ASTR100 (Spring 2008) Introduction to Astronomy Discovering the UniverseHow can we know what the universe was like in the past?Example:Definition: A light-yearAt great distances, we see objects as they were when the universe was much younger.Can we see the entire universe?Why can’t we see a galaxy 15 billion light-years away? (Assume the universe is 14 billion years old)Slide 8How do our lifetimes compare to the age of the universe?Slide 10Spaceship EarthHow is Earth Moving in Our Solar System?Slide 13How is Earth Moving in Our Galaxy?More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy…dark matter!How do Galaxies Move Within the Universe?PowerPoint PresentationAre we ever sitting still? No!Patterns in the Night SkyWhat are constellations?Thought QuestionSlide 22The Celestial SphereSlide 24The Milky WaySlide 26How do we locate objects in the sky?Slide 28Angular MeasurementsSlide 30Slide 31Why do stars rise and set?What moves? The Earth or the sky?Celestial SphereSlide 35Our view from EarthEnded Here 1/31/08ASTR100 (Spring 2008) Introduction to AstronomyDiscovering the UniverseProf. D.C. RichardsonSections 0101-0106How can we know what the universe was like in the past? Thus, we see objects as they were in the past:The farther away we look in distance, the further back we look in time.Destination Light travel timeMoon 1 secondSun 8 minutesSirius 8 yearsAndromeda Galaxy 2.5 million years Light travels at a finite speed (c = 300,000 km/s).Example:This photo shows the Andromeda Galaxy as it looked about 2½ million years ago. Question: When will be able to see what it looks like now?M31, The Great Galaxy in AndromedaDefinition: A light-yearThe distance light can travel in one year.About 10 trillion km (6 trillion miles).Distance = Speed x Time= (300,000 km/s) x (1 yr) x (31,557,600 s/yr)= 9,500,000,000,000 km!= 9.5 x 1012 km• At great distances, we see objects as they were when the universe was much younger.• Can we see the entire universe?A. Because no galaxies exist at such a great distance.B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.C. Because looking 15 billion light-years away means looking to a time before the universe existed.Why can’t we see a galaxy 15 billion light-years away?(Assume the universe is 14 billion years old)A. Because no galaxies exist at such a great distance.B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see.C. Because looking 15 billion light-years away means looking to a time before the universe existed.Why can’t we see a galaxy 15 billion light-years away?(Assume the universe is 14 billion years old)How do our lifetimes compare to the age of the universe?The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year.New Year’s Day: The Big BangMilky Way formsSun & planets formOldest known life (single-celled)First multi-cellular organismsHow do our lifetimes compare to the age of the universe?The Cosmic Calendar: a scale on which we compress the history of the universe into 1 year.Spaceship EarthHow is Earth Moving inOur Solar System?The Earth rotates around its axis once every day.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.How is Earth Moving inOur Solar System? The Earth orbits the Sun (revolves) once every year.How is Earth Moving inOur Galaxy? The Sun moves randomly relative to other nearby stars, and orbits the galaxy once every 230 million years.More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy…dark matter!Most of Milky Way’s light comes from disk and bulge ……. but most of the mass is in its haloHow do Galaxies Move Within the Universe?Galaxies are carried along with the expansion of the universe.Part CThe following statements describe ways in which the analogy might apply to the real universe. Which statements are correct? A. Both the raisin cake and the universe have a well-defined inside and outside. B. Raisin 1 is near the center of the cake, just as our galaxy is near the center of the universe. C. The temperature starts low and ends high in both the raisin cake and the universe. D. The raisins stay roughly the same size as the cake expands, just as galaxies stay roughly the same size as the universe expands. E. The average distance increases with time both between raisins in the cake and between galaxies in the universe. F¡. An observer at any raisin sees more distant raisins moving away faster, just as an observer in any galaxy sees more distant galaxies moving away faster.Enter the letters of all correct statements in alphabetical order (without spaces). For example, if statements C and E are correct, enter CE.DEFCorrectLike any scientific model, the raisin cake analogy has limitations, but it gives us a good overall picture of how the universe is expanding.Are we ever sitting still? No!Earth rotates on axis: > 1,000 km/hrEarth orbits Sun: > 100,000 km/hrSolar system moves among stars: ~ 70,000 km/hrMilky Way rotates: ~ 800,000 km/hrMilky Way moves in Local GroupUniverseexpandsPatterns in the Night SkyWhat are constellations?A constellation is a region of the sky. 88 constellations fill the entire sky (North & South).Thought QuestionThe brightest stars in a constellation…•all belong to the same star cluster.•all lie at about the same distance from Earth.•may actually be quite far away from each other.Thought QuestionThe brightest stars in a constellation…A. all belong to the same star cluster.B. all lie at about the same distance from Earth.C. may actually be quite far away from each other.The Celestial SphereStars at different distances all appear to lie on the celestial sphere.The ecliptic is the Sun’s apparent path through the celestial sphere.The Celestial SphereA band of light making a circle around the celestial sphere.What is it?Our view into the plane of our galaxy. The Milky WayThe Milky WayHow do we locate objects in the sky?An object’s altitude (above horizon) and direction (along horizon) specify its location in your local sky.We measure the sky in angles...blankAngular Measurements• Full circle = 360º• 1º = 60 (arcminutes) • 1 = 60 (arcseconds)Thought QuestionThe angular size of your finger at arm’s length is about 1. How many arcseconds is this?•60