Astronomy 1021 Lecture Notes - Lecture 12: Star Cluster, Main Sequence, Stellar Classification
3 Apr 2016
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Document Summary
Hydrogen, helium, no more than 2% of elements heavier than helium. We need to disinguish between a star"s brightness in the sky and the actual amount of light it emits. Apparent brightness is the amount of starlight reaching earth. It"s how bright it seems to us when we look. Luminosity is when we talk about how bright stars are in an absolute sense, regardless of their distance. The total amount of power that a star emits into space. A stars apparent brightness in the sky depends on both its true light output, or luminosity, and its distance from us. The apparent brightness of a star or any other light source obeys the inverse square law with distance. Doubling the distance to a star would decrease its apparent brightness by a factor of 2^4, or 4. We can measure the distance to a nearby star by observing how its apparent locaion shits as earth orbits the sun.
