Transcript lecture2
Announcements Class’s web site no live! www.astro.umass.edu/~mauro/astro100 Syllabus is on line, check it! Lectures #1 and 2 are on line Scales of the Universe Course Web Site: www.astro.umass.edu/~mauro/astro100 Today’s Assigned Reading Chapter 1, all Chapter 2, all Appendix A Now… on to scales in the Cosmos… Let’s explore the universe in steps of increasing magnification, from small to big At each step, let’s change scale by TWO orders of magnitude at a time and see what happens. 52 feet across 1 mile across = 6000 feet =1.6 km 160 kilometers across 1 mile = 1.61 kilometers Infrared photo 12,756 km 1,600,000 km =1.6 x 106 km Moon 1.6x108 km =1.6x1011m 1AU = 1.5x1011 m AU is a distance unit 1.6 x 1010 km, or a trillion (1012) times wider than the first picture! 32 AU 100 AU 10,000 AU Stars are so far apart from each other! If the Sun is a golf ball in NYC, the nearest star is a golf ball in Chicago!!! 106 AU = 17 ly 1 ly =63,000 AU = 1013 km 1700 ly 170,000 ly 1.7 x 107 ly The local group of galaxies 1.7 x 109 ly Announcements Class’s web site is live! www.astro.umass.edu/~mauro/astro100 Syllabus is on line, check it! Lectures #1 and 2 are on line Basic classes of celestial objects: Comets and Asteriods Planets Earth as one Stars Sun as one Galaxies Milky Way as one Their sizes Comets and Asteroids: 10-100 km Earth: 104 km Sun: 106 km Milky Way 1018 km dust poppy seed grape fruit Earth-Sun distance A Sense of Space 1. The Sun would hold 1.3 million Earths. i.e. the radius of the Sun is about 100 times that of the Earth. 2. There are ~100 billion "Suns" in a galaxy like our own Milky Way Galaxy. 3.Astronomers can see billions of galaxies. Your local address: What is our address in the Universe? •UMass, Amherst, Massachusetts, USA •Earth, •Solar System •Milky Way Galaxy, •Local Group, •Local Supercluster •Our Universe •Are there other universes? Most likely yes! •Multi-verses as opposed to Uni-verse So…How Big is the Observable Universe anyway? …about 14 billion-billion-billion centimeters in diameter or 14,000,000,000,000,000,000,000,000,000 cm or 1.4x1028 cm or 14 billion ly or 6000 Mpc The finite speed of light lets us “look” into the past! Supernova Earth 100,000 l-years now 97,997 BC Consider this: If you had friends throughout the galaxy, how could you create a “live” TV show? If the Sun were a grapefruit on one side of the room, what would be Earth? 1) A poppy seed on the other side. 2) Another grapefruit on the other side. 3) A poppy seed in my office. 4) An apple on the other side. If the Sun were a grapefruit on one side of the room, what would be Earth? 1) A poppy seed on the other side. (Earth-Sun distance~100x radius of Sun~ 100 radius of Earth) 2) Another grapefruit on the other side. 3) A poppy seed in my office. 4) An apple on the other side. What would then be the nearest star? 1) A poppy seed at the Amherst downtown. 2) A poppy seed on the west coast 3) A grapefruit at the Amherst downtown. 4) A grapefruit on the west coast. What would then be the nearest star? 1) A poppy seed at the Amherst downtown. 2) A poppy seed on the west coast 3) A grapefruit at the Amherst downtown. 4) A grapefruit on the west coast. (Nearest star is 4Ly ~ 300,000 AU) A Sense of Time If we were to compress the time since the Big Bang into one year, and make the time of the Big Bang January 1, The Earth was formed in mid-September. The mammals appeared on December 26. All human prehistory (from the first known stone tools) and history have occurred in the last ½ hour of New Year's Eve. All of human history is but a fleeting instant on the cosmic timescale. So MANY objects….How Do We Make Sense of it ALL?? Our investigation of the universe has become quite sophisticated… Here is the NASA/ESA Hubble Space Telescope (HST) The Sky and its Cycles Goals: To describe and locate objects in the sky To understand the apparent motions of celestial objects To introduce the seasons How Do We Locate Objects on the Sky? The Celestial Sphere (C.S.) Stars, planets, the Sun , external galaxies, are at different distances from us, yet for studying patterns in the sky, we represent them in projection onto the C.S. Angular Size The Sun is physically 400 times larger