Science Lecture Notes Review

Science and the Scientific Method

• Science is the methodical study of natural phenomena.
• The sciences are commonly divided into social sciences and natural sciences.
  • Social sciences deal with human and social relationships, and include the disciplines of anthropology, economics, history, political science, psychology, and sociology.
  • Natural sciences deal with the physical world and include biology, physics, chemistry, geology, and astronomy.
• Astronomy: study of planets and their adornments, stars and star systems, nebulae and galaxies, and evolution of the universe
  • Some common branches of astronomy:
    • Astrophysics: applications of physics laws and theories to astronomical objects and observations
    • Cosmology: science dealing with the evolution of the universe
    • Astrobiology: interdisciplinary study of life in the universe
• Universe: totality of all energy, matter, time, and space
• Scientific Method: Science is conducted according to an organized procedure called the scientific method.
  • This procedure consists of systematic observation, experimentation, and formulation of a hypothesis, followed by testing and possible modification of the hypothesis.
  • A hypothesis is an objective proposition based on verifiable data that describes natural phenomena and is capable of being proven false by testing.
    • Requirements of a hypothesis:
      • Objective (not subjective) statement
      • Make a prediction that can be tested and proven false

Kepler’s Laws

• Ptolemaic (Geocentric) Models
  • Prior to the 16th century, the prevailing models of heavenly motion (i.e., planet and star motion) were ones where Earth was at the center. These are called geocentric models.
  • A person often associated with geocentric models is Claudius Ptolemy, and as such Earth-centered models of planetary motion are often called Ptolemaic models.
  • Claudius Ptolemy was a Greco-Egyptian writer, mathematician, astronomer, geographer, and astrologer.
  • Using 800 years of recorded observational data, he predicted the future location of the five naked eye planets (Mercury, Venus, Mars, Jupiter, and Saturn).
  • A fundamental assumption in his analysis was that planets and stars move in circular orbits around Earth.
• Copernican (Heliocentric) Models
  • Nicolaus Copernicus was a German mathematician and astronomer, as well as physician, politician, and economist.
  • He recognized discrepancies in Ptolemy’s predicted planet locations that led him to propose a model of the universe where Sun was at the center. Sun-centered models are called heliocentric models.

States of Matter and Temperature

• Matter
  • Matter is a substance or object that occupies space and has mass.
  • There are four states of matter:
    • Solid: object of definite volume and definite shape
    • Liquid: substance of definite volume and variable shape
    • Gas: substance of variable volume and variable shape
    • Plasma: high temperature gas of electrons and ions
      • An ion is an atom that has an excess of electrons (negative ion) or deficit of electrons (positive ion) compared to the electrically neutral atom.
      • Molecules can also be ions. In very high temperature plasmas, all electrons are removed from the atoms, resulting in a gas of negatively charged electrons and positively charged nuclei.
  • The change from one state of matter to another is called a phase change or phase transition.
  • A fluid is a substance of indefinite shape that flows easily. Liquids, gases, and plasmas are fluids. Solids can also sometimes be modeled as fluids.
• Temperature
  • Temperature is often defined as a measure of hotness or coldness.
    • A more insightful and physical definition is that temperature is a measure of the average kinetic energy of atoms or molecules in an object/substance.
  • Kinetic energy: energy of motion, manifested by the movement of an object, substance, or particle

