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TheTrashFactory
2014-Sep-19, 04:08 PM
Hello,

Can someone please tell me if astrometry using simple software like Salsa J () can be done for galaxies as well as stars? I.e. Can you measure the intensity of light from an entire galaxy as a whole or do you need the number of stars and an average star magnitude?

Thanks!

StupendousMan
2014-Sep-20, 12:51 AM
Your question doesn't quite make sense. You ask about "astrometry", which is the measurement of _positions_ of astronomical objects, but then your specific questions are all about "photometry", which is the measurement of _brightness_ of astronomical objects.

Can you please clarify? Do you have some specific goal in mind?

Hornblower
2014-Sep-20, 02:49 AM
The total amount of light from an extended source such as a galaxy can be determined by adding up the amounts from the individual pixels in a modern photodetector. If I am not mistaken a pixel will give the same response to a portion of a galaxy as it does to a single star that is focused on it and delivering the same amount of light.

We estimate the number of stars in a distant galaxy by measuring its total brightness, estimating its distance by whatever means the experts do so, and finding the absolute magnitude as a result. Once we have that, our knowledge of the average luminosity of a star from studies within our galaxy gives us a rough estimate of the number of stars.