Objectives:Objectives:

Describe the importance/value/benefit of Astronomy

Clearly describe our place in the cosmos and the universe at various scales

Overview of AstronomyOverview of Astronomy

1/ What is the value/benefit of Astronomy?1/ What is the value/benefit of Astronomy?

Scientific and TechnologicalPhysics Laws; Satellites and Rockets;

Materials, etc.

PracticalGeography; Time-keeping; Religious

Injunctions; etc.

PhilosophicalOur place in the Cosmos; Creation; etc.

Astronomical distances and sizes are so Astronomical distances and sizes are so large that astronomers use special unitslarge that astronomers use special units

Astronomical Unit (AU) – applicable inside the solar system

= average distance between Sun and Earth

= 150,000,000 km = 1.5 x 108 km

Light-Year (ly) – applicable between stars and galaxies

= distance light travels in one year

= ~ 9 x 1012 km

The Solar System:The Solar System: The Sun: Our nearby star, at 1 AU = 150

million km away, or just 8 minutes of light travel time, and 100 times the diameter of the Earth; the energy we use on Earth initially comes from nuclear reactions in its core.

• Four Small, Rocky Terrestrial Planets+ asteroids (belt)

• Four Large, Gaseous Jovian Planets + “Dwarf Planets”: Pluto + Eris + Sedna…

Scale of the Universe and Scale of the Universe and Overview of ContentsOverview of Contents

Outskirts of the Solar System

• Comets: Icy objects, in elliptical orbits. They can enter the inner Solar System and sometimes hit planets.

Stars and Other Things in the GalaxyStars and Other Things in the GalaxyStars: From dwarfs to supergiants; the nearest one is

Alpha Centauri, a triple star system at 4.34 ly = 40,000 billion km (~ 300,000 AU), visible from the Southern

Hemisphere (only).

Between stars: Nebulae (clouds) and interstellar matter.

Do those stars have planets? So far, we've found more than 400, and we've just begun (since 1997).

Our “country”: The Milky Way

100,000 light years across, contains ~ 200 billion stars.

A useful analogy: Universe and Earth Geography• City and Neighborhood• Country• Continents…

Our “city”: The Solar System

Downtown (Sun) and Neighborhoods (Planets).

Other Galaxies and Other Galaxies and the Rest of the Universethe Rest of the Universe

Other Galaxies: Spiral, elliptical, or irregular. How many are there? About 100 billion in the visible

universe, arranged in clusters and superclusters and moving away from us.

Our neighbor countries: The Local Group (of ~ 20 galaxies), ~ 3 million light-years across.

The Andromeda Galaxy is our nearest neighbor, about 2 million light-years away.

The Universe is also expanding, everywhere, in all directions, since the Big Bang, and the most recent observations seem to imply that it will probably expand forever…

(More on this later.)

• The Universe: A size of ~ 1023 km!

A “size” of the Universe can be determined from its age and from knowing how fast light travels from one region to another (speed of light = 300000 km/s). The most recent age of Universe (determined in January 2003) is 13.7 billion years...

Graphical Summary of the scale levels of the Universe, from the Earth, to the Solar System, to the Galaxy, to the Universe as a whole!

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