ASTR 151 Final Exam Study Guide
ASTR 151 Final Exam Study Guide ASTR 151 001
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Wesley Fowler ASTR 151 Final Exam Study Guide Practice Questions Chapter 1 Introduction to Astronomy What is Astronomy the study of? What are the three qualifications of a valid scientific theory? What is the celestial sphere? What is the Zodiac? What are constellations, and how many are there in the Celestial sphere? What is right ascension, and what is declination? What are the phases of the moon? (Be able to draw them in order) Eclipses and Parallax What’s the difference between synodic, sidereal, solar, and tropical time periods? What is retrograde motion? What is prograde motion? When is a planet at opposition to Earth? When is it a conjunction? What characteristic of a planet determines it having seasons or not? What is an eclipse? What is a solar eclipse? What is a lunar eclipse? What is parallax, and how is it used to calculate distance? Chapter 2 Ancient Astronomy What is Geocentricism? How was it described? Which scientist is credited with the creation of the Ptolemaic Universe theory? What is Heliocentrism? How was it described? Which scientist is credited with the Heliocentric model of the Solar System? Name three other scientists associated with Heliocentrism’s development. Kepler, Newton, and Gravity What are Kepler’s three laws of gravity? What is the Astronomical Unit? What are Newton’s three laws of gravity? Which equation explains why gravity gets weaker over longer distances? What does escape velocity mean? Chapter 3 Wave Motion Define frequency, wavelength, amplitude, and period. How are wavelength and frequency related to each other? What is electromagnetism? What is an electrical field? What is a magnetic field? What is the electromagnetic spectrum? (Be able to draw it) Light and Radiation Explain how light is a form of radiation, and define radiation. Define temperature. What is a blackbody? What is Wien’s law? What is Stefan’s law? What is the Doppler Effect? Chapter 4 Spectroscopy What is spectroscopy? What is a continuous, emission, and absorption spectrum? What are Kirchhoff’s three laws of spectroscopy? What are the three components of an atom? What happens when an electron jumps or drops an energy level? Spectral Lines and Ionization What does it mean for an atom to be ionized? Be familiar with the Lyman and Balmer series. How can an atom’s electrons become excited? What are spectral lines, and how do they broaden? What does it mean for a star or object to be “reder” or “bluer” Chapter 5 Telescopes What are the primary components of a telescope? What are the differences between a refracting and reflecting telescope? What is angular resolution? What is a photometer? What is a Charge-coupled device? Spectral Telescopes Why is radio astronomy ideal for observing deep space objects? Describe radio telescopes. What is interferometry? Where is infrared astronomy typically done? Chapter 6 Introduction to the Solar System and Planetology What are the order of the planets? What is the Kuiper Belt? Where is it located in the solar system? What are some of the key terms used in planetology? Which planets are terrestrial, and which ones are Jovian gas giants? Describe the density gradient of the solar system. Describe the temperature gradient of the solar system. Solar Formation What are the stages of the Nebular Theory? What is the Condensation Theory? What is the Ice line? (Know the exact temperature) What are the steps of Planet building? What is the Core-Accretion Theory? What is the Gravitational Instability Theory? Chapter 7 Introduction to the Earth Describe the Earth’s interior. What is the primary component of Earth’s atmosphere? What is photodissosociation? What are the layers of Earth’s atmosphere? (know the order) Why is the sky blue during the day? What is it red at sunset? How does the atmosphere contribute to the Earth’s temperature? The Earth’s Condition What is the difference between weather and climate? How is our climate changing? What are Primary waves, and what are secondary waves? How are P and S waves useful in studying Earth? What are the layers of Earth’s interior? (Know the order) What are plate tectonics, and tectonic motion? What is the Dynamo Theory? What is the Magnetosphere? What are Van Allen Belts? Why does the Earth have tides in its oceans? How is the Tidal force a differential force? What is the Tidal Bulge? What is Tidal Locking? Chapter 8 Introduction to the Moon What are the two sides of the moon? Why do we say that the moon is in synchronous orbit? What are the two dominate surface features of Moon? Where are they most present? Why are craters important in understanding the moon and other space objects? What is Regolith? What are crater chains? What are Volcanic Rilles? Formation of the Moon Describe the interior of the moon What are the four theories of the Moon’s formation? Which one is most supported by scientists today? How did the Moon evolve after its formation? How does the near-side crust of the moon compare to the far-side crust? Chapter 9 Venus Describe Venus’ rotation and orbit. Does it have retrograde or prograde rotation? What are longer, Venus’ days or years? What are the two continent-sized highlands on Venus’ surface? What are the other dominant surface features of Venus? What are shield volcanos? What are lava domes? What are Coronae? How does Venus’ atmosphere compare to Earth’s? Describe it’s speed. What is the primary component of Venus’ atmosphere? How does being closer to the Sun affect Venus’ atmosphere? What is a Runaway Greenhouse Effect? Chapter 10 Mars How does Mars’ orbit compare to Earth’s? How does Mars’ northern region compare to its southern region? What are the dominant surface features of Mars? What are runoff channels? What are outflow channels? What are other surface features that support the existence of water-ice on Mars Describe Mar’s polar ice caps. How did Mars loose its permanent atmosphere? What is the component of Mars’ atmosphere? What storms frequently occur on Mars? What are the differences between Mars’ two moons, Phobos and Deimos? Chapter 11 Jupiter How does Jupiter’s size and rotation compare to the other planets in the solar system? What do these two features affect Jupiter’s magnetic field? What does it mean that Jupiter has a differentiated rotation? What does it mean that it has a differentiated radius? What is the primary component of Jupiter? What is the difference between Jupiter’s belts and zones? What is the Great Red Spot? Why does Jupiter have liquid metallic hydrogen beneath its apparent surface? Wesley Fowler ASTR 151 152 Intro to Astronomy Astro-Star Nomy-Law Oldest of the sciences Football stadium scale of the Solar System: The sun is a nickel, Earth is a grain of salt 5 1 Kilometer = 1,000m = 10 cm 11 1 astronomical unit (AU) = 1.496 x 10 m (avg distance between the earth and the sun) 1 light year (ly) = 9.46 x 10 m = 9.45 x 10 km = 63,200 AU (distance