Wednesday, May 11, 2011

Stars Delivered in 10 seconds a lesson in Magnitude

Welcome back to another astronomy blog after a long week.   On the right there is an image of the sky taken with my SCT 12" telescope and my main CCD camera. The exposure was 10 seconds long in the neighborhood of the North Star, Polaris.   The first feature I would like to point out on this image is the light bleeding out from Polaris into the image.   Even though Polaris itself is not on the image you can still see the light streaking out from the left side of the image.  This is because Polaris has an apparent magnitude of 1.97.  That means that it is a very bright star. For comparison, a full Moon has an apparent magnitude of -12.7.   The more negative the magnitude the brighter the object is.

The image is also inverted in color space.  What this means is that the black dots are the stars.  It is sometimes easier to see them this way.  Now lets talk about the stars on the image.  The brightest of them is the one that look the largest on the image, slightly right off center.  This is HR 286 a 6.47 apparent magnitude main sequence star.  Now, try this out, find the dimmest star on the image.  There are several very dim stars on this image.   Their apparent magnitude is around 14.

But what does this apparent magnitude means?  It means that a star of 0 magnitude is 100 times brighter than a star of 6th magnitude.   In our previous example HR 286 is approximately 1000 times brighter than the dimmest star in the image.   That is a LOT brighter!

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