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 = mc 2 (the speed of light is very large, so there is a lot of energy in even a tiny mass)
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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.
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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 = mc2
(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
Most stable element in the universe
Small harvest by decay
Big harvest by fusion
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
Heat GravityGravity
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
• 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?– 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– Earth = a grain of sand
– The Earth orbits the Sun at a distance of one meter