light travels in a year) 16 1 Parsec (pc) = 3.09 x 10 m = 206,000 AU = 3.26ly “One parsec corresponds to the distance at which the mean radius of the earth's orbit subtends 8n angle of one second of arc” 3.0 x 10 m/s = speed of light Giga: Billion Mega: Million Kil: Thousand Centi: .01 Micro: 10^-6 Nano: 10^-9 Earth= 12,800 km 10 Solar System= 1018m Milky Way= 10 km= 100,000 ly Virgo closest to milkyway, millions of light years? Everything is being drawn towards “The great attractor” which is made of dark matter and galaxies 100 Billion stars in milky way galaxy 100,000 light years across the galaxy 7 Seconds in a year = x 10 sec Scientific theory: Observation-theory-predictions/hypothesis-observation… Data-hypothesis-test-results Wesley Fowler ASTR 151 152 Chapter 1 Motions of the Moon Average distance between the Earth and Moon is 384,500 Km Synodic Month: Full cycle of moon phases (29.5 days) Sidereal Month: Time for moon to revolve around Earth 360 degrees (27.5 days) Synodic Period: 4 weeks The moon is always half lit, acting as a reflecting “mirror” of sunlight. Eclipses Eclipses occur when the Sun, Moon, and Earth form a straight line. When the shadows align. - Lunar Eclipse: The Earth is between the sun and the moon. Total or partial. Full Moon - Solar Eclipse: The Moon is between the sun and the earth. New Moon o Penumbra: Sun partially visible. Outer shadow o Umbra: Sun completely obscured. Inner shadow Corona: Crest of the sun. HOT Diamond Ring affect: Seeing the rim of the sun during an eclipse just before or after an eclipse. Eclipses do not occur every month because the Moon’s orbit is inclined 5.2 degrees with respect to Earth’s orbital plane. Tilted tiles. Distance Measurement What do you need? - Baseline distance - An angle () Parallax: The measurement of angular distance (apparent motion, arcsec) relative to two different background vantage points. The earth’s diameter is too short of a baseline to calculate parallax, the diameter of Earth’s solar orbit is though! Largest we can possibly have, June December. greater distance = smaller parallax angle Eratothanes’ Method: Deduced the Earth’s circumference to be 39,000km Actually 40,075km Diameter = Distance x (Angular Diameter/57.3 degrees) Wesley Fowler ASTR 151 152 Reading 1.1-1.3 Scientific Method Scientific method: The process of testing a theory via a controlled and measured experiment Theory: “The framework of ideas and assumptions used to explain some set of observations and make predictions about the real world” (8 McMiillan). Theoretical model: Result of theory, a representation. Qualifications for a theory: -Testable -Continually tested -As “simple” as possible -elegant Mapping the Universe Planet-star-system-galaxy-cluster-universe Universe: The totality of space Astronomy: The study of the universe 1 Light year= 10 trillion kilometers or 6 trillion miles -speed of light= 300,000 kilometers per sec Fixed background: The stars appear to hold relative fixed positions in the sky Constellations: Ancient observations of how stars connect into configurations. Used to pinpoint the location of stars. Smaller groupings of stars with casual names are called asterism. For example the “Big Dipper”. Celestial sphere: The simple concept that earth resides in a celestial sphere in which stars travel in a fixed relative location. Earth’s poles coincide with the universe’s poles. Divided into 88 constellations. - Right Ascension: Equates to longitude for celestial sphere, horizontal - Declination: Equates to latitude celestial sphere, vertical - The grid of locating stars and objects on the celestial sphere http://www.daviddarling.info/encyclopedia/C/celsphere.html The zodiac is comprised of the constellations that are aligned on the ecliptic. The ones that the sun passes through. Eath’s Motions Diurnal: Daily - Rise in the east, set in the west Annual: Yearly - Due to earth’s revolution, or orbital motion Sidereal: Rotation or revolution of 360 degrees. Rotation: Earth is tilted at 23.5 degrees with respect to its orbital path. In summer, the Sun rises north of east, travels through the southern sky, and sets north of west. In winter, the Sun rises South of East, travels through southern sky, and sets south of west Solar Day: Noon-to-noon (24 hours) Sidereal Day: Time for 360-degree rotation (23 hours 56 minutes) Revolution: 0.986 degrees per day If revolution increases in speed, the solar day becomes longer (catching up to noon) Seasons are determined by the Earth’s 23.5 axial tilt! The planet is closest to the sun during January, but due to the concentration of sunlight, it’s the coldest season in the north and the warmest in the south. Procession Tropical Year: Calendar year (365.2422 days) time between two Vernal Equinoxes Sidereal Year: Time needed to complete one orbit around the sun (365.256 days) Wesley Fowler ASTR Ancient Astronomy Stonehenge: An ancient astronomical observatory of sorts. Helps determine what time of year it is…useful to plan harvesting and planting crops Diurnal Motion: Sun, moon and Stars Annual Motions: The Ecliptic Prograde motion: night-to-night eastward motion -> (rights are pro) Retrogade motion: night-to-night westward motion <- (lefts are retro) Mercury and Venus are “tethered” to the sun Mars, Jupiter, and Saturn exhibit retrograde motion Inferior planets: Between earth and sun Superior planets: Outside earth from sun Opposition occurs at a planet’s closest approach to earth (Inf/superior) Conjunction occurs when a planet appears closest to the sun from the earth’s perspective (etc) Aristotle: Geocentrism. Not an experimentalist at all - The Ptolemaic Universe: Earth at center, perfect orbs and orbits. Circular orbits called epicycles. Lots of problems, planets too close to sun, etc. Copernicus: Heliocentrism (borrowed from Aristarchus) - The Copernican universe: Observed Mar’s orbital motion in comparison to Earth’s, found that Mars had a retrograde motion Retrograde motion: *Point “B” is the opposition. In line with each other 1. Earth is not at the center 2. Center of Earth is the center of the Moon’s orbit 3. All planets revolve around the sun 4. The stars are much farther away than the sun 5. The motion of the stars in the sky is due to Earth’s orbit 6. The motion of the sun in the sky is due to Earth’s orbit 7. Retrogade motion of planets is due to Earth’s motion around the sun - Still believed in perfect orbs and orbits The Team: Heliocentrism’s Establishment Tycho Brahe (1546-1601) The data collector Galileo Galilei (1564-1642) The observer Johannes Kepler (1571-1630) The analyst Galileo Used the pre-invented telescope to observe the heavens. Drew the moon. - Observed craters on the moon. Observed sun spots on the sun. Thus disproving “perfect sphere” theory - Observed phases of Venus - Observed Jupiter’s moons in orbit Tycho Brahe - Took very accurate measurements for decades (~1 arcsec) and preserved them well. - Copper nose, mysterious death. Wesley Fowler ASTR Kepler’s Laws Johannes