Force and Energy

• Force and energy are different physical quantities that scientists created to describe nature.
  • Because they are different, they offer alternative ways of describing natural phenomena.
  • Sometimes it is easier to use force concepts; other times it is simpler to use energy concepts.
  • The two are not independent, however, they are related to each other mathematically (we will not go into the math details).
• Forces
  • Force can be defined as a push or pull.
  • If you were to push a book across a table, you would be exerting a force on the book. If you were to drag a tree branch across the lawn, you would be exerting a force on the branch.
  • There are four fundamental forces in nature: strong, electromagnetic, weak, and gravity. These are listed in the order of decreasing strength, i.e., from strongest to weakest.
    • The strong force holds nuclei of atoms together.
    • The electromagnetic force causes oppositely charge objects ( + and – , or – and + ) to be attracted to each other and like charged objects (+ and +, or – and –) to be repelled.
    • The weak force is responsible for a radioactive decay process called beta decay.
    • Gravity, the weakest of the four fundamental forces, is the force that causes all matter to be attracted to all other matter.
• Fundamental Forces in Nature and Their Characteristics
  • Strong: Where Relevant - Holds particles together in nuclei, Relative Strength - 1, Range - Short (nuclear dimensions)
  • Electromagnetic: Exists between charged particles, Relative Strength - 10^{-2}, Range - Long (infinite)
  • Weak: Responsible for beta decay, Relative Strength - 10^{-15} to 10^{-7}, Range - Short
  • Gravitational: Exists between all objects of mass, Relative Strength - 10^{-40}, Range - Long
• Newton’s Laws of Motion
  • Newton’s First Law: it a takes a net force to change the velocity of an object. The velocity of an object is its speed (i.e., how fast it is moving) and the direction of motion.
    • The net force that acts on an object is the sum of all forces that act on the object. This addition must be performed carefully because the directions of the forces must be considered. For instance, two forces that act on an object in the same direction result in a different outcome than if the two forces are act in opposite directions. If just one force acts on an object, it is the net force.
• Acceleration is the rate of change of velocity with respect to time, or equivalently, the change in velocity per change in time.
  • An object is accelerating if its speed is increasing, decreasing, and/or its direction of motion is changing. Situations where you are undergoing an acceleration include:
    • When you push down on the accelerator pedal of an automobile, you undergo a positive acceleration.
    • When you push down on the brake pedal of an automobile, you undergo a negative acceleration (a deceleration).
    • When you go around a curve with constant speed (perhaps on a highway ramp), you are still accelerating because your direction of motion is changing.
  • Newton’s Second Law: F{net} = m a Newton’s second law is a statement of the relationship between the net force that acts on an object (F{net}) and its acceleration (a): the net force is directly proportional to the acceleration. The proportionality constant is the mass of the object (m). mass quantity of matter
    • If two objects of different mass are acted on by the same force, the more massive object will have a smaller acceleration than the less massive object. This makes sense; which is easier to move, a Mack truck full of stone or a wagon carrying a toddler?
    • The net force and acceleration are typed in bold face font to indicate that forces and accelerations depend on direction (mass does not).
    • The net force acting on an object always produces and acceleration in the same direction. We feel forces, and therefore accelerations.
  • Newton’s Third Law: F{A on B} = - F{B on A}
    The above mathematical statement of Newton’s third law can be stated as, “the force that object A exerts on object B is equal in magnitude to and opposite in direction of the force that object B exerts on object A.” An alternative statement is that forces occur in action/reaction pairs of the same magnitude, but opposite direction.
• Energy
  • Energy can be defined as a property to cause motion. If something can cause motion, it has energy.
    • A firecracker has energy - when ignited it explodes, creating motion of the fragments. A person running has energy - imagine what would happen if they ran into you.
  • Energy is often divided into two broad categories, potential and kinetic. Potential energy (PE) is energy by virtue of position. Examples of potential energy include gravitational, electrical, and nuclear. Kinetic energy (KE) is energy due to motion. All objects in motion have kinetic energy.
• The total energy of individual atomic and molecular motion within a substance is a form of kinetic energy called thermal energy (or internal energy).
  One of the foundations on which all of science is based is energy conservation.
  law of conservation of energy energy cannot be created or destroyed, only transformed from one form to another
  There are no known violations of energy conservation.
  A guiding principle in nature is that systems left on their own evolve towards a lower potential energy state. For example, when you drop a book, it falls to the floor. The gravitational potential energy of the book on the floor is less than the gravitational potential energy of the book before it was dropped.
• Forces and Energy
  Force and energy are different physical quantities. The force of gravity is not the same as gravitational potential energy, just as a Pepsi is not New Zealand. Force and energy, however, are related mathematically. Problems in astronomy (and all of science) can in theory be solved using force methods (Newton’s laws) or energy methods (energy conservation). Which approach to use is generally dictated by the questions to be answered and the mathematical sophistication required (Occam’s razor – use the simplest method).
  Let’s revisit the dropped book mentioned above. We stated that the book fell to the floor to get to a lower potential energy state. Is this a violation of energy conservation? The gravitational potential energy in the final state (on the floor) is less than the initial state (book held above the floor). Energy conservation is not violated. You need to consider the entire system and all energy involved. The decrease in gravitational potential energy as the book is falling is offset exactly by an increase in kinetic energy of the book, so the total energy remains constant. What about when the book strikes the ground? Just prior to impact the book has maximum kinetic energy and minimum gravitational potential energy. After impact the book no longer has kinetic energy; where did it go? It caused the atoms and molecules on the book’s surface and the floor to move back and forth more; i.e., it increased the thermal energy of the book and floor. Total energy was conserved throughout the process, just transformed from one form to another.
  From the viewpoint of forces, the book was pulled to the floor by the force of gravity (our next topic of discussion). The book stopped falling when it struck the floor because the downward force of gravity on the book (i.e., the weight of the book) was opposed by the upward force of the floor on the book.

