Study Guides (247,973)
Canada (121,209)
Astronomy (142)
AS101 (137)

AS101 Midterm Study Guide

16 Pages
Unlock Document

Victor Arora

A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 1 Scale of the Universe The Cosmos - Mostly vacuum (empty space) - Everything we can possibly see if a tiny fraction - Why does it seem like there are so many objects in space? - “dark matter” = means not visible - We only really know what atoms are (the 5%), we don’t even know what dark matter/energy is, it’s invisible The Sun is our Star - Massive ball of glowing gas that generates energy through nuclear fusion - About 100x as wide as the Earth - Source of almost all energy on Earth - Mostly of hydrogen gas - The thing that’s holding the sun together is gravity Planets are less massive than stars (their difference) - Planets are either rocky or gaseous (Earth is a rocky planet) - How are planets different from stars o Planets are less massive o Non-luminous (don’t glow on their own) and spherical  Reflect light instead from the sun  Can only see the planets and moon from reflected light o In orbit around a star (Need to be in orbit) o “Cleared the neighbourhood” of other objects  You kind of dominate the neighbourhood  All of the little objects go away, disappear  Why Pluto is not a planet anymore, hasn’t cleared the neighbourhood  There are several hundred objects out there that’s the same size as Pluto o At least that’s what we think Exoplanets - Over 800 planets confirmed to be orbiting other stars - Thousands of “candidate” exoplanets, observed by the Kepler Space Telescope - Kepler Space telescope is the most recent telescope - Several methods have to have access to a theory before it can be confirmed Some planets have satellites - Satellite – an object in orbit around a planet, anything that orbits around a planet - Natural satellites are also known as “moons” - Saturn doesn’t have the most, Jupiter has more Other objects in the solar system - Asteroid: a small, rocky object orbiting a star - Comet: a small, icy object that orbits the sun - Comets can grow tails near the Sun - If a thing isn’t quite spherical, then gravity didn’t form it o Gravity pulls on all sides equally, so therefore should be a sphere - Comet’s tails can be very long, ironic to call it a small icy object Eris - The planet that knocked Pluto out of being classified as a planet - Slightly bigger than Pluto Planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune - Earth is the massive out of the terrestrial planets - Venus is the sister planet of Earth o Just because almost the same size (just a little bit smaller) A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 2 Galaxies: “cities” of the Cosmos - A large system of dark matter, stars, gas, and dust, all bound together by their combined gravity - Pictured is the spiral galaxy M83 - Not all galaxies are spirals - Galaxy – where a star is born and dies Nebulae - Nebulae – Clouds of gas and/or “dust” within galaxies - Raw materials for new stars from previous generations - They recycle; stars die, the matter erupts, the same material is used again for others stars Star clusters - Different types of clusters: open and global - Open clusters: 1000s of stars - Globular clusters: 100000s of stars - Our Sun probably formed in an open star cluster but has since “moved out” Galaxy groups and clusters - A group of galaxies (a few dozen up to thousands) all held together gravitationally - Galaxies go with each other Many clusters form a supercluster which make up the cosmic web - Supercluster – from a bunch of clusters Summary: we are the 5% - The Universe is mostly made of invisible matter and energy - The Universe if mostly empty space - But the Universe is a very big place - Walls – made up of cluster of galaxies - We are the 5%, we are the atoms - Galaxies made up of stellar systems, nebulae, and star clusters - Walls SuperclustersGalaxy clustersGalaxies(Nebulae, star clusters) Stellar systems stars, planets, small objects Powers of 10 Models in science - In general, a model is something that represents reality - A scientific model is a hypothesis that describes reality and has withstood observational/experimental tests - We can also develop conceptual models to help us think about how nature works - We will be looking at some scale models (miniature or shrunk down versions) Scale model: the solar system - How do the sizes of the planets compare to the distances between them? - What if we shrunk the solar system by a factor of about 45 billion? - Sun  radius = 15mm - Earth  radius = 0.1mm - Jupiter  radius = 1.6mm - Neptune  radius = 0.5mm - At this scale, from the Sun: Earth is 3m away, Jupiter is 17m away, Neptune is 99m away - The sizes of the planets are tiny compared to the distances between them The astronomical unit - We can specify distances in the solar system by comparing them to the average Earth-Sun distance - 1AU = 150,000,000km = 150,000,000,000m - A planet twice as far from the Sun would be 2AU away There has to be a better way - What if we just specified how many zeroes a number has? 1 2 3 - This is known as scientific notation. For example: 10 = 10 , 100=10x10=10 , 1000=10x10x10=10 - 1 Googol = 10 100 A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 3 Representing