ASTR 10, Vista College, Spring 2004
Instructor: Dr. Korpela
What Was Important - Material for Exam #2
These notes may seem long, but I've tried to summarize the ideas,
instead of just saying "know this", especially where the information
is a little scattered in the text.
Solar System Formation
- It is important that you have some idea of the true scale of
the solar system. Review the solutions to the in-class activity
from the first class, where the Sun is represented by a volleyball.
- You should understand that scientific theories are stories we tell
to fit the observations, but these stories must be supported by
evidence, and follow clear, logical steps.
- Know the Overall Characteristics of the Solar System, and how we
- common plane and direction of orbit/spin in solar system
- there are exceptions to the above
- there are three kinds of planets: terrestrial, gas giants (Jovian) and tiny
icy planets (Pluto). You should understand what they are made of, and the
basics of their internal structure.
- terrestrial: small, dense, rocky, inner solar system, atmospheres of CO2
- gas giants: large, gaseous, low-density, outer solar system, all have
moons and rings, mostly H2 and He.
- icy planets: Pluto and Charon, maybe the moons of the gas giants are in
- understand why the interiors of the planets have a layered structure.
- cratering of solid surfaces is common
- space debris: asteroids, comets, meteors
- common ages (~4.5 billion years) of solar system bodies
(Sun, moon, Earth, meteorites)
- The idea behind radioactive dating, with which we determine
the ages of rocks, is important.
- Know the main steps in the theory of solar system formation, and how they
explain the observed characteristics of the solar system. This theory
unifies many details about the solar system into one overall picture.
- planets formed naturally along with the Sun, from the same
interstellar cloud (explains common ages)
- angular momentum conservation resulted in a spinning dist of gas/dust
around the protosun (explains disk shape and common direction of motion)
- solids condensed out of the nebula as it cooled - in the hot parts
of the nebula, only the densest materials could be solid (explains
why the densities of the terrestrial planets decrease as you move away
from the Sun; explains why the gas giants are less dense than the
- planets grew as the solids stuck together (terrestrial planet formation)
- gas giants became big enough to gravitationally acquire gas
from the nebula - the solar nebula contained a higher concentration
of elements that make ices (H2O, etc) than metallic/rocky elements (Silicon,
Iron), so the gas giants got more massive from solid-material accretion
and began to attract gas directly from the nebula
(explains why Jupiter/Saturn rich in H, He; explains why gas giants so
much more massive and made of such light elements => have such low densities)
- planets swept up much of the debris of the nebula (explains the
cratered solid surfaces we see virtually everywhere in the solar system;
asteroids between Mars/Jupiter and comets at outer edge of solar system
are debris left behind)
- radiation pressure and the solar wind pushed gas/dust out of the solar
system (explains the empty space between planets)
- In particular, understand how the condensation sequence above
explains the two types of planets.
- You should know the exceptions to the general characteristics of
the solar system: Mercury is even more dense than the theory predicts,
and its orbit is tipped by 7 degrees, Venus rotates backwards, Earth's
Moon is unusual and large for a terrestrial planet, Uranus rotates on
its side, Pluto's orbit is tipped by 17 degrees, and is elliptical. We'll
discuss a few more next week. Some of these exceptions (and possibly all)
can be explained by giant impact between large bodies late in the
formation of the planets.
- The theory above indicates that at the end of planet formation,
all terrestrial surfaces should be heavily cratered. We believe that
slow surface evolution/volcanic resurfacing explains why Earth, Venus, and Mars
are not as heavily cratered as Mercury and the Moon.
- It turns out that the terrestrial planets with the most evidence
for geological activity (such as earthquakes, volcanoes) have the fewest
craters on the surface. Geological activity requires internal heat,
and the least massive planets cool the most quickly, and have less
resurfacing. We don't have time to discuss the evidence for this.
- Geological evidence supports the hypothesis that the Earth's Moon
formed as a result of a Giant Impact with a Mars-sized body late in
formation. We don't have time to discuss the evidence and virtues of
this theory relative to other possibilities.
- Know that atmospheres are very thin layers on the surface!
- Know which terrestrial planets have the thickest (most dense) atmospheres,
and which have no atmospheres.
- Know that the atmospheres were originally due to gases baked
out of the planet's rocks, and volatile materials from impacting icy
planetesimals. This means the original atmospheres were mostly
CO2, N2, water vapor, and ammonia.
