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Glossary - About the Universe

The Big Bang
Early Universe
Life and Death of Stars
Galaxy Formation
The Solar System
Exotic objects
End of the Universe
Who created the universe?
What is Time?
Life beyond Earth
NASA Missions
Particle Map
Glossary
Sources
Accretion
The process by which planets are formed, in which gravity is the driving force. Accretion begins when there is an asymmetry of density in a region of space, giving gravity a foothold. Clumping of matter occurs in the high-density areas as the clumps collide with other clumps. The gravity of the clump increases with ever-increasing mass and the process continues until the clump eventually becomes a rock, then a boulder, growing ever bigger until the gravity is adequate to start drawing matter to it. Eventually, the clump becomes a sphere and, ultimately, a planet.
Antimatter
Antimatter is made of antiparticles just as matter is composed of particles. Antiparticles have the opposite charge of their partner particles, thus an electron with a negative charge has as its partner a positron, an antiparticle with a positive charge. Antiparticles are the same as their partner particles in all respects except for the charge that they carry. Importantly, when matter and antimatter are mixed or particles and antiparticles are mixed, both are annhilated, releasing high energy photons such as gamma rays.
Aphelion
The point in its orbit where a planet is farthest from the Sun. When referring to objects orbiting the Earth the term apogee is used.
Astronomical Unit
The distance from the Sun to the Earth or 93 million miles is called an astronomical unit and is used as a standard measure of astronomical distances.
Atom
The smallest bit of an element. Every element can be divided down into an atom of that element. An atom consists of a nucleus containing a specific number of protons (the atomic number), neutrons (sometimes) and electrons orbitting the nucleus in a series of energy shells. Atoms are infinitessimally small. How small? There are more than 100,000 atoms in the period ending this sentence.
Chromosphere
The layer of the Sun above the photosphere where the temperature begins to rise with altitude.
Compound
A substance made of atoms of two or more elements bound together by chemical bonding of electrons.
Conservation of angular momentum
Conservation of angular momentum is the principle that the angular momentum of an object remains constant as long as no external torque, or moment, acts on that object.
Convective zone
The layer of the Sun above the radiative zone in which energy transfer continues through convection, in the same way as bubbles transfer energy through boiling water.
Core of the Sun
The region of the Sun extending from the center outward to approx. .20 of the radius. The core is the nuclear furnace of the Sun, where hydrogen is fused into helium.
Corona
The layer of the Sun above the chromosphere which is paradoxically hotter than the Sun's surface and which is visible as a bright halo surrounding the Sun disk only during total solar eclipses.
coronal mass ejection
An explosion that takes place in the corona that ejects particles into the solar wind, caused by an energy release in the Sun's magnetic field
Dark Matter
In astronomy and cosmology, dark matter is matter that is inferred to exist from otherwise unexplained gravitational effects on visible matter and background radiation, but is undetectable by observable electromagnetic radiation.
Doppler Shift
An observer of an object moving toward him or herself will see a blue shift of the object, caused by the squeezing of the light waves emitted by the moving object. As the light approaches, the frequency of its light waves increases, that is they get shorter. When the object is observed moving away, the opposite occurs and the observer sees a red shift, as the light waves are stretched (the frequency decreases and the waves get longer)The same thing happens with sound, as sound also travels in waves.
Electromagnetism
One of the four forces of nature defined as a radiant stream of particles (photons) that that is arranged in the electromagnetic spectrum, from least energetic, longest wave lengths to highest energy, shortest wave lengths: (1) Radio waves, (2) Microwaves, (3) Infra-red - heat energy, (4) Visible light - , (5) Ultraviolet - , (6) XRays - energetic, radioactive rays, (7) Gamma rays - The most energetic, most lethal rays.
Element
A substance consisting of one or more instances of a specific atom.
Four forces of nature
Gravity, Electromagnetism, Nuclear strong force, Nuclear weak force
Galaxies
(1) Elliptical Galaxy - A cluster of stars that exist in a primarily elliptical shape and a smooth brightness profile. There is little dust or gas in elliptical galaxies, thus very little star formation takes place. It is a population of older stars. (2) Spiral arms Galaxy - A huge cluster of stars encircled by spiral arms full of stars, all arranged in one plane. If viewed edge-on, it appears as a bulging center of intense brightness. There is a large supply of dust and gas and star births are common in these "star nurseries".
Gas Giants