than the Moon. Why is their angular size is the same? S a arad = S/D Angle diameter (in radians) = diameter/distance Full circle = 2 (in radians)=360 degrees=360o 1o=60 arminutes = 60’ 1’ = 60 arcseconds = 60” ; 1 rad = 206,264.81” Angular Size Some Examples: Horizon to zenith (point overhead) 90o Your fist at arm's length 10o Your fingernail at arm's length 1o = 60' Sun or Moon seen from Earth 0.5o = 30' Smallest detail visible to naked eye 1' = 60'' Smallest detail visible by a single telescope from Earth's surface: 1'' Angular size of Pluto: 0.15” Betelgeuse (largest star) seen from Earth 0.004'' The Orion Constellation Constellations 88 official constellations divide the sky into areas with clearly boundaries. The names of constellations are in Latin. But most bright star names derived from ancient Arabic. The original constellations were invented by farmers over 5000 years ago. Remember that the constellations are not real! They are just imaginative figures “invented by ancient cultures Projection Big Dipper ---- an asterism Humans have logarithmic senses Our senses are logarithmic, i.e. respond logarithmically to stimuli. If a stimulus I is exerted on our senses (vision, hearing, touch, all work the similarly)… … our body DOES NOT sense I, but sense Log(I) instead For example, if one doubles I (e.g. turn the volume of the sound from 3 to 6), we only perceive an increment of sound of Log(2)=0.3, i.e. 30% louder Same thing for vision Luminosity and Magnitude Log(I) is what we sense we our eyes mV = -2.5*Log(I) + C I is the true intensity of the light m1 – m2 = -2.5*Log(I1/I2) If I1=2xI2, then m1 – m2 = -2.5*Log2 = -0.75 If I1=10xI2, then m1 – m2 = -2.5*Log10 = -2.5 If I1=100xI2, then m1 – m2 = -2.5*Log100 = -5 I1/I2 = 10-0.4*(m1 – m2) = 2.512(m1 – m2) The Motions of Earth 1. The Earth rotates – this leads to day and night. 2. The Earth revolves around (orbits) the Sun – this leads to the seasons. 3. The Earth and Sun revolve around (orbit) the center of the Galaxy. 4. The Galaxy moves through the Universe. Latitude and longitude make a convenient coordinate system for locating objects on the Earth. Altitude and Azimuth give us the direction to look on the celestial sphere. Time Zones and Universal Time Time We live on a rotating sphere (Earth) that receives lights only from one source (the Sun). If here is day, on the other side of Earth it is night. So, how do we synch the time across the planet? The time for many astronomical events is given in Universal Time (UT), which is (approximately) the local time for Greenwich, England --- the Greenwich Mean Time or GMT. Cycles of the Sky Earth's rotation causes the sun and stars to rise in the east and set in the west daily. But stars and the Sun do not set and rise daily everywhere! Near the poles, the sun rises and sets every half year, and circumpolar constellations such as Ursa Major, never set. What other cycles do you know? There are two ways to think about the motion of the skey: - the C.S. rotates - or the Earth rotates while the sky sits still. Sometimes one works better than the other … The altitude of Polaris above the horizon is approximately the same as the observer's latitude in the Northern Hemisphere. Are there stars in the sky during the daytime? How long to go all the way around? 1 day b. 1 week c. 1 month d. 1 year Earth also orbits around the Sun. Takes 1 year for one round. Note the tilt between the ecliptic and Earth’s rotation axis Ecliptic The constellations along the ecliptic are called the zodiac Everything in the Universe is Moving! … but … the further away from us something is, the more it has to move to be noticeable. the motion of the Earth rotating on itself (day/night) is the easiest to see Followed by the motion of the Moon around the Earth (~29 days) followed by the motion of the Earth around the Sun and finally the motion (over tens or hundreds of years) of close stars What causes the seasons? What are the differences between days in summer and winter? Longer days stronger Sun in summer than in winter Seasons are due to the tilt of the earth. Are the seasons due to the changing distance from the sun? NO! (Not to scale) The Earth-Sun distance does NOT matter with the seasons!