constituents and interactions. The most commonly known application of nuclear
physics is nuclear power generation, but the research has led to applications in many
fields, including nuclear medicine and magnetic resonance imaging, nuclear
weapons, ion implantation in materials engineering, and radiocarbon dating
ingeology and archaeology.
The field of particle physics evolved out of nuclear physics and is typically
taught in close association with nuclear physics.
The history of nuclear physics as a discipline distinct from atomic physics
starts with the discovery of radioactivity by Henri Becquerel in 1896,
a year later was an indication that the atom had internal structure. At
"plum pudding" model in which the atom was a positively charged ball with smaller
negatively charged electrons embedded inside it.
In the years that followed, radioactivity was extensively investigated, notably
by the husband and wife team of Pierre Curie and Marie Curie and by Ernest
Rutherford and his collaborators. By the turn of the century physicists had also
discovered three types of radiation emanating from atoms, which they named alpha,
beta, and gamma radiation. Experiments by Otto Hahn in 1911 and by James
Chadwick in 1914 discovered that the beta decay spectrum was continuous rather
than discrete. That is, electrons were ejected from the atom with a continuous range
of energies, rather than the discrete amounts of energy that were observed in gamma
and alpha decays. This was a problem for nuclear physics at the time, because it
seemed to indicate that energy was not conserved in these decays.
The 1903 Nobel Prize in Physics was awarded jointly to Becquerel for his
discovery and to Pierre Curie and Marie Curie for their subsequent research into
radioactivity. Rutherford was awarded the Nobel Prize in Chemistry in 1908 for his
"investigations into the disintegration of the elements and the chemistry of
In 1905 Albert Einstein formulated the idea of mass–energy equivalence.
While the work on radioactivity by Becquerel and Marie Curie predates this, an
explanation of the source of the energy of radioactivity would have to wait for the
discovery that the nucleus itself was composed of smaller constituents, the nucleons.
κόσμος, kosmos "world" and -λογία, -logia "study
of"), is the study of the origin, evolution, and eventual fate of the universe. Physical
cosmology is the scholarly and scientific study of the origin, evolution, large-scale
structures and dynamics, and ultimate fate of the universe, as well as the scientific
laws that govern these realities.
The term cosmology was first used in English in 1656 in Thomas Blount's
Wolff, in Cosmologia Generalis.
Religious or mythological cosmology is a body of beliefs based on
Physical cosmology is studied by scientists, such as astronomers and
physicists, as well as philosophers, such asmetaphysicians, philosophers of physics,
and philosophers of space and time. Because of this shared scope with
philosophy,theories in physical cosmology may include both scientific and non-
scientific propositions, and may depend upon assumptions that can not be tested.
Cosmology differs from astronomy in that the former is concerned with the Universe
as a whole while the latter deals with individual celestial objects. Modern physical
cosmology is dominated by the Big Bang theory, which attempts to bring together
observational astronomy and particle physics;
parameterization of the Big Bang with dark matter and dark energy, known as the
Theoretical astrophysicist David N. Spergel has described cosmology as a
"historical science" because "when we look out in space, we look back in time" due
to the finite nature of the speed of light.
Physics and astrophysics have played a central role in shaping the
cosmology was shaped through both mathematics and observation in an analysis of
the whole universe. The universe is generally understood to have begun with the Big
Bang, followed almost instantaneously by cosmic inflation; an expansion of space
from which the universe is thought to have emerged 13.799 ± 0.021 billion years
Cosmogony studies the origin of the Universe, and cosmography maps the
features of the Universe.
In Diderot's Encyclopédie, cosmology is broken down into uranology (the
science of the heavens), aerology (the science of the air), geology (the science of the
continents), and hydrology (the science of waters).
Metaphysical cosmology has also been described as the placing of man in the
observation that a man's place in that relationship: "He who does not know what the
world is does not know where he is, and he who does not know for what purpose the
world exists, does not know who he is, nor what the world is."
and chemistry "to ascertain the nature of theheavenly bodies, rather than their
positions or motions in space."
galaxies,extrasolar planets, the interstellar medium and the cosmic microwave
spectrum, and the properties examined include luminosity, density, temperature, and
chemical composition. Because astrophysics is a very broad subject, astrophysicists
typically apply many disciplines of physics, including mechanics,electromagnetism,
statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and
particle physics, and atomic and molecular physics.
In practice, modern astronomical research often involves a substantial amount
of work in the realms of theoretical and observational physics. Some areas of study
for astrophysicists include their attempts to determine: the properties of dark matter,
dark energy, andblack holes; whether or not time travel is possible, wormholes can
form, or the multiverse exists; and the origin and ultimate fate of the universe.
Topics also studied by theoretical astrophysicists include: Solar System formation
and evolution; stellar dynamics andevolution; galaxy formation and evolution;
magnetohydrodynamics; large-scale structure of matter in the universe; origin of
cosmic rays; general relativity and physical cosmology, including string cosmology
and astroparticle physics.