Kepler - “Acquires” Brahe’s data and creates “Kepler’s Laws of Planetary Motion” 1. Planetary orbits are elliptical. An oval, not circular. The sun is the focus 2. Equal Areas: equal times. Fast when close to sun, slow when far away. 3. The square of the period (P, years) is proportionate to the cube of the semi-axis major (a, astronomical units) P (years) = a (AU)3 st 1 law Elliptical Orbit Perihelion: Closest to sun Aphelion: Farthest from the sun Eccentricity: How “out of circular” the ellipse is (e) Semi-major axis: Radius of major axis (a) nd 2 Law Equal areas, equal times. When plant is farthest away from the sun, it moves the slowest. When it’s closest it moves the fastest. - Fastest at perihelion and slowest at aphelion rd 32 Law 3 P (years) = a (AU) Where (P) is period and (a) is Semilunar Axis AU is defined as the Semilunar axis of Earth The Astronomical Unit Giovanni Cassini measured the earth’s semi major axis via parallax, and was pretty close! One Astronomical Unit is the distance between the Sun and Earth. *1.5 x 10 km is the AU (for this class)* - Force: (N) An influence that tends to change motion - Inertia: (kg*m ) The resistance of an object to change its speed or direction, inertia. - Mass: (kg) The amount of matter - Weight: The gravitational force exerted by an object. How mass is changed by gravity. - Speed: Distance divided by the time it takes to cover it. - Velocity: Speed and direction - Acceleration: (m/(s ))The rate of change of velocity, meters per second per second Newton’s Laws Isaac Newton: Invented Calculus, and fathered Physics. 1. Newton’s first law of motion: An object will maintain constant velocity, or its state of rest (v=0), unless acted upon by an outside force. 2. Newton’s second law of motion: An enacting force is equal to the mass times acceleration of the object it’s pushing against. F = ma - Constant Force: The greater the mass, the harder it is to accelerate the object - Constant Mass: The greater the force applied the larger the acceleration 3. Newton’s third law of motion: When one body exerts a force on the second body, the second body exerts a force equal in magnitude, but opposite in direction of the first body Gravity Th2 acceleration due to gravity’s pull towards the center of Earth: g = 9.8 m/ (s ) F = (Gm m1)/2 2 This is the “Inverse Square Law”. The farther away the object is, the weaker gravity becomes. - The gravitational force is proportional to the product on the two objects mass’s divided by the square of the distance between the two objects (Definition) Gravitational constant (G) = 6.67 x 10 N m /kg 2 2 Planets are constantly being pulled towards the sun by the sun’s gravity Orbiting masses are localized to a common focus, at the center of mass Wesley Fowler ASTR 152 Newtonian Mechanics Planetary Motion Planets don’t shoot strait ahead due to the pull of the sun’s gravity. Thus they orbit. Due to its mass, the sun is the dominant influence over planetary orbit in our solar system. - The earth orbits the sun at a speed of 30km/sec Newton’s corrections to Kepler’s first and third laws: - The earth does not orbit around the center of the sun, but rather around the center of mass between the earth and sun. The “average” position of all the matter. This is the true common focus. This point is still in the sun for the Earth, because it’s so massive. *Mass of sun: 2.0 x 30 kg - P = (a )/(M ) P-orbital period a-semimajor axis M-combined total mass of two objects in solar units Escape Velocity If an object escapes the gravitational pull of an object, then it is unbound. The orbit is no longer an ellipse! Orbital Speed: GM vorb R √ Escape Velocity: Wesley Fowler ASTR Chapter 3 Light and Radiation Thermal Radiation: Temperature and energy - Temperature is the measure of the average microscopic motion of particles, which collide, releasing energy, thus heat - Absolute zero: No motion, 0 Kelvin - Kelvin = Degrees Celsius + 273 (No degrees!) The average motion of particles Blackbody curve/spectrum: - Blackbody: An ideal object that absorbs and reradiates all forms of light perfectly (Coal is close, it absorbs 95% of light) The relationship between the intensity, or the amount of radiation (y axis), and frequency (x axis) - As temperature of blackbody increases, the peak of emission decreases to shorter wavelengths (“bluer”) Wien’s Law When things heat up to a certain temperature, they often produce a visible color. Wein’s Law: The peak of wavelength emission is inversely proportional to the temperature of the object, and is directly proportional to frequency. - /T(K) peak - “Cooler stars have longer wavelength peaks” Wavelength: Lambda () Intensity refers to “the amount of color” Equations (Don’t need to be memorized): peakm(microns)) = (2900)/T(K) peaknm(nanometers)) = (2900)/T(1000s of K)) peakmm(millimeters)) = (0.29)/T(K) The color of a star reflects its surface temperature! Hotter stars are blue, cooler ones are red. Stefan’s Law: As temperature increases, energy output increases - The total energy emission per unit of time (Flux) is proportionate to temperature 4 4 Flux = T Flux = Power/Area -8 2 4 = 5.67x10 W/m K (don’t need to memorize this) Power is energy per second. For Emitting objects this can be called Luminosity - For example, if you double the temperature of an area, the output of energy increases by a factor of (2)^4 = 16 **Both of these laws are used together in certain problems!** Example: If star 1 is 60K and star 2 is 600K… - Star 1: (m(microns)) = 2900/60 = peak 48m - Star 2: peakm(microns)) = 2900/600 = 4.8m - Star two’s flux increases by a factor of 10 4 Colors are both reflections of visible light, as well as emissions of light due to temperature. The Doppler Effect Relative motion influences how light and sound are perceived. Has to be towards or away from you (radial motion) - If an object is moving towards you, wavelengths seem shorter, and sound higher and appear “bluer”. - If an object is moving away from you, the wavelengths seem lower, and appear “redder” as they spread out over distance. This also applies if you are moving towards a stationary object. (Apparent wavelength/True wavelength) = (True frequency/Apparent frequency) = (Recession velocity/wave speed) - Depends on the relative motion between the source and observer, and it must have radial motion. It must move towards or away from the observer. It does not depend on distance! Wesley Fowler ASTR Chapter 3 Properties of Light Light is a form of radiation. It is the reason why we know anything about the universe. Light is also called electromagnetic radiation. Light is both a wave and a particle. Radiation is any way in which energy is transferred through space without physical connection. - Opacity: The extent as to how much radiation is blocked due to an object’s material. All light travels at the speed of light, no exceptions! (3.00x10 km/s) Wave Motion Frequency: Number of crests that pass a given point per second. Kind of like speed (hertz) - Frequency = (1/Period) Period: The time it takes for the crest of a wave to travel to another crest’s former position (s) Amplitude: The maximum departure or size of a wave from its undisturbed state Wavelength and frequency are inversely related. Big waves have small frequencies. Wavelength units: -9 Nanometer (nm) 1.0x10 m = 1nm Micrometer (m) 1.0x10 m = 1m Angstrom (Å) 10 m = 1 Å “The Single Slit Experiment” Thomas young - Proves that light can be a wave. The shadow from the experiment was fuzzy due to diffraction, it lost clarity. This shows that light can bend around corners, thus it is an electromagnetic wave. Charged Particles and Waves Electromagnetism is one of the four fundamental forces, along with gravity. Unlike gravity, electrical forces can either attract or repel objects. Light, as an electromagnetic wave, requires no medium to travel through. It is created by accelerating charged particles (protons and electrons attract, protons and protons repel) Any charged particle has an electrical field - Decreases with distance, same as gravity Any changing electrical field has a magnetic field - Moving charges create magnetic fields - Magnetic fields exert force on moving electrical charges Electrical and magnetic fields are two different aspects of Electromagnetism. They both comprise an electromagnetic wave. 5 - All electromagnetic waves travel at the speed of light (3.00x10 km/s) The Electromagnetic Spectrum: Spectrum: The division of light by specific wavelengths. In the visible spectrum, larger wavelengths are “reder”, shorter wavelengths are “bluer” - “Ultraviolet is bluer than visible light” Wavelength is typically measure in nanometers (nm) - Nano = 10 , 1 nanometer = 1x10 m (1 billion nanometers in a meter) -10 - Angstrom: (1 A = 10 m = 0.1 nm) Electromagnetic waves require no medium, unlike other types of waves. Created by accelerating charged particles. It can travel through a vacuum just fine. Atmospheric Opacity: How opaque the atmosphere is in allowing light through. 0% means unhindered passage, 100% means absolutely blocked. Wesley Fowler ASTR Chapter 4 Spectroscopy Spectroscopy is the study and analysis of how matter emits and absorbs radiation. It is the study of how light interacts with matter. - Radiation is analyzed by a spectrometer Three types of Spectra Continuous spectra: Full emission. From a blackbody source that is dense enough. Emission Spectra: Distinct lines of color in “emission” Absorption: Distinct lines of color removed by absorption - Every element has a unique set of absorption lines The radiation given off by certain gases, as observed by a spectroscope, is seen as a few narrow emission lines. These are all the colors/wavelengths not emitted. - The frequency or wavelength cannot be altered! Absorption Line: Black lines/gaps represent wavelengths that have been absorbed. Kirchhoff’s Three Laws of Spectroscopy 1. A luminous solid, liquid or gas with high-density emits a continuous spectrum of radiation (Blackbody) 2. Hot gases with low-density emit an emission spectrum with a series of bright emission lines, unique to each gas 3. Cool thin gases (along line of sight of the emitting object) emit the absorption spectrum, leaving dark absorption lines in place of certain wavelengths Atoms and Radiation Light is also a particle Quantum mechanics: The branch of physics governing the balancing of atoms and subatomic particles. Protons: Positive charge Electrons: Negative charge Neutrons: Neutral charge *The Bohr model is outdated; the electron cloud model is current. Excited state: When an electron orbiting an atom occupies a farther than normal orbital. Contains a greater amount of energy than normal. Ground State (of an atom): State of lowest energy. “Normal” When an electron achieves its maximum energy it breaks free from the nucleus’ pull, and the atom is ionized. An ion is an atom missing one or more of its electrons. When an electron drops closer to an energy level closer to the nucleus it emits energy. If it jumps away from the nucleus it absorbs energy. Atoms can only absorb or emit specific energies; each electron has an associated energy level. - The amount of energy absorbed or emitted is directly proportional to the energy difference between two electron orbitals Photons A particle of “electromagnetic energy”, the packet of energy equal to the difference between two electron orbitals. Photon energy is directly proportionate to radiation frequency, and thus inversely proportional to wavelength. Intensity is not a factor in electron energy. Shorter wavelength (blue), more energy. Longer wavelength (red) less energy E = hf = (hc)/ -34 - Planck’s constant: h= 6.63 x 10 Joule seconds (JS) - E: Photon energy (in electron volts, eV) E(eV) = 1240/((nm)) eV: The energy gained by an electron accelerated through an electric potential of one volt. - 1 eV = 1.60 x 10 -19J Wesley Fowler ASTR Chapter4 Spectral Lines and Ionization The frequencies of gases are not continuously spread out, rather they vary depending on their atomic structure. Emission lines correspond to the energy differences in an atom’s electron orbital levels. This helps us understand the composition of the universe. The energy of any state (n): E n1 3 6 .1 eV. n2 (Don’t need to memorize this) - E: Energy - eV: Electron Volt (unit) The minimum amount of energy needed to ionize hydrogen from its ground state is 13.6eV (Ionize is to free an electron from its local nucleus) The Lyman Series: Transitions starting or ending at the ground state. Ultraviolet Lines - L = 121.6nm (“Lye” on the ground) 10.2eV The Balmer Series: Transitions starting or ending at the 1 excited state. Visible Lines. - H = 656.3nm 1.9eV There are two ways that atoms/molecules can become excited - Direct excitation: Electron goes from ground state, to 1 excited state, back to ground. This releases a UV photon. nd - Cascade De-excitation: Electron goes from ground state, to 2 excited state, then either drops directly back to ground state emitting a UV photon, or it cascades down the 1 excited state to the ground state, releasing a visible and then UV photon. These lines make up the emission and absorption lines that can be seen in a spectrum! - If you view a gas cloud along sight with continuous spectrum (light bulb) we see Absorption Spectrum. - If viewed from a side angle without the continuous spectrum, we see Emission Spectrum. Hydrogen is the simplest element to teach this concept. The more electrons, neutrons, and protons in an element, the more complicated it becomes due to there being more spectral lines. Molecular Motion Electron transitions -> Visible and ultraviolet spectral lines (largest energy changes) Changes in vibration -> Infrared spectral lines Changes in rotation -> Radio and microwaves lines The strength of a spectral line (how bright or dark it is) is dependent on the amount of atoms, or concentration. It is also dependent on the temperature of the gas containing the atoms because temperature determines how atoms transition