Temperature

• Common temperature scales used today are Fahrenheit, Celsius and Kelvin temperature scales. The degree symbol is not used with the Kelvin scale, it is just “kelvin” (the temperature unit is all lower-case letters). The Kelvin scale is a thermodynamic temperature scale because it has a meaningful (i.e., natural) zero. Zero kelvin corresponds to the temperature at which all classical motion ceases to exist. This temperature is called absolute zero. This definition of absolute zero is consistent with our definition of temperature as a measure of average kinetic energy, i.e., energy of motion. If an object were at 0 K, it would have no KE and therefore no classical motion. Comments:
  • There is still motion at 0 K. Electrons are still moving around nuclei, and protons and neutrons in nuclei are still in motion. Also, there is quantum mechanical motion (that’s why the phrase “classical motion” was used above).
  • The temperature of “outer space” is 2.7 kelvin, the same temperature as the cosmic microwave background (this is remnant light from the Big Bang).
  • Stars form in molecular clouds where the temperatures are about 10 to 20 K.
• Maxwell-Boltzmann Distribution of Particle Speeds Temperature is a measure of the average kinetic energy (KE) of atoms or molecules in an object/substance. The average KE is specified because not all particles of gas type have the same KE. There is a distribution of kinetic energies centered around the average KE, and therefore a distribution of particle speeds around the average speed (since KE = ½ mv2).
  • This distribution of particle speeds is called the Maxwell-Boltzmann distribution.
    For a given gas type, hotter gas particles have a higher speed (on average) than colder gas particles.
    At a given temperature, small mass gas particles have a higher speed (on average) than large mass gas particles.
    The distribution means that (for a system of very many gas particles), some gas particles are traveling very slow and some are traveling very fast