- Notice that Venus and Mars have atmospheres with similar compositions
(mostly carbon dioxide), but very different densities/pressures. Notice
also that although Earth and Venus have similar sizes and masses, their
atmospheres are very different. Earth's atmosphere is mostly Nitrogen
and Oxygen! Venus' mostly CO2 atmosphere is 90 times thicker at the surface!
- Understand why (in general), the least massive planets have the
least atmosphere. The gases escape more easily, because the escape
velocity due to gravity is smaller. This explains why Mercury and the Moon
have no atmospheres, and Mars has only a thin atmosphere.
- Venus, although slightly less massive than the Earth, has a much
thicker atmosphere. This is an exception to the above rule!
- Understand why Venus and Earth are so different: liquid water
and the Greenhouse Effect!
- Understand how the greenhouse effect works - CO2 and water vapor in
the atmosphere allow visible light to come in, which warms the surface, but
don't allow infrared radiation to escape, making the surface warmer (heat
March 11-The cycles that shape planets
This class focussed on the processes that shape the the surfaces and
atmospheres of the terrestrial planets. We discussed how certain cases
apply specifically to certain planets.
Meteors and Meteorites:
- We believe that the meteors seen in meteor showers are dust/debris
from comets because we see meteor showers when the Earth passes near the
orbit of a comet.
- Most meteorites (rocks that reach the ground) are believed to come
from asteroids (icy comet debris is too weak to survive passage through
- There's plenty of geological evidence supporting our theory
that meteorites were once part of larger rocky bodies, and that they
are debris from asteroids which have been broken apart in collisions.
(No details required).
- We believe that asteroids formed between Mars and Jupiter,
but couldn't coalesce into a single planet due to Jupiter's gravitational
influence. This is supported by the fact that the density of asteroids
is higher in the inner parts of the asteroid belt than
it is in the outer parts (condensation sequence in the solar nebula).
- We think some asteroids were once part of larger bodies
(geology/spectroscopy - No details required).
- Asteroids are heavily cratered, some
are double asteroids (so there have been collisions); this supports our
theory of asteroid evolution and meteorite origin.
Rings: Recent Debris
- Dirty snowball model: lump of dirty ices that vaporize, releasing
gas and dust, as the comet comes close to the sun. See only
when close to the sun.
- The orbits of comets imply 2 populations - "Kuiper Belt" and "Oort Cloud"
- "Kuiper Belt" is a thick disk between Neptune (30 AU) and 100 AU
from the Sun. These comets formed where they are now.
- Pluto/Charon are believed to be the largest of the "Kuiper Belt"
- Evidence for KB: have seen ~60 of these objects in the KB,
and also have seen dust disks around other stars that are believed
to be produced by dust released from comets.
- The "Oort Cloud" is a spherical swarm of comets 10,000 to 100,000 AU
from the Sun! Comets in the Oort Cloud formed between Saturn and Uranus
- ejected from solar system by gravitational interactions with giant planets.
- Jupiter's rings are made out of tiny dust particles.
We know this from the way they scatter light. These
particles would have been blown out of their orbits
by the solar wind and sunlight if they had been around since
the beginning of the solar system. Instead, we think they
are debris from impacts between the moons and asteroids.
- Saturn's spectacular rings are made of ice: dust-sized
to house-sized particles. The ice would have melted in
the heat of formation, and darkens with time due to
micrometeorite impacts. They are icy and bright, so
are very fresh. We believe they are debris from impacts
between Saturn's moons (dust) and comets (ice).
- Uranus and Neptune have rings black as coal.
- Know why the orbits of Pluto/Charon and Neptune's
moons, as well as the rotation of Uranus and its moons/rings,
are evidence for giant impacts early in the solar system.
In the previous lecture I also mentioned some evidence for
such impacts from the inner solar system.
- Know that a comet hit Jupiter in 1994!
- Know that craters can form from much smaller objects.
- Know that the Earth has been hit in the past by
giant asteroids/comets: We believe that climate
changes due to this caused the death of the dinosaurs.
- EVIDENCE: Layer of Iridium (rare on Earth,
common in asteroids) in sediments 65 million years ago.
Soot layer (from forest fires?) also in sediments.
Plentiful dino fossils under this layer (earlier),
and no fossils above this layer (later). NOTE THAT
WE CAN'T BE CERTAIN OF THE REASON FOR THE CORRELATION.
- There are many near-earth asteroids that could
hit the Earth, but sizable ones aren't expected very
often. On the other hand, we haven't identified
them all, so we might not know it is coming.
Geologic and Atmospheric processes that shape the planets.
