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Astonomy introduction page Astronomy - Basic Definitions, 1 of 5 Basic definition, page 2

Astronomy is an enormous subject. Only a selection of glossary is included mainly to help those who are new to astronomy to understand the subject published in this web site.

absolute brightness - The apparent brightness an object would have if it were placed at a distance of 10 parsecs (33 light years) from Earth.

angular resolution - The ability to distinguish two adjacent objects. Often measure in arc distance, a unit of angular measure. There are 360 arc degree in a full circle; 60 arc minute in one arc degree and 60 arc seconds in one arc minute (or 3600 arc seconds in one arc degree). It is also used to describe the extent of fine details of an image attainable (synonmous to clarity or sharpness).

angular resolution

angular size - The apparent size of an object, measured by the angle the object subtends, in arc distance units.

apparent brightness - The brightness of an object as it naturally appears in the sky.

binary stars - A star system consists of two stars orbit about their common point (center of mass), held together by their mutual gravitational attraction.

blue giant - Very hot star (temperature at least 15000C) which appears blue. Its size is much larger than the Sun.

brown dwarf - Clouds of gas and dust that collapse under gravity to form a star. However, there is not enough mass to initiate core nuclear fusion (a process that cause stars, and our Sun, shine and give out heat). As a result, these 'failed stars' form compact, dark (not necessary brown!) objects. Brown dwarfs are usually small, comparatively cold (probably less then 1500C) and very difficult to detect.

color index (B - V) - Numerical measurement of a star color. The V (visual magnitude) is the magnitude of the apparent brightness as seen by the naked eye and can be measured by photometry. In early days, photometry was carried out by measuring the exposure of a star on a photographic plate that is more sensitive to the blue than it is to red. This gives a measurement of star brightness weighted towards the blue end of the spectrum. This is the B or photographic magnitude. The difference B - V is also known as the B - V color index. Bluish stars tend to have negative numbers while red stars have positive numbers. Table below shows a list of color indices correspond to the colors.

Color index

Color (approximate)

Spectral type



















corona - The outermost atmosphere of a star, consists of very hot gas heated to temperatures millions of degrees.

Sun's corona

Our Sun's corona is clearly visible when the Moon completely covers the sun during total solar eclipse.The temperature of the surface of the sun is about 6000C, whereas corona temperature can reach up to 1 000 000C. There is still no satisfactory explanation why the corona is so much hotter than the sun's surface.

cosmic microwave - It is the microwave background radiation emanating from space in all direction. It is believed to be the 'left over' from the early stages of the Universe's formation.

astronomy introduction Basic definition, page 2


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