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Department
Astronomy
Course
AS101
Professor
Victor Arora
Semester
Fall

Description
AS101 MIDTERM NOTES Lecture 1 to 8 (not 3) – ch. 1 to 3 Lecture 1 - universe is mostly empty space – various astronomical objects, gasses, dust and energy within and in between galaxies - over 100 billion stars in our home galaxy (Milky Way) - 1 light year (distances to stars) = 63 000 AU (distances in solar system) – (9.45 trillion km) - 1 AU = distance between Earth to Sun (150 million km) - smallest to largest: planet (Earth) star (Sun) solar system galaxy (Milky Way) – formed of billions of stars filament / walls outlining voids (largest) - spiral galaxy – has spiral arms & stars inside of it, but is inside of galaxy clusters & filaments - astronomers use metric system for calculations - star: a large, glowing ball of gas that generates energy through nuclear fusion - planet: a moderately large object which orbits a star; it shines by reflected light - may be rocky, icy, or gaseous in composition. - moon / satellite: an object which orbits a planet - asteroid: a relatively small and rocky object which orbits a star - comet: a relatively small and icy object which orbits a star - star system: a star/stars and all the material which orbits it – including planets, moons, comets & asteroids – ex: Solar System - galaxy: a large group of stars in space, all held together by gravity and a common center - nebula: an interstellar cloud of gas and/or dust - cluster: a group of galaxies all held together by gravity - supercluster: concentration of clusters – form filaments and walls around voids - outermost planet is always the same (Neptune) - Earth rotates counterclockwise when viewed above the North Pole  sunrise on west - rotates east – sunset on east - Earth / most other planets orbit the sun in nearly circular elliptical orbits - Picture on right shows stars’ apparent brightness - # of galaxies in the universe = # of stars in the Milky Way - 100 billion galaxies (star systems) in the universe What makes up the Universe ? - The universe is made up of all matter and energy within and in between galaxies - Matter and energy can interact with each other, and can also be converted from one to another Where are we in the Universe ? - The earth is a planet in the solar system  consisting of objects orbiting the sun - The solar system is part of a galaxy called the Milky Way - The Milky Way is part of a group of 40 galaxies called the Local Group. - The Local Group belongs to the Local Supercluster (also called Virgo Supercluster) - The Local Supercluster belongs to the Universe How far is the nearest star system? Let’s build a scale model – scaled down by a factor of ten billion sun is the size of a large grapefruit (14 cm) earth is sesame seed (100 times smaller) Jupiter is 10 times smaller than sun – In this scale model, how far is the nearest star system Alpha Centauri? 4400 scaled km On this scale, the distance to Alpha Centauri is the roughly distance between Toronto and Vancouver ! Lecture 2 - speed of light: 300, 000 km / second – 18 million km / minute AU - earth’s orbit around the sun is elliptical – hence the distance between the earth and sun varies at different points in its orbit - perihelion: minimum distance between earth and sun (148 million km) - aphelion: maximum distance between earth and sun (152 million km) - average (au): 150 million km - size of observable universe: roughly 8.67 trillion au / 14 billion ly Scientific Notation - # . # # (one number, then decimal place) Example: 4812 – bigger to smaller = 4.812 (move decimal over L 3) = 4.812 x 103 positive exponent Example: 0.0000312 – smaller to bigger = 3.12 (move decimal over R 5) = 3.12 x 10-5 negative exponent - earth is roughly 4 billion years old Light Light takes: - 8 minutes to reach us from the Sun - 8 years to reach us from the star Sirius (8 light-years away) - 1,500 years to reach us from the Orion Nebula - we see the sun as it was 8 minutes ago - Sirius as it was 8 years ago - Orion as it was 1500 years ago the farther out we look into the universe, the farther back in time we see! When did we come to be? - One month on “cosmic calendar” = approx. 1 billion years - Human civilization is just a few seconds old on cosmic calendar How did we come to be? • The Big Bang theory of an initial cosmic explosion is the most widely accepted theory of the origin of the universe - Matter and energy was hurled in all directions - The simplest elements hydrogen and helium initially dominated - Gravity caused matter to be drawn together to create stars, galaxies and clusters - Stars manufacture more complex elements such as nitrogen and oxygen - When the star dies, they are expelled into space.... to form new stars and planets - Most of the atoms in our bodies were created in the core of a star! What is our motion in the Universe ? - Earth rotates around it’s axis once every day from west to east - we do not feel this motion because we are moving with the earth - at equator: you will travel 1670 km/hr! - Earth orbits around the sun once every year - Earth’s axis is tilted by 23.5 degrees relative to the plane of its orbit - Sun and stars of the local Solar neighborhood (our solar system) orbit around the center of the Milky Way Galaxy every 230 million years! - Galaxies outside our Local Group appear to be moving away from us - farther away they are = faster they are moving  the universe is expanding Lecture 4 Milky Way: band of light making a circle around the celestial sphere  it is our view into the plane of our galaxy Constellations - ancient civilizations named groups of stars called constellations - names were based on ancient heroes, gods, animals, shapes  mythology was associated with them - star patterns today originated 5000 years ago with the Babylonians, Egyptians & later Greeks - International Astronomical Union (IAU) established 88 official constellations – with clearly defined permanent boundaries that together cover the entire sky - map of earth: divided into countries – map of sky: divided into constellations - constellation represents group of stars and section of the sky (viewing direction)  (any star within the region belongs to only that constellation) - asterisms: star groupings in sky ex: Big Dipper  in constellation Ursa Major (the Great Bear) Great Square of Pegasus  3 stars (top left - Alpheratz, other 2 - Pegasus) -- now belongs to Andromeda only constellations the big dipper great square of pegasus - the brightest stars in a constellation may actually be quite far away from each other - most stars in constellations are not physically close to one another - some stars may be moving through space in different directions - only thing in common: they lie in approx. same direction from Earth Star Names - most individual star names  derived from ancient Arabic, Greek or Latin (altered over centuries) ex: Sirius: ‘glowing’ in greek Spica: ‘spica virginis’ in Latin, means ‘ the ear of wheat of Virgo ’ Aldeberan: ‘follower’ in Arabic Vega: ‘landing’ in Arabic OR - assign Greek letters to the bright stars in a constellation  (in approx. order of brightness) ex: alpha (α) is brightest, beta (β) 2 , gamma (γ) 3 , delta (δ) 4 , epsilon (ε) 5 etc. – there are some exceptions! The Brightness of Stars - magnitude scale  measure brightness of stars - ancient astronomers: divided stars into 6 brightness groups brightest: first-magnitude stars dimmest: sixth-magnitude stars  faintest visible to human eye - greek astronomer Hipparchus: believed to have compiled the 1 star catalog may have used the magnitude system in the catalog - Egyptian-Greek astronomer Claudius Ptolemy: used magnitude system in his own catalog - star brightness: apparent visual magnitude (m V)  how stars look to human eyes - flux: total light energy hitting one square metre per second  can then be used to calculate apparent visual magnitude - some stars are so bright  negative magnitudes - magnitude larger than 6  detected by telescopes apparent visual magnitude is based only on visible light and does not say anything about actual energy output of star (b/c distance isn’t included) The Celestial Sphere - look at sky, can’t tell how far different stars are from us - ancient astronomers: thought sky was giant celestial sphere, all stars attached - celestial sphere is useful for describing the location (not distance) of stars - stars at different distances all lie on the celestial sphere celestial sphere Ecliptic: sun’s apparent path through celestial sphere
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