Astrophysics can be studied at the bachelors, masters, and Ph.D. levels in
physics or astronomy departments at many universities.
Although astronomy is as ancient as recorded history itself, it was long
separated from the study of terrestrial physics. In the Aristotelianworldview, bodies
in the sky appeared to be unchanging spheres whose only motion was uniform motion
in a circle, while the earthly world was the realm which underwent growth and decay
and in which natural motion was in a straight line and ended when the moving object
reached its goal. Consequently, it was held that the celestial region was made of a
fundamentally different kind of matter from that found in the terrestrial sphere; either
Fire as maintained by Plato, or Aether as maintained by Aristotle.
century, natural philosophers such as Galileo,
material and were subject to the same natural laws.
Their challenge was that the
For much of the nineteenth century, astronomical research was focused on the
routine work of measuring the positions and computing the motions of astronomical
A new astronomy, soon to be called astrophysics, began to emerge
when William Hyde Wollaston andJoseph von Fraunhofer independently discovered
that, when decomposing the light from the Sun, a multitude of dark lines (regions
where there was less or no light) were observed in the spectrum.
physicist, Gustav Kirchhoff, and the chemist, Robert Bunsen, had demonstrated that
the dark lines in the solar spectrum corresponded to bright lines in the spectra of
known gases, specific lines corresponding to unique chemical elements.
elements in the Solar atmosphere.
In this way it was proved that the chemical
Among those who extended the study of solar and stellar spectra was Norman
Lockyer, who in 1868 detected bright, as well as dark, lines in solar spectra. Working
with the chemist, Edward Frankland, to investigate the spectra of elements at various
temperatures and pressures, he could not associate a yellow line in the solar spectrum
with any known elements. He thus claimed the line represented a new element, which
was called helium, after the GreekHelios, the Sun personified.
In 1885, Edward C. Pickering undertook an ambitious program of stellar
spectral classification at Harvard College Observatory, in which a team of woman
computers, notablyWilliamina Fleming, Antonia Maury, and Annie Jump Cannon,
classified the spectra recorded on photographic plates. By 1890, a catalog of over
10,000 stars had been prepared that grouped them into thirteen spectral types.
Following Pickering's vision, by 1924 Cannon expanded the catalog to nine volumes
and over a quarter of a million stars, developing the Harvard Classification Scheme
which was accepted for world-wide use in 1922.
In 1895, George Ellery Hale and James E. Keeler, along with a group of ten
associate editors from Europe and the United States,
Journal: An International Review of Spectroscopy and Astronomical Physics.
was intended that the journal would fill the gap between journals in astronomy and
physics, providing a venue for publication of articles on astronomical applications of
the spectroscope; on laboratory research closely allied to astronomical physics,
including wavelength determinations of metallic and gaseous spectra and experiments
on radiation and absorption; on theories of the Sun, Moon, planets, comets, meteors,
and nebulae; and on instrumentation for telescopes and laboratories.
In 1925 Cecilia Helena Payne (later Cecilia Payne-Gaposchkin) wrote an
influential doctoral dissertation at Radcliffe College, in which she applied ionization
theory to stellar atmospheres to relate the spectral classes to the temperature of
principal components of stars. This discovery was so unexpected that her dissertation
readers convinced her to modify the conclusion before publication. However, later
research confirmed her discovery.
By the end of the 20th century, further study of stellar and experimental spectra
All matter, whether living or nonliving, is made of the same tiny building
blocks, called atoms. An atom is the smallest basic unit of matter. All atoms have the
same basic structure, composed of three smaller particles.
• Protons: A proton is a positively charged particle in an atom’s nucleus. The
nucleus is the dense center of an atom
. • Neutrons: A neutron has no electrical charge, has about the same mass as a
proton, and is also found in an atom’s nucleus.
• Electrons: An electron is a negatively charged particle found outside the
nucleus. Electrons are much smaller than either protons or neutrons. Different types
of atoms are called elements, which cannot be broken down by ordinary chemical
means. Which element an atom is depends on the number of protons in the atom’s
nucleus. For example, all hydrogen atoms have one proton, and all oxygen atoms
have 16 protons. Only about 25 different elements are found in organisms. Atoms of
different elements can link, or bond, together to form compounds. Atoms form bonds
in two ways.
• Ionic bonds: An ion is an atom that has gained or lost one or more electrons.
Some atoms form positive ions, which happens when an atom loses electrons. Other
atoms form negative ions, which happens when an atom gains electrons. An ionic
bond forms through the electrical force between oppositely charged ions. 
