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Department
Astronomy
Course
AS101
Professor
Victor Arora
Semester
Fall

Description
Cosmos: all of the order in the universe 5% of matter is atoms, visible Dark matter- 26.8% Dark energy- 63.8% Dark- means not visible Sun, mercury, venus, earth, mars, Jupiter, Saturn, Uranus, neptune -the universe is mostly vacuum, empty space The sun is a massive ball of gas that generates energy through nuclear fusion -100 times as wide as the earth -indirect source of almost all energy on earth Exoplanets: over 900 planets confirmed to be orbiting other stars, thousands of candidate exoplanets observed by Kepler telescope Star: globe of gas held together by its own gravity and supported by the internal pressure of its hot gases which generate energy by nuclear fusion -stars rise 4 min earlier each night, stars rise 2 hours earlier per month -stars rise and set like the sun, they are up in the day we can’t see them Sidereal day: time between successive meridian crossings for any other star (23h 56m) Northern hemisphere stars never appear to set, located near a point called NCP north celestial pole. These are circumpolar stars because they appear to circle the pole and never set NCP is near the star called Polaris, other stars make circles but part of their path is below the horizon Stars reach their highest point when they cross the meridian Satellites: an object in orbit around planets, natural satellites are known as moons Asteroid: small rocky object orbiting a star, not spherical Comet: like asteroid but have an icy shell, leave tails near the sun, orbits the sun Trans-neptunian objects: larger than asteroids, not large enough to be a planet, orbit the sun (Pluto), dwarf planets Galaxy: large system of dark matter, stars, gas, dust, and nebulae orbiting a center of mass bound by gravity Nebula: clouds of gas or dust within galaxies, raw materials for new stars from previous generations Star clusters: Open clusters: 1000s of stars Globular clusters: 100 000s of stars, sun probably formed in an open star cluster but has since “moved out” Galaxy groups and clusters: group of galaxies (a few dozen up to thousands) all hld together gravitationally Milky way: the hazy band of light that circles the sky, produced by the glow of our galaxy Milky way galaxy: the spiral galaxy containing our sun, visible in the night sky as the Milky Way Spiral arms: long spiral pattern of bright stars, star clusters, gas and dust that extend from the center to the edge of the disk of spiral galaxies Superclusters: made up of galaxy clusters Walls: made of superclusters Voids: space between superclusters Constellations: one of the stellar patterns identified by name, usually of mythological significance and the region of the sky containing that pattern. There are 88 official constellations with clearly defined permanent boundaries that cover the entire sky. Asterism: named grouping of stars that aren’t part of a constellation Another way to identify stars is to assign greek letters to the bright stars in a constellation in approximate order of brightness Magnitude scale: the astronomical brightness scale from first to sixth (first being brightest) apparent visual magnitudes (m ),vsome stars are so bright they are in negative numbers, some so faint they can only be seen with a telescope and they extend past sixth. Each step is 2.5x brighter, if we don’t know how far away the star is its apparent brightness does not tell us about its true power output Solar day: average time between successive meridian crossings for the sun (24h) Scientific model: hypothesis with reasonable validity Conceptual model: help us think about how nature works 8 Astronomical unit= average distance from earth to the sun 1.5x 10 km 2AU=twice as far, 3AU=3 times as far Earth is 15 billion m from the sun 14 (about 4 light years away) Alpha Centauri: The closest star system is 4.1 x 10 Andromeda Galaxy 2 million light years away Light has a finite speed 300 000 km/s, the star is so far away that we are seeing it as it looked in the past A light year is the distance light travels in a year about 10 km The further an object is back in space the further we are looking back in time To multiply two or more numbers written in scientific notation 1. Multiply the coefficients 2. Add the exponents Earth: -revolves every 24hrs (around the sun) -rotation takes 365 days (on its tilted axis) 23.5 degrees from vertical -seasons due to the projection of Earth’s orbit on the celestial sphere, changes in the amount of solar energy that Earth’s northern and southern hemispheres receive at different times of the year because of the tip of the Earth’s equator, furthest north at summer equinox and furthest south at winter solstice -perihelion: Earth’s closest point to the sun (1.7 percent closer than average) -aphelion: Earth’s most distance from the sun (1.7 percent more than average) -3.4% variation in distance from the sun -sun rises in the east and sets in the west -moves 32km/s -to a stationary observer stationary objects move in the opposite direction as the observer’s motion -earth rotates from west to east, counter clock-wise -looking down from north pole it moves counter clockwise Precession: the slow change in orientation of the Earth’s axis of rotation, one cycle takes takes nearly 26 000 years, we adjust constellations 1 degree every 72 years sun and moon unevenly pulling on the slight bulge around the Earth’s equator, causes a very slow but regular wobble of Earth’s axis, orientation changes but it remains tilted at 23.5 degrees, slowly changes position of NCP, catalogues of stellar coordinates are slightly adjusted only every 50 years Celestial sphere: conceptual model of the sky (imaginary sphere of very large radius surrounding the earth, objects in the sky appear to be attached to the rotating celestial sphere, we can see exactly one half of the sphere at a time, the rest is the ground (horizon separates them> Zenith is right above us. North and South celestial poles are directly above the earths north and south poles Celestial equator divides north and south Meridian: line passing through the north point on the horizon through the zenith to the south (separating north and south half) The sun reaches noon as it crosses the meridian Am- antemeridian (before) Pm post meridian- after noon -on the earth lines of longitude and latitude create a grid Celestial coordinates: we specify positions in the sky using Right Ascension (RA) and Declination (Dec) You can measure the angular sizes and separations on the celestial sphere using degrees. Meridian spans a distance of 180 degrees -rule of pinky: pinky held at arms length is 1 degree -fist held at arm length is 10 degrees Right ascension: in hours east of the Vernal Equinox (0h-24h) Declination: in degrees North + or south – of celestial equator (+90 to -90) The half of the sky that we see at a given time of night changes depending on the position of the earth along its annual orbit and the latitude of the observer The phases of the moon are caused by the moon’s position in its orbit, different parts of the side become illuminated by the sun, rises and sets at different times depending on where it is in its orbit The moon rotates, we only ever see one side of the moon, its librations allow us to see 59% of the surface over a lunar cycle (it wiggles a little) Moon orbit in slightly elliptical path once in approximately one month, appears to move eastward against background of stars Orbital period of moon around earth is not the same length as moon phase cycle There is no permanently dark side of the moon, only a near side and a farside, moon experiences a month long day and month long night One full moon to another full moon is actually a little longer than how long it takes for the moon to cycle the earth, (about 3 days longer) Phases of moon are consequence of the position along its orbit with respect ti the sun, has nothing to do with the earth’s shadow -line that divides day and night on the moon terminator -orbits of sun and moon are almost in line but not quite, if the Earth, sun and moon line up there is an eclipse Lunar eclipse: moon and sun on opposite sides, earth in the middle, moon is full, can only occur when the moon is crossing the ecliptic and has the right phase, when moon moves through the shadow of the earth, receives some sunlight refracted through earth’s atmosphere which can turn the moon copper red, easier to see a lunar eclipse because it is visible all across the night sky, the shadow of the Earth on the moon is much larger Solar eclipse: moon is between earth and sun, shadow of moon is on the earth, only visible where the shadow falls, angularly moon appears the same size as the sun, diamond ring effect- sun showing around the moon as a glow and a crater shows a larger glimpse, making it look like a ring Annular eclipse: when the moon is slightly further from the earth, more of the sun shows, the moon is in a silhouette, solar photosphere appears around the edge of the moon in a bright ring or annulus -in the future the earth will slow down, the moon will drift further from the earth and the only eclipses visible will be annular, moon will only be visible from one side of earth Umbra: region of total shadow Penumbra: partial shadow Saros cycle: an 18 year and 11 ½ day period after which the pattern of lunar and solar eclipses repeats Aristotle: geocentric model of the universe, Earth at the center, the heavens are perfect Ptolemy: Aristotle’s model doesn’t account for planets seemingly going backwards, moves Earth a little off center in the universe and slightly varies planets speeds, planets have small circles (epicycles) and move around a larger circle (deferent) Copernicus: heliocentric