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red bar Astrophysics
The part of astronomy that deals principally with the physics of stars, stellar systems, and interstellar material

Light radiated by ions and atoms in the Earth's upper atmosphere, mostly in polar regions, the result of bombardment by energetic electrically charged particles from the magnetosphere
a. Aurora Australis - South Pole
b. Aurora Borealis - North Pole

A reddish layer in the Sun's atmosphere, the transition between the photosphere and the corona

The central, energy-producing region of the Sun where it's energy is produced by the fusion of hydrogen nuclei into hydrogen nuclei

The outermost layer of the Sun's atmosphere, visible to the eye during a total solar eclipse; it can also be observed through special filters and best of all, by X-ray cameras aboard satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the solar wind.

Coronal Mass Ejections (CME)
A huge cloud of hot plasma, occasionally expelled from the Sun. It may accelerate ions and electrons and may travel through interplanetary space as far as the Earth's orbit and beyond it, often proceeded by a shock front. When the shock reaches Earth, a magnetic storm may result.

Convection Zone
The region of the Sun between the Radiative Zone and the Photosphere where energy from the solar interior is transported outwards by the convective motion of the solar gases

A lightweight particle, carrying a negative electric charge and found in all atoms. Electrons can be energized or even torn from atoms by light and by collisions, and they are responsible for many electric phenomena in solid matter and in plasmas.

A rapid outburst on the Sun, usually in the vicinity of active sunspots. A sudden brightening (only rarely seen without special filters, isolating the red light of hydrogen) may be followed by the signatures of particle acceleration to high energies--x-rays, radio noise and often, a bit later, the arrival of high-energy ions from the Sun.

Interstellar Space
The space between the stars

The highest region of the Earth's atmosphere containing free electrons and ions

Speed of Light
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.

Magnetic Field
A region in which magnetic forces can be observed.

Magnetic Poles
Either of two limited regions in a magnet at which the magnet's field is most intense. The two regions have opposing polarities, which we label "north" and "south", after the two poles on the Earth.

Magnetic Storm
A large-scale disturbance of the magnetosphere, often initiated by the arrival of a plasma cloud originating at the Sun. A magnetic storm is marked by the injection of an appreciable number of ions from the tail regions of the magnetosphere into] the near-Earth magnetosphere, a process accompanied by increased auroral displays. The injected particles cause a worldwide drop in the equatorial magnetic field, taking perhaps 12 hours to reach its greatest intensity, followed by a more gradual recovery.

The region of space in which the magnetic field of an object (e.g., a star or planet) dominates the radiation pressure of the stellar wind to which it is exposed

Near-Earth Space
A region of space surrounding Earth interior to the orbits of the geosynchronous satellites (approximately 40,000 km from the surface).

Nuclear Fusion
The process of releasing energy by combining hydrogen atoms to form helium, or more generally, to combine light nuclei into heavier ones. Nuclear fusion appears to be the source of the energy of the Sun and of stars.

The smallest (quantum) unit of light/electromagnetic energy. Photons are generally regarded as particles with zero mass and no electric charge.

The layer of the Sun from which all visible light reaches us. The Sun is too hot to have a solid surface and the photosphere consists of plasma at about 5500 degrees centigrade.

A low-density gas in which the individual atoms are ionized (and therefore charged), even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality

A particle with a positive charge commonly found in the nucleus of atoms

Energy radiated in the form of waves (light) or particles (photons)

Radiative Zone
The region of the Sun between the Core and the Convective Zone where energy from the Core is transported outwards towards the surface by electromagnetic radiation (mostly gamma rays and X rays) rather than by convection

Radio Waves
Electromagnetic waves of relatively low frequency

A body that revolves around a larger body. For example, the moon is a satellite of the earth.

Solar Cycle - (or Sunspot Cycle)
An irregular cycle, averaging about 11 years in length, during which the number of sunspots (and of their associated outbursts) rises and then drops again. Like the sunspots, the cycle is probably magnetic in nature, and the polar magnetic field of the Sun also reverses each solar cycle.

Solar Flare
Violent eruptions of gas on the Sun's surface

Solar Maximum
The month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a maximum. The most recent solar maximum occurred in July 1989.

Solar Minimum
The month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a minimum. The most recent minimum occurred in September 1986.

Solar Wind
A fast outflow of hot gas in all directions from the upper atmosphere of the Sun ("solar corona"), which is too hot to allow the Sun's gravity to hold on to its gas. Its composition matches that of the Sun's atmosphere (mostly hydrogen) and its typical velocity is 400 km/sec, covering the distance from Sun to Earth in 4-5 days.

Space Weather
A collection of processes and events often driven by solar activity, which can effect interplanetary space and the near-Earth environment

A large ball of gas that creates and emits its own radiation

An intensely magnetic area on the Sun's visible face. For unclear reasons, it is slightly cooler than the surrounding photosphere (perhaps because the magnetic field somehow interferes with the outflow of solar heat in that region) and therefore appears a bit darker. Sunspots tend to be associated with violent solar outbursts of various kinds.

Ultraviolet Light
Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.

Van Allen Radiation Belts
A collection of high-energy particles (mostly protons and electrons) trapped within the EarthÍs magnetic field which form a doughnut-like cloud in its equatorial plane

X rays
Electromagnetic waves of short wavelength, capable of penetrating some thickness of matter. Medical x-rays are produced by letting a stream of fast electrons come to a sudden stop at a metal plate; it is believed that X-rays emitted by the Sun or stars also come from fast electrons.

Many of the above definitions were provided by the following websites:

From Stargazers to Starships: Author and curator: David P. Stern

Imagine the Universe! : Project Leader: Dr. Jim Lochner

Responsible NASA Official: Dr. James Thiemann
Web Manager/Curator: Troy Cline
Web Designer/Curator: Lori Ann Persichitti Lopez
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