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The apparent shift in a nearby star's position on the celestial sphere resulting from the Earth's orbit around the sun; the motion of nearby stars among the background of more distance stars due to Earth's motion around the Sun Example: Proxima Centauri is closer to us than other stars, and its position among the background stars changes as the Earth orbits the sun |
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Formed between your eyes and the object; lets your brain judge how close you are to the object Distance to a star in parsecs d=1/p d=distance to stars in parasecs p=parallax angle of star in arcseconds precision of stellar parallax measurements is limited by angular resolution of telescope parallax angles smaller than .01 arcsecs difficult to measure from Earth |
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Brightness of stars without regard to their distances;a measure of the brightness of light from a star or other object as seen from Earth Brightness of star indicated by a number, brightest stars were 1: example: m=+1 etc. Later, astronomers discovered some stars to be brighter than the original apparent magnitude (or the +1 stars) and so, negatives came into play Sirius, the brightest star in the universe, has m=-1.44, Venus m=-4.4, Moon m=-12.6, and Sun m=-26. |
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The apparent magnitude that a star would have if it were 10 parsecs from Earth If you have one star, and another that is twice the distance from the first, the star twice the distance will be much dimmer light moves outward from a source, and spreads out over increasingly larger areas of space, decreasing its brightness (this is shown by the inverse-square law) Absolute magnitude determined by calculations based on apparent magnitudes -first measure the apparent magnitude -then find distance by measuring its parallax angle -combining these numbers gives the star’s absolute magnitude Absolute magnitudes range from M=-10 to M=+17 (dimmest) |
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The gravitational attraction between two objects and the apparent brightness of a ligth 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
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Total amount of electromagnetic power (energy emitted each second); the rate at which electromagnetic radiation is emitted from a star or object; total amount of energy emitted by a star each second Luminosity of Sun represented by L Luminosities range from 106L to 10-5L Depends on stellar size and temperature -higher luminosity, brighter an object is (smaller, or more negative a stars absolute magnitude is, the greater its luminosity) -luminosity expressed in multiples of the Sun’s luminosity |
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| Stars behave much like black bodies, and so surface temperatures determines their color -Cool stars peak at long wavelengths, so they look red -Hot stars peak at short wavelengths, so they look blue Wien’s Law (wavelength of emitted by a blackbody varies with its temperature) But, both are affected by absorption lines in the spectrum and extinction |
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The measurement of light intensities telescope collects starlight that is then passed through one of a set of colored filters and recorded on a CCD, providing the peak of the star’s blackbody spectra, and hence the temperature of the star -Filters light through different radiation (such as ultraviolet , blue filters etc.) -Hotter stars are bright in ultraviolet-filters, dim in blue, and dimmer in yellow -Cool stars visible in red filters (dimmer in yellow and blue) |
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The study of properties of stars encoded in their spectra; determining a stars temperatures by studying its spectrum -begin by taking the spectrum of star and then identifying and discarding spectral lines due to interstellar gas and Earth’s atmosphere---left are spectral lines created in star’s atmosphere -hydrogen makes up about ¾ of a stars mass, but they don’t always show up strongly on the spectrum because the strength depends on the star’s temperature |
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visible hydrogen lines -explained because balmer lines are produced when photons excite electrons in the second energy level to a higher level. Stars with higher temperatures absorb enough energy from photons to go through this process, producing the strongest balmer lines -but balmer lines are dim (or weak) also if the temperature exceeds 10,000 K because high-energy photons simply strip away electrons from most of the hydrogen atoms |
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The grouping of similar spectra into classes types today are : OBAFGKM “Oh, Be A Fine Guy(Girl), Kiss Me!” -hottest are type O, M are coolest Each Spectral type is broken into 10 temperature subranges (0 hottest, 9 coolest) Example: A8 is hotter than A9, which is hotter than F0 Sun is a type G2 star
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Hertzsprung-Russel Diagram or H-R diagram |
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Graphs of stellar lumniosity (or absolute magnitude) against surface temperature shows surface temperature (spectral type) and luminosity are correlated, not random bright stars near top, dim near bottom, hotter stars to the left, cooler to the right |
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A grouping of stars on the H-R diagram extending diagonally across the graph from hottest, brightest stars to the dimmest, coolest stars most of the stars we see in the night time sky on the H-R diagram extends from hot, bright, bluish stars in upper left down to cool, dim, reddish stars in lower right number of main-sequence stars decreases with increasing temperature -M, K, G stars most common, O rare |
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A star, fusing hydorgen to helium in its core, whose surface temperature and luminosity place it on the main sequence on the H-R diagram 90% of stars The greater the temperature the more luminous the star spans range from hot, bright stars to cool, dim stars. |
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A star whose diamter is roughly 10 to 100 times that of the Sun
stars that are both bright and cool, indicating that they must be enormous to emit so much more energy for their grouping |
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A star of very high luminosity; a star even bigger than a giant star make up top of the H-R diagram combined with giant stars make up less than 1% of the stars in our vicinity very bright and very large stars across the range of temperatures |
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A low-mass stellar remnant that has exhausted all its thermonuclear fuel and contracted to a size roughly equal to the size of the Earth stars that are hot, dim, and tiny compared to the sun. roughly the size of earth, can only see with the aid of telescope only remnants of stars |
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A dark line in a continuous spectrum created when photons are removed from the continuum Studying absorption lines more accurate than luminosity and temperatures because absorption lines are affected by density and pressure of the gas in a star’s atmosphere, both of which determined what type of star it is |
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