Globular Clusters
Globular clusters are compact, nearly spherical, groups of hundreds of thousands of stars. The stars in globular clusters are among the oldest stars in the Galaxy, with ages greater than 10^10 years. The stars in a globular cluster share a common chemical composition.
Gravity
One of the four forces of nature. Gravity is directly proportional to the mass of an object - the heavier an object is, the more gravity it has. Gravity manifests as an attraction for other objects within the field of influence of the gravity. In space, objects that are under the gravity of a close by object will tend to orbit the source object, providing the velocity of the orbiting object is not adequate to break free. When an object's orbital velocity exceeds a certain threshold, the gravitational attraction is broken and the orbitting object will fly away from the source in a new trajectory.
heliopause
The heliopause
heliosphere
The heliosphere
Hubble Deep Field
The Hubble Deep Field is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area that is two parts in a million of the whole sky, which is equivalent in angular size to a 65 mm tennis ball at a distance of 100 metres. The image was assembled from 342 separate exposures taken with the Space Telescope's Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and December 28, 1995.
Ion
An atom with either extra electrons or less electrons. Ions are charged with either a negative (extra electrons) or positive charge (less electrons.
The speed of light
186,000 miles per second. The universal speed limit.
Main sequence star

Matter
The stuff of the universe. Any substance that has mass and that occupies a portion of space. Matter is made of atoms or other subatomic particles.
Metallic-Hydrogen dynamo
A theory called the magnetic dynamo theory says that the magnetic field is produced by swirling motions of liquid conducting material in the planet interiors. Materials that can conduct electricity have some electrical charge that is free to move about. Such materials are called metallic and are not necessarily shiny solids like copper, aluminum, or iron. Jupiter and Saturn have a large amount of hydrogen that is compressed so much it forms a liquid. Some of that liquid hydrogen is in a state where some of the electrons are squeezed out of the atoms and are free to move around. A moving charge will produce a magnetic field. The liquid conducting material in a planet's interior can be made to swirl about if the planet is rotating quickly enough. The faster a planet rotates, the more the material gets stirred up and, therefore, the stronger the generated magnetic field.
Molecule
A group of at least two atoms held together by covalent chemical bonding. A molecule may either contain atoms of a single element, such as O2 or of different elements, such as H2O.
nuclear fusion
The process by which atoms of a particular element combine with atoms of another element, creating a new heavier element. A heavier element contains more protons in its nucleus than a lighter element. Fusion is an exothermic (heat producing) process that emits electromagnetic radiation, such as infrared light (heat), XRays (energetic particles) and gamma rays (the most energetic, deadly particles).
nucleons
The sub-atomic particles that make up the nucleus of an atom, otherwise known as protons and neutrons.
neutrons
The neutral sub-atomic particles that make exist in the nucleii of some atoms. A particular atom that contains varying numbers of neutrons in their nucleii define the isotopes of that atom. Not all atoms have isotopes.
perihelion
The point in its orbit where a planet is closest to the Sun. When referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies.
Photosphere
The layer of the Sun above the convective zone where light is emitted as visible wave lengths.
Photosynthesis
A metabolic process where by plants convert carbon dioxide into oxygen, powered by sunlight.
Planck time
Named after German physicist Max Planck, the founder of quantum theory, a unit of Planck time is the time it takes for light to travel, in a vacuum, a single unit of Planck length. Planck lenghth is about 10^-20 of the diameter of a proton. Short beyond description.
Prominence
A mass of cloud-like gas that rises in a magnetic loop from the Sun's chromosphere, rising up to several hundreds of thousands of km above the surface of the Sun. They are both dramatic and spectacular in appearance.
Protogalactic Clouds
Galaxies are believed to have formed from large clouds of gas (mostly hydrogen) that slowly condensed due to their own gravity until they were dense enough to begin star formation. Those large masses of gas are protogalactic clouds.
protons
The positively charged sub-atomic particles that exist in the nucleus of every atom are protons. The number of protons in an atomic nucleus defines the atomic number and, by definition, the element that the atom is.
Radiative zone
The layer of the Sun above the core from which the energy produced by the Sun is radiated outward into space in the form of photons.
Solar wind
A continuous stream radiating outward from the Sun in all directions, consisting of
Strong force
One of the four forces of nature defined as the force that binds protons and neutrons together to form the nucleus of an atom. On the smaller scale, it is also the force that holds quarks and gluons together to form the proton, the neutron and other particles. In the context of binding protons and neutrons (nucleons) together to form atoms, the strong interaction is called the nuclear force (or residual strong force).
Sunspot
Spots on the Sun that are dark, surrounded by lighter regions that are cooler than the photosphere.
Weak force
One of the four forces of nature: It is responsible for the radioactive decay of subatomic particles and begins the process known as nuclear fusion.