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Basic Definitions


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Basic definition, page 1 Astronomy - Basic Definitions, 2 of 5 Basic definition, page 3

declination - Celestial coordinate system used to measure latitude above or below the celestial equator on the celestial sphere.

Doppler effect - Apparent change in wavelength of the radiation from a source due to its relative motion away from or towards the observer.

escape velocity - The speed necessary for an object to escape the gravitaional pull of an object. Anything that moves with more than the escape velocity will never return.

event horizon - Imaginary spherical surface surrounding a black hole within which no event can be detected by the outside observer. Even light that enters this region will never escape.

fusion - A nuclear reaction process where two or more light nuclei are combined, or fused into heavier ones, release intense radiation and energy in the process. This is the process that takes place at the core of a star (including the Sun) and causes it to shine. Under tremendous pressure due to strong gravitaional pull, fusion reaction at the core of the Sun begins with the fusion of two hydrogen nuclei or protons to form a deuteron or deuterium nuclues. Fusion of a deuteron with another proton forms helium-3 and finally fusion of two such helium-3 to give an ordinary helium-4 nucleus.

gamma-ray burst - An outburst (lasts for a few minutes) of gamma-ray and X-ray which contains energy equal to or greater than a supernova.

giant molecular cloud (GMC) - Huge region of cool clouds of dust grains, and gas (usually in molecular forms). It appears where most of the stars are formed in galaxies.

globular cluster - Tightly bound, roughly spherical collection of hundreds of thousands, and sometimes millions, of stars spanning about 100 light years.

G1 cluster

This is the Hubble Space Telescope (HST) image of G1 globular cluster, consists of at least 300 000 stars. G1 orbits the Andromeda Galaxy (M31) and located 130 000 light-years from the Andromeda nucleus.

Credit: M. Rich, K. Mighell, and J. D. Neill (Columbia University), and W. Freedman (Carnegie Observatories) and NASA

infrared - Region of the electromagntic spectrum located just outside visible light, correponding to a light of a slightly longer wavelength than red light.

irregular galaxy - A weird shaped galaxy, often rich in interstellar matter, but does not belong to any of the major classes of spiral or elliptical galaxies.

Local Group - The small galaxy group that includes the Milky Way Galaxy (our galaxy), the Andromeda Nebula, and about 20 smaller galaxies.

luminosity - Defined as the total energy radiated by a star at all wavelengths, in a second. It is one of the basic properties for star characterization. A luminosity classification scheme is used to compare the brightness of stars with the same temperature or spectral class.

Luminosity class

Star type


largest supergiants




bright giants










white dwarfs

Luminosity class V stars are also known as main-sequence stars because most of the stars fall on this category.

Basic definition, page 1 Basic definition, page 3


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