large numbers - The Earth is on average 150,000,000,000m from the Sun - In scientific notation we would write this as a coefficient between one and ten, multiplied by the appropriate power of ten - Hint: just count the numbers of places the decimal would move - 150,000,000,000m=1.5x10 11 Doing math with exponents - To multiply two (or more) numbers written in scientific notation, just 1. Multiply the coefficients (if there are any) 2. Add the exponents 1 1 2 - For example: 10x10=10 x10 =10 10 x10 =10 5 Clicker - There are an estimated 100 billion galaxies in the universe with, on average, 100 billion stars per galaxy. How many stars are there in the universe? o 10 22 Clicker - Eta Carinae is a massive star located 10 000 light years away and is expected to undergo a violent explosion known as a supernova. If this star went supernova in 2000 BCE, how long would it be from today until we know about it? o 6000 years How far away are the stars? - The nearest star system is Alpha Centauri: 4.1x10 km 13 - Using our scale from our model solar system: 60 billion actual km = 1 scale km, puts Alpha Centauri at 683km away, in NYC Light-travel and look-back time - But light has a finite speed: 300,000km/s - In other words, this star is so far away that we are seeing it as it looked in the past - Imagine you can walk at rate of 3km/hr; in one hour, you will walk 3km o Let’s call this a “walk-hour” - In one year (8760 hours), you will walk 26280km o Let’s call this a “walk year” 13 - A light year (is the distance light travels in one year) is about 10 km (ten trillion km!) - So Alpha Centauri is about 4 light years away Distances in the universe - The further an object appears to be in space, the further we are looking back in time o The Sun: about 8 light minutes o Alpha Centauri: 4 light-years o Andromeda Galaxy: 2 million light years o Observable universe: 13.82 billion light-years - Galaxy is 2 million light-years from the Earth - This means that it takes 2 million years for its light to reach us - This means that this picture shows this object as it was 2 million years ago Summary - The stars, planets, and galaxies are separated by fantastic distances - We can use models, scientific notation, and appropriate distance units to tame these large numbers - We see the stars and galaxies as they were, not as they are Observing Part 1 Recap and preview - There are different objects at different scales in the visible universe, but it is mostly invisible - Objects in the universe are separated by astronomical distances - Our place in the cosmos is our observatory of the universe A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 4 How the earth is moving - Perihelion: nearest points to the Sun (147.1 million km) - Aphelion: furthest point from the Sun (152.1 million km) - Average distance: 1AU = 149.6 million km - Two basic motions: 1. Revolves around (“orbits”) the Sun 2. Rotates on its (tilted) axis - Rotation is responsible for rising and setting of the sun The rotating Earth and you! - The Sun rises in the East and sets in the West - To a moving observer, stationary objects appear to move in the opposite direction as the observer’s motion - The Earth rotates from west to east - The Earth rotates counter-clockwise when viewed from the North Pole (you’re on Earth) The celestial sphere - A conceptual model of the sky: o An imaginary sphere of very large radius surrounding the Earth o A very large crystal sphere that completely surrounds the Earth - Assumptions: every model has ssumptions o Objects in the sky appear to be attached to the rotating celestial sphere - Allows us to specify the direction to an object in the sky and how it moves from night to night The celestial sphere model - From the inside, we can see exactly one-half of the entire sky at any given time, from the horizon up to the zenith (directly overhead) - The North and South Celestial Poles are directly above with the Earth’s North and South poles - The Meridian is the line passing through the North point on the horizon, through the Zenith, and to the South point - All of the points don’t move on the celestial sphere as long as you don’t move Celestial sphere: inside view - In the northern hemisphere, stars that never appear to set are located near the: North celestial pole - These are called circumpolar stars because they appear to circle the pole - Stars go in a circle motion o Some appear to set and rise because they go below the horizon - The North Celestial pole is near the star Polaris - Stars reach their highest points when they cross the: meridian o True for any star even the Sun o Called the high noon o Where am and pm comes from o Am is when the sun is before the meridian and pm is when after Terrestrial coordinates - We specify locations on Earth using latitude and longitude - Longitude: in degrees East or West of the Prime Meridian - Latitude: in degrees North or South of the Equator - On the Earth, intersecting lines of longitude and latitude form a grid - Imagine this grid projected outward onto the celestial sphere - Prime meridian = the dotted line in the middle of diagram Celestial coordinates - We specify positions in the sky using Right Ascension (RA) and declination (Dec) - Right ascension: in hours East of the Vernal Equinox (from 0h to 24h) - Declination: in degrees North (+) or South (-) of the Celestial equator (from +90 degrees to -90 degrees) - Vernal equinox – right ascension is 0 A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 5 Latitude affects what you can see - Recall: we can only see one-half of the sky at one time - The half that we see at a given time of night changes depending on: o The position of the Earth along its annual orbit o The latitude of the observer Clicker - Which of the following positions is closest to the South celestial pole? o RA = 14h, Dec = -88 degrees - You’re stranded on a deserted island and forgot your astronomy textbook! You locate the pole star Polaris at 15 degrees above the northern horizon. What is your latitude? o 15 degrees north o If you see Polaris (north star), you must be in the north Summary: part one - Objects in the night-sky rotate due to the Earth’s rotation on its axis - The celestial sphere is a model that helps us map out the relative positions and the nightly motions of these objects - What we see in the night-sky depends on our latitude Observing Part 2 Recap - The Earth’s rotation on its axis causes objects in the night-sky to rotate as well - We can specify the positions of these objects and visualize their motion using the celestial sphere model - The stars that we can see, and how they move over the course of a night, both change with latitude Precession: a slow wobble - The sun and moon are unevenly pulling on the slight (43km) bulge around the Earth’s equator - This causes a very slow but regular wobble of the Earth’s axis o The orientation of the axis changes but it remains tilted at 23.5 degrees - The star Polaris will trace the circle that Earth slowly makes (26 000 years to complete one full circle) Precession changes the NCP - This very slowly changes the position of the NCP, and all the other coordinates - Catalogues of stellar coordinates are slightly adjusted only every 50 years The constellations: then - The constellations are arbitrary patterns of stars invented, mostly, by ancient cultures The constellations: now - Today, astronomers officially recognize 88 constellations - The sky is divided into parcels that each contain the constellation and other stars of objects in the same part of the sky Stars in the constellations - The stars in a constellation are not at the same distance from the Earth, just in the same direction Measuring angular distances - You can measure the angular sizes and separations on the celestial sphere using degrees - The angular size of an object decreases the further you are separated from it - The Meridian spans a distance of 180 degrees - Rule of pinky: pinky held at arm length = 1 degree - Rule of first: fist held at arm length 10 degrees Measuring apparent brightness - Astronomers sometimes specify how bright an object would appear the naked eye using the magnitude scale - For historical reasons, the scale runs backwards - Our eyes are non-linear detectors: each step is a factor of 2.5x brighter - The professional astronomers measure the “flux” (the rate of light energy collected per unit area) A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 6 - If we don’t know how far away the star is, its apparent brightness does not tell us about its true (absolute) power output - The smaller the number is, the brighter the star is - The dimmest thing you can see with the naked eye is 6 magnitude - We can’t tell the star’s brightness power if we don’t know the distance Sidereal and solar days - Recall: the two basic motions of the Earth are rotation and revolution - How can we define what a “day” should be? Two ideas o Solar day: the average time between successive Meridian crossings for the Sun (24h) o Sidereal day: the time between successive Meridian crossings for any other star (23h 56m) - A solar day is longer than a sidereal day because the Earth has moved along its orbit so it takes a bit longer for the observer on the Earth to line up with the Sun again - Sun and stars are the highest when they cross the meridian - For any other star, it takes shorter than the Sun to cross the meridian Yearly variation - This slight difference means that the stars rise about 4 min earlier each night - Over the course of a year, the constellations we see at night time change - The underlying cause of the change is the Earth’s revolution around the Sun - The stars are moving each night - Have to go through one year for the constellations to go back to the same spot (the same view of the night sky) Clicker - What is the brightest star in the night sky? o Sirius (It is “seriously” bright) - Sirius is further away than Alpha Centauri; which star has a greater absolute power output? o Sirius Summary: Part 2 - Constellations are invented patterns used to help demarcate and find our way around the sky - The apparent brightness of stars is measured using a non-linear scale that runs backwards - The Earth’s revolution causes different constellations to be visible through the year Seasons Recap: - Objects in the sky move in circles around the celestial poles due to the Earth’s rotation on its axis - What we see in the night-sky depends on our latitude Recall: the orbit of the Earth - The Earth’s orbit is nearly-circular - By what percent