between orbitals. - At higher temperatures, more atoms are in an excited state. The radial velocity of an object can be determined by comparing the pre- determined wavelength of the element to the presently-observed color. By seeing how “reder” or “bluer” it is, astronomers can determine the radiations’ radial motion. The environment in which absorption or emission occurs causes spectral lines to broaden. Measuring the width of expansion can help determine the speed of particles. Rotation and turbulence can also be used to measure speed. Additionally, these factors also broaden spectral lines: 1) Thermal motions of particles 2) Rotation of a macroscopic object 3) Collisions between atoms 4) Magnetic Fields – Zeeman Effect Electrons and particles are moving randomly like crazy, so their observational frequency is also affected by the Doppler Effect too. The amount of collisions also alters the apparent energy, collisions dependent on the density and pressure of the gas. The more rapid a star’s motion, the broader its spectral line appears. The half rotating away is “reder”, rotation towards is “bluer”. The Zeeman Effect: The effect of splitting spectral lines into multiple components in the presence of a static magnetic field. Wesley Fowler ASTR Chapter 5 Telescopes Telescopes gather photons together for observation, and thus make faint objects brighter. - Not about magnification, but capturing light. If the size is doubled, image is 4 times brighter. Lens: A medium for light to travel through which bends light rays towards a central focus. Made out of plastic or glass. Focus: The point where all light rays converge Focal Length: The distance between the primary mirror and the focus Telescope Diameter: Edge-to-Edge length the mirror. Determines area and resolution of image Telescope Area/Collecting Area: Area of mirror, controls brightness and exposure time - Area = πr 2 Collecting area: The total area capable of gathering radiation (duh) - “The observed brightness of an astronomical object is directly proportional to the area of the telescope’s mirror and therefor to the square of the mirror diameter”. Refracting telescopes Uses the refraction of light through a lens to gather and concentrate a beam of light. Images can end up inverted. Refraction Anatomy: Prime mirror->eyepiece Problems with large scale-refractors: 1. Chromatic Aberration: “Color Error”. Each color is bent differently via refraction (hence the rainbow from a prism). This doesn’t happen with mirrors. 2. Through-put: “Light Loss”. Light is absorbed since it passes through the medium (lens). Mirrors do not absorb nearly as much light. 3. Weight deformation: Large lenses are very heavy, and can only be supported around the edges. It will stretch and deform under its own weight. Mirrors are supported at the base of the telescope; this is not a problem. 4. Polishing: A lens has two sides to be polished, while mirrors only have one Reflecting telescopes Uses a series of mirrors to reflect and focus incoming light. Light travels to primary mirror, reflects off its curved surface to secondary mirror, which then reflects to eyepiece. Reflecting Anatomy: Prime mirror->Secondary mirror- >eyepiece Types of reflecting telescopes: - Prime focus: Observe via the prim focus at the top. You stand in the path of the incoming light. Radio telescopes. - Newtonian telescope: Small mirror directs light out to the side in a small hole 90 degrees out. Invented by Newton. - Cassegrain telescope: Small mirror directs light towards the base of the telescope, in-between the primary mirror. - Nasmyth focus/coude room: Three mirrors allow for flexibility. - Gregorian: Two primary mirrors allow gap at base for observation from secondary mirror. Photometry: Angular resolution: The ability of an object to distinguish or separate objects close together in one field of view. Larger telescopes have better angular resolution. (Arcseconds, ArcMinutes, Degrees…) - Smaller number for resolution means more fine detail. Photometer: A device that measures the total amount of light received in all of or the specified area in the field of view. - To determine a star’s photometry, astronomers add up the values of all the CCD pixels focused on that star. Sounds simple, but images often overlap with each other. To counteract this, filters are used to distinguish particular wavelengths. Charge-coupled devices (CCD): Hundreds of millions of pixels (made of silicon) are electrically charged whenever light hits. The amount of charge is directly proportional to the amount of photons striking each pixel. - They record 90 percent of photons striking the area - What’s in our phone cameras - Large dynamic range, see faint and bright objects - Gives actual data to work with Observation “Seeing” refers to the angular resolution of a telescope, which is usually 1 arc second. Seeing disc: The circle over which a star’s light is spread for observation. Altered by limiting factors. Limiting Factors: *Turbulence interferes with telescopes and early image processors (pre- 1990) because the wind blurs the images of stars! For high resolution images, must be precise. *Light pollution, upward directed light from streets scatters from air dust to telescopes obscures seeing discs. Why astronomers go to remote areas. Solutions: - Space telescopes/high elevations - Avoid light pollution - Active Optics: Controlling the environment in which telescopes operate (ie temperature) - Adaptive Optics: A computer intentionally deforms the mirror’s shape to to compensate for the atmospheric turbulence. Angular resolution as low as 0.06” Wesley Fowler ASTR Chapter 5 Radio Astronomy The atmosphere is no hindrance to long-wave radiation, they’re safe for us and are easy to observe from earth. - Karl Jansky is the man, started radio astronomy in 1931: Observed “hisses” in radio interference every sidereal day. Those turned out to come from the center of the galaxy Incoming radiation bounces off of collecting area/dish, to focus, down to detector, which goes to a computer - Have to be large to get a sufficient amount of data *Works 24 hours a day, even when it’s cloudy, or when it’s storming. *Radio waves are generally unaffected by intervening matter *Poor resolution though Radio waves can travel through interstellar dust, revealing images behind the cloud. Greenbank Radio Telescope: Largest steerable single dish, best for 1cm radiation - Arecibo Telescope: Largest single dish telescope in the world -> - The Haystack Observatory: 37m radio telescope *Longer wavelength telescopes do not require as smooth surfaces Interferometry: Using two or more radio telescopes to observe one object at the same wavelength, then sending the combined data to one central computer. Can span huge distances - Resolution is equal to a single telescope with a diameter of the largest distance between two of the telescopes. This applies to more than radio astronomy! Interferometry has application to other/higher wavelengths, since technology