Complete course notes part 1

Page 1

Luminosity of stars Stars show different luminosities (Luminosity = emitted power, [energy][time]-1). This is due to: - different distances - different intrinsic luminosities Is it possible to quantify the apparent luminosity? A scale of magnitudes from 1 to 6 was already introduced around 150 b.C. (Ipparcus, Tolomeus): - mag1: the brightest stars; - mag6: the faintest stars, just visible at naked eye Since the human perception of light brightness is logarithmic, we introduce a logarithmic scale of luminosities: where fi is the flux (luminosity) and mi the corresponding magnitude. If the flux increases, the magnitude decreases. The faintest stars have highest magnitudes. Previous formula can be inverted: Examples: 1) Find the luminosity ratio between the faintest star detectable at naked eye and the faintest one visible using a powerful telescope. Ans. ≈ 108 2) A flux ratio is 106. Find the corresponding difference in magnitudes. Ans. 15 ------------------------------------------------------ Stars and, more generally, astronomic objects, emit not only in the visible but in other regions of the electromagnetic spectrum. Relationship between frequency ν and wavelength λν: λ ν =V where V is the velocity of the wave. In a vacuum (EM waves) V = c = 299792458 m/s ≈ 3 108 m/s. Region Wavelength Frequency Radio > 10 cm < 3 109 Microwaves 0.1 mm – 10 cm 3 109 – 3 1012 Infrared 700 nm – 0.1 mm 3 1012 – 4.3 1014 Visible 400 – 700 nm 4.3 1014 – 7.5 1014 Ultraviolet 10 – 400 nm 7.5 1014 – 3 1016 X-rays 0.1 – 10 nm 3 1016 – 3 1018 Gamma rays < 0.1 nm > 3 1018 Visible light is only a tiny portion of the visible spectrum, but very important for us (eye sensitive only to visible; most of the Sun’s emission is in the visible). A few definitions: - Flux: energy/time/surface (W m-2) -…