Kepler’s Laws

• The Copernican model had Sun not only at the center of the solar system, but also at the center of the universe. It is accepted knowledge today that Sun is at the center of the solar system but not the center of the universe.
• Interestingly, the planet position predictions of Copernicus were not much better than Ptolemy’s because he (Copernicus) also assumed planet orbits were circular.
• Tycho Brahe (1546-1601)
  • Tycho Brahe was a Danish nobleman and astronomer. He is sometimes described as the first skilled experimental astronomer.
  • Along with his sister Sophie, he made systematic naked eye (i.e., without the aid of a telescope) measurements of the positions of 777 stars.
  • He was perhaps the first person to record the observation of a supernova and its lack of parallax, indicating that it was much farther away than Moon.
  • Using his observations of a comet, he was able to prove that the comet was well beyond Earth’s atmosphere (Aristotle and others argued that comets were associated with Earth’s atmosphere).
  • Brahe proposed a solar system model (not widely accepted) where Sun orbited Earth, while the other planets orbited Sun.
• Johannes Kepler (1571-1630)
  • Johannes Kepler was a German mathematician, astronomer, and astrologer (there was no clear distinction between astronomy and astrology at the time).
  • In 1601 he became an assistant to Tycho Brahe. Following Brahe’s death, Kepler was appointed to his mentor’s position.
  • He spent the next decade trying to mathematically (and physically) make sense of Brahe’s observations and resolve (the very small) discrepancies between predicted planet locations and their actual locations in the sky. In doing so, Kepler realized that he had to abandon the idea of perfectly circular orbits. He proposed instead that the planets orbited around Sun in slightly elliptical orbits. He also departed from Brahe in asserting that Earth, as well as the other planets, also orbited Sun.
• Solar System Motion Terminology
  • All planets orbit Sun in the same direction in nearly circular orbits in nearly the same plane. The direction of planetary motion is counterclockwise (CCW) when viewed from above and looking down on the North Pole of Earth. CCW is the same direction that most races are run and is opposite the rotational motion of hands on a clock.
  • Planetary orbits are nearly circular, so their size is usually characterized by their average distance from the Sun. This distance is also called the average orbit radius or mean orbital radius, or just orbital radius. The orbital radius of Earth is 1.50 x 10^8 km, meaning that the average distance between the center of Sun and center of Earth is 1.50 x 10^8 km. This distance defines the astronomical unit, abbreviated au: 1 au = 1.50 x 10^8 km = 1.50 x 10^{11} m = 93 million miles
  • Planets orbit Sun in nearly the same plane.
    The ecliptic plane is the plane formed by the path of Earth orbiting around Sun. A related term is the ecliptic, which is Sun’s apparent path against the backdrop of the stars.
  • A period of motion in science is the time that it takes for a complete cycle of motion. The orbital period of a celestial body is the time that it takes the body to complete one orbit around another body. The orbital period of Earth around Sun is 1 year. The rotational period of a body is the time that it takes the body spin completely around its axis of rotation. The rotational period of Earth is 1 day (or 24 hours).
• Kepler’s Laws *Johannes Kepler deduced three rules of planetary motion that we now call Kepler’s laws.
  • These laws are empirical ,i.e., they are based on experimental data and not an underlying theory of motion. Isaac Newton provided the theory, and we will discuss this later (Kepler’s laws are consequences of Newton’s laws of motion and universal gravitation).
• Aside: Kepler’s laws are valid for all objects orbiting another object when acted on by an inverse square law force, such as gravity and the electric force.
• Kepler’s First Law Planets orbit Sun in elliptical orbits with Sun at one focus.
  Elliptical orbits are the only stable orbits (a circular orbit is a special case of an elliptical orbit with an eccentricity of 0).
• Kepler’s Second Law Planets sweep out equal areas in equal times.
  Orbiting objects travel faster when they are closer to the body pulling them around than when farther away.
• Kepler’s Third Law The distance of the planet from the Sun cubed is proportional to the orbital period of planet squared.
  Mathematically this can be written as (distance)^3 = (period)^2 when the period is expressed in years and the distance is in astronomical units.

Solar System Census

• The solar system contains a star (Sun), eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), a handful of known dwarf planets (Pluto, Ceres, etc.), moons and rings, and asteroids and comets.
  star glowing ball of gas and plasma that derives its energy from nuclear fusion
• In 2006 the International Astronomical Union (IAU), an association of professional astronomers, redefined the definition of a planet.
• To be considered a planet today, a world must:
  • Orbit a star
  • Be mass enough that its self-gravity has pulled it into a spherical shape
  • Be massive enough that its gravity has cleared away other objects of similar size (3) Pluto was once a planet but failed to meet the third criterion and was reclassified as a dwarf planet.
    dwarf planet world that orbits a star and is spherical shape, but has not cleared its neighborhood of debris
• Six of the eight planets (and some dwarf planets) have moons orbiting them.
  moon a large natural satellite that orbits a planet
  satellite a smaller object that orbits around a larger object; there are natural satellites (created by nature) and artificial satellites (man-made)
• Asteroids and comets are leftovers from the planet formation process.
  asteroid a small rocky object (typically hundreds of meters to a thousand kilometers in size) that orbits the Sun
  comet a small icy and rocky object (typically hundreds of meters to a thousand kilometers in size) that orbits the Sun
  Most asteroids orbit Sun between the orbital paths of Mars and Jupiter in a region called the asteroid belt.
  Comets exist in two regions in the solar system, in the Kuiper belt and in the Oort cloud. The Kuiper belt is a disk-shaped region that extends from 30 au (Neptune’s orbital radius) to about 100 au. Comets in the Kuiper belt orbit the Sun in nearly circular paths.
  The Oort cloud a huge spherical region surrounding the Sun, extending out to 50,000 to 100,000 au (about half-way to the next nearest star) where perhaps trillions (10^{12}) of comets orbit with random inclinations, directions, and eccentricities.