Understand the difference between positive and negative feedback, and how one
leads to a stable situation and how one leads to an out of control situation.
- Convection drives many of the processes that shape the surfaces of planets.
You should understand the basics how convection works, and how it transports
energy from one place to another. You should also understand the following
convective processes: Plate Tectonics, Atmospheric Circulation, Storms, and
- Plate Tectonics
- Heat is generated in the cores of planets from radioactive decay from heavy
elements. (Understand why the heavy elements are in the cores of planets!)
- The mantle is "plastic." That means that even though it is solid, it can
flow over long periods of time. (Think about how ice in a glacier flows).
- The lower portion of the mantle is heated and starts to rise.
- Cold mantle near the survace starts to sink.
- The crust is carried along with the mantle.
- Understand how this relates to earthquakes and volcanoes
- On Venus and Mars, the mantle is not plastic. Because of this heat must get
out in another way.
- Heat trapped in the planet can melt rocks.
- On Venus, the trapped heat is released planet wide in a surge of vulcanism.
- On Mars, there are only a few "hot spots" that result in volcanos. These
may all be too cool to erupt in the future.
- Atmospheric Circulation (Winds)
- Global wind patterns are driven by cold are descending at the poles and warm
air rising at the equator.
- Understand how rotation makes this pattern more complicated.
- Be able to make diagrams showing which direction the surface and high
altitude winds would be blowing. Understand why the surface winds blow from
cold regions toward hot regions.
- Remember that evaporating water cools the air, condensing water into a cloud
heats the air.
- A storm is an example of "positive feedback." As warm moist air rises, it
cools causing water droplets to condense. This heats the air causing it to rise
further and more water to condense. The rising air draws more warm moist air in
at the bottom of the storm.
- A hurricane is a storm taken to extremes. A storm sitting in still air over
warm water as a very large amount of energy available. (The warm water
continually heats and humidifies the air, which feeds the storm.)
- Ocean Currents
- Ocean currents are also driven by convection cold water in the arctic
regions sinks, warm water in the tropics rises.
- The overall effect of the currents is to transport warm water toward the
poles, and cold water toward the equator. This is called the "global conveyor
belt" It helps keep Europe and Australia warm.
- Because fresh water is less dense than salt water, if the polar ice caps
melt and dump a large amount of fresh water into the arctic, the conveyor could
stop. This would trigger severely cold winters in the high latitude regions.
Parts of Europe might be snow covered all year round.
- The Water Cycle
The water cycle is just another way of thinking about how atmospheric circulation
and storms transport energy and water. One thing to remember when
thinking about storms, atmospheric circulation, ocean currents and the
water cycle is that they are all powered by the sun.
- The Carbonate-Silicate cycle
- This cycle is an example of negative feedback. It helps to maintain the
Earth in a narrow range of temperatures.
- It is related to both the water cycle and plate tectonics.
- Carbon dioxide is a greenhouse gas. It is opaque to infrared rays which
traps heat in the atmosphere and results in increased temperatures.
- Rain falling on the land erodes silicate rocks releasing ions into the
- The water ends up in the sea, increasing the concentration of ions in the
sea (making it saltier).
- Carbon dioxide in the atmosphere dissolves in the sea to make carbonic acid.
- Carbonic acid and the ions combine to build the shells of sea creatures and
other carbonate minerals.
- These carbonates collect on the sea floor.
- Plate tectonics causes some of the carbonate materials to be accumulated
deep in the crust of the earth.
- Volcanoes release some of the carbon dioxide from the carbonate minerals
into the atmosphere.
- The negative feedback arises because an increase in the amount of carbon
dioxide in the atmosphere results in an increase in temperature. Increased
temperature increases evaporation of water from the oceans which increases the
amount of rainfall. Increased rainfall increases the amount of ions in the
water, which increases the rate at which carbon dioxide is incorporated into
shells which eventually reduced the amount of carbon dioxide in the atmosphere.
This allows the Earth to keep a relatively stable temperature over geologic
time even though the sun has gotten significantly brighter
- There are limits to the feedback. Water vapor is also a greenhouse gas. If
the amount of water vapor in the atmosphere gets too high, it will become warmer
which will cause more water to evaporate. This is a positive feedback
mechanism, called the "runaway greenhouse."
- On Venus, this positive feedback occurred, water vapor in the atmosphere
increase the temperature until the oceans had entirely evaporated. Water in the
atmosphere was broken into hydrogen and oxygen by UV light. The light-weight
hydrogen escaped. The oxygen combined with minerals in the crust. Without
liquid water, there was no way to make carbonate minerals so all the carbon
dioxide in the crust was eventually released into the atmosphere.
