What keeps the sky from being lit up by stars at night ?

1  Human generated ambient light. Nearby lit areas are easier for our eyes to catch than more distant light. The nearby light drowns out more distant light which is why many call it light pollution.

2  Time. Light has a speed limit. While some of it is making its way towards us a lot of it is still on its way here (some of the stars that we see light coming from have actually burn out). Every star is being shown to us at a different time period.  We only see light from parts of the universe that are less than 15 billion years old.

3  Olbers Paradox states that the universe expansion is what causes light to be redshifted which lowers its frequency and energy.

Assumption:  A bright light should be seen in every direction.  Objects farther away are fainter but more numerous which should yield the same overall brightness.

Resolved !  The expansion of the universe known the cosmological constant, resolves the paradox. As the universe expands, the light waves are stretched out reducing the energy sent out from galaxy to another. Also, the time to receive the light is lengthened over the time it took to emit the photon. Because luminosity = the energy/time, the apparent brightness is lowered enough by the expansion to cause the sky to be mostly dark.
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4  Light from distant stars is less intense because the photons being emitted are emitted over a larger sphere area, fewer enter people's field of vision. There's also gravity (curves space) and interstellar gas like hydrogen and helium (blocks light).  General relativity shows how gravity is a curvature in spacetime.

5  Distance !  proven by the inverse square law

Why does a planet like Earth appear to not fall ?

Its orbit (which contributes to the magnetic field) gives the Earth angular momentum. The earth travels at enough speed while in orbit so that it doesn't fall into the sun, but not so much speed that it gets flung into deep space. Ironic since the earth's gravitational force is also being affected by the sun's mass. In the bigger picture (galaxies) there's also dark matter that exerts a gravitational force that gives larger clusters of bodies their integrity.

Climate change enlarges total fertile area while reducing prime locations, C4 replaces C3

  Climate change negatively affects the growing season for areas in the mid latitudes traditionally associated with farming & wheat production (in part due to more super wet regions, change in night temperature, wild shifts in temperature over a short period of time) but it does enlarge the total land area globally able to be used for growing through a warming of the northern latitudes (shifts precipitation over to rain from snow).

In Western Canada for example in 2010 more than half of the lower latitude regions of the key provinces of Manitoba and Saskatchewan received more than two feet of water in the fall alone, that's more than what all of Western Canada usually averages per year. This presents new problems; In Canada for example by 2050 optimal growing conditions will shift to as far north as the Canadian Shield where for the last century the area has been infertile or at the very least, difficult to grow vegetation on. Photosynthesis shifts from C3 to C4 allowing for better water efficiency use and faster carbon dioxide intake (important since CO2 levels are also rising). Under phosphorous stress both C3 and C4 plants show similar biomass production even though the root to shoot ratios are not the same.

C3 - Only enzyme used (rubisco) is also involved in CO2 uptake. Better than C4 under temperate conditions (cool/avg light intensity).

C4 - CO2 initially incorporated into 4 carbon compound. Differs from C4 since it makes use of both rubisco and pep carboxylase (pep is responsible for both the uptake and delivery of CO2). rubisco ends up preocupied with CO2 deliveries from pep and so it does not have the opportunity to also take in o2 for photorespiration. pep is also faster than rubisco at taking in co2 and that means stomata aren't open as long (less water ends up transpired and so C4 is more efficient at photosynthesis in less moist conditions).

-More co2 is absorbed, less water is used, photosynthesis only takes place within a specialized part (kranz cells), an anatomy feature unique to C4 plants; that makes C3 photosynthesis simpler and less energy intensive.
-C4 plants end up with a higher shoot to root ratio (better suited to high light intensity/competition for light becomes more of a factor).

root (supportive tissues)/shoot (growth tissues) ratio - decreases with plant size, changes according to nitrogen availability (fertilizer).