universe, with the Sun in the center, uniform motion of the planets Kepler: orbits are ellipses (a closed curve around two points, foci, such that the total distance from one focus to the curve and back to the other foci remains constant. Semi-major axis a: half the longest diameter. Eccentricity (e) : a number between 1 and 0 that describes the shape of an ellipse, near 1 is elongated, near 0 is circular. Kepler’s Laws: 1. Orbits of the planets are ellipses with the sun at one focus 2. A line from a planet to the sun sweeps over equal areas in equal intervals of time 3. A planet’s orbital period squared is proportional to its average distance from the sun cubed P ya 3AU Galileo: moon is not perfect it has mountains and craters showing heavens were not perfect, Venus went through phases so it is impossible for earth to be in the center of the universe, Jupiter has moons that orbit, so it is possible for Earth to have an orbital moon -all objects near the surface of a celestial body have the same gravitational acceleration Velocity: speed with direction Acceleration: rate of change in velocity (can change speed, direction) Force: push or pull in some direction -all objects near the Earth’s surface fall at an acceleration of 9.81m/s towards the center of the earth -acceleration is net force divided by mass a=fnet/m Newton: mass a measure of matter, weight the force that gravity exerts on an object. Gravitational force between two bodies depends on the masses of the bodies and the distance between them, the force of gravity decreases as the square of the distance between the objects increases (inverse square relation), moon orbits as it is continually falling towards the centre of the earth Newton’s Laws: 1. A body continues at rest or in uniform motion in a straight line unless acted upon by some force (unbalanced) the “keep doing what you are doing” law Inertia has kept 1977 voyager mission still moving in space 2. A body’s change of motion is proportional to the force acting on it and is in the direction of the force 3. When one body exerts a force on a second body the second body exerts an equal and opposing force back on the first body if an object pulls on another object then the second object pulls on the first in an equal and opposite direction, a curved path(changing direction) implies an acceleration caused by a net force The moon exerts the same amount of force on the moon as the moon does the earth All objects in the universe with mass attract all other objects with mass Mass: the amount of matter in another object (in kg) Weight: the force that gravity exerts on an object (n neutons) Always attractive, applies to any pair of objects with mass, including dark matter The gravitational force between two objects depends on the product of their masses -The gravitational force between two objects also depends on the inverse square of their separation eg twice as far away is one quarter the force -Orbiting bodies actually revolve around a balancing point between them called centre-of-mass, centre of mass closer to the more massive object and in the case of the sun, sometimes within it. -planets move faster in orbit when closer to the sun -tides are caused by small differences in gravitational force, the side of Earth closer toward the moon is a bit closer, the moon pulls on it more strongly and that pulls up a bulge, as the Earth rotates there are two bulges and two high tides each day Spring tide: ocean tide of large range that occurs at full and new moon Neap tide: ocean tide of small range that occurs at first and third quarter moon Northern summer solstice: sunlight hits the surface at nearly perpendicular angle, energy is more concentrated, start of summer for northern hemisphere. For southern it is more spread out, it’s the start of winter. Equinoxes: roughly equal amounts of daylight and darkness everywhere on Earth, mark the beginning of spring (vernal) or fall (autumnal) Ecliptic: apparent yearly path of the sun among the stars (moves eastward in respect to background stars) Sun spends half the year north of the celestial equator and half the year south Vernal equinox: sun crosses CE heading north Summer solstice: sun at furthest point north from CE Autumnal equinox: sun crosses CE heading south Winter solstice: sun at furthest point south from CE After midterm Light is both a particle and a wave Waves: any repeating motion that can carry energy without carrying matter along with it Frequency: how often it goes by a fixed location Wavelength: how far between two peaks Speed: frequency x wavelength Hz hertz Light: combination of oscillating electric and magnetic fields that can travel through empty space, also known as electromagnetic radiation Radiation: anything that spreads outward from a force -light interacts with matter through electromagnetism Photon: a quantum of electromagnetic energy -energy