Characteristics of a good hypothesis:

* Based on verifiable data
* Consistent with data
* Not a tautology (i.e., a statement that is true by virtue of its logical form)
* Explain a general phenomenon (not a specific event)
* Limited in scope (not vague with many possible outcomes)
* Stated in its simplest form (Occam’s razor)

Pseudoscience is a collection of beliefs or practices mistakenly regarded as being based on the scientific method. Pseudoscience is not science because it lacks supporting evidence based on the scientific method.
Astrology (not astronomy) is pseudoscience.
Astrology is the belief that the position of celestial objects influences human behavior and terrestrial events. Astrology is not a science; it is a pseudoscience because it does not stand up to the scrutiny of the scientific method.

Structure of Science

• Laws form the foundation of science.
  A scientific law is a well-tested hypothesis that is a fundamental pillar of science and for which there are no known violations. Ex: conservation of energy, momentum conservation, law of universal gravitation, etc.
• A theory is a collection of laws and ideas that explain a significant aspect of nature. Ex: Dalton’s atomic theory; Einstein’s theory of special relativity, Darwinian evolution
• Principles are general statements about how nature behaves.
  • Ex: cosmological principle:
    • The same laws of science apply throughout the universe and for all times
    • On average and sufficiently large scales, matter is distributed uniformly throughout the universe → there are no special places in the universe → the universe has no center or edge
  • Ex: Occam’s razor:
    • No more assumptions should be made than are necessary
    • All things being equal among two hypotheses (explanations), the simplest one is usually the most likely
    • “Extraordinary claims require extraordinary evidence” [source: Carl Sagan]
• Scientists
  *A scientist is a person who uses the scientific method to investigate nature.
  Scientists today are sometimes classified into one of three broad categories:

Dissemination of Science Information

• Scientists present their findings at discipline specific conferences and in peer reviewed journals.
  • Presentations at meetings provide for immediate dialogue about the material, but this information exchange is often limited to those in attendance. A much wider audience is reached when scientific results are published in journals. Most scientists publish their results in peer reviewed journals.
• Peer review evaluation of scholarly or professional work by experts (peers) in the same field.
• The peer review process is designed so that published results are sufficiently meritorious, relevant for the journal’s target audience, analyzed correctly, and interpreted properly. This process also helps to ensure that credit is given to others who have laid the foundation for or contributed to the current work.

Gravity

• Gravity
  The force of gravity (or just gravity) is the weakest of the four fundamental forces of nature, yet gravity is the force responsible for the structure of the universe, galaxies, and the solar system.
  Gravity is an attractive force; there is no repulsive gravitational force. Gravity acts on all matter.
  matter physical substance that has rest mass and occupies space (i.e., has a volume)
  mass (i) quantity of matter; (ii) quantitative measure of inertia
  inertia tendency of an object to resist a change in its state of motion
• Newton’s Law of Universal Gravitation The law of universal gravitation appeared in Isaac Newton’s Philosphae Naturalis Principia Mathematica (what we today call the Principia), first published in 1687. In words this law states that every particle is attracted to every other particle with a force that is directly proportional to the product of the particle masses and inversely proportional to the square of their separation distance. The magnitude (number and units; no direction) of this force can be written more compactly with math as: F{grav} = G \frac{m1 m_2}{d^2}
  • Where m1 and m2 are the masses of particles 1 and 2, respectively; d is the separation distance between the particles, and G is a proportionality constant called the universal gravitation constant.
  • The numerical value of G and its unit depend on the units of mass and distance used. For a given system of units, however, the value of G has the same value for all objects regardless of mass (galaxies, stars, planets, gas, and dust; it does not matter – no pun intended).
    There are three important implications of the universal gravitation law that we will make repeated use of:
  • All matter is attracted to all other matter, particles, and objects.
  • The force of gravity is proportional to the product of the masses of the objects. The more massive the objects (one or both) the stronger the gravitational pull.
  • Gravity is an inverse square law force. Qualitatively this means that the force decreases as the separation distance increases, or equivalently, that the force increases as the separation distance decreases. If two objects are approaching each other, the force of gravity increases and the acceleration of the objects towards each other increases (a consequence of Newton’s second law).
    Weight is the downward pull of gravity on an object. We have weight on Earth because of the gravitational pull created by very massive Earth (more correctly stated, we are in Earth’s strong gravitational field). Let’s use the universal gravitation law to determine a mathematical expression for determining our weight.

