Modern Astronomy

Lecture 1

August 26, 2002

 

Initial pleasantries:  name of course, name of instructor, importance of web site.  

 

This should be the most interesting course you take in college.   Every year, new example.  Here goes for this year. 

  • One of the most interesting questions in astronomy is the existence and nature of planets around other stars. 
  • In the Fall of 1994, I described planets in other solar systems as a highly speculative matter; they probably were out there, but it would be decades or centuries until we were able to know if they were really there.
  • The situation nowadays: over 100 planets known around stars like the Sun. You can look up a list of them on the Internet   http://exoplanets.org/almanacframe.html
  • Based on their properties, scientific artists have drawn imaginary pictures of what these other solar systems look like    http://antwrp.gsfc.nasa.gov/apod/ap020614.html
  • The stars which these planets orbit are all fairly similar to the Sun, but some are believed to be much older than the Sun, while others (like ε  Eridani,) are thought to be much younger.  All of this is food for thought about the nature of life in the universe. 
  • Stay tuned, we’ll be talking about this at length. 

 

►  Now back to tedious details: the syllabus.  Look at it carefully and be sure you are familiar with all its information, especially the test days. 

 

1.  Incipitur…. Let’s get started, Overview of the Solar System

 

            This class deals with stars, the Milky Way galaxy, other galaxies, and the whole universe.  The solar system is a huge topic in its own right, and is dealt with in another course we offer. 

            Planetary systems are probably a common attribute of star systems, and we will be talking about things like life in the universe, so it is worthwhile to get a sketchy view of the solar system. 

 

A little bit of physics

 

To get a real sense of the nature of the solar system, we need two physical concepts and their units, length or distance and mass.  Length is a measure of an object’s extent in space, and we measure it in meters.  Mass is a measure of the amount of matter in an object, and is measured in kilograms. 

 

► Demo with meter stick and kilogram mass.

 

            We will frequently employ the kilometer as a handy unit that we have an intuitive feeling for.  It is roughly half a mile.  More precisely, 1 kilometer=0.6214miles (see Appendix 3 in your book). 

            The diameter of the Earth is 12756 kilometers.  That is close to the distance you fly from Los Angeles to Sydney, Australia. It takes 14 hours to fly this distance in a Boeing 747.   So the Earth is really large by our everyday standards, but not mind-bogglingly so. 

 

►  Demo:  Point suggestively at a globe

 

            The mass of the Earth is known very precisely to be 5.9742  X 1024 kilograms. 

→  Question for the august assembly: how could we possible know that?  

 

            The closest object to us in outer space is the Moon.  Its average  distance is 384,400 kilometers, or 239,000 miles, or 60 times the radius of the Earth.  If our globe model is a foot in diameter, and we had a scale model of the solar system, the Moon would be 30 feet away. 

 

            The  most prominent object in the sky, and next in line,  is the Sun.  With the Sun, the vastness of astronomy begins  to really come out.  The average distance to the Sun is 149.6 million kilometers.  In terms of our basic unit, the meter, the average distance is 1.4960 X 1011   meters.  This distance is really important in astronomy, so we have a technical term for it.  It is the astronomical unit. 

            This distance is really big, as can be grasped by the following comparisons.

  • One astronomical unit is 23456 time the radius of the Earth. 
  • The distance to the Sun is 390 times the distance to the Moon.  In our scale model, if the Moon is 30 feet away, the Sun would be 2.2 miles away. 
  • The Boeing 747 that took us across the diameter of the Earth in 14 hours would take 18 years to reach the Sun! 

 

Given its huge distance, it is not surprising that the Sun is an extravagant  object in all respect.

·        It has a radius of 695,990 kilometers (6.960 X 108 meters).  That is 109 times the radius of the Earth.  If we return to our 1 foot diameter Earth globe, to scale the Sun would be 36 yards in diameter: a sphere that would occupy a third of the football field at Kinnick.

·        The mass of the Sun is (known very precisely) 1.989 X 1030  kilograms.  This is such a big number that it is hard to relate to.  It is 333,000 times the mass of the Earth. 

·        Finally, to top things off, the temperature of the surface of the Sun is about 9950 degrees Fahrenheit (we’ll come up with a better system for describing temperature later. 

 

►      Picture of the Sun today   http://sohowww.nascom.nasa.gov  

 

            These facts tell you something very important about astronomy right from the start.  The Earth is a pretty puny little object next to the Sun.  We will see later that the other stars in the night sky are similar to the Sun.  So the Earth is an extremely minor object astronomically.  

 

The Planets

 

            The Earth moves on a roughly circular path through space around the Sun.  It takes one year to complete its path.  More on that in a couple of lectures.  It is not the only object in the solar system to do so.  There are seven other major planets, tens of thousands of minor planets (one of which barely missed the Earth a week ago) and billions of comets.  Let’s look at a diagram indicating the arrangements of planets in the solar system.