decreases as wavelengths increase, increases as frequency increases E=hf H is planck’s constant Visible light: 400nm-700nm Wavelength also describes colour - 700nm-400nm, a prism can spread white light into the visible spectrum - We can’t see all types of magnetic spectrum wavelengths - We can use electronic detectors (eg. Cameras) to see other types of electromagnetic radiation - Spectrum is arranged in order of increasing wavelength Doppler effect: a change in wavelengths caused by relative motion toward or away from the source of waves, observer perceives a different pitch Two objects approaching eachother emits lightshift to shorter wavelengths blue shift Two objects receding eachother emits lightshift to longer wavelengths red shift Size of shift tells you relative radial velocity of two objects, Doppler shift for light is usually too small to discern visually Shortest to longest: Xray Gamma ray UV Visible Infrared Microwave Radio How light can interact with matter: Emission- energy in matter can be converted to EM radiation Absorption- EM radiation can absorb light energy in matter and convert it to internal energy or re-emit it Reflection/ scattering- light can simply bounce off matter in a direct manner (reflection), or random direction (scattering) Transmission- light passes through unimpeded -we can directly see objects that emit their own light or reflect another object’s light A diffraction grating reflects (or transmits) light at different angles for different wavelengths but with greater dispersion (bending) -3 categories of spectral features can be present -spectrum of an object gives clues about how the light was produced -massive compact nucleus made of + protons, neutral neutrons and surrounded by low mass – electron cloud -every element has a different number of protons -an isotope of that element has a different number of neutrons -the number of electrons is equal to the number of protons to the atom is neutral, if not it is an ion -particles that make up an object are constantly moving -the particles have thermal energy -averaging the random motion of these particles is one way to define the temperature of an object -this is why the Kelvin scale starts at absolute zero (-273 degrees C) when there is absolutely no random particle motion -the simplest way to produce electromagnetic radiation is to increase the temperature of an object, dense objects produce a smooth mixture of colours, a thin gas will produce light at specific colours ex. Neon sign -dense object glows because greater temperature creates increase in particle motion, part of the energy of motion is converted to light -a hot opaque object emits thermal or blackbody radiation (blackbody= perfect absorber and perfect emitter) Wien’s Law: the wavelength of maximum emission for a glowing object decreases as its temperature increases. If an object becomes hotter; particles move faster, collisions are stronger, electromagnetic radiation is more energetic. Therefore the colour of a star tells us its temperature. -hotter objects also emit a greater overall flux than cooler objects, more frequent collisions and more energetic radiation is emitted more frequently Stefan-Boltzmann Law: as the temperature of a glowing object increases the blackbody curve rises everywhere, a small increase in temperature causes a big increase in energy emitted Bohr model of the atom: -imagined negative electrons orbit positive nucleus because of an attractive force (Coulomb force) -only specific orbits are permitted, no in between -each orbit is better called an energy level -the pattern of specific energy level is unique for each element, isotope and ion Conservation of energy: energy is never truly created or destroyed, but can be transformed from one type to another Excitation: -to become an ion the atom must be hit with an amount greater than the binding energy for an electron in a particular level -electrons can also move up to another energy level if exactly enough energy is supplied to make up the difference between levels – an electronic transition -photons of the wrong wavelength are transmitted -if the wavelength of a photon corresponds to the exact amount of energy needed for a transition, the atom absorbs the photon and becomes (temporarily) excited, excitation by collision is also possible -the extra energy is released as new photons after a short while the electron falls back to a lower level -each neutral atom/isotope/ion emits or absorbs at unique wavelengths -characteristic wavelengths of light absorbed or emitted show up as lines in a spectrum and can be used to identify elements present -we can measure Doppler shift of individual lines 1. A hot dense object emits a continuous spectrum, low density gas produces emission line spectrum, low density gas in front of a hotter blackbody source shows ab
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