- Positive feedback in the opposite direction is also possible. Ice is a good
reflector of sunlight. If sunlight isn't absorbed by the surface, little
infrared is radiated, so surface ice decreases the effectiveness of greenhouse
gasses. That results in more cooling and more ice build up. If this isn't
balanced by an increase in greenhouse gasses, eventually the planet could get
cold enough from carbon dioxide to freeze out of the atmosphere. With no carbon
dioxide in the atmosphere there would be little chance of warming up again.
- Understand how and why more carbon dioxide in the Earth's atmosphere will
affect our global climate.
Mar 18 -
Life in the Universe.
Properties of life on Earth
- Composed of Cells
- Not necessarily a universal property of life
- Some non-living things have cell like structures
- Uses energy
- Many things that are not life also use energy
- Self Replicating
- Based upon a stored pattern or template (DNA)
- Some non-living chemicals can be considered to be self replicating.
- Capable of responding to the environment.
- There are non-living things that also respond to the environment.
- Maintains homeostasis (internal balance)
- Also present in some non-living things (i.e. climate on the Earth).
- Capable of evolution due to imperfect replication.
Evolution is the process by which life changes over time. It is due to
imperfections in the replication process.
- DNA is the molecule that contains the information used to construct
- DNA consists of strings of base pairs of nucleic acids.
- A group of three base pairs (a triplet codon) codes for an amino acid.
- A gene is a string of triplet codons that is a list of the amino acids used to build a protein.
- Large portions of the DNA in an organism are unused.
- Mutations are the primary mechanism by which the DNA molecule
- Mutations can be beneficial, neutral, or harmful. Most are neutral or
- You probably have 10 unique mutations in your DNA. Three are probably
harmful. The rest are likely to be neutral.
- What is neutral to you may be harmful or beneficial to your distant descendants.
- Selection is the means by which genes are removed from a population.
- This occurs if an organism dies before reproducing, cannot produce viable
offspring, or cannot attract a sexual partner.
- The combination of selection and mutation cause isolated populations of
organisms to become dissimilar.
The history of life on earth
- The oldest definite fossils are bacterial fossils about 3.5 Gy old.
- There are chemical hints that life may be as old as 4.2 Gy.
- Abiogenesis: the formation of life from non-living things.
- Organic molecules, including amino acids, are found in star forming regions,
meteors, asteroids and comets and could have arrived on Earth during
- Amino acids could also have been created by UV light or electrial discharge
in the atmosphere of the early earth.
- Given enough time, and the right conditions, long chains of these molecules formed through chemical evolutions. Eventually one capable of
self-replication formed and biological evolution took over.
- Some astronomers theorize that bacterial life arrived on Earth from another
solar system. This theory is called panspermia.
- The first organisms probably got their energy through chemosynthesis
rather than photosynthesis. You should understand the difference.
- Know the timeline for the formation of life.
- Life took between 0 and 700 My to get started after the oceans formed.
- For 3 Gy, life on earth was dominated by bacteria and other single
- Complex multi-cellular organisms have only been around for 550 My.
- We can use the timeline of life on earth to help us understand how
difficult it is for certain steps to take place.
- Think of playing Yahtzee. Rolling a 6 is easy, it might happen on the first
roll. Rolling a yahtzee (5 of a kind) is far more difficult. That makes it
likely to happen later in the game. It might not happen at all. But then
again, sometimes yahtzee happens on the first roll.
- Based on the timeline we think making single celled life is fairly easy, but
getting complex multi-cellular organisms is probably hard. Once you've got
them though, getting intelligence seems fairly easy.
Life elsewhere in the solar system
- Earthlike life requires: A planet or moon, assorted elements like carbon,
oxygen, nitrogen, and metals, a liquid solvent (water), and an energy source.
- Venus and mars have no liquids
- Mars probably had liquid water in the past. A meteorite believed
to have come from Mars shows carbonate deposits, complex organic
molecules, and tiny "fossil-like" structures that MAY have been
produced by bacteria. HOWEVER, the water that deposited the
carbonates was probably too hot for life, the organic molecules
may not be from life, contamination may have occurred, and
the "fossils" are quite probably natural mineral features.
- Venus may have had liquid water for a short time.
- Gas giants could conceivable have life floating in the atmosphere or oceans.
