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Chapter 11 Book Notes

Chapter 11 Book Notes - Astronomy 180 Chapter 11...

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Astronomy 180 Chapter 11: Characterizing Stars (Pages 292-316) Stellar evolution – changes in size, luminosity, temperature, and chemical composition that occur as a star ages I. Distances to nearby stars are determined by stellar parallax The closest star other than the Sun: Proxima Centauri (in Centaurus), about 40 trillion km away (~4LY); its position moves amongst the background stars as the Earth orbits the Sun because of its proximity Stellar parallax – the apparent shift in a nearby star’s position on the celestial sphere resulting from the Earth’s orbit around the Sun Parallax cannot be used for very distant stars, it is only accurate within about 100 pc II. Magnitude Scales: Apparent magnitude measures the brightness of stars as seen from Earth Apparent magnitude (m) – a measure of the brightness of light from a star or other object as seen from Earth Originally, the brightest stars were classified as first magnitude denoted by m = +1. Ones half as bright as these were second magnitude m = +2, and upwards to +6 Later, in order to classify stars actually brighter than the original first magnitude , astronomers resulted to negative numbers; also, dim stars only visible through telescopes have stretched the magnitude scale to +30 III. Magnitude Scales: Absolute magnitudes and luminosities do not depend on distance Absolute magnitude (M) – the apparent magnitude that a star would have if it were 10 parsecs from Earth Inverse-square law – the gravitational attraction between two objects and the apparent brightness of a light source are both inversely proportional to the square of its distance Apparent brightness decreases inversely with the square of the distance between the source and the observer Absolute magnitudes range from roughly M = -10 to M = +17 Absolute magnitudes give us comparisons between the energy outputs of stars Luminosity – the rate at which electromagnetic radiation is emitted from a star or other object Stellar luminosities are expressed in multiples of the Sun’s luminosity Ranges from 10 6 L to 10 -5 L IV. The Temperatures of Stars: A star’s color reveals its surface temperature The hotter the star, the shorter the wavelength, so cool stars have long wavelengths (red) and hot = short (blue) Photometry – the measurement of light intensities V. The Temperatures of Stars: A star’s spectrum also reveals its surface temperature Stellar spectroscopy – the study of the properties of stars encoded in their spectra The strength of an absorption line depends on the star’s temperature Really hot and really cold stars produce dimmer spectral lines VI. The Temperatures of Stars: Stars are classified by their spectra Spectral type – a classification of stars according to the appearance of their spectra OBAFGKM sequence – the sequence of stellar spectral classification from hottest to coolest stars
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