Ages of Things
The age of the Universe 13.7 +- 0.2 Billion Years ( 13.7 Gy), source MAP spacecraft mapping the temperature fluctuations in the cosmic background microwave radiation, date reported in 2003.
Galaxies form shortly after the Universe begins from large gas clouds which collapse under gravity leading to the formation of stars. Out galaxy formed within 1 billion years of the big bang. (this comes from the ages of the globular clusters gravitationally bound to the Milky Way.)
The Sun and solar system
Stars are formed by the gravitational collapse of dense cool interstellar clouds. The star is a millionth the diameter of the gas cloud which collapses to form the star. Stars live out their lives converting hydrogen to helium, carbon, oxygen etc on up the periodic table to iron. Then, if they are massive enough, they explode as supernovas which create elements heavier than iron and spread the resulting atoms into interstellar clouds which collapse to form new stars.
The age of the sun and solar system is obtained by radioactively dating meteorites, using Rubidium-strontium dating. It is 4.6 +- 0.1 Billion Years. (4.6 Gy) this is the date of formation of the sun.
Radioactive dating dates the age of crystallization of minerals within a rock.
Meteorites contain magnesium 26 which came from radioactive aluminum 26 with a half life of 720,000 yr. Which means that they formed with a few million years of the creation of the radioactive elements by supernovae. The amount of radioactive daughter products (such as Xenon 129) found on earth indicate that it was formed within 100 My of the supernovae.
As the hot 2000 K gas of the solar system cools, first the highest vaporization temperature materials form such as nickel irons at 1400 K (found in the earth's core) and silicates (found in the earth's mantle), then cooler materials such as aluminum silicates (feldspars found in the earth's crust) At cooler temperatures carbon compounds formed, and at 500 K water began to react with silicate minerals. At 200 K water ices began to form. These particles then agglomerated to form planets.
The age of the earth from uranium-lead and Thorium lead radioactive decay is 4.5 +- 0.5 billion Years (4.5 Gy) Each of these decays produce an isotope of lead (Lead 206 for Uranium and lead 208 for Thorium) whereas lead 204 is not produced by radioactive decay. Thus the amount of lead 204,206 and 208 found on earth gives an age of formation of the earth.
The earth has a density of 5250 Kg/m^3, the moon has a density of 3340 Kg/m^3. The earth has an iron core the moon does not. The moon is made of material like that of earth's mantle. It was formed by the collision of a large Mars sized object with the earth after the formation of the earth's core. The collision ejected mantle material into orbit where it recondensed to form the moon.
HALF-LIFE of some geologically interesting Isotopes
Uranium-235 Lead-207 0.704 billion years
Uranium-238 Lead-206 4.47 Gy
Potassium-40 Argon-40 1.25 Gy
Rubidium-87 Strontium-87 48.8 Gy
Thorium-232 Lead-208 14.0 Gy
USGS Age of the Earth Website
Moons and Planets William K. Hartmann 3rd ed.
Scientific Explorations with Paul Doherty
28 July 2003