Lecture25.doc

Lecture25.doc - STARS Goal is to measure intrinsic...

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Unformatted text preview: STARS Goal is to measure intrinsic properties of stars. These include: Radius Luminosity How much total energy a star puts out per second Temperature Of the photosphere! Mass Chemical Composition Of the surface! To measure these one needs to know the distance s to the stars. Distances For nearest stars can be measured by trigonometric parallax . d = 1/p where d is measured in parsecs and p in arcseconds. There are 206,265 arcseconds in one radian. Thus there are 206,265 x 1AU = 3 x 10 13 km = one parsec. NB : Astronomers never use light years to measure distances. Reason: Light-years cannot be measured directly, but parsecs can. However, 1 light-year = 3.26 pc Nearest star is Alpha Centauri at a distance of 1.3 pc. (actually a companion, Proxima Centauri gets a bit closer) Question: what is the parallax angle of Alpha Centauri? First parallax was measured by Friedrich Bessel in 1838. Accuracy depends on how accurately one can measure stellar positions. With ground based telescopes, limitation is atmospheric (seeing). Can measure relative position of stars to about 1 10% of the effective resolution. Until space telescopes and adaptive optics, this was limited to 0.02 arcseconds, corresponding to a distance of 50 pc. Special purpose space satellites (HIPPARCOS) can now measure angles to about 0.002 arcseconds (2 milliarcseconds) thus get trigonometric parallax to distances of 500 pc (1 kiloparsec = 1 kpc). Brightness Apparent Brightness (or apparent magnitude) How bright a star apears from Earth Absolute Brightness (or absolute magnitude) How bright a star actually is We use the term luminosity to describe the absolute brightness of a star. Luminosity, L, is the number of ergs/sec a star sends into space. Luminosity is energy emitted per unit time....
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Lecture25.doc - STARS Goal is to measure intrinsic...

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