Structure of Matter

• Length Scales
  Objects can be grouped based on their size. Five possible categories (from largest to smallest) include astronomical, macroscopic, microscopic, atomic, and nuclear. These scales are summarized in categories.

• Astronomical Objects: star systems, galaxies, and nebulae.

• Macroscopic Objects: plant seeds, sports balls, sailboats, and airports

• Microscopic Objects: bacteria, cells, and hair

• Atomic Objects: atoms and molecules

• Nuclear (or Subatomic): electrons, protons, and neutrons; quarks
  Viruses, for instance range in size from about 20 nm to hundreds of nm (SARS-CoV-2 is about 100 nm in diameter).
  Let’s now look at a piece of chalk on these length scales. An ordinary piece of chalk is a macroscopic object. We can see it without a microscope, and it fits in the palm of our hand. Now we hit it repeatedly with a hammer until it is pulverized into fine dust. At this point we may want to use a magnifying glass or microscope to see what the microscopic pieces of chalk look like. If we keep hitting the chalk dust with a “mental” hammer, the next structure we encounter is a molecule, most likely calcium carbonate.
  molecule a group of atoms chemically bound together and representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction
  atom (i) basic building block of matter; (ii) the smallest stable entity of matter
  Calcium carbonate is a chemical compound with the chemical formula CaCO3.
  Ca, C, and O are chemical symbols for the elements calcium, carbon, and oxygen, respectively.
  element collection of atoms that all behave the same chemically.

• Atoms and Atomic Structure
  There are about 100 different chemical elements. They are often arranged in order of increasing atomic number and organized so that elements with similar chemical properties are in the same column. This general presentation is called the periodic table of elements, or just periodic table. Periodic tables are used to display chemical element symbols, names, atomic numbers, atomic masses, states of matter as well material properties like electron configuration, density, melting point, boiling point and crystal structure.
  The atomic number of an atom is the number of protons in the nucleus. It is atomic number that determines element type. All hydrogen atoms have one proton in their nucleus and all carbon atoms have six protons in their nucleus. Atomic numbers are always positive integers since every atom has a nonzero, integral number of protons.
  The unit on these masses is atomic mass unit, abbreviated u. One atomic mass unit is equal to 1/12 the mass of a carbon-12 atom at rest, approximately 1.66 x 10^{-27} kg.
  An alternative (but equivalent) interpretation of the atomic mass is that it is the molar mass (mass in grams per mole of substance) of the element.
  I want to make an important point – nobody has ever seen an atom, and nobody ever will. They are too small to be seen even with the best optical microscope that could ever be invented. The wave nature of light and wavelengths of light that we can see impose natural limitations on how small an object we can see. All pictures of atoms that you have seen are based on inferential techniques and theory.
  The atom shape is roughly spherical and is determined by the outer electrons (it is the outer electrons of atoms that participate in chemical bonding). At the center of the sphere is a massive nucleus that contains protons and neutrons.
  The following things were “known” about atoms by the early 1900s (there is some redundancy in these statements):
  * the nucleus is positively charged and contains most of the mass of an atom (> 99.9%)
  * all nuclei contain at least one proton; some nuclei contain neutrons
  * protons are positively charged; neutrons are electrically neutral
  * protons and neutrons have about the same mass, about 1 u
  * electrons are negatively charged, with a charge is exactly opposite the charge of a proton
  * the mass of an electron is about 1/2000 the mass of a proton (or neutron)
  * the radius of the nucleus (if modeled as a sphere) is about 100,000 (10^5) times smaller than the radius of the atom
  Let’s use this knowledge to talk about the most common atom types of the first three elements on a periodic table (these are hydrogen, helium, and lithium). The most stable state of an isolated atom is for it to be electrically neutral (i.e., not be an ion). Hydrogen has 1 proton in its nucleus, so the most stable state for a H atom is to have 1 electron orbiting it. Helium has an atomic number of 2, so how many electrons orbit a stable He nucleus? The answer is 2. Helium also has 2 neutrons in its nucleus. The third element in the periodic table is lithium, it has an atomic number of 3 and an atomic mass of 6.94 u. So, what does this neutral atom look like classically? 3 protons in the nucleus means there are 3 electrons in orbit around it. Since the mass of a proton is about the same as the mass of a neutron (about 1 u), an atomic mass of 6.94 u suggest that the nucleus contains 7 nucleons (6.94 u rounds to 7 u).
  nucleon a particle in the nucleus of atoms, either a proton or neutron
  *3 of the nucleons in lithium are protons, so the remaining 4 nucleons must be neutrons.