Though fundamentally different Bosons and Fermions sometimes overlap

  Fermions are matter, bosons are just force carriers however some fermions (coupled electrons collecting into the same energy state at low temperature) can exhibit characteristics normally seen only in bosons (Bose–Einstein statistics, idential particles occupying the same energy states at the same time). Diagram: Neutron atom's only down quark decaying into an up quark (to conserve charge, also remember that the process produces a proton which requires the down quark ---> up) and a W boson which then goes on to further decay into an electron and electron neutrino. Also shown is the muon and muon neutrino (produced only through decay)

Baryon - Are those particles composed of 3 quarks (compared to mesons which have 2 quarks). They participate in the strong interaction (strong interaction is the force that keeps components of the nucleus together for example quarks within protons but also protons/neutrons within the nucleus; there's also a residual force mediated by the mesonic gluons that transmits the nuclear force). Baryons are fermions (1/2 integer spin as opposed to Bosons with integer spin) while mesons are bosons. Differ from bosons since they obey the pauli exclusion principle. A neutron is 1 up quark (+2/3 charge) and 2 down quarks (down quark has a charge of -1/3). In the case of neutron decay a down quark ----> up quark, what the W boson does is take -1 charge away in the process turning the down quark into an up quark -1/3---->+2/3 = net gain of +1 charge turning it into a proton.

Hadron family (quarks makes them distinct) - baryon (3 quarks), protons, neutrons, mesons (pi, rho). Composed of quarks. Quarks/Hadrons/Baryons are associated with matter. All hadrons have integer charges but quarks have fractional charges meaning the number of quarks/antiquarks used to create baryons (3) or mesons (quark+antiquark) must add up to integral charge. Quarks can't exist on their own, only in baryons and mesons.

Bosons - Associated with force carrying; ones with the same energy can occupy the same quantum state as opposed to the pauli exclusion principle. Photons, Mesons are examples, they all have integer spin (0, 1, 2, etc). Because they can occupy the same state, at low temperatures they can collect into the same energy state which is called condensation; that is illustrated for example in superfluidity. W bosons can decay into up and down quark (meson) or a lepton and a neutrino. The W boson plays a key role in muonic decay Unlike photons, bosons have mass which means the force they transmit (weak nuclear) operates on a smaller distance. W bosons - flavour and charge changes between quarks and leptons (quark/anti-quark or lepton + neutrino/anti-neutrino). In the case of Z bosons (no change in flavour or charge) the boson decays into lepton/anti-lepton pairs.

Muon - Elementary particle (lepton) that's heavier than the electron. Muons also help W bosons escape the nucleus, neutron during beta decay. The rate at which muons transition into lighter particles during decay is governed by the available energy of the reaction (depends also on how heavy the products are). Muons and electrons both carry the same negative electric charge. W bosons play a key part in muonic transitions, they are absorbed and emitted by muons and that's what gives the muons the extra charge needed to transition into muon neutrinos.

Fermion - Electrons, Protons. Associated with matter (only one kind can occupy a particular space, if more than one they must differ in terms of their properties). Half integer spin. 24 different fermions 12 of them being anti particles of quarks and leptons. Fermions are also produced through Z boson decay (into a fermion and its anti particle).

Leptons - Six main types of elementary particles categorized based on their flavours/generations (generations are how elementary particles are grouped). Only electrons are charged because electrons are the lighest (mass) they are the most stable. Many of the other leptons can only be produced in particle accelerators due to their instability. They are subject to all forces (weak inter - decay, EM force) except the strongest one (strong nuclear force). Though when paired, coupled electrons can be made to act like bosons, in which case they can be made to display zero electrical resistance (happens when they collect into the same energy state). Electrons, muons and tau differ significantly in terms of their mass (tau is 15X muon, muon is 200X electron).

Neutrinos - 1/2 integer spin (fermionic), no mass, no charge, subject only to the weak force. Electron antineutrinos are produced through beta decay (beta + and - decay so called based on the type of emission (+=positron, - =electron). Feynman diagrams are used to illustrate the process by putting into visual form the decay mechanism and its associated energy transmission & products.

Weak Interaction - This is where beta decay happens (heavy particles either absorb or emit bosons in the process altering their states). For example, leptons absorb bosons producing neutrinos; quarks absorb or emit bosons (W or Z) and convert into a different kind of quark (up and down are only 2 of the 6 different kinds of quarks however they have the least mass and are the most stable and so are more commonly encountered). Responsible for radioactive decay through the exchange of W and Z bosons which converts one quark (up) into another (down).

Strong Nuclear Force - Gives integrity to the structure of protons and neutrons. It is this force which creates color attraction between quarks of different charges. Force is carried by gluons. The strong force is associated with the color force.

Gravity - The only force that every particle is subject to. Also differs in the sense that different sources of gravity can reinfornce one another to create a bigger effect.

Some great sources for more info: The Four Forces and Hadrons, baryons, mesons
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