- Jupiter's moon Europa my have a liquid water ocean under
its icy crust (remember that tidal heating keeps this moon somewhat warm).
- Titan's moon is this cold because of an "inverse Greenhouse
Effect": the atmosphere lets infrared (heat) out, but doesn't
let new light from the Sun in. (Dust/soot particles can have
this effect, which is why large volcanic eruptions or a nuclear
war might produce a cool climate on Earth).
- Titan's atmosphere contains a smog of organic molecules
(produced in reactions with sunlight) that settle to the
ground in a tarry goo.
- Saturn's moon Titan may have methane oceans on the surface,
and has an organic tarry goo settling out of its atmosphere. But
the surface is COLD.
Life in other solar systems
Drake's Equation: A way to estimate the number of communicative
civilizations we expect. Lists all the factors involved so
we can evaluate which are the most important.
The factors are: (1) number of stars in the galaxy (2) probability
of planets in stable orbits (3) number of planets with orbits in
the "life zone" (4) probability that life will originate if the
conditions are suitable (5) probability life will evolve to be intelligent
(6) fraction of star's lifetime during which the life form is
The fraction of a star's lifetime during which a life form (if
it exists) is communicative is the most uncertain!. Technological
survival between 100 years and 1 million years gives a different
answer by a factor of 10,000!
Optimistic estimates give 750,000 communicative civilizations in
our galaxy, while pessimistic estimates give only 2 communicative civilizations
PER 4 million galaxies!
- We have detected planets in other solar systems by detecting motion of the
stars caused by the gravitational pull of the planets
- We can only detect Jupiter or bigger, can't detect earth sized planets
- Stars have a "habitable zone", a range of distances at which an earth sized
planet could support earthlike life.
- How long the habitable zone lasts is a function of the type of star
Mar 25-The properties of stars
- Stars: Hot dense object producing a continuous spectrum, with a
cooler outer atmosphere which produces absorption lines.
- The presence of particular absorption lines can tell us something about
which elements are present (the absence of a line doesn't insure the absence
of the element, however).
- The strength of particular absorption lines depends on how many
electrons are in the relevant energy level. If the gas is too hot the
electrons will be out of the atoms (which are therefore ionized), and
you might not see the lines. If the gas is too cool, the electrons will
all be in the lowest energy levels, and you won't see transitions involving
the higher energy levels. In both cases, the lines will be weak. The
strength of absorption lines therefore indicates the temperature of the gas.
The colour (wavelength of spectrum peak) also gives a cruder indication of
the temperature, but the peak wavelength is not always in visible light.
- Parallax is very important. All distances to stars and
galaxies are based on these measurements! Understand the idea of parallax,
and that the measurement is smaller for more distant stars. We can
only measure this for stars nearer than about 500 pc. Know the definition
of a parsec!
- The Doppler effect is also important, in that it is how we measure
motions along the line of sight between us and a star. Understand how
the effect works for sound waves, and how we use it to measure radial velocity.
- Notice that we also need the transverse velocity to completely know
the star's motion. To do this we measure a star's proper motion (or
apparent change in direction with time), and combine that with distance.
- The inverse square law is also very important. The
luminosity or intrinsic brightness of a star is the
energy per second coming from the star. How bright the star really is.
The apparent brightness is how bright it looks to you, or the
energy per second per unit area that you see. The apparent brightness
of a star depends on the inverse square of the distance to the
star. In other words it is proportional to 1/(distance)2.
For two identical objects, something twice as far away looks 1/4 as bright.
Something 10 times as far away looks 1/100th as bright.
- Be very familiar with the HR diagram, as we will use it for the next
couple of weeks! A plot of luminosity vs. temperature, it shows that
stars fall into distinct groups. Be able to recognize these groups.
- Understand that we can use the orbits of binary stars (two stars moving
around each other) to estimate the masses of the stars. But we need to
know everything about the orbits before we can use gravity to
determine the masses! Finding masses is hard!
- Know the 3 different ways we can see a binary star system (measuring
motions on the sky, seeing changing Doppler shifts in the absorption lines,
and seeing a periodic dip in the light from eclipsing binaries).
- For the first case, a visual binary, only a few nearby systems with
widely separated stars can be easily measured. And it takes a long time
to determine the orbits!
- Understand that for the second case, a spectroscopic binary, we can
only measure motion towards or away from us, so can't be sure we are
measuring the whole velocity! Only if an eclipsing binary (where the
orbit must be edge-on) is also seen as a spectroscopic binary can we
figure out the whole orbit.
