In the above image, the entire spherical shell isn't illustrated, just a small section. As that energy gets emitted, you can picture it passing through spherical shells centered on the star. This idea is illustrated in this figure:Īgain, think of the luminosity-the energy emitted per second by the star-as an intrinsic property of the star. Why do light sources appear fainter as a function of distance? The reason is that as light travels towards you, it is spreading out and covering a larger area. There is a mathematical relationship that relates these three quantities–apparent brightness, luminosity, and distance for all light sources, including stars. An intrinsically faint, nearby star can appear to be just as bright to us on Earth as an intrinsically luminous, distant star. The variation in their brightness is caused by both variations in their luminosity and variations in their distance. Stars have a wide range of apparent brightness measured here on Earth. If you put an automobile headlight 10 feet away and a flashlight 10 feet away, the flashlight will appear fainter because its luminosity is smaller. However, not all light bulbs are the same luminosity. To think of this another way, given two light sources with the same luminosity, the closer light source will appear brighter.
However, when the car passes you within 10 feet, its lights may appear blindingly bright. When the car is far away, even if its high beams are on, the lights will not appear too bright.
So, everyone will measure a different apparent brightness for the same star if they are all different distances away from that star.įor an analogy with which you are familiar, consider again the headlights of a car. However, apparent brightness is not an intrinsic property of the star it depends on your location. Another way to look at these quantities is that the luminosity is an intrinsic property of the star, which means that everyone who has some means of measuring the luminosity of a star should find the same value. The difference between luminosity and apparent brightness depends on distance. The luminosity of a star, on the other hand, is the amount of light it emits from its surface. When I say apparent brightness, I mean how bright the star appears to a detector here on Earth. I am purposely being careful about my choice of words. Perhaps the easiest measurement to make of a star is its apparent brightness.
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