How is stars brightness measured

We measure the brightness of these stars using the magnitude scale. The magnitude scale seems a little backwards. The lower the number, the brighter the object is; and the higher the number, the dimmer it is. This scale is logarithmic and set so that every 5 steps up equals a times decrease in brightness. So magnitude 10 is times dimmer than magnitude 5, which is times dimmer than magnitude 0. Perhaps the easiest measurement to make of a star is its apparent brightness.

I am purposely being careful about my choice of words. When I say apparent brightness , I mean how bright the star appears to a detector here on Earth.

The luminosity of a star, on the other hand, is the amount of light it emits from its surface. The difference between luminosity and apparent brightness depends on distance. 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.

However, apparent brightness is not an intrinsic property of the star; it depends on your location. So, everyone will measure a different apparent brightness for the same star if they are all different distances away from that star.

For an analogy with which you are familiar, consider again the headlights of a car. When the car is far away, even if its high beams are on, the lights will not appear too bright. If you have observed the night sky, you have noticed that some stars are brighter than others.

The brightest star in the northern hemisphere winter sky is Sirius, the "Dog Star" accompanying Orion on his nightly journey through the sky. In the constellation of Lyra the Harp, Vega shines the brightest in the summer sky. How bright is Sirius compared to its starry companions in the night sky?

How does it compare to Vega, its counterpart in the summer sky? How bright are these stars compared to the light reflected from the surface of the Moon? Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Prev NEXT. Photo courtesy NASA. Stefan-Boltzmann Law.

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