does the distance from the Sun vary through the year? - (152.1-147.1)/147.1=0.034 or 3.4% - Is that enough to make a difference? Compare ideas to “real-world” - In January, the average daytime high temperature in o Waterloo (latitude 43 degrees North) is: -3 degrees C o Christchurch, New Zealand (latitude 43 degrees South) is: 23 decrees C - What might we conclude about the seasons in opposite hemispheres of the Earth? o At the same time (and, therefore, distance from the Sun), opposite hemisphere have opposite seasons - Also: since the Earth’s orbit is pretty much a circle, the distance is almost the same year round anyways - Conclusion: distance from the Sun does not cause the seasons Annual motion of the Earth - The Earth revolves once around the Sun in 365.25 days and remains tilted at 23.5 degrees - The ecliptic is the plane of the Earth’s orbit around the Sun - The amount of solar energy received in either hemisphere varies through the year due to the tilt A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 7 - The Earth’s orbit is more of like a circle rather than an oval Northern summer solstice: 6/21 - Northern hemisphere: sunlight hits the surface at a nearly perpendicular angle o Energy is more concentrated o Start of summer, first day of summer o The longest day of the year o When the sun shines on Earth at a perpendicular angle (less likely for shadows) - Southern hemisphere: sunlight hits the surface at a grazing angle o Energy is spread out o Start of winter Northern winter solstice: 12/21 - Northern hemisphere: sunlight hits the surface at a: grazing angle o Energy is spread out o Start of winter, first day of winter o South is more faced on compared with the Sun - Southern hemisphere: sunlight hits the surface at a: nearly perpendicular angle o Energy is more concentrated o Start of summer The equinoxes: 3/21 or 9/21 - Days with roughly equal amounts of daylight and darkness, everywhere on Earth o The same amount of day time hours and night time hours anywhere on Earth - Summer = shorter shadows, winter = longer shadows - Mark the beginning of Spring (vernal) or fall (autumnal) - On the equinox, the sun is crossing the celestial equator Apparent annual motion of Sun - Recall: the stars rise 4 min earlier every night  they move westward - On the celestial sphere, the Sun moves Eastward with respect to the “background stars” - We can also think of the ecliptic as the apparent yearly path of the Sun amongst the stars - The Sun spends half the year north of the celestial equator (CE) and half the year south of the CE o Vernal equinox: Sun crosses CE heading North o Summer solstice: Sun at furthest point north from CE o Autumnal equinox: Sun crosses CE heading south o Winter solstice: Sun at furthest point south from CE Length of days - The Sun rises and sets at different times through the year - In the Northern hemisphere, there are more daylight hours when the Sun is further North - More daylight hours in the summer means more energy from the Sun is received at the surface per day Summary - Observations: o Earth keeps nearly the same distance from the Sun o The seasons are opposite in opposite hemispheres - Model: the Sun is higher in the sky in Summer o More efficient warming when the sunlight is nearly perpendicular to the Earth’s surface o More daylight hours means more time for warming - The longest day of the year is the Summer solstice - As the Earth now revolves around the Sun, the Sun migrates southward in the sky and the amount of daylight decreases - On the autumnal equinox, the amount of daylight is equal to the amount of darkness - On the winter solstice, the sun is the furthest south in the sky; it is the shortest day of the year - As the Earth continues to revolve around the Sun, the Sun migrates northward in the sky and the amount of daylight increases - On the Vernal Equinox the amount of daylight is equal to the amount of brightness A S 1 0 1 M i d t e r m E x a m N o t e s E d w i n a C h e u n g | 8 Lunar Phases and Eclipses The Earth-Moon system - The moon orbits the Earth in a slightly elliptical path once in approximately one month (“moon-th”!) How does the moon shine? - The moon shines by reflected light from the Sun - Only half of the spherical moon is illuminated at any given time (just like the Earth) Describing the phases The moon rotates - This must be the case since we only ever see same side of moon o There is no permanently “dark side” of the moon, only a near-side and a far-side - The moon’s librations allow us to see 59% of the surface over the course of a lunar cycle How long is a moon-th? - How long does the moon take to orbit the Earth once? o Reference point: background stars o Time: 27.32 days  sidereal month - How long does the lunar phase cycle take? o Reference point: the Sun o Time: 29.53 days  synodic month Summary so far - The phases of the Moon are a consequence of its position along its orbit with respect to the Sun - Different parts of the near-side of the Moon become illuminated at different times in its orbit - The Earth’s shadow plays no role o Consider the shape of the line dividing day and night Earth-Moon system and the Sun - Both orbits lie in
More Less

Related notes for AS101

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.