• Covalent bonds: A covalent bond forms when atoms share one or more pairs
of electrons. A molecule is two or more atoms that are held together by covalent
2.1 Laws of Chemical Combination—The basic laws of chemical combination
multiple proportions. Each played an important role in Dalton’s development of the
2.2 John Dalton and the Atomic Theory of Matter—Dalton developed his
atomic theory to account for the basic laws of chemical combination. The theory
centered around the existence of indivisible small particles of matter called atoms and
addressed the unique nature of chemical elements, the formation of chemical
compounds from atoms of different elements, and the atomic nature of chemical
2.3 The Divisible Atom—Of the fundamental particles found in atoms, the
three of most concern to chemists are protons, neutrons, and electrons. Protons and
neutrons make up the nucleus, and their combined number is the mass number, A, of
the atom. The number of protons is the atomic number, Z. Electrons are found outside
the nucleus, and their number is also equal to the atomic number. The negative
charge on an electron is equal in magnitude to the positive charge on a proton. All
atoms of an element have the same atomic number, but they may have different
numbers of neutrons and hence different mass numbers. Atoms containing the same
number of protons (atomic number) but different numbers of neutrons (mass number)
are isotopes of an element. Chemical symbols for isotopes are commonly written in
with Abeing the mass number and Z the atomic number of the element E.
2.4 Atomic Masses—The atomic mass of an element is a weighted average
value calculated from the masses and relative abundances of its naturally occurring
isotopes. Theatomic mass unit represents the standard unit of measure of atomic
masses; it is exactly
of the mass of a carbon-12 atom.
2.5 The Periodic Table: Elements Organized—The periodic table is an
arrangement of the elements by atomic number into rows and columns. This
arrangement places elements having similar properties in the same vertical groups
(families). This arrangement also allows for the classification of elements as metal,
nonmetal, or metalloid.
2.6 Molecules and Molecular Compounds—A chemical formula, the generic
term for the various notations used to represent compounds, indicates the relative
numbers of atoms of each type in a compound. An empirical formula expresses the
simplest atom ratio, and a molecular formula reflects the actual composition of a
molecule.Structural formulas describe the arrangement of atoms within molecules.
Molecular models are also used to represent the structure and shape of molecules. For
example, for acetic acid:
A molecular compound consists of molecules. In a binary molecular
compounds, the numbers of atoms in the molecules are denoted by prefixes; the
names also feature -ide endings.
= nitrogen triiodide S
2.7 Ions and Ionic Compounds—Ions are formed by the loss or gain of
electrons by single atoms or groups of atoms. Positive ions are cations, and negative
ions areanions. An ionic compound is made up of cations and anions held together by
electrostatic attractions. Chemical formulas of ionic compounds are based on an
electrically neutral combination of cations and anions called a formula unit, such as
The names of some monatomic cations include roman numerals to designate
elements, modified to an -ide ending. Polyatomic ions contain more than one atom.
For polyatomic anions, the prefixes hypo- and per- and the endings -ite and -ate are
commonly used. A hydrate is an ionic compound that includes a fixed number of
water molecules associated with the formula unit.
in water and a base produces OH
. A neutralization reaction between an
acid and a base forms water and an ionic compound called a salt. Binary acids have
hydrogen and a nonmetal as their constituent elements. Their names feature the prefix
oxoacids have oxygen as an additional constituent element, and their names use
prefixes (hypo- andper-) and endings (-ous and -ic) to indicate the number of O atoms
carbon. Hydrocarbons contain only hydrogen and carbon. Alkanes have carbon atoms
joined together by single bonds into chains or rings, with hydrogen atoms attached to
the carbon atoms. Alkanes with four or more carbon atoms can exist as isomers,
which are molecules that have the same molecular formula but different structures
Although atoms are the smallest unique unit of a particular element, in nature
only the noble gases can be found as isolated atoms. Most matter is in the form of
ions, or compounds.
Molecules and chemical formulas
A molecule is comprised of two or more chemically bonded atoms. The atoms
may be of the same type of element, or they may be different.
Many elements are found in nature in molecular form - two or more atoms (of
the same type of element) are bonded together. Oxygen, for example, is most
commonly found in its molecular form "O
" (two oxygen atoms chemically bonded
Oxygen can also exist in another molecular form where three atoms are
chemically bonded. O
is also known as ozone. Although O
compounds of oxygen, they are quite different in their chemical and physical
properties. There are seven elements which commonly occur as diatomic molecules.
These include H, N, O, F, Cl, Br, I.
An example of a commonly occurring compound that is composed of two
different types of atoms is pure water, or "H
O". The chemical formula for water
atoms of hydrogen and one atom of oxygen (the "1" subscript is omitted) in the
compound known as "water". There is another compound of Hydrogen and Oxygen
with the chemical formula H
both compounds are composed of the same types of atoms, they are chemically quite
different: hydrogen peroxide is quite reactive and has been used as a rocket fuel (it
powered Evil Kenievel part way over the Snake River canyon).