has been improving over the years. Low Frequency Astronomy Infrared Astronomy: Observes wavelengths that are in-between visual and radio frequencies. Often use balloons, airplanes, rockets to observe above the atmosphere. Orbit in space too! - Allows observations of dust structure, instead of just a blur +Spitzer Space Telescope: Wavelengths of 3.6-160 (um) cooled to 4 kelvin with liquid Helium +Herschel Space Observatory: 50-700 um Can penetrate interstellar dust! Outlines dust. Microwave Astronomy: Good for viewing cold interstellar dust High Frequency Astronomy Ultra-Violet Astronomy - Primarily used to observe gases between stars - Good for young, very hot stars as well as star formation regions X-Rays and Gamma Ray Astronomy: These wavelengths do not reflect off any surfaces easily. They tend to pass straight through, or be absorbed. Useful for studying extremely hot gases. - CCD’s do not work well for these at all X-Ray telescopes use grazing incidence by using cylindrical mirrors to focus light. Individual photons must be counted by orbiting devices, and then transmitted down to a computer. - The number of photons in the universe appear to be inversely related to their frequency Wesley Fowler ASTR Chapter 6 Planetology Terminology: - Semimajor Axis (a) distance from sun. [AU] P =a 2 3 - Orbital Period (P) (Sidereal) Time to revolve around the Sun [years] P =a 3 - Radius (R) Center-to-surface distance [km or m] F = (Gm m )/r 1 2 2 - Mass (M) Number of kilograms of matter [kg] F=ma - Rotational Period: How long each day is (hours or days) - Average Density: How much mass is in a standard volume [kg/m or 3 g/cm ]3 – Density = Mass/Volume Inner Planets: Terrestrial Worlds Outer Planets: Jovian Worlds Outer Solar System: Icy Worlds Terrestrial Planets: Within 1.5 AU of the Sun - Solid surface - Mercury, Venus, Earth and Mars - High density average Jovian (Gas Giant) Planets: Beyond 1.5 AU of the Sun - Gas surface, almost all hydrogen and helium - Jupiter, Saturn, Uranus, and Neptune - Low average density Comparative Planetology: The study of understanding planets based on how they are similar to each other. - Differences between planets are rarer than similarities, and provide very specific and substantial information for study - Mechanisms of our solar system can be applied beyond our solar system - Gives insight to formation of the solar system All four terrestrial planets have very different atmospheres - Mercury barely has an atmosphere - Venus has an atmosphere with 10x more pressure than Earth’s - Earth is the only planets with Oxygen (O ), but is primarily Nitrogen 2 (N2) - Mars has a thin atmosphere of Carbon Dioxide (CO ) 2 Rotation Speed (Sidereal Day): - Earth 23h 56min - Mars 24h 40min - Mercury 58 days - Venus 243 days (rotates clockwise, backwards to Earth and Mars) Earth has one moon, Mars has two, Venus and Mercury have none. Earth and Mercury have measurable global magnetic fields, but Venus and Mars do not. Density (kg/m ): Density decreases as the distance from the Sun increases Mercury: 5300 Venus:4400 Earth: 4400 Mars: 3800 Terrestrial planets are closer together than Jovian planets Jovian planets are massive compared to Terrestrial planets Terrestrial planets are entirely solid, while Jovian planets are gas with a solid core Magnetic field of Jovian planets are stronger than Terrestrial planets Jovian planets have many more moons than Terrestrial, they are Icy worlds All Jovian planets have ring systems Jupiter: - Most massive planet in the solar system, more than 2.5 massive than all other planet’s mass combined Outer Icy Worlds: Asteroids: - Small, rocky bodies: Smaller = lumpier - Main Asteroid Belt: Small objects mostly of rock and some metal, between Mars and Jupiter - Jupiter Trojan Asteroids: Little is known of these objects Comets: - Highly eccentric orbits - Rock, ices and organics - Originate from Kuiper Belt and Oort Cloud Wesley Fowler ASTR Chapter 4 The Solar System The Sun makes up 99.8% of the Solar System’s mass The Solar System is primarily empty space! It has a range of 30.1 AU from the Sun to Uranus Mercury-Venus-Earth-Mars-Jupiter-Saturn-Uranus-Neptune 5 Dwarf Planets: Ceres (Asteroid belt), [Pluto, Haumea, Makemake, Eris (Kuiper Belt Objects)] 181 Moons Estimated millions of Asteroids (600,000 discovered) Estimated billions of Comets (4,000 discovered) The Kuiper Belt: The outer region beyond Neptune, filled with meteoroids, interplanetary dust, and pervasive solar wind. 30-2,000AU from Sun is the Kuiper Belt. 0-30: Planetary Realm 30-2,000: Kuiper Belt + Scattered Disk 2,000-20,000: Inner Oort Cloud 20,000-50,000: Outer Oort Cloud 1955 planets beyond the solar system (exoplanet/exosolar) - Most are bigger than Jupiter, and orbit closer to their Sun than Mercury Solar System Characteristics All planets, except Mercury, orbit nearly circularly, lying on the same orbital plane. All planetary objects orbit counter- clockwise on the plane of our solar system (ecliptic) Properties (1 – 4) all refer to orbits of planets 1. Each planet is relatively isolated in space. 2. The orbits of the planets are nearly circular. 3. The orbits of all the planets all lie in nearly the same plane. 4. The direction in which all the planets orbit the Sun (counterclockwise when viewed from above Earth’s North Pole) is the same as the direction in which the Sun rotates on it’s axis. Properties (5 – 8) refer to chemical make up of SS 5. * There is a chemical and density gradient from inner (metals + rocks; high density) to outer (gas and ice; low density) outer planets 6. * Asteroids are very old; most lie between Mars and Jupiter; and most of the material expected to exist is missing. - The meteorites we find are very primitive in composition 7. The Kuiper Belt exists and is a collection of small, rocky-icy bodies beyond the orbit of Neptune 8. The Oort Cloud comets are primitive, rocky-icy bodies that do not orbit in the plane of the SS (ecliptic) and reside primarily at large distances (~20,000 -50,000 AU) Wesley Fowler ASTR Chapter 6 Nebular Theory The nebular theory claims that the existing solar system was formed by the collapse of a giant cloud of interstellar gas and dust. 1. A cloud of gas and dust (nebula) exists 2. The nebula is compressed (by gravity, shockwave?) 