Chapter 1Our Place in the Universe Chapter Outline Our Modern View of the Universe The Scale of the Universe Spaceship Earth 1/25/2006 1:12 PM
Why is the universe comprehensible?What does comprehension of the universe have to do with Darwinian survival of the fittest? • My answer is absolutely nothing.
What types of questions the answers to which define a worldview? • Origins • Meaning • Morality • Destination
1.1 Our Modern View of the Universe A. What is our place in the universe? B. How did we come to be? C. How can we know what the universe was like in the past? D. Can we see the entire universe?
Planets A moderately large object which orbits a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition. Mars Neptune
Stars Like Our Sun Self-gravitating, self-luminous spheres of gas, composed primarily of hydrogen, that generate heat and electromagnetic radiation through thermonuclear fusion.
A Universe of Galaxies Typical large galaxy contains hundreds of billions of stars, and the universe contains hundreds of billions of galaxies. Andromeda Galaxy, 2.5 million light years
Formation of an Expanding Universe Galaxies like our Milky Way recycle matter: Stars form from clouds of gas and dust, and stars return matter to the interstellar medium when they die. Stars form from collapsing clouds of gas and dust, and planets may form in the spinning disk that surround them. In less than a billion years after the Big Bang, gravity caused matter to collapse into galaxies and clusters of galaxies as spacetime in the universe expand. Earth formed from hydrogen chemically enriched by heavier elements that result from the death of massive stars. Stars generate radiant energy by thermonuclear fusion of light elements into heavy elements and massive stars scatter these new elements back into space when they die.
Lookback Time • Refers to the amount of time since the light we see from a distant object, such as a galaxy or a cluster of galaxies, was emitted. • Example: if a galaxy is 400 million light years distant, then light has been in transit for 400 million years, or we see the object as it existed 400 million years ago and not as it exists now.
Look-Back Times in the Universe • Earth-Sun – 8 light minutes (light travel time) • Earth-Pluto – 4.5 hours • Nearest Star-Sun – 4.5 years • Galactic Center-Sun – 28,000 years • Andromeda Galaxy – 2.25 million years • Virgo Cluster of Galaxies – 64 million years • Clusters of Galaxies – 100s millions of years separation • Most Distant Galaxies – 14 billion years
1.2 The Scale of the Universe A. How big is Earth compared to our solar system? B. How far away are the stars? C. How big is the Milky Way Galaxy? D. How big is the universe? E. How do our lifetimes compare to the age of the universe?
Celestial Sphere Nearby Star Parallax Angle Earth Earth Sun Parallactic Distances • Parallax - apparent shift in position of foreground object relative to background objects due to motion of observer. • Parallactic shift can be used to determine distances for nearby stars (triangulation as is surveying). • Parallactic shifts are extremely small even for nearby stars because they are so far away. • Parallactic distances are the basis for all distance determination techniques in astronomy.
Astronomical Units of Distance • Astronomical Unit (AU) - mean distance between Earth and Sun • Light year (ly) - distance light travels in a vacuum in one year • Parsec (pc) - distance at which two objects separated by one AU subtend an angle of one second of arc
1.3 Spaceship Earth A. How is Earth moving in our solar system? B. How is our solar system moving in the Milky Way Galaxy? C. How do galaxies move within the universe? D. Are we ever sitting still?
A Rotating/Precessing Earth Earth rotates about an internal axis. Depending on the distance from the axis of rotation, the surface is moving at a rate dependent on its latitude. Earth’s axis of rotation sweeps through space describing a cone, like a spinning top.
Earth’s Orbit Because Earth’s orbit is elliptical, its distance from the Sun varies over the orbital period. Also, Earth’s axis of rotation is inclined to the orbital plane by 23.5o.
Our Galaxy’s Structure Halo Sun’s Location Galactic Disk Nucleus
Motion in Our Galaxy The Sun moves relative to nearby stars and thegroup of nearby stars orbits the center of our Galaxy.
/hr /hr /hr /hr /hr Expanding Universe as an Expanding Raisin Cake Galaxies are carried along with the expansion of spacetime in the Universe.
The Big Picture • Earth is not the center of the universe, but orbits a modest star in a modest galaxy. • Life cycle of stars is the source of the chemical evolution of the universe making possible our existence. • Various scale sizes in the universe cover an immense range from the fundamental particle scale to the large scale structure in the universe. • Evidence suggests that the age of the Solar System, 4.6 billion years, is about 1/3 the age of the universe, about 14 billion years. • To understand the universe, we must know something about the four topics: (a) motion, (b) matter and light interaction, (c) space, and (d) time.