University of California, San Diego
Physics 7 - Introduction to Astronomy

• The Atom -- the basic unit of every chemical element is the atom, consisting of the nucleus, composed of protons and neutrons held together by the Strong Force, with electrons "orbiting" the nucleus, attracted by the Electrical Force.

  Atomic Particles
  Particle	Symbol	Charge	Mass (g )	Mass (amu )	Family
  proton	p+	+1	1.673 x 10-24	1.00727	baryon
  neutron	n0	0	1.675 x 10-24	1.00866	baryon
  electron	e-	-1	9.109 x 10-28	5.485 x 10-4	lepton

  • Atomic Nuclues -- ~10^-13cm.
  • Size of Atom -- the radius of the first electron "orbit" in the Hydrogen atom is 1/2Å = 0.5 x 10^-8cm.
    
  
  The ratio of the size of the atom to the size of the nucleus is about 100,000; if the atomic nucleus were a pingpong ball (r ~ 1cm) its electrons would be in orbits about a kilometer away.

• Molecules - Atoms build themselves into molecules (e.g. H₂O) with sizes ranging from a few Å to the largest molecules, such as DNA,at up to 10 cm.

• Cells - 10^-2 cm.

• Humans - The largest humans who don't play basketball are about 2 meters in size; Professor Smith is 186 cm in height.

• The Earth - The earth is about 25,000 miles around, implying a radius of about 4000 miles. More precisely, the equatorial radius, R_E = 6378 km = 6.378 x 10⁸ cm. In other words it would take about 7 million people to span the distance from San Diego to China through the center of the Earth.

• The Sun - the Sun, our nearest star is 93 million miles or about 150 million km = 1.5 x 10¹³cm away. It has a size, R_S = 7 x 10¹⁰cm.

  If the sun were a large grapefruit (r ~ 7cm) the earth would be a pinhead 15 meters away.

• The Stars - the nearest star, Alpha Centauri, a triple star system visible from the Southern Hemisphere in the constellation Centaurus, is 4.34 light years = 4 x 10¹⁸cm away.

  A light year is the distance light travels in a year = 3 x 10¹⁰ cm/second x 60 seconds/minute X 60 minutes/hr x 24 hrs/day x 365 days/yr ~ 10¹⁸ cm. If the sun were a grapefruit in San Diego, the next grapefruit would be 2500 miles away.

• The Galaxy - The Sun and Alpha Centauri are among about half a trillion stars in the Milky Way Galaxy, which is about 100,000 light years = 10²³cm across.

• Other Galaxies - The Andromeda Galaxy, our nearest sizeable neighbor is about 2 million light years = 2 x 10²⁴cm away.

• The Universe - 10²⁷cm: there may not be a "size" to the Universe; it may be infinite, but a crude estimate of the scale of the Universe can be made from the fact that the Universe began with the Big Bang some 10 - 20 billion years ago. Thus the most distant objects that we can see are about 10 billion light years = 10²⁷cm away.

From the size of the atomic nucleus, important for the fusion energy of the stars, at 10^-13cm, to the scale of the Universe as a whole ~ 10²⁷cm, the distance scales of interest in Astronomy span a factor of about 10⁴⁰ -- 40 orders of magnitude!