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Astronomy 1021
Chris Racknor

Jan 17 12 StarsThings We Want to Know About Every Star Distance from usTemperatureSizeMass Age CompositionBecause we cannot get close enough to stars to really measure the above we have to just work with what we can see from earth Our snapshot of the stars in the Milky Way galaxy is based on what we can actually observe right now which are points of light which can be faint or bright We see that measuring two quantities is enough for now and those are the luminosity total energy output of stars and the surface temperature of the stars As we have seen for the sun the total energy power output is called luminosity which we abbreviate to L For what follows generally energy output will be compared to the suns we use the unit solar luminosity L sunTherefore two stars that have the same luminosity have the same total energy output and thus are producing the same amount of energyBrightness More energy output means intrinsically brighter stars We can very accurately measure the apparent brightness or magnitude of stars Therefore on first sight measuring stellar luminosities is easy because you just measure how bright the stars appear The further away a star is the fainter it looks The same amount of starlight passes through each sphere The surface area of a sphere depends on the square of its radius distance from the star so the amount of light passing through each unit of area depends on the inverse square of distance from the star The apparent brightness follows an inverse square law just like gravity If you double the distance the brightness will decrease by a factor of four and if you increase the distance by a factor of ten the brightness will decrease by a factor of 100 A brighter star might have a higher luminosity but it may just be closer byHow We Determine Distance There are many complicated way of doing this Parallax is the most direct and easiest method As earth orbits the sun the position of a nearby star appears to shift against the background of more distant stars Parallax is measured in arcseconds 1 arcsecond of movement1 parsec away d1p 1 parsec326ly A circle has 360 degrees 1 degree60 minutes of arc 1 arcminute 1 arcminute60 seconds of arc 1 arcsecond 1 arcsecond11296000 of a circle which is very small Some other ways of determining distance are RR Lyrae variables Cepheids Novae XRay Bursts and Galactic RedshiftApparent magnitude is how bright a star appears in the sky Absolute magnitude is how bright the star would look if you put it where the sun isIt turns out that many nearby stars are really dim while more distant stars appear much brighter Stellar luminosities span a wide rangeStellar Luminosities Intrinsically dimmest stars have a luminosity of about 110000 L while the intrinsically brightest stars have a sunluminosity of about 1 million L sunOur suns output is pretty average The brightest stars have an energy output that is 10 billion times higher than the dimmest stars
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