- In these cases, the maximum velocity of the stars, combined with the
orbital period, can give us the size of the orbit, and so we can
use gravity to find the masses. In these special cases, as a bonus, the
time that the star spends in eclipse, plus the speed of the stars, gives us an
measure of the radius of stars!
- Understand that for a blackbody, the energy emitted per second per
unit of surface area is proportional to (temperature)4. Something
twice as hot emits 16 times as much energy in each square meter of its
surface. So if two stars are the same size, but one has twice the
temperature, that one has 16 times the luminosity (or intrinsic brightness).
- Understand that this fact allows us to estimate the sizes of stars.
By measuring the temperature, we know how much energy is emitted in each square
meter of the star. By measuring the luminosity, we know how much total
energy the star is emitting. Comparing the two numbers tells us how much
surface area the star must have. Bigger stars are brighter. If two stars
of the same temperature, but one has twice the radius of the other, then that
one emits four times as much total energy.
- The above methods are fairly "direct". But once we accept the
HR diagram, we can estimate the luminosity of a star just by measuring
its temperature and which 'luminosity class' it falls into on the HR diagram.
(These are both possible through a detailed analysis of the spectrum).
Then we can use the HR diagram to estimate the luminosity. Comparing
this with a measurement of the apparent brightness also gives an estimate
of the distance. This is how we determine the distances to stars
that are more distant than about 500 pc! (Remember the Milky Way Galaxy
is up to 30 000 pc across!
- Handout with summary of how we measure stellar properties
- Temperature: color of star, and strength of absorption lines
- Distance: parallax is the only direct method
- Luminosity: measure apparent brightness and distance, calculate luminosity
- Which class on HR diagram (ie main sequence or giant or white dwarf): width
of absorption lines
- Distance: Measure temperature, and whether the star is on the main
sequence or a giant or a white dwarf. Assume that it would fall in the
same place on the HR diagram as other stars of its type. This gives you
the luminosity. Measure the apparent brightness. Comparing the two gives
an estimate of the distance.
- Radius or Diameter: Measure the temperature, and the luminosity.
The temperature tells you how much energy is being emitted per second per
unit surface area of the star. The luminosity is the total energy per second.
Comparing the two gives you the surface area, which is proportional to
- Masses: Measuring the orbit of binary stars and applying Newton's
generalization of Kepler's Laws (which follows from the law of gravity)
allows you to estimate the masses of the orbiting stars.
- Understand how we use our knowledge of blackbody radiation to
estimate the radii (sizes) of stars.
- Know that intrinsically bright (luminous) stars are rare, but can
be seen for a long ways. The most common stars are the dim red
main-sequence stars. Most nearby stars are dim.
- Know that due to differences in size (radius), giants and supergiants
are much less dense than main sequence stars, while white dwarfs are much
- Know what I mean by the Mass-Luminosity relation: looking ONLY
at main-sequence stars, the most massive ones are the most luminous.
- Recognize that the Mass-Luminosity relation tells us something about how
long a star will live. A star 10 times the mass of the sun has 10 times as much
fuel. But it has 10,000 times the luminosity, so it's burning it's fuel 10,000
times faster. That means a 10 solar mass star will only live about 1/1000 as
long as the sun.
April 1- Stellar Structure
Hydrostatic (aka Gravitational) Equilibrium is the balance between pressure and
gravity, and is extremely important to understand. It is what keeps a
star the size that it is.
A star is shining because it is hot, and so is losing energy. To remain
stable in size, it must have an internal energy source. This source is FUSION.
Know the conditions required for fusion (very high temperature and pressure), and why it results in energy.
Also know that the rate of fusion depends upon temperature and pressure and
why this is so.
Know that fusion turns 4 H atoms into 1 He atom, and releases
neutrinos, positrons, and gamma ray photons. Don't stress over the
details of the fusion reaction chains (understand that a CHAIN is required).
Understand the results of the solar neutrino experiment, and what
it tells us about our models of the Sun, and about particle physics. Note
that studying the way the sun vibrates (helioseismology) also provides
supporting evidence for our model of the Sun.
The "Pressure-Temperature Thermostat" regulates the rate of
fusion burning. This is another example of "negative feedback".
Know and understand the ingredients that go into the calculation of a
stellar model - there are only four! Mass, Energy Generation, Hydrostatic
Equilbrium and Energy Transport
Understand how gravitational equilibrium explains the Mass-Luminosity
relation of main sequence stars
and why there is a lower limit to the
mass of main sequence stars.