3. Conservation of angular momentum causes nebula to rotate faster 4. The rotation speed causes the nebula to flatten into a disc called the Solar Nebula. It has a large central mass called the protosun 5. The dense materials (dust) in the Solar Nebula accrete together into planets and other solar system bodies Nebular theory is well supported by visual observation! Condensation Theory Condensation refers to the changing of phases, typically from gas to liquid. However, when dealing with the solar system, it’s from gas to solid. The planets and objects farther away from the sun are composed of more materials than the ones closer to the sun - This is called the compositional gradient of the solar system The planets and objects farther away from the sun have lower temperatures than the ones closer to the sun - This is called the temperature gradient of the solar system The temperature gradient of the early Solar Nebula explains why rocky planets formed close to the sun, while planets farther away remained gaseous. - Solid materials require higher temperatures to condense - Solid materials have higher densities than gaseous ones Hot: 1200-1500 K Warm: Around 500 K Cool: 200-300 K Cold: Around 50 K Ice Line: (T = 273 K) - Boundary in which icy grains can form Planet Building 1. Condensation of solids (Condensation Theory): Two grains of ice for each grain of rock - Inner SS: Rock and Metal grains available - Middle SS: Metal, Rock, and High T ices (e.g., water) - Outer SS: Metal, Rock, High and Low T ices 2. Accretion of solids: Grains clump together in the protostellar cloud - More material is available farther from the sun due to lower density, thus larger planets 3. Collection of solid grains into planetesimals: Grow from cm to km - Gravitational attraction begins when planetesimals reach 10-100km in size - Leftovers of these are asteroids! 4. Formation of protoplanets out of planetesimals: Begin to have strong gravitational force - 100 – 1000+ KM in size 5. Combination of protoplanets via collision - How we think the moon was formed, why Venus rotates “backwards” This process explains how terrestrial planets, rocky cores of gaseous planets, and other SS objects are formed. The sequence takes about 100 million years. Core-Accretion Theory Much larger protoplanets form beyond the ice line due to the abundance of materials that have not condensed. - Gas giant planets have solid cores that have an immense gravitational force. These cores attract huge amounts of gas from the nebula itself, and thus become massive. The core-accretion theory is disapproved of by many scientists, as the time required for it exceeds beyond the lifetime of solar nebula. Gravitational Instability Theory The giant gaseous planets formed in a very similar way that the Solar nebula did, with gases from the original nebula condensing into planets. The gravitational instability theory is disapproved of by many scientists because there is simply not enough mass in the solar disc to cause this type of gravitational collapse. The core-accretion theory is currently better supported than the gravitation instability theory. Clearing the Disk Strong solar winds blow interstellar dust, gas, and leftover planetesimals out of the solar system. This isolates and defines the specific planets and interstellar objects Wesley Fowler ASTR Chapter 7 The Earth’s Structure Earth is the most massive of the terrestrial planets, it has very dense materials in its interior, it is the only planet with liquid water on its surface. • Inner Core: 0 - 1300 km (solid) • Outer Core: 1300 – 3500 km (liquid) • Mantle: 3500 – 6350(ish) km • Crust: 5 – 50 km thick – Oceanic Crust: ~5 km thick – Continental Crust: ~ 30 – 50 km thick • Hydrosphere: All water • Atmosphere: Crust – 100 km (Point at which flight is not possible) • Magnetosphere: >100 km The Earth’s Atmosphere The outgassing of water vapor, methane, CO , and2nitrogen compounds from the Earth’s surface is the origin of the atmosphere. These compounds are altered by the Sun’s UV light in a process called photodissociation, which breaks up nitrogen compounds, isolating nitrogen. Nitrogen clumps together to form N gas2 which is very inert and hard to break up. - Remaining compounds absorbed into rocks Oxygen enters the atmosphere from photosynthetic organisms in the ocean. Nitrogen 78% Oxygen 21% Argon 0.9% CO ~ 0.03% 2 Troposphere: Lower level of atmosphere. All weather occurs here. (0-17kn) Stratosphere: Where the ozone layer is, and where planes fly. (17-50km) Mesosphere (50-80kn) Ionosphere/Thermosphere: Ionized molecules and free electrons here. (80+km) Temperature Inversions define the boundaries between layers - Temperature increases in ozone layer with ascension due to o 3 absorbing sunlight, Troposphere: Air is constantly being moved around through convection, which is driven by Erath’s warm surface due to sunlight. - Convection is the transfer of heat from one pace o another through the movement of a gas or liquid. Astrosphere: Ozone layer is an excellent absorber of UV radiation, why there is the temperature inversion The ozone hole: Large hole in the ozone layer above Antarctica, caused by pollution of chlorofluorocarbons (CFCs) in Antarctica, which kills ozone cells. The sky is blue because blue light is scattered in the atmosphere of of air molecules. Blue light is scattered more than red light due to its shorter wavelength (400nm), and appears to come from all directions as a result. Most red light is not scattered in the atmosphere. “Rayleigh scattered” 9.8 times more efficiently scattered than red light Mie scattered 1.75 more efficiently than red light At sunrise and sunset, the sunlight has more atmosphere to travel through, and blue light scatters out of line of sight along with greens and yellows, leaving mostly red light to be seen. Surface Heating Earth’s surface absorbs about 70% of the incident solar radiation. - Were it not for the atmosphere, the surface temperature would be 250K or -23C Greenhouse gases are molecules and compounds that efficiently absorb the infrared radiation. Include CO 2 methane, and water vapor. Warms up the -23C to 14C (57.2F) Wesley Fowler ASTR Chapter 7 Climate Change Climate: Long timescale characteristic of the environment (i.e. a dessert, a rainforest) Weather: Temporary condition (it is raining) - Climate change: The change of the average condition of the Earth. Human activity is increasing the amount of CO in t2e atmosphere, thus increasing the amount of the atmosphere’s greenhouse gases which trap thermal radiation. Thus the temperature of the world is increasing with the rise of CO emission because less and less thermal radiation is escaping 2 through the atmosphere. Possible outcomes of climate change: - Increased surface and ocean temperatures - Rising sea levels - Longer and more intense periods of severe weather - Changes in atmospheric and oceanic circulation patterns - Increase of desert area in equatorial regions - Increase in ocean acidity and larger and more numerous “dead zones” The Earth’s Interior In order to determine the internal structure of the Earth, scientists use seismic waves generated during earthquakes. - P-waves (Primary Waves) Pressure waves that travel fastest, causing vibration in the direction of motion. These can travel through solid, liquid and gases. - S-waves (Secondary Waves) Shear waves that travel slower, causing vibration perpendicular to the direction of motion. These can only travel through solids, and be absorbed by liquids and gases. By observing the difference between these two wave speeds, scientist can determine the density of the rock in the Earth’s interior. - Since S waves cannot pass through liquids, we