Measures of Astronomical Distances Let s view the Universe in terms of kilometers The size of the observable universe is roughly 130,000,000,000,000,000,000,000 km! - Obviously measuring astronomical distances in km is inconvenient. - We need much BIGGER units of distance: Some commonly used distance units: 1 ly (light year) = Distance traveled by light in 1 year = 9.46 trillion km 1 au (astronomical unit) = average distance between the sun & earth = 150 million km Reading: Chapter 1 Measures of Astronomical Distances The light year (ly) 1 ly is the distance light travels in 1 year Speed of light = 300,000 km/s In 1 second light travels 300,000 km In 1 minute light travels 300,000 km x 60 = 18 million km In 1 hour light travels 18 million km x 60 = 1.08 billion km In 1 day light travels 1.08 billion km x 24 = 25.92 billion km In 1 year light travels 25.92 billion km x 365 = 9.46 trillion km 1 ly = 9.46 trillion km = 9,460,000,000,000 km 1
Measures of Astronomical Distances The astronomical unit (au) The earth s orbit around the sun is elliptical. Hence the distance between the earth and sun varies at different points in its orbit. Minimum distance between earth & sun (Perihelion) = 148,000,000 km Maximum distance between earth & sun (Aphelion) = 152,000,000 km 1 au = Average distance between earth & sun = (Min. distance + Max. distance) /2 = (148000000km+152000000km)/2 = 150,000,000 km = 150 million km Measures of Astronomical Distances The size of the observable universe is roughly 130,000,000,000,000,000,000,000 km! What is the size of the observable universe in astronomical units? 150,000,000 km = 1 au 130,000,000,000,000,000,000,000 km = (1au/150,000,000 km) x 130,000,000,000,000,000,000,000 km = 8,666,666,666,667 au = roughly 8.67 trillion au 2
Measures of Astronomical Distances The size of the observable universe is roughly 130,000,000,000,000,000,000,000 km! What is the size of the observable universe in light years? 9,460,000,000,000 km = 1 ly 130,000,000,000,000,000,000,000 km = (1ly/ 9460,000,000,000km) x 130,000,000,000,000,000,000,000 km = 13,742,071,882 ly = roughly 14 billion ly Scientific Notation Scientists use a convenient shorthand for expressing very large or very small numbers: Every number can be expressed as a multiple of powers of 10: Example: 4812 = 4.812 x 1000 = 4.812 x 10 3 Example: 0.0000312 = 3.12 x 1/100000 = 3.12 x 10-5 Rules for expressing any number in scientific notation: 1. Place the decimal after the first nonzero digit 2. Count the number of places that the decimal point moved. This gives the power of 10. 3. If the decimal moves to the left, the power is positive. If the decimal moves to the right the power is negative. 3
How old is the Universe? Let s build a scale model: The Cosmic Calendar Say we speed up our clocks by a factor of 13.8 billion. - Then the entire age of the Universe would be one calendar year. - one month would be approximately 1 billion real years What date on the cosmic calendar would these events occur? Big Bang : 13.8 billion years ago Milky Way Galaxy disk forms: 8.8 billion years ago Sun and planets form: 4.6 billion years ago First life forms: 3.8 billion years ago First dinosaurs: 300 million years ago First mammals: 200 million years ago Extinction of dinosaurs: 65 million years ago First apes: 15 million years ago How old is the Universe? Let s build a scale model The Cosmic Calendar Say we speed up our clocks by a factor of 13.8 billion. - Then the entire age of the Universe would be one calendar year. - one month would be approximately 1.15 billion real years http://school.discoveryeducation.com/schooladventures/universe/itsawesome/cosmiccalendar/ 4
How old is the Universe? On the cosmic calendar, Earth was formed in mid September. How old is the earth? On the Cosmic Calender the Earth is 3.5 months old. 1 yr on the cosmic calendar = 13.8 billion actual years Thus 3.5 months on the cosmic calendar = (1.38 x 10 10 yrs / 1 year) x (3.5 mths x 1 yr/12mths) = 4.02 x 10 9 actual yrs The earth is roughly 4 billion years old Fundamental Questions of Modern When did we come to be? if the entire age of the Universe were one calendar year Human civilization occurred in the last minute of December 31st. We are newcomers to the universe! The Egyptian pyramids were built about 10 seconds ago Copernicus and others convinced humanity that the Earth orbits the Sun about one second ago Elizabeth II became Queen of England about 0.14 seconds ago You were born about 0.04 seconds ago (assuming your age is 18) Canada won the Olympic Gold Medal in Women s and Men s hockey in 2002 about 0.023 seconds ago; You started reading this about 2 millionths of a second ago; And it is now exactly midnight on December 31 - Happy New Year!!! 5
Looking back in time Light, although fast, travels at a finite speed. 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 Thus 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! Looking back in time Ultra Deep Field image of about 10,000 galaxies taken by the Hubble Telescope Lookback time: 13 billion years! 6
Fundamental Questions of Modern 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 Fundamental Questions of Modern How did we come to be? - 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! http://chandra.harvard.edu/photo/2007/g292/ 7
Fundamental Questions of Modern What is our motion in the Universe? The 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. http://www.physicalgeography.net/ Fundamental Questions of Modern What is our motion in the Universe? If you are located at the equator, you will travel 40,070 km in 24 hours at a speed of 1670 km/hr! http://www.physicalgeography.net/ 8
Fundamental Questions of Modern What is our motion in the Universe? The Earth orbits around the Sun once every year! The Earth s axis is tilted by 23.5º relative to the plane of its orbit Fundamental Questions of Modern What is our motion in the Universe? Our Sun and the stars of the local Solar neighborhood orbit around the center of the Milky Way Galaxy every 230 million years! 9
The Expansion of the Universe Galaxies outside our Local Group appear to be moving away from us. The farther away they are, the faster they are moving. This means that the universe is expanding Summary What makes up the Universe? The universe is made up of various astronomical objects, gasses, dust and energy within and in between galaxies. Where are we in the Universe? Earth is a planet in a solar system, which is one of some 100 billion star systems in the Milky Way Galaxy, which is one of about 40 galaxies in the Local Group, which is part of the Local Supercluster, which is part of the Universe. What is the scale of the Universe? The observable Universe is about 14 billion ly in size and hence 14 billion years old. It is vastly bigger and older than human scales. 10
Summary When did we come to be? On a cosmic calendar that compresses the history of the Universe into one year, human civilization is just a few seconds old. How did we come to be? The Universe began in the Big Bang, which produced Hydrogen & Helium. Gravity caused formation of stars and galaxies. The more complex elements of life were produced by stars. What is our motion in the Universe? Earth rotates on its axis once each day and orbits around the Sun once each year. The Solar System orbits the center of the Milky Way Galaxy every 230 million years. Galaxies in the Local Group move relative to each other. Other galaxies are moving away from us with expansion of the Universe. 11