Quarks and the Universe 

Until the early part of the nineteenth century it was generally thought that what we now call the atom was the smallest constituent of matter. The word "atom" comes from the Greek for "uncuttable". The work of Wilhelm Roentgen, Marie Curie, Joseph Thomson, Ernest Rutherford, Niels Bohr and others led to the discovery of even smaller particles: the electrons, protons, and neutrons.

Recent work carried out at the sites of the largest particle accelerators has confirmed that these three atomic particles are themselves composed of combinations of even smaller constituents which we call "quarks." (The name quark comes from the novel Finnegan's Wake, by James Joyce.)

Quarks
There are six "flavors" of quarks that combine to make sub-atomic particles. They are named: up, down, charm, strange, bottom and top. If you think the names show that physicists have a sense of humor, you should also know that the unit of nuclear neutron-cross-section is the barn, because hitting tiny nuclei with neutrons is as easy as hitting the broad side of a barn. Quarks are studied in high energy physics.
Subatomic particles
Classified as baryons, mesons, leptons, and bosons, only the proton and electron have both mass and stability. Neutrons are stable only while inside nuclei of atoms of elements up to and including those of lead. The study of these is particle physics.
Atoms
Atoms have nuclei containing protons and neutrons, and have electrons in the surrounding space. The nucleus is only 1/100,000th the diameter of the atom (like comparing the size of a baseball to that of a ball park) and yet nearly all the mass of the atom is in that tiny nucleus (the baseball is as heavy as all of the major and minor league ball parks, combined!) Atoms of each element (silver, oxygen, iron, mercury, etc) are essentially identical. There are just over one-hundred elements (numbers of protons per nucleus). Each element may also have several isotopes (different numbers of neutrons), but generally only a few will be stable (not radioactive). Heavy atoms tend to be radioactive. A billion atoms in a row would make a line only a few centimeters long. This study is nuclear physics and quantum physics.
Molecules
When atoms bond together by sharing their electrons they form molecules. Water molecules each contain two hydrogen atoms and one oxygen atom. Some proteins contain thousands of atoms per molecule, largely carbon, oxygen, hydrogen, and nitrogen. The properties of molecules (color, melting temperature, hardness, solubility, etc) are due not only to what atoms they contain, but also to how they are arranged. This study is called chemistry.
Substances
Substances are classified in several ways. They may be solids, liquids, or gases (physical states). They may be metals or non-metals (metals usually conduct electricity). They may be pure or impure (pure substances generally have sharp melting temperatures). They may be crystalline or non-crystalline. They may be stable or unstable. Instability may be chemical or nuclear or both. All substances are composed of molecules. The study of materials is covered in geology.
Living things
It was once thought that living things were made out of organic "stuff," and non-living things were made out of inorganic "stuff." The understanding that all things are made from the same atoms began with the synthesis of urea by Friedrich Wohler in 1828. The study of living things is biology.
Planets
The planets are generally solidified composite bodies made of heavy elements. Only the largest planets have enough gravity to hold on to the light gases hydrogen and helium. The earth has a mass of 5 983 000 000 000 000 000 000 000 kilograms, which gives it a gravitational acceleration at the surface of almost 10 meters/second/second. This study is covered in orbital mechanics.
Solar systems
Every star has the opportunity to collect the gases, dust, and particles that enter its gravitational field. The particles that fall into it cause heat and will eventually start a thermonuclear reaction that will give off both heat and light. Some objects may miss the star and continue to orbit it, gathering in meteorites and dust as well. These may become asteroids, comets, planets, or planetary moons. There are probably planets orbiting many of the stars in the universe. The distances make it almost impossible to search for dark objects. We have not even counted all of the asteroids between mars and jupiter, and we still are not certain that neptune is the last planet in our own solar system. Making the study of astrophysics even more exciting is the known existence of white dwarfs, black holes, and quasars.
Galaxies, or nebulae
There are about 50 billion enormous groups of stars and solar systems called galaxies. There may easily be a billion stars in our own galaxy, the milky way. The number and variety of galaxies seems to be endless, and yet there is evidence that the size of this collection of stuff which we call our universe is limited. The radius of the spherical space occupied by this material in the universe is probably about 13 billion light years (a light year is about six trillion miles). This is the subject of astronomy.
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Updated July 15, 2009
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