AST201/A02 Ch-6.docx

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Astronomy & Astrophysics
Dr.Parandis Khaveri

Astronomy ch-6.1 Stellar parallax (p) – the small apparent shift in position of a nearby star relative to distant background objects due to earth’s orbital motion Parsec (pc) – distance to hypothetical star whose parallax is 1 second of arc Intrinsic brightness – a measure of amount of light that a star produces Flux – measure of flow of energy out of surface, usually applied to light Absolute visual magnitude (Mv) – intrinsic brightness of a star – the apparent visual magnitude the star would have if it were 33ly away Luminosity (L) – total amount of energy a star radiates per second at all wavelengths Ch-6.1 Triangulation method: - To find distance of a star from earth, use baseline as the size of earth’s orbit (1AU) - Parallax refers to common experience of an apparent shift in position of a foreground object due to change in location of observer’s viewpoint. (thing appearing in foreground seems to have changed it location whereas the real change is in position of observer) - Stellar parallax – the angle representation of stars in arc seconds due to very far distance, technically, half the total shift of star or shift seen across baseline of 1AU - Measuring parallax is difficult due to very small angle - Two units used to measure the distances of stars are: o Light year – amount of distance light travels in one year o Parsecs – measure the distance of star using its parallax, hence units called parsec, mean parallax per second - 1 parsec = parallax of 1 arc sec = 206.265 AU and 3.26 Ly - Parallax of 0.006 is maximum that can be observed from earth – uncertianity is 30%, max acceptable; 006 arc seconds is distance of 550 ly Ch-6.2 Brightness, Intrinsic and Luminosity Apparent brightness/magnitude – measure tells how bright star appears on Earth - light depends upon distance Intrinsic brightness – a measure of amount of light a star produces; tells true nature of star - a very bright star may appear faint, only because its located very far away Brightness and distance:- - Brightness is related to flux of energy entering the eye - Flux is measure of flow of energy out of surface, measured in joules (J) per second falling on 1 square metre --> 1 Joule per second = 1 watt - Flux (energy) receiving from light is inversely proportional to square of distance to source. Move object 2 times far away, light falling on object would cover 4 times larger area. Absolute visual magnitude:- - (Mv)Intrinsic brightness of a star. The apparent visual magnitude would be 33ly, use this as standard distance - V stands for visual magnitude, the wavelength that enters your eye. Electromagnetic spectrum such as infrared and ultraviolet radiation is used for other stuff that does not reach eye Luminosity:- - Extreme bright star would have absolute magnitude of -8, located 33 ly away from earth and appear as bright as moon and emit 100 000 times more energy than Sun - Absolute magnitude refers to visible light – Hot stars emit ultraviolet radiation while cool stars emit infrared radiation - Luminosity – total amount of energy a star emits per second at all wavelengths - total electromagnetic radiation that star output, that depends on its temperature - Luminosity of Sun is 4x10 watts; two way to measure luminosity of other stars: o Compare it with Sun as 100 times more luminous than sun o Or express real energy units by multiplying the luminosity of Sun Ch- 6.3 - Surface is photosphere, which is the limit of our vision into the star from outside but is not an actual solid surface. - Hydrogen Balmer absorption lines are produced by hydrogen atoms with electrons initially in second energy levels o If surface of star is too col
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