know that there is a liquid outer core surrounding the inner core with a radius of 3500 km. Surface Activity Plate Tectonics: The oceans and continents are floating on top of a highly viscous convective mantle of in a series of plates - Earth is the only planet known to have plate tectonics. Lithosphere: The Earth’s crust and upper solid mantle made up of the Earth’s plates. - 100 km thick on average Asthenosphere: Upper mantle below the Lithosphere, a highly viscous convective mantle. - Not molten, but flowing Tectonic Motion: Plates floating on top of convection cells in the mantle. - Magma seeps to surface at cracks between plate boundaries. Volcanoes are very common in these areas. - Plates grinding or colliding against each other produces earthquakes. Pangaea Pangaea refers to the supercontinent that housed the current continents. Pangaea explains fossil evidence, and its breakup explains why the continents drift about 2cm ever year. Earth’s Magnetic Field Requirements for a planetary magnetic field: 1. Convection of conducting liquid in interior - Needs to be large enough 2. Sufficiently fast rotation rate - Lower than ~25 days Dynamo Theory: The combination of these two requirements creates an electrical generator which produces an electrical field. - Earth fits this theory, and has the strongest field of all the terrestrial planets Magnetosphere: The region in space influenced by Earth’s magnetic field - Charged particles of the Solar Wind are deflected by Earth’s magnetic field Van Allen Belts: Regions in the magnetosphere perfect for trapping charged particles, which send them into the upper atmosphere, creating the aurorae (northern lights) Tides Earth’s tides are caused primarily by the moon’s gravitation force, as well as by a force from the sun that is twice as weak due to distance. There are two low and two high tides per day. - Tides are a differential force, meaning that the gravitational force from the moon is stronger on the closer side of Earth, and weaker on the farther side. (a) Tidal Bulge: The near-side’s ocean is lifted towards the moon, while on the far side the land is being pulled away from the far side’s ocean. This is why we have low and high tides. (b) - High tide is the side closest and farther from the moon, low tide in on the caps of the bulge. Determined by the tidal bulge. Spring tides are the strongest tides, and occur at a new or full moon. Neap tides are the weakest tides, and occur at a 1 or 3 quarter moon. Tidal locking: The moon’s motion is causing the Earth’s rotation to gradually slow, making the days longer by +2.3 milliseconds per century - Furthermore, the moon is retreating from Earth at a rate of 4cm per year Wesley Fowler ASTR Chapter 8 The Moon’s Interior 3 Average Density: 3300kg/m - Lower than Earth’s, but higher than the Moon’s surface. Thus an iron core. Total Radius: 1,738 km Layers: Solid iron inner core: 240km (small) Fluid outer core: 330Km Molten inner mantle Large, rocky outer mantle: Out to 1600Km Thin rocky crust No plate tectonics No electromagnetic field Maria is believed to be made of lunar mantle material mixed with the crust Origins of the Moon Earth’s mantle and crust are very compositionally similar to the Moon. They are all made up of similar rocky and metallic materials. - The Moon lacks a substantial metal core 1. Co-formation (sister) Theory: Earth and Moon formed separately, but at the same time, out of the same material. - The moon’s deficiency of iron refutes this theory. 2. Capture Theory: Earth and Moon formed separately apart from each other, with the Moon being captured by the Earth’s gravitational force, causing it to orbit. - The physics required for the Earth to capture the Moon in its orbit are not possible. 3. Fission (Daughter) Theory: Earth and Moon form as single object, and the Earth and moon split up due to a very high rotation rate. - The physics required for this split are not possible 4. Giant Impact Theory: When the Earth was mostly molten and young, a grazing collision happened with a “Mars-sized” protoplanet caused a chunk to separate from Earth. - This physics required are possible, and the theory explains compositional similarities Evolution of the Moon Oldest rocks dated: 4.4 billion years ago (age of highlands) - Crust must have formed by this time Shortly after formation, the Moon’s surface was entirely molten, it was covered by what is called the lunar magma ocean. Lunar Magma Ocean Sequence: 1) Formation result of hot accretion 2) Light material (anorthosite) floats -> Crust, such that thinner crust formed on near-side 3) Dense materials sink 4) Large impacts form huge basins during Late Heavy Bombardment – Precursors to maria 5) Volcanism over next several hundred million years (3.9 – 3.2 Gya) filled impact basins to form maria with mantle compositions 6) No significant geologic activity after 3.2 Gya; Surface evolution dominated by the steady of impacts (generating new craters and regolith) Near-side crust is thinner than the far-side, and thus can flood with lava. Wesley Fowler ASTR Chapter 8 The Moon Distance from Earth: 384,000 km Radius: 1,738 km (0.27 Earth radii) Mass: 7.3 x 10 22kg (~1/8 Earth Masses) Orbital Period: 27.3 days Rotational Period 27.3 days - The moon has no atmosphere and no global magnetic field The side of the moon that can be seen from Earth is called the Lunar near side, while the face that cannot be seen is the Lunar far side. We can see about 59% of the moon due to its slightly elliptical orbit, which causes a lunar libration. - Lunar libration: The oscillation of the moon, or its rocking back and forth. The moon’s rotational period is equal to its orbital period around Earth, both being 27.3 days. This is why only the lunar near side is visible from earth. The moon is in synchronous orbit. Tidal locking: When an orbiting moon is in synchronous orbit with its associated planet. - Most moons in the solar system are tidally locked. The Moon’s Surface Two dominate surface features: 1. The Lunar Maria: The younger, smoother surface that has fewer craters. Darker in color due to high amount of iron in rock. 2. The Cratered Highlands: Elevated above maria by several kilometers. Older surface with much more craters, a lighter color. The region is filled with mountains. The Lunar near side is mostly maria with highlands at the southern tip, while the far side is mostly highlands with some scattered maria. Impact craters: Ring-like impressions on the moon created from meteor impacts. Simple craters: Clear and singular bowl shape. Simple physics. Complex craters: Large craters that have a central peak. Complex physics. Ejecta: The material ejected from the moon due to a meteor strike. - Ejecta blanket: White surrounding material around crater. - Ejecta rays: White streaks from craters that stretch across the moon. The older surfaces of the moon have more craters than younger surfaces. Superposition: What is on top is you
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