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Basic Definitions


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Basic definition, page 2 Astronomy - Basic Definitions, 3 of 5 Basic definition, page 4

magnetosphere - A zone of charged particles trapped by a planet's or star's magnetic field, lying above the atmosphere and extends far into space. It can be regarded as the region of space in which a planet's magnetic field dominates that of solar wind.

magnitude - The method used to compare the apparent brightness of stars and other celestial objects. It was first conceived by Hipparchus in second century BC. In 140 AD, Ptolemy refined the method and used 1 to 6 to compare star brightness. The brightness decreases (becomes fainter) as the number increases and the 1st magnitude is 100 times brighter than the 6th magnitude stars. In other words, the difference of one magnitude corresponds to a brightness difference of about 2.5. Table below lists magnitudes of some selected objects.


Apparent magnitude



Full Moon


Sirius (brightest star)




Naked eye limit


HST limit


Another method of measurement is the absolute magnitude, which is the apparent magnitude of an object as it is placed at a distance of 10 parsecs (33 light years) from Earth. The Sun has an absolute magnitude of 4.85.

neutron star - A dense ball of neutrons that remains after a supernova has destroyed the rest of the star. Typically neutron stars are about 20km across, and contain more mass than the Sun.

nova - A star that suddenly flare up, often by a factor of as much as 10 000. It can remain bright for a few days or even months before slowly fades back to its original luminosity. It is often the result of an explosion on the surface of a white dwarf star, caused by matter (such as hydrogen) falling onto its surface from the atmosphere of a binary companion star. Nova generally appear in regions close to the Milky Way. Nova may occur more than once on a particular star. These stars are known as recurrent nova.

open cluster - Loosely bound collection of tens to hundreds of stars, and spans over a hundred light years across. It is usually found in the plane of the Milky Way.

planetary nebula - The ejected envelope of a red giant star, spread over a volume roughly the size of our solar system. The ultraviolet radiation from the exposed red giant core causes the surrounding matter to glow. Eventually, it collapses to form a white dwarf.

MyCn18 hourglass nebula

HST image of MyCn18, a young planetary nebula. The image reveals the delicate detail of ejected matter resembles an 'hourglass'. Composed of three separate images, the red represents nitrogen; green hydrogen and blue oxygen.

Credits: R. Sahai and J. Trauger (JPL), the WFPC2 science team, and NASA

pulsar - Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Generally used to describe the pulsed radiation from a rotating neutron star.

quasars - A distant, mysterious and highly luminous object that looks like a star. Recently, there is strong evidence to show that a quasar is produced by gas falling into a supermassive black hole in the center of a galaxy.

Basic definition, page 2 Basic definition, page 4


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