• Proton-Proton Chain a series of three nuclear reactions that convert four H nuclei into a He nucleus and other particles with an energy release of about 27 MeV per reaction.
  positron electron neutrino gamma ray

Light

• Light
  Telescopes are the primary tool used by astronomers to investigate celestial objects beyond Earth. A telescope an optical instrument that makes distant objects appear closer, and therefore larger than would be seen with the naked eye. Telescopes in use today span the entire electromagnetic spectrum of light.
  light electromagnetic radiation
  Light to an astronomer (and scientist, in general) is more than just the visible light that we see. Light is electromagnetic radiation. The word radiation may conjure an image of nuclear radiation – that is not what we are talking about here. Radiation means emission. i.e., “giving off”.
  With particle emission (such as in some nuclear processes), matter is ejected. Matter has mass; light has no mass. With light, a bundle of energy in the form of electric and magnetic fields is emitted.

Light as a wave is characterized with the same terminology as any other wave:
amplitude (i) maximum deviation from the average value, (ii) peak value of a sine wave
wavelength (\lambda) minimum repeat distance
period (of a wave, T) minimum repeat time
frequency (of a wave, f) number of complete wave cycles per time

Planck Spectra

Planck spectrum (also called blackbody spectrum) the spectrum (I.e., all wavelengths) of light emitted by a blackbody per unit area per unit time
black body an idealized physical object that absorbs all incident light, regardless of wavelength and incidence angle
Inspection reveals that the wavelength yielding peak intensity decreases as the temperature of the radiating body increases (the peak intensity at a given temperature is the
largest intensity on the Planck curve at that temperature).
A decrease in wavelength means a shift towards higher frequency blue light if we happen to be talking about visible light. This makes sense; a blue gas flame is hotter than orange flames (similarly, blue stars are hotter than red stars).
The total area under each curve is indicative of the amount of energy radiated per time. This physical quantity is power.
power (P) energy per time
The SI unit of energy is the joule, abbreviated J (1 J = 1 kg m2 / s2). One Joule per second is defined to be one watt: 1 W = 1 J/s.
The results are Wein’s law and the Stefan-Boltzmann law.
Wein’s Law states that the peak wavelength in the electromagnetic spectrum of an object is inversely proportional to the temperature of the object. The peak wavelength (\lambda{max}) is the wavelength that corresponds to the peak intensity. Mathematically Wein’s law can be written as: \lambda{max} = \frac{0.0029 K m}{T},
where T is thermodynamic temperature (i.e., temperature measured with the Kelvin scale).
The Stefan-Boltzmann law states that the total radiated power P (I.e., energy per time) from a surface is proportional to the fourth power of its temperature T. Mathematically this can be written as:
P = \sigma A T^{4},
where A is the surface area of the radiating object and \sigma is the Stefan-Boltzmann constant (\sigma = 5.67 x 10^{-8} W / m^2 K^4).
For spherical objects (like planets and stars), A = 4 \pi R^2, where R is the radius of the object.
Note: The total radiated power is proportional to T^{4}, so doubling the temperature results in 16 (2^4 = 16) as much radiated energy per time.

• Semiquantitative summary of Planck spectra:
  • Higher T ↔ smaller \lambda_{max} ↔ bluer light
  • Lower T ↔ larger \lambda_{max} ↔ redder light
  • Thermal energy radiated per time (power) is proportional to the radiating body’s surface area
  • power radiated is proportional to temperature raised to the fourth power
• Qualitative summary of Planck spectra:
  • An object radiating blue light is hotter than an object radiating red light
  • For two same sized objects, the hotter object radiates more thermal energy than the cooler object
• Wave speed (v) how fast a wave disturbance propagates
• wave frequency and period are inversely related: f = \frac{1}{T}, or equivalently, T = \frac{1}{f}.
• Wave Speed
  *Wave speed is how fast the disturbance propagates. This is not the same as how fast the wave moves up and down. Take the example of a stone dropped into a pond. Wave speed corresponds to how fast the ripples move away from where the stone hit the water, not how fast the water moves up and down