star with d given
in lightyears, use d=1/p but
convert ly to pc.
• Remember: 1 degree = 3600”
• Note: p is half the angle the star
moves in half a year
Our Stellar Neighborhood
Scale Model
• If the Sun = a golf ball, then
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Earth = a grain of sand
The Earth orbits the Sun at a distance of one meter
Proxima Centauri lies 270 kilometers (170 miles) away
Barnard’s Star lies 370 kilometers (230 miles) away
Less than 100 stars lie within 1000 kilometers (600 miles)
• The Universe is almost empty!
• Hipparcos satellite measured distances to nearly 1
million stars in the range of 330 ly
• almost all of the stars in our Galaxy are more distant
Reminder: Three Things Light Tells Us
• Temperature
– from black body spectrum
• Chemical composition
– from spectral lines
• Radial velocity
– from Doppler shift
Luminosity and Brightness
• Luminosity L is the total power
(energy per unit time) radiated
by the star, actual brightness of
star, cf. 100 W lightbulb
• Apparent brightness B is how
bright it appears from Earth
– Determined by the amount of
light per unit area reaching Earth
– B  L / d2
• Just by looking, we cannot tell
if a star is close and dim or far
away and bright
Brightness: simplified
• 100 W light bulb will look
9 times dimmer from 3m
away than from 1m away.
• A 25W light bulb will look
four times dimmer than a
100W light bulb if at the
same distance!
• If they appear equally
bright, we can conclude that
the 100W lightbulb is twice
as far away!
Same with stars…
• Sirius (white) will look 9
times dimmer from 3
lightyears away than from 1
lightyear away.
• Vega (also white) is as
bright as Sirius, but appears
to be 9 times dimmer.
• Vega must be three times
farther away
• (Sirius 9 ly, Vega 27 ly)
Distance Determination Method
• Understand how bright an object is
(L)
• Observe how bright an object appears (B)
• Calculate how far the object is away:
B  L / d2
So
L/B  d2 or
d  √L/B
Homework: Luminosity and Distance
• Distance and brightness can be used to find
the luminosity:
L  d2 B
• So luminosity and brightness can be used to
find Distance of two stars 1 and 2:
d21 / d22 = L1 / L2 (since B1 = B2 )
i.